Silent modded Laney troubleshooting

[Matthews Guitars]

So for the first time since my good friend/amp guru died, one of his creations has come to roost upon my workbench in need of repair.

It's a modded Laney AOR, originally a 100 watter, converted to 50 watts, with a lot of other mods done as well. Basically as I understand it, it's been converted to a circuit similar to that of a 2204 but with an added tube and two added gain stages. I can't verify how much was done to the in-chassis wiring, but the PC board is close to stock. (I've got spare AOR PC boards taken from other amps in the past to use as a reference.)

Symptom is no audio from the guitar inputs, but jacking into the FX loop input shows that the power amp is working.

I'm still figuring out the actual circuit topology. I have no documentation (that I have found yet) on this modified amp. I'm learning as I go. That includes digging in the filing cabinet that has my friend's stored documentation which SHOULD include the docs on this amp.

To do rapid and effective signal path tracing, I use an audio generator and an oscilloscope. This has caused me to find that on V2B, there is signal input on the grid but no signal output on the plate or on the cathode bypass capacitor either. . Plate voltage is healthy at around 230 volts. Tube socket is good, known good tubes have been checked in the socket.

I'm thinking that the output coupling cap on the output of the cathode follower has failed. I'll be testing this diagnosis tomorrow.

The weirdest thing about the mod is that the two stages that comprise the cathode follower and gain stage span across two separate tube sockets. The plate-to-grid jumper wire runs from socket V3 to socket V4.

 

[Tatzmann]

Can you post some pictures of
the unit? Since it has been modded
the good introductionary description
wont help much im afraid. I'd like to help.

Is it an 8 knob unit or a 6? The year of
the built is crucial too, they changed
things couple of times. The schematics
are also not 100% conclusive at all
times.

Series 1 (Pro-Tube AOR) or Series 2?
Even within series multiple changes
occured which were not always
documented in schematics.

 

[South Park]

That jumper wire is a bypass that did not work out that amp has to much gain with that extra tube . You might be able to bypass the stages that don’t work

 

[Tatzmann]

Wait a minute.

If this is a 8 Knob Series 1 Pro-Tube AOR,
wheater modded or not, there is a couple of
things that should be understood by the
operator.

Guitar into low input: Master v. up to 4 atleast
Now pre-amp 2 Volume up full.
Eqs all at 5.

Sound or not?

Guitar into high input without boostchannel:

Master v. up to 2

Preamp 2 volume up to atleast 6
Preamp 1 volume up to atleast 6

All eqs 5.

Do you get sound?

Guitar high input with boost (red light on):

Master volume 2

Preamp 1 lev full
Preamp 2 vol atleast 6
Preamp 1 vol atleast 6

Do you get sound?

 

[Matthews Guitars]

It's at my shop and I'm at home, 20 miles away. I thoroughly tested it and it makes no sound via the front panel inputs, but using the fx return jack I can hear it run so the power amp is OK.

It's an 8 knob version.

I'm reasonably confident that it's a coupling cap issue. I'll know when I get back on it tomorrow. All tubes are OK, sockets are good, I've got continuity, no visibly damaged parts, no open resistors as of yet.

 

[Tatzmann]

Ok, the 8 knob series 1 can be confusing at
first so i wanted this out of the way.

I'll watch out for your tomorrow findings.

 

[Matthews Guitars]

Got it licked. Found a shorted cathode bypass capacitor and also some issues with some very crusty power tube pins. The sockets are fine but the pins needed a good cleaning.

 

[Matthews Guitars]

What was interesting is that the output was very low, and when I went to check on the signal input lines to the power tubes (green and orange wires on a Marshall), I found that one had a signal, and the other had none. The badly out of balance signal was the cause of the very low output. It was a combination of both corroded tube pins AND the shorted cathode capacitor that was silencing the amp. I also tend to think that there was another bad connection that I wasn't fully aware of, but which got cleaned up during my troubleshooting and cleaning process.

Sometimes you end up fixing something and you aren't 100 percent sure exactly how you fixed it. I admit, this is one of those times.


And another amp came in for me to work on. A Crate Stealth 50 that had been messed with. The power switch wasn't even connected properly. I had to figure out the correct wiring. But as I'm inspecting the amp, it wasn't too hard to figure out what else was wrong with it.

Some yahoo had put three EL84s in, in place of the 12AX7 preamp tubes! And the power tube sockets were full of JJ 6V6 tubes!

Obviously an EL34 is not going to work in place of a 12AX7. And I would not expect a 6V6 to survive for very long in an EL34 configured output stage.

After putting the right tubes in, the amp just worked. Thankfully. The customer will be thrilled that it wasn't a lot of labor to set it right.

 

[Tatzmann]

So does the modded Aor sound good?

I guess its too far gone to put back to
stock?!?

Or leave as is because your buddy modded
it?

 

[Matthews Guitars]

The owner/customer likes it as it is. It does sound very good. I'd see no reason to return it to stock.

 

[Matthews Guitars]

I got done with that Stealth 50 and discovered the reverb sounded HORRIBLE. I went to replace the reverb tank,
and guess what? At the factory, whomever had installed the reverb tank had never bothered to remove the foam pad that
keeps the springs from bouncing around during shipping! The pad that has a label on it that clearly states "REMOVE THIS PAD BEFORE INSTALLING REVERB TANK."

That has to be part of why Crate is gone. With quality control like that, who'd be surprised they failed?

And today I fixed a rebuilt Danelectro external reverb unit, model 9100, I think. That should bring me a few hundred bucks.

 

[Matthews Guitars]

Today's repair success: Recapping and testing an Ampeg V2 that was in the collection I inherited.

I'd never played with one of these before. I didn't have high expectations. I was very surprised and pleased at its tonal range and the sounds you can get out of it. I wasn't sure what I was expecting but it's well above that!

It'll be an easy sell when I take it to the guitar expo in Orlando, FL at the end of this month. I just need someone to play it and people will pay attention to the tones it gets. It'll sell.