Ashdown ABM500 EVO III Signal Lost In Preamp - No Schematic

[Relsom]

Hello all. First post here. I just came into a UK made ABM 500 EVO III. It was broken when I got it so anything is possible since a previous repair was attempted. The layout for the EVO III is different than the EVO II and so far I have not been able to locate a schematic for the APC056 Issue 2 2008 preamp. pcb. Believe me, multiple searches have been done. I have a request in at Ashdown for it.

The Good:
Power amp and transformer check good.
Test signal output at speaker good and clean when signal is applied at power amp section from preamp
+/- 15vdc is good
High Voltage to preamp tube is good.
VU meter responds nicely
I can trace a test signal with scope cleanly through the eq and at tuner jack.

The Bad:
A couple of sections of the common reference lands were flaked off on rear I/O pcb and on the preamp. No indication of anything burnt or arced. Looked more like poor adhesion of the copper to the glass. I have repaired those with wire.
The DI XLR hums when connected to am outboard headphone amp. No test signal present.
No test signals present at FX send or Line Out
No amplification of test signal when applied to the FX Return or Line In Jacks.

I have come to the conclusion that the preamp is the culprit but none of the schematics available in the web are helping out. I really don't want to go in and shotgun this if I can avoid it. Seeking help and advice.
Thanks,
....Chris

[Relsom]

Follow Up (With Schematics)

In case anyone is interested in helping me sort this out, I've finally gotten the schematics for the EVO III preamp. Fire away!
....Chris

[Relsom]

IC14 Pin 6 test signal is good. Pin 7 signal is very weak, even with PL-5 header cable disconnected. Signal still weak with the header plugged in. The FX return on pin 2 of that header is about the same. Signal is totally lost on the other side of R128 where it joins RV5 pin 3.

[The Dude]

Welcome to the place!
Can you list the DC voltages on IC14?

[Relsom]

Gladly! These were takren with reference to the common buss, no signal applied:
1 -13.42
2 -2.4
3 0
4 -14.85
5 0
6 -5.33
7 -13.44
8 +14.99
Thanks!

[Dave H]

These were takren with reference to the common buss, no signal applied:
1 -13.42
2 -2.4
3 0
4 -14.85
5 0
6 -5.33
7 -13.44
8 +14.99

Looks like IC14 is dead. With no signal 1,2,3,5,6,7 should all be at 0V.

[Relsom]

So, if I replace IC14, would a TL082 in that position be ok, at least temporarily? The Shack doesn't stock TL072.

[Dave H]

So, if I replace IC14, would a TL082 in that position be ok, at least temporarily?

Should be OK. TL082 is general purpose, TL072 is low noise.

[Relsom]

OK Replaced IC14. All voltages at 0 as stated, with exception of 4 & 8. Good strong signal on the FX send/Return header pins 1 & 2. Signal gets weak on the RV5 side of R128. Signal present but weak on IC14 Pin 1 and is variable with RV5. No signal at either the output to power amp or the line out. I'm thinking TR18 or TR19 but unclear. Mute switch doesn't do anything other than turn the LED on and off.

[The Dude]

Most likely one or both of those muting FET's is shorted. Check to see if either is shorted. You could also remove them temporarily or unsolder pin 3 of each to see if your output starts working. You can do one at a time to see which is shorted. They don't need to be there for the amp to work. They simply ground the signal when mute is engaged.

[Enzo]

TR18,19 are JFETs. By their nature, they are ON until turned off with a voltage on their gate. Do the gates of TR18,19 have a voltage on them? Does the mute switch turn that voltage on and off? Is ther a positive voltage at the collector of TR16? At R105? Does teh mute switch toggle that off and on?

You said R128 had a weak signal. Does that signal get stronger as you turn the output control DOWN? (Loaded by TR19.)

[Relsom]

This amp needs to be completely disassembled to do anything on the preamp board. I'll tear it down again tomorrow. In the mean time....the gates of TR18 & TR19 switch from .023v to .020v when activating the mute. Pin 3 of each are pretty much at 0v as well. TR16 collector is about +14.98v either way. R105/R114 junction goes from +14.31 to 0 when activating the mute. Signal on R128 near RV5 does indeed diminish as RV5 is rotated clockwise. R128 nearest the I/O header pin 2 remains steady.

[g1]

Like The Dude said, but you could clip (rather than unsolder) pin3 of TR18, TR19 mute Fet to test without disassembly.

[Enzo]

TR18,19 are wired in parallel, so one of them has a gate shorted to an end terminal. One side of each transistor. R105 toggles, so the switch is working and the TR16 voltage tells me the voltage supply for the TR18,19 gates is present. The short prevents those transistors from turning off, your amp is stuck in mute.

[Enzo]

To me, parts are a LOT cheaper than labor. I charged a dollar a minute labor rate. How many minutes would I take to decide if a 40 cent transistor was bad. So in my shop, if I was pretty sure TR18 or 19 were bad, I'd just snip both off at the legs and see if the amp worked now. I then can fix anything else going on, and leave the new TR18,19 for when I replace other parts.