Marshall tsl 100 problems

Marshall tsl 100 problems DEFAULT

Marshall JCM DSL/TSL Bias Drift Issue

The Marshall JCM has been released in several configurations: the Dual Super Lead and the Triple Super Lead. Both models offer W (watt) and 50W versions, available as a head or combo. The early versions of the JCM were prone to an issue that has been coined the &#;bias drift&#; issue. It&#;s been discussed at length on countless fan forums and since around mid the issue was resolved in manufacturing by Marshall.

The issue that causes the bias to drift is related to several factors: The type of components (resistors) that were used to populate the board and how they respond to heat, and the routing of power traces near sensitive components.

The Symptom:

The longer the amplifier is played, the more the heat builds up. As the components start to heat up, they drift causing audible volume changes that are caused by the components as they drift in value. This results in the voltages shifting and forcing the output tubes&#; bias point to shift off target, oftentimes causing the tubes to shut off. It has been known to cause them to take more current then they can handle and cause red plating, which can result in tube failure and potentially costly repairs if left untreated. Many times, if only played for a short amount of time, the issue is never noticed. It often takes well over an hour of solid playing at gig levels before the heat can build up enough to cause an issue. Once the amplifier is shut off and cools down, it returns to normal and the cycle repeats itself the next time it&#;s played.

The Solution:

If you have a Pre JCM, you have a couple options to remedy the drift issue. Marshall offers a full card replacement now that is corrected and populated with higher heat resistant components. The other option is to modify the card to perform correctly. This involves removing the card, destructively modifying some of the traces, and then repopulating a large number of components. Both options have proven to give the same successful results.

If your JCM is pre and you are experiencing issues, contact Desert Amplifier Repair for options on getting your amplifier serviced. If you are unsure of the production year of your amplifier, it should be part of the serial number which you will find located on the rear of the chassis.

Sours: https://desertamplifierrepair.com/project/marshall-jcm/

The Marshall TSL / TSL thermal bias drift repair page

Many TSL (TSL ) users have reported and complained that the amp is not "stable": When the amps becomes warm, the current from (one or more of) the ELtubes to the output-transformer is getting higher and higher until the amp collapses. The ELtubes are getting red hot inside. Then hopefully one fuse blows. The reason for that is a phenomenon, called the "thermal bias drift". JC Maillet has made a great site explaining and discussing the problem:
The Marshall TSL JCM Repair/Mods Page

There are several posts on the Marshall-forum about this problem. The conclusion in general is to buy a new motherboard from Marshall. This might be easy and affordable in the UK, but not in other countries in the world. In Germany for example the Marshall distributor wants Euros for that. Why spend Euros, if you can repair the beast quite easily?

Here is the analysis of the problem and here are the pictures, what to do. In brief:

- Yes, it is in the first place a problem of the faulty epoxy-material of the motherboard.

- Yes, it also is a problem of the cheap resistors in the bias circuit (maybe in the phase-splitter also).

- The layout of the heater-supply of the EL34s should be enhanced, as you work inside the amp anyway.

Marshall has admitted, that the motherboard epoxy material ages and becomes faulty during normal use. The motherboard starts to act like a NTC (a resistor with a Negative Temperature Coefficient), which means that the isolating capabilities of the board-material are reduced once the board becomes hot. In other words: Some megaohms occur, where they definitely should NOT be This is not a problem in most places of the motherboard. It happens mostly in the hottest spots of the motherboard. The hottest spots are the four areas where the tube sockets are soldered into the board. Let us check the situation there:

Assuming the motherboard had thermal stress over the years and had become faulty. Now there are megaohms between the pins of the tube sockets once the amp is getting hot. Let us find out first, where these bad megaohms are NOT a big problem:

It is obviously clear, that these megaohms do not or not much affect

- the area between pin 1 (Surpressor Grid - connected to GND) and pin 8 (Cathode - connected to GND), because these pins are connected anyway.

- the area between pin 1 (Surpressor Grid - connected to GND) and pin 2 (Heater), because there are Ohms only (the Ohms resistor that balances the AC voltage of the tube heaters to GND),

- the area between pin 8 and pin 7, because there are also Ohms only (the other resistor that balances the AC voltage of the tube heaters to GND)

- the area between pin 7 and 6, because pin 6 is not existing,

- the area between pin 3 and 4, because these pins are connected via the 1 Kiloohm screen-grid-resistor.

