Marshall TSL122 DIY BIAS Probe Snafu

[mcmailman]

I have a Marshall TSL122 that had a hacked up, burnt up main board so I ordered a new one. I got it installed and it worked pretty good but I wanted to check and set the BIAS correctly. I made a pair of octal bias probes with a 1 ohm resister on pin 8. Problem is that somehow put the resister connection on Pin 1 instead of 8 which was bad enough, but then it turns out that on one of the probes the resister or pin legs shorted out Pin 1 (Grid #3) and Pin 2 (Heater). It blew the main fuse right away (not the HT fuse) and kept blowing fuses until I removed the BIAS board. Got a different BIAS board which stopped it from blowing fuses, but now it makes a loud humming sound and all the tubes go in to thermal overload within 60 seconds...on there way to getting red hot, before I turned the overheated beast off. I did manage to get a quick BIAS measurement and it was reading over 350mv.

I've done a thorough visual inspection and nothing looks fried or unusual. Where do I start to troubleshoot this issue? Is there a list somewhere of even ballpark voltages for key areas to check that I should reference? With the tubes pulled, will I be able to at least get a sense of direction by doing targeted voltage checks? Any help would be greatly appreciated...I am a novice/hack at tube amp repair but have measured and set BIAS voltage before on the Marshall TSL122 but I am in deep keeshee now and really need some guidance. I've attached a couple of pictures of the main board model number in case that is of interest.

Thanks in advance to everyone!!

Carl

[Mick Bailey]

With the power tubes removed check for bias voltage on pin 5. You should see a negative voltage of around -38v to -48v or thereabouts. Depends where your bias pots are set.Check each power tube socket. If this is missing your tubes will pull a heavy current and hum loudly due to the excessive power supply ripple.

A short from HT to heater will usually fry the 100R heater ground-reference resistors R8 and R78. This will also cause hum.

Also, check the preamp DC heater supply for AC. Often the bridge rectifier BR1 will short due to excess voltage. The associated smoothing capacitors (C1 C2 C3 C4) can sometimes fail at the same time. Again, another source of hum.

[Dave H]

The first thing I'd do is (with the power tubes removed) measure the bias voltage on pin 5 of the octal sockets.

Edit: Mick said it better

[mcmailman]

Can't tell you how appreciative I am for you giving me some guidance. I am going to troubleshoot this a little at a time as I have a funky work schedule right now. This is what I found:

With power tubes pulled out and amp turned on here are some readings I got so far:

Preamp Tube Heaters:

V1 and V2 - Pins 4, 5 and 9 = .076 volts AC

V3 and V4 - Pins 4, 5 and 9 = 3.5 volts AC on

(All the pre-amp tubes are lit up though)

Power Tubes: V5, V6 V7 and V5 show zero volts at pin 5 which doesn't look good.

I have a good schematic to work on...ideas for next steps would be appreciated!

Thanks so much!

Carl

[g1]

Can you post the schematic you are using?
V1 & V2 heaters are probably running on DC.

[Dave H]

V1 & V2 heaters are probably running on DC.

Correct, if these are the right schematics.

[mcmailman]

The TST100/122 Schematics I am using

Can you post the schematic you are using?
V1 & V2 heaters are probably running on DC.

These look like the same/correct schematics. Let me know what you think.

[Mick Bailey]

You need to establish whether the bias board is getting a negative supply. Check for voltage at the junction of C42/R77. If you have bias voltage then then check the other side of R77 and trace the voltage through the connector to the bias board. The return side is R67/R69.

If there's no voltage present at C42/R77 then the fault lies with the bias supply circuit.

[g1]

to the bias board.

Am I blind, or is this mythical board not shown on the schematics?

[mcmailman]

No sir, you are not blind. They documented everything but the BIAS board. When I received this amp the BIAS board was hacked and had only one pot on it. I made my own out of heavy duty CEM-1 Prototyping Silver tinned prototyping board I found at a local electronics recycle/re-sell place called Surplus Gizmos. I copied the trace patterns from the old board by holding it up to the light.

[Mick Bailey]

See post #2 here for schematic;

http://music-electronics-forum.com/t14863/

[g1]

Thanks Mick, I'll post it here for posterity. Guess it would have been way to hard to include it on the main board schematic with dotted lines showing it was on a separate board.

[mcmailman]

No, still not working. I am going to pull the main boards mounting cover tonight, gingerly isolate it so it doesn't short out, then get some measurements from the under side: i.e. see if the caps C1 C2 C3 C4 and the bridge rectifier BR1 are working.

[mcmailman]

Thanks for posting this...helpful to have.

[Jazz P Bass]

The bias voltage is derived from D7, 8, 9 & 10 which gets it's voltage from W11, 12 & 13 /Volts ac from the power transformer).

The bias board connector:
The bias voltage must go to Pin #1.
Pin #2 & #6 go to the EL34/ pair grids. (with the bias voltage ridibg on them)
Pin #3 & #5 are the 1 ohm sense resistors to ground.
Pin #4 is chassis ground.
Pin #7 is the bias load resistor. It should read 33K to ground.

Verify all of this.