Marshall tsl122 Bias problem

[sebagitara]

Hi all, i have a marshall tsl122 combo amp that i bought second handed. Amp is about 12 years old and my bias is constantly rising as the amp worms up but not on the both sides but only on the side closer to the power transformer. It starts ar about 73 mV on that side and it is constantly rising while the other side stays at about 75-76 mV. I have talked to a marshall authorised local tech about the bias drifting of this amp models but he said it was internet rubbish so i really dont know what to do. I do study electronics and i already made a 15 watt homemade tube amp but i really dont know where to go further with this marshall it was working fine and everything was great the suddenly when i was recording my ht fuse broke and i changed the tube that was malfunctioned and it still keep on rising the bias. Guys please help i really like this amp and i dont have enough money to buy another one. They are rather expensive here where i live so please help if you can. Thanks in advance )

Ok now i spotted that the other side is rising too, it went from 75mV to about 80 mV and the othr side closer to PT went to about 95-100 mV, just a reminder it was set to 75mV when i started the amp.

[Lee_ranaldo]

Hi all, i have a marshall tsl122 combo amp that i bought second handed. Amp is about 12 years old and my bias is constantly rising as the amp worms up but not on the both sides but only on the side closer to the power transformer. It starts ar about 73 mV on that side and it is constantly rising while the other side stays at about 75-76 mV. I have talked to a marshall authorised local tech about the bias drifting of this amp models but he said it was internet rubbish so i really dont know what to do. I do study electronics and i already made a 15 watt homemade tube amp but i really dont know where to go further with this marshall it was working fine and everything was great the suddenly when i was recording my ht fuse broke and i changed the tube that was malfunctioned and it still keep on rising the bias. Guys please help i really like this amp and i dont have enough money to buy another one. They are rather expensive here where i live so please help if you can. Thanks in advance )

Ok now i spotted that the other side is rising too, it went from 75mV to about 80 mV and the othr side closer to PT went to about 95-100 mV, just a reminder it was set to 75mV when i started the amp.

There is an issue with some marshall amps and their bias resistors. They tend to drift in value as soon as the chassis starts getting warm. Just replace the bias resistors shown in the schematic with a higher capacity ones (1/2 watt resistors) or metal film around the same power handling capacity. That solved the problem in most TSL amps that had that .

Also check if the bias voltage keeps rising with no power tubes installed after you change those bias resistors.

Let us know how it goes.

[sebagitara]

Bias circuit is connected to G2 of every power tube right, that's the middle grid right? Can you please tell me what which resistors are those, i'm having trouble finding them in the schematic. Thanks in advance.

[Lee_ranaldo]

Sure sebagitara

Replace these resistors

R68=33k
R77=10k
R67=220k
R69=220k

They are part of the bias circuit in your TSL amp., you can use anything sturdier (i will go with at least 1/2 watt or 1 watt to be sure). You can even use metal film resistors for more accuracy.
Some resistors installed in the factory dropped their values when they were exposed to heat (That's a NTC behaviour). Also they are famous for becoming faulty since the epoxy used in their motherboards became a bit conductive within years of use.

I would also check the bias variable resistors on the separate board, Make sure you clean them with some Contact cleaner to avoid dust inside the carbon track that might alter the values of the bias for each pair ot power tubes.

[sebagitara]

Sure sebagitara

Replace these resistors

R68=33k
R77=10k
R67=220k
R69=220k

They are part of the bias circuit in your TSL amp., you can use anything sturdier (i will go with at least 1/2 watt or 1 watt to be sure). You can even use metal film resistors for more accuracy.
Some resistors installed in the factory dropped their values when they were exposed to heat (That's a NTC behaviour). Also they are famous for becoming faulty since the epoxy used in their motherboards became a bit conductive within years of use.

I would also check the bias resistors on the separate board, Make sure you clean them with some Contact cleaner to avoid dust inside the carbon track that might alter the values of the bias for each pair ot power tubes.

Ok i'll replace those trimmer pots on the small board just to be sure that every measures are taken. Thank you very much for your help.

[Jazz P Bass]

As an aside, faulty power tubes can display the same symptom.

[acorkos]

this solved the bias drift on 2 TSL122 amps i worked on:

The Marshall TSL122 TSL100 thermal bias drift repair page

[sebagitara]

Sure sebagitara

Replace these resistors

R68=33k
R77=10k
R67=220k
R69=220k

They are part of the bias circuit in your TSL amp., you can use anything sturdier (i will go with at least 1/2 watt or 1 watt to be sure). You can even use metal film resistors for more accuracy.
Some resistors installed in the factory dropped their values when they were exposed to heat (That's a NTC behaviour). Also they are famous for becoming faulty since the epoxy used in their motherboards became a bit conductive within years of use.

