Marshall TSL122 DIY BIAS Probe Snafu

[Steve A.]

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.
[ATTACH=CONFIG]40326[/ATTACH]

Let me get out my magnifying glass!



Steve Ahola

P.S. I looked through the 6 pages of schematics last night but found no hide nor hair of the bias board...

[Dave H]

P.S. I looked through the 6 pages of schematics last night but found no hide nor hair of the bias board...

Same here, I looked for the mating half of the CON1 7 pin and found nearly every page has a CON1 with a different number of pins! I've no idea how it all connects together.

[Jazz P Bass]

Con 1 is located at the rear of the amp.
Right where the bias board sits.

On the schematic, it's on page 1.

[mcmailman]

Took the metal cover off of the main board and suspended it using non conductive material (chop sticks) so I could measure voltages from the underside of board.

With power tubes removed and amp on standby I am getting the following voltages:

At The Bias Board Connector:
Pin #1: I measure -52.8 vdc
Pin #2 & #6 : I measure -48 vdc and 48.4 vdc and at pin 5 of EL34's I measure -39.4 vdc
Pin #3 & #5: I measure 1.5 ohms
Pin #4: Verified chassis ground
Pin #7: I measure ~ 23k ohms which is what the resister is marked on this new board (5 Band resister is red, red, black, red, brown)

Main Board:
At C42 I measure -58.9 vdc
At R77 I measure -52.3 vdc

Also, I did what Mike suggested and checked BR1 and smoothing capacitors (C1 C2 C3 C4) which all tested good. I kind of knew they were fine as all my 12Ax7's where lighting.

Where to go next? Again, very much appreciate the fantastic help this forum has given me so far.