Conclusion: Between these above mentioned points additional megaohms will not do much trouble. So let us find out finally, where these megaohms are a BIG problem. There are only two problem-zones left:

- the area between pin 2 and 3, because the high voltage above Volts on pin 3 (anode) can affect the 6,3 V AC on pin 2. My tests show, that this area causes not the problem we are talking about.

- the area between pin 4 (high screen-grid-voltage above Volts DC) and pin 5 (the negative bias-voltage of approx. Volts DC).

Here - between pin 4 and pin 5 - is the big problem. Just imagine the horror that occurs to the negative bias-voltage, if there is any leakage going on from the high and positive screen grid voltage to the negative bias-grid-voltage. Even some megaohms between these two pins will affect the bias-voltage - and in these amps they do just that. The leakage causes the bias-voltage to drift to zero or even to a positive value. The effect is a tube-collaps. You can compare it with a water-pipe-burst in your kitchen.

The easy solution is made with a dremel-tool. The surrounding area of pin 5 is milled away. After this procedure you can easily desolder the remaining stuff at all pins 5 of the sockets. Pin 5 of each socket has now a perfect isolator - air.

the air is the best isolator

The pic shows that the cheap resistors in this area (R66 - R70) are replaced by Metal-Film-Resistors. You see that R66 and R70 are directly connected to the pins of the tube sockets. These resistors were k - a wrong value. The schematic (and my experience) tells that the right value for these resistors (bringing the sound to the output tubes and preventing FM-Radio interference) is 5k6 Ohms. The same thing has to be done with (not in the pic) the resistors R7 and R

Yes, I changed the resistors in the phase-splitter area also (no pic). But I assume that these resistors are not responsible for the thermal bias drift problem we try to solve here.

One word about the pic on the left side: You see a "bad-dremel-job". My dremel-tool got out of control and damaged the outer part of the hole around pin 5. The copper-strip next to the hole was damaged. I am glad it happend, because you can now replace this copper strip with a normal wire (seen completely in the last pic at the end of this text). The wire supplies GND (the ground of the negative bias-voltage). It is a good thing to have GND far away from the high voltages at pin 4 of the tube socket.

Ok. The thermal bias drift problem is fixed now.

Since you have the motherboard on your workbench anyway, you may want to improve the overall stability of the motherboard. I have seen TSL amps with a lot of damage at the edge of the motherboard - damage caused by the weak connectors supplying the heater-current. One EL34 sucks ca. 1,6 Amp. heating current (x 6,3 Volts = 10 Watts!) out of the power supply. So all four EL34s suck more than 6 Amperes. That is a lot. It needs to be observed.

Marshall has decided to save costs - that means here: to use copper-strips on the motherboard as the wire for the heater current. Look at the motherboard and let light shine on it and through it: You might see - like I did - that the copper strips have partly chanced their color a little bit. This discoloration happens mostly at the edge of the motherboard where the connectors (to the power-transformer) are located. It is obvious, that these points are "weak" and need a better connection to deal with more than 6 Amperes heater current.

The following two pictures show what I did. I scraped off a little bit of the green protective lacquer and soldered a little wire between the post and the copper-strip. This prevents further damage to the motherboard and gives the tubes the "food" that they need.

improving the connection at the soldering sideimproving the connection at the components side

To do this job on the component side of the motherboard you need to desolder the Ohms resistor R This resistor and the resistor R8 (also Ohms) balance the heater-filaments to GND. I decided not to solder these resistors back into their former place. I soldered them to the sockets at tube V5 - as seen in the pic below. reinstalling the symmetry-resistors R78 and R8

Finally - here is a pic of the complete repaired and modified motherboard. I reassembled the amp and gave it back to my friend Ralph. He uses this amp almost daily now - with a better sound (due to the 5k6 resistors mentioned) and without any thermal drift problems.

Keep rocking!

Fred

overall view of the repaired motherboard

Sours: http://www.hullerum.de/Marshall/TSLrepair.html
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fix-Marshall JCM Bias Drift

Marshall JCM DSL amp image

The early () Marshall JCM DSL50, DSL, TSL and TSL amps are often troubled by an unstable bias (drifting bias).