I would also check the bias variable resistors on the separate board, Make sure you clean them with some Contact cleaner to avoid dust inside the carbon track that might alter the values of the bias for each pair ot power tubes.

I replaced those resistors and alos changed the value of grid stoppers to 5.6k because they are obviously wrong valued resistors but the bias still rises on the side closer to PT, and i tried to use different sets of tubes but it still happens, also when i try to measure the bias with no tubes inserted it's shows 0 which is normal with no tube inserted.

[oc disorder]

Hi Sebagitara firstly you may know some of this so just ignore it if you do !
I give Marshalls at least 5 mins on a low bias setting ie approx 25mA per tube with a signal into a dummy load for a warm up before I start to set the bias.
As the bias pots may be damaged or dusty try at a lower setting - I'm assuming its a 100w with 4 tubes so aim for approx 50mA per pair and see if it settles.
Also an amp like that has to have a load or speaker connected when its on.
If not there's a chance the output transformer may take a beating and end up with a shorted turn.
Check the screen grid 1k resistors too while you're there. The big oblong white ceramic type.
Check the output transformer too, between the center tap that goes to the power supply and the anodes on alternative sides. With the power off and amp discharged you should get a similar ohms reading from the two primary sides to the C.T.
Jazz P Bass's comment is worth remembering and sometimes new tubes can be faulty as well.
Also if one tube shorts (you mentioned it blowing the HT fuse) there could have been lack of bias on its pair which is now deteriorating rapidly but I guess you covered this with new tubes. In my case (in Aust.) the amps I've seen haven't required a new board or extensive modification.
Keep us posted on developments !

[km6xz]

The suggestion to check the bias voltage at the sockets with the tubes removed was not to check the cathode current, there should be any, it was to measure the DC voltage at the control grid with the tubes removed. Watching that to see if there is any drift is a way of eliminating the bias supply as source of the problem. If the bias divider resistors are of the same type, and have the same temperature coefficient, their values would drift in proportion so the ratio between their values will be relatively stable. If they are different types of resistors with different temperature coefficient, replace them with resistors of the same type.
You should be able to monitor all the voltages in the bias circuit over time/temperature to see what is drifting. New tubes drift this way also so set the bias low for a while and run it into loads to burn in the tubes. See if the drift rate changes or stabilizes.
A few measurements of the bias supply should pin-point whether the supply or the tubes are drifting, and to confirm the source of the drift, use a hair drier to raise the temperature of the suspected components to reproduce the drift from a cold start. Conversely, a shot of freeze mist on suspected resistors should drop the idle current back to that of a cold start, if in fact that resistor targeted is the culprit.
Good luck

[Jazz P Bass]

What version number is the main board?
If it is version 5, that is the bad board.

[sebagitara]

So i measured voltage on the resistor R77, i clipped one lead of the voltmeter on the ground and the other one on one side of the R77 and i got a reading of -46 V and i used a blow drier and blowed the hot air on to the board and the voltage dropped for only 1V and both the amp and the board were pretty much hot, so i guess it is stable on that resistor though i didnt cheked the other side of that resistor while the amp was hot whne i started it the other side had -57V, i'll try to eliminate other components further but i don't know should i suspect that the tubes were the source of the problem. could it be that tohse two bias trimmer resistors have NTC and can they cause the problem cause they do get quite how because of their position?

[sebagitara]

And now i measured the voltage on resistor R10 which i changed to 5K6 like it says in all the schematics while blowing hot air inside and i got a voltage drop of about 3V, the starting voltage was -40V and the final was -37V. All the measurements were done with no power tubes inserted.

[drewl]

What version number is the main board?
If it is version 5, that is the bad board.

Yup, and what acorkis said.

"Internet Rubbish" my arse!
I've had several with this problem and so have many others.
Why do you think they made a new replacement board for these?

I've had them with varying degrees of the bias drift.
Some were able to be repaired by replacing the bias resistors, and some I had to remove the bias trace from the board and jumper across a resistor as shown on the TSL repair page.

[Jazz P Bass]

I have also seen a few of these.
All but one I installed the new board.
The one that I repaired I did the 'router out the bias pin' thingy.
It solved the bias drift but still, there was an increasing amount of hum as the board heated up.
The customer was o/k with it, as he did not play it loud or very long.
(I still say it needed a new board).
Granted, not all of the rev 5 boards act up.
It is the ones that show up on the bench that most likely do.