04/09/2011, Brendan Donnelly ...

Hi JC,

I stumbled across your forum years ago, and since then I have developed a repair that seems to work quiet well. I have repaired countless TSL100's and DSL 100's that suffer from the bias drift cook valve destruction problem. As all your other emails state there are several issues.

Cutting to the chase, the repair that seems to work for me is. To remove all power tube sockets. Drill out PIN 5 on the PCB (control grid) so it totally clears the PCB/pin of the tube socket when it is reinstalled. Then point to point the complete bias/and coupling section with new resistors and correct values. I usually start from the coupling caps from the driver tube, and keep totally free from the PCB. There are many ways of mounting the components of the board, (tab mounts etc). This mod ensures a clean path to the control grid and avoids the voltage drift problems. I point to point with 1 watt resistors.

I have also had other failures due to tracking. Again you can follow the same idea. (I had the heaters tracking on one repair, so again I point to point wired them)

In the worst case I believe I could cut the whole power tube section out of the PCB and point to point the whole power section. But so far I have been lucky and have not had to do this. I live in Australia and the price for a complete PCB was over $350 dollars. I have replaced with new PCB's before for customers who have requested this, but many of them are not so wealthy musicians, so the mod is a little cheaper for them.

I had to develop these mods as I was getting so many of these amps in for repair. I have none come back as yet. So I believe that the fix seems to work.

hope this helps other techs. Thanks for the great forum which helped me on the path to get things sorted.

regards, Brendan

\\\ update: Fall 2011 ///

Fred Skenderbeg's Solution ...

I recently did a search on the net to see if anything new had surfaced and found a thread at the Marshall Amp forum pointing to a most ellegant solution that I had missed (dated 12/04/2010) :

!! -->

Note: I'm reproducing Fred's page verbatim (without permission) just in case his page were to disappear one day ...

(apologies in advance, I don't mean to step on any toes here ...)

The Marshall TSL122 TSL100 thermal bias drift repair page

Many TSL122 (TSL 100) users have reported and complained that the amp is not "stable": When the amps becomes warm, the current from (one or more of) the EL34-tubes to the output-transformer is getting higher and higher until the amp collapses. The EL34-tubes 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 TSL122 JCM2000 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 200 Euros for that. Why spend 200 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 100 Ohms only (the 100 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 100 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 400 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 400 Volts DC) and pin 5 (the negative bias-voltage of approx. -40 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 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 220k - 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 R10.

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 TSL122 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.

To do this job on the component side of the motherboard you need to desolder the 100 Ohms resistor R78. This resistor and the resistor R8 (also 100 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.

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

\\\ update: Spring 2013 ///

here's Glenn Martin's take on things ...!!

(*!*)

I've worked on a few of these monsters. The major issue you are missing (and I have missed for years) is not on the power amp board. It is on the overdrive channel front control Board. (dwg TL1--612-02.dgm). The capacitors in the circuit in the upper right hand of this drawing (C36 & 37) are connected via Con 20 to Con 12 on the power amp board and the right sides of resistors 4 and 5 on the power amp board. These capacitors are 100n at 63 volts. They invariably cannot handle the onrush voltage from V4, eventually becoming leaky and pull down the bias on the output tubes. This initiates all the troubles in these units. When these are replaced with 250 volt capacitors, the bias stabilizes under all temperature conditions, and the unit will function as advertised (after all the major repairs in your article have been accomplished). ALSO. I replace C46 on the power amp board with a 9 inch length of open terminated RG58. This has about 25 pf of capacitance, but can handle the voltage and heat in this unit much better than that small capacitor.

... (ed. I think he means TL01_61_02.pdf)

Hi JC. Im just using the TSL 122 schematics found on the net. You'll notice that I referred to the Schematics by the drawing numbers found on the diagrams, and by their names. I dont know why , but I suspect that the problem isnt really solved by the mother board swap, and that most people just give up. There is No way the problem can be solved without replacing these capacitors. take a serious look and it will become apparent to you too. Its just Basic electronics. Once they begin to leak, they will draw down the bias on whatever tube pair they are connected to. Follow the instructions I gave you. I also change the minimum bias set resistor (R69 on drawing TL10-60-02). from 22k to 39 k. This increases the bias on the tubes and decreases the bias current. I set bias to 70ma per pair. Since R69 is now larger , as the internal temperature of th eamp increases, the resistance of this resistor increases positively IN GREATER PROPORTION TO THE OTHER RESISTORS IN THE BIAS NETWORK. This causes the bias to increase as the internal temperature increases, and the bias current to decrease slightly, a much safer situation. Try it and see.

Glenn Martin Martek Mississippi Electronic Repair Biloxi, MS

... thanks Glenn !!!

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