These are nice amps and I have one myself

This can be difficult to diagnose correctly, as it can take half an hour or more of testing and/or playing before this Marshall JCM bias drift shows up. . Due to this drift the bias current of the output valves  will gradually increase, and keep increasing, until the EL34 glow bright red/orange ( “red plating” and this can also be bad enough to blow the HT fuse. When this phase has been reached the EL34&#;s  are usually defective and are now ready to go in the trash can..

The Tung Sol El34B is a very good replacement. Here are our kits

For matched pairs &#; 50 watt amps: JJ EL34, Electro Harmonix EL34EH, Tung Sol El34B

For matched quads &#; watt amps :JJ EL34 II, Electro Harmonix EL34EH, Tung Sol El34B

The cause of the problem.

The primary cause of the Marshall JCM Bias Drift is to be found in the main printed circuit board (PCB). That is the big PCB where all the valves are.

If the code on this PCB is either JCM or JCM, then it is likely you already have been, or soon will be, confronted with this problem. Due to the heat produced by the tubes the PCB material becomes ever so slightly conducting. The specific layout of this PCB is such that some circuit tracks at the HT potential, (usually around V or so in these amps) run right next to the tracks for the negative grid (bias) voltage.

At one location, the distance between these two tracks is less than 1mm! The HT voltage will &#;pull up&#; the negative grid voltage, thus increasing the bias current. This is a runaway process and it will only stop due to the EL34’s liiting the current and getting cooked in the process

This issue is seen on the JCM and the JCM PCBs. These PCBs are also often fitted with the incorrect control grid resistors . 

On the board numbered JCM R55, R57, R62 & R

and board numbered , JCM R7, R10, R66, R70

 often have  kΩ resistors fitted while they should be .

It is worth checking your board, to see if it has the correct resistors were fitted during manufacture.  Wrong resistors will magnify the drifting bias problem. The later TL PCBs have not shown the drifting bias problems as yet, and they also have the correct control grid resistors fitted.

Marshall offers a new complete main circuit board as a &#;solution&#; for this problem. Not exactly a cheap option,  and we believe  that these new circuit boards are troubled by the same problem!  Not really surprising, as the PCB layout of the problem areas is the same as the old boards.

The solution.

 We have a kit which solves this Marshall JCM Bias Drift problem permanently.

Marshall DSL Bias board Image

This is a little kit which replaces the bias circuitry. It is small board that is mounted on the main pcb ( mountings supplied).

You do have to remove the main board in the amp. It is not for the inexperienced user. You will need to drill out one pin on each EL34 socket and be able to solder onto the main board. We supply the drill bit and all connectors with the kit. The kit is assembled by us here in the UK.

The kit comes with printed instructions and everything needed to fit the board.

Marshal DSL Bias bord close up image

You can order the kit on our website.  From only £ including delivery in the UK. ( price correct on 19th January,)

We export these kits all over the world and the website is multi-currency. At checkout you can select the delivery option for shipment to your country.

We will have stock of our UK assembled DSL Boards by 27th January, We are accepting back orders.

Click here  to order the kit for the DSL family of amps on this website.

 

 

 

 

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Sours: https://valvetubeguitaramps.com/marshall-jcmbias-drift/

** The Marshall TSL JCM Repair/Mods Page **

Photos/Text JC Maillet (c) //

A little background check doesn't hurt, go to Harmony Central and read what players have to say about these amps

I've worked on a bunch of JCM's now and I've seen two sets of problems crop up. In this one TSL I'm about to describe the mother board was found to be populated with negative Temp-coefficient resistors. This sent the bias voltage for a loop when the amp got hot - I'm sure it must be an accident at the factory or something. In a DSL I didn't find that specific problem but in all JCM's so far (TSL's, DSL's) I've found k grid block resistors on the power tubes instead of the customary 5k6 as indicated in all schematics - converting them to stock values definitely makes an audible change !

series TSL schematics



Hot Rat's Nest - The TSL Thermal Runaway Saga

This amplifier first came in with a pair of crisped-out power tubes on one side of its output stage and some nagging microphonic problem. Having never seen one of these before I assumed that the amp had no major design or manufacturing flaws and likely had faulty power tubes put in and/or the bias hadn't been adjusted properly previously

I biased in a new set of EL34's to around 35mA per tube (B+ just below vdc) found/fixed a cold solder around the B+ connector (W5 blue) on the mother-board rebiased, let the amp idle for a while as I normally do, double checked bias a few times more using the bias pins (which I usually don't use but at ohms they give a conservative reading anyway) played the a bit more and sent the amp back to the owner

The amp resurfaced a week later with a pair of tubes fried on the other side of the output stage, and the HT fuse again did not blow - something I couldn't figure out. All this prompted a serious in-depth study of the overall design.

I began to notice strange things happening while the amp was idling - basically a form of runaway bias drift was taking place over the course of an hour or so, something I hadn't seen before. The mother-board showed severe temperature stress on all four power tube sockets which are directly mounted to the mother board - a potential source of untold and incurable problems on the one hand. Had the amp been under warranty I would have turned back at this point and had the customer send his amp back to the factory or ask for an exchange. But, that wasn't the case so trudging on

The bias circuit was obviously a first place to look for leading clues in the mishaps. Being of the dual type (not a good idea to begin with) in which two trimpots mounted on an external board connected to the mother board via a multi-wire connector (another not-great idea) adjust bias on each pair/side of the output stage - this off-board arrangement invites lots of trouble. My original beef with general dual-biasing is outlined in my Tube Amp book (IF&MTA), but the beef I have with mutually interactive versions of that idea is way worse.

Siding with reason and caution I decided to hard wire a standard/classic single 10k bias pot directly to the mother board (all other bias circuit component values remain the same). I went to single control because having two degrees of adustability made it difficult to isolate the source of "long term" bias drifting I was observing on the bench - basically I wasn't sure if grid current draw on one pair of power tubes was affecting the bias on the other side of the output stage. Installing it as such elliminates that 2-way variable and, more importantly, renders the bias circuit as a whole more immune to circuit "opens" via vibrationary action and faulty/intermittent connectors which I also had to rule out as a potentially likely possibility for frying tubes. Who knows about these connectors, I guess they're ok now that I've tugged at them while the amp is working with no apparent sign of weakness - still, the fact that the bias lines are going through two sets of connectors before making it to the main circuit is pretty balsy, almost contradictory to some other overkill safety features in the amp.

Scraping the enamel off the traces corresponding to pins 2 and 6 on the bias board connector and mounting a support connector to connect both traces together into a common bias feed to the power tubes. This doubles the total bias feed filter cap value to uF which lowers output stage hum at the expense of slightly longer charging times - not worth fussing over.

The new bias pot (10k - 1/4watt) is mounted via two standoffs on pins 1 and 7 of the bias board connector. The old bias pots are removed from the board and only the bias measuring pins remain active in the circuit "just in case" they're needed in the future by someone. Or I'll remove the bias board altogether - I don't use the bias pins for biasing anyway. I reconnected everything together and was ready to do some more testing.


one may wonder if the location for the new bias pot is optimal - personally I don't like the idea of snaking lines around in a high gain amplifier potential for added radiation-prone interference increases rapidly with distance so the safest thing is to instal the trimmer right over the connector, in effect shortening the wiring distances (always a good thing if anything) I'm not worried about heat too much here since the circuit is shielded by a metalplate and the board lies well below the tube sockets about access, a long-neck screw driver will do the trick but I agree it's a PITA to have the control on the opposite side to tranny access but that's how it is for now based on the other considerations mentioned before suggestions are welcomed


Note-book Diagram of my TSL Bias Circuit Mod

The following diagram was drawn by a buddy while I'm shouting out label tags at him from the other side of the bench I know it's ugly and shouldn't be posted but it's there just in case it might help someone who forgot to label their wires maybe someone will send in a better diagram for me to post.


Wrong Value resistors + Thermally Weird Components on Motherboard

This is where the weirdness begins. Having gone back to the usual tranny-shunting for measuring plate currents I discovered that a matched quad of tubes would kreep to arbitrarily high currents over a period of an hour or more this occured regardless of initial bias setting and then a spread between sides would develop as the increase took place. Two things I noticed were of importance here, (i) if I swapped tubes the spread tendency remained the same socket-wise, pointing to the circuit and not the tubes, and (ii) if I turned off the amp once it was well warmed up and measured the bias feed resistor values I found that one measured around k instead of its nominal k and the other laid around k something funny going on (!)

I pulled both resistors out and tested them under a blow-drier and sure enough both these critters dropped their values when heated. that's Negative TempCo shit. Could it be that all resistors on the mother-board had negative TempCo's ??? I wondered how they would affect the bias circuit, and other things not to mention To answer part of that question I replaced all resistors on the mother board that were part of the bias circuit (R68=33k, R77=10k, R67=k, R69=k) by 1watt film resistors - they hardly budge in value when placed over the flame of a lighter. This made a big difference - the tendency towards drifting was muchly slowed down and at some point it peaked to a semi-stable value unlike before. The only thing to exlain the residual bias drift and divergence towards a semi-stable value was the burnt sockets and oxidized contact with the mother-board. My advice here is to give the amp about an hour to settle following this mod to make % sure all is fine btw, as reference, I found the TSL sounded good with these GT-JJ's tubes biased quite colder than what I would mormally shoot for, namely around 54mA per side total by tranny shunt.

The other thing I noticed was grid blockers (R7, R10, R66, R70) lying at k instead of the usual 5k6 this finally explained the choked clean sound and a muddy/grainy power distortion tone. By replacing them with 6k8's (closest I had) the classic Marshall sound came out more, it was slightly more alive and clearer in clean mode and the distortion tones were less grainy - to my ears anyway.


Scoping the High-Tension Circuit

One good thing about 1 ohm resistors in the cathode circuits is in giving the ability to scope the pentode plate currents indirectly through them. A mV peak cathode voltage swing translates into a total mA peak current sink per push-pull side, this justifies the standard use of a 1A fast-blo fuse in the B+ circuit of watt EL34 amps, right in series with the output transformer feed bring a scope to your gig and hook up :)~

I think this mother board could have suffered damage from several previous harsh malfunctions around the power tube sockets and I guess it's only a matter of time to see if these corrections will allow the amp to hold up over time (unlikely I'm afraid - that's PCB based amps for ya). Not a job I could guarantee, so I warned the customer about it and he was ok about it.



DSL check-up

This DSL mother board had k grid blockers, I piggy-back them with 6k8's as I did in the TSL and the bias circuit resistors were changed to 1% metal film yes, I'm just about to change that 10k bias circuit resistor in the back the bias resistors on this amp also exhibited NTC behavior which again was confirmed OFF-BOARD using a multimeter and w hair drier



Note: A few guys I talked with were thinking of drawing up replacement boards - for anyone nuts enough to tackle this re-design I would suggest doing a board with a cut-out around the power tube section and mount the power tube sockets to a metal bracket instead of the PCB board - I got an email from a dude who says he did it successfully the only resistors I think need changing are in the bias circuit and the grid-blockers - they should be 5k6 and not k all other resistors, incl. screen block power resistors, I wouldn't touch unless there's a reason

Much appreciated Alan !

Victor Soto's Follow Up


More


updates 07/09


The Marshall TSL TSL thermal bias drift repair page

Many TSL (TSL ) users have reported and complained that the amp is not "stable": When the amps becomes warm, the current from (one or more of) the ELtubes to the output-transformer is getting higher and higher until the amp collapses. The ELtubes are getting red hot inside. Then hopefully one fuse blows. The reason for that is a phenomenon, called the "thermal bias drift". JC Maillet has made a great site explaining and discussing the problem:
The Marshall TSL JCM Repair/Mods Page

There are several posts on the Marshall-forum about this problem. The conclusion in general is to buy a new motherboard from Marshall. This might be easy and affordable in the UK, but not in other countries in the world. In Germany for example the Marshall distributor wants Euros for that. Why spend Euros, if you can repair the beast quite easily?

Here is the analysis of the problem and here are the pictures, what to do. In brief:

- Yes, it is in the first place a problem of the faulty epoxy-material of the motherboard.

- Yes, it also is a problem of the cheap resistors in the bias circuit (maybe in the phase-splitter also).

- The layout of the heater-supply of the EL34s should be enhanced, as you work inside the amp anyway.

Marshall has admitted, that the motherboard epoxy material ages and becomes faulty during normal use. The motherboard starts to act like a NTC (a resistor with a Negative Temperature Coefficient), which means that the isolating capabilities of the board-material are reduced once the board becomes hot. In other words: Some megaohms occur, where they definitely should NOT be This is not a problem in most places of the motherboard. It happens mostly in the hottest spots of the motherboard. The hottest spots are the four areas where the tube sockets are soldered into the board. Let us check the situation there:

Assuming the motherboard had thermal stress over the years and had become faulty. Now there are megaohms between the pins of the tube sockets once the amp is getting hot. Let us find out first, where these bad megaohms are NOT a big problem:

It is obviously clear, that these megaohms do not or not much affect

- the area between pin 1 (Surpressor Grid - connected to GND) and pin 8 (Cathode - connected to GND), because these pins are connected anyway.

- the area between pin 1 (Surpressor Grid - connected to GND) and pin 2 (Heater), because there are Ohms only (the Ohms resistor that balances the AC voltage of the tube heaters to GND),

- the area between pin 8 and pin 7, because there are also Ohms only (the other resistor that balances the AC voltage of the tube heaters to GND)

- the area between pin 7 and 6, because pin 6 is not existing,

- the area between pin 3 and 4, because these pins are connected via the 1 Kiloohm screen-grid-resistor.

Conclusion: Between these above mentioned points additional megaohms will not do much trouble. So let us find out finally, where these megaohms are a BIG problem. There are only two problem-zones left:

- the area between pin 2 and 3, because the high voltage above Volts on pin 3 (anode) can affect the 6,3 V AC on pin 2. My tests show, that this area causes not the problem we are talking about.

- the area between pin 4 (high screen-grid-voltage above Volts DC) and pin 5 (the negative bias-voltage of approx. Volts DC).

Here - between pin 4 and pin 5 - is the big problem. Just imagine the horror that occurs to the negative bias-voltage, if there is any leakage going on from the high and positive screen grid voltage to the negative bias-grid-voltage. Even some megaohms between these two pins will affect the bias-voltage - and in these amps they do just that. The leakage causes the bias-voltage to drift to zero or even to a positive value. The effect is a tube-collaps. You can compare it with a water-pipe-burst in your kitchen.

The easy solution is made with a dremel-tool. The surrounding area of pin 5 is milled away. After this procedure you can easily desolder the remaining stuff at all pins 5 of the sockets. Pin 5 of each socket has now a perfect isolator - air.

the air is the best isolator

The pic shows that the cheap resistors in this area (R66 - R70) are replaced by Metal-Film-Resistors. You see that R66 and R70 are directly connected to the pins of the tube sockets. These resistors were k - a wrong value. The schematic (and my experience) tells that the right value for these resistors (bringing the sound to the output tubes and preventing FM-Radio interference) is 5k6 Ohms. The same thing has to be done with (not in the pic) the resistors R7 and R

Yes, I changed the resistors in the phase-splitter area also (no pic). But I assume that these resistors are not responsible for the thermal bias drift problem we try to solve here.

One word about the pic on the left side: You see a "bad-dremel-job". My dremel-tool got out of control and damaged the outer part of the hole around pin 5. The copper-strip next to the hole was damaged. I am glad it happend, because you can now replace this copper strip with a normal wire (seen completely in the last pic at the end of this text). The wire supplies GND (the ground of the negative bias-voltage). It is a good thing to have GND far away from the high voltages at pin 4 of the tube socket.

Ok. The thermal bias drift problem is fixed now.

Since you have the motherboard on your workbench anyway, you may want to improve the overall stability of the motherboard. I have seen TSL amps with a lot of damage at the edge of the motherboard - damage caused by the weak connectors supplying the heater-current. One EL34 sucks ca. 1,6 Amp. heating current (x 6,3 Volts = 10 Watts!) out of the power supply. So all four EL34s suck more than 6 Amperes. That is a lot. It needs to be observed.

Marshall has decided to save costs - that means here: to use copper-strips on the motherboard as the wire for the heater current. Look at the motherboard and let light shine on it and through it: You might see - like I did - that the copper strips have partly chanced their color a little bit. This discoloration happens mostly at the edge of the motherboard where the connectors (to the power-transformer) are located. It is obvious, that these points are "weak" and need a better connection to deal with more than 6 Amperes heater current.

The following two pictures show what I did. I scraped off a little bit of the green protective lacquer and soldered a little wire between the post and the copper-strip. This prevents further damage to the motherboard and gives the tubes the "food" that they need.

improving the connection at the soldering sideimproving the connection at the components side

To do this job on the component side of the motherboard you need to desolder the Ohms resistor R This resistor and the resistor R8 (also Ohms) balance the heater-filaments to GND. I decided not to solder these resistors back into their former place. I soldered them to the sockets at tube V5 - as seen in the pic below. reinstalling the symmetry-resistors R78 and R8

Finally - here is a pic of the complete repaired and modified motherboard. I reassembled the amp and gave it back to my friend Ralph. He uses this amp almost daily now - with a better sound (due to the 5k6 resistors mentioned) and without any thermal drift problems.

Keep rocking!

Fred

overall view of the repaired motherboard



Sours: http://www.lynx.net/~jc/TSLhtml

100 problems tsl marshall

Vaughan B. is a loyal regular customer on the NSW Central Coast since way back when he was using a Rocktron rig. In more recent times he updated to a Marshall JCM TSL head, which we also have been servicing for him. A couple of months ago the amp &#;just stopped&#; for no apparent reason, so back to the workshop it came.

We already had a few TSL&#;s in the workshop with similar symptoms, so we had a pretty good idea what might be wrong. Preliminary examination of Vaughan&#;s amp revealed that the 2 amp slow-blow mains fuse was blown and one pair of the existing EL34 output valves had experienced some kind of melt-down. In other words they had begun to draw more & more current, increasing out of control until finally the fuse blew.

Unfortunately, the fuse didn&#;t blow quickly enough to save the amp from further (expensive) damage, as further testing revealed that the primary winding of the Dagnall power transformer had gone S/C (short-circuit). So far we will have to replace the quad of EL34 output valves, plus the power transformer. What could have caused such a scenario ? From past experiences with this specific model we have learned (the hard way) that some of the main boards have left the factory with inappropriate components loaded, and/or some boards have developed leakage paths which significantly upset the normal DC voltage conditions.

Before you all start panicking, this scenario does not apply to every single TSL, just some of them, and they are likely to have been manufactured earlier in the life cycle of this model, rather than later. Looking at the photo to the right, you can see that the main board is the one that all the valves (tubes) plug into, ie the valve sockets are p.c.b. mounted. Most of the valve circuitry is mounted on the board, in particular all of the phase-inverter and power amp valve circuitry.

The problem with Vaughan&#;s board is that a leakage path developed between the high voltage tracks & pads and the bias supply to one pair of EL34&#;s in particular. We have previously proven this to be true by powering up a faulty amp with the EL34&#;s removed, and a DVM (digital voltmeter) connected between pin 5 of one of the EL34 sockets & earth (ground). Over a period of time it was easy to observe the bias voltage to one pair of the EL34 sockets gradually drop quite significantly. This would result in a runaway condition for that pair of valves, just drawing more & more current.

You can see from the photo to the right that components/tracks/pads are packed tightly together in this design, regardless of the design rules concerning high voltage on p.c.b.&#;s. Nevertheless, in the numerous TSL amps where we have replaced the main board altogether, the repair has been successful, so the latest issue of this board appears to have had the problem corrected. But &#; back to Vaughan&#;s amp: we therefore ordered in a new power transformer, a new main board, and a matched quad of the very nice Mullard (reissue) EL34&#;s from New Sensor.

To sum up, we replaced the transformer, board & output valves in that order, with a successful rebias of the EL34&#;s and conducted the usual power output tests (30V/8 ohms = watts), burn-in tests & electrical safety test. The final cost of this major repair was significant &#; almost the resale value of this amp in the current economic climate, but at least Vaughan now has a reliable TSL to gig with. Vaughan B. is a very decent guy, and was happy to pay the estimated cost of repairs in full prior to commencement, so that we wouldn&#;t be out of pocket. We truly appreciate that !

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Sours: https://ivanrichards.com.au//07/01/vaughans-jcmtslmajor-repair/
Marshall TSL100 head. Noise problems!!!!

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