Ok, I bought a non-working Laney VC50 at a good price. The plan is to troubleshoot the thing myself and get it to working condition. Sick Marshally combo for cheapz and some elbow grease. Trannys supposedly were checked ok

This is the first amp I have ever troubleshot so I could use some help and a plan on how to attack this beast.

Schematic: http://www.laney.co.uk/manuals/VC50-100 ... %201.1.pdf

So say you are given an amp to troubleshoot, no tubes. Where do you start?

I would probably sell it to some schmuck who would try to fix it up, then use the money towards buying a new map.

First thing, i would try to find a schematic for the thing.

Check fuses, look for burnt things.

If you don't see anything wrong with either, you could go the careful way or the quick way.

Careful way: Same as a new build, check voltages with no tubes, or just the rectifier tube if tube rectified. Make sure you have heater voltage, and just check to see that B+ isn't crazy high or crazy low. If you have a lightbulb limiter or something like that, use it to see if you are drawing wildly too much current.

If nothing seems overtly screwy, start plugging in some known good tubes and see what you get. If you have a signal probe you can use it to see where the signal is getting lost.

The quick way: if i own the amp, and can take a risk with it, i will perhaps skip most of the careful steps. Basically, i might check fuses and signs of fire or shorts/sparks, then i would just take a set of tubes that i know work, but that aren't too precious to me, and i would go straight to the fire-it-up-and-see-whathefuck-happens stage. If nothing pops immediately and i don't have sound, i would probably takes some quick basic voltages, then maybe try to signal probe it to see where i might be running into a problem.

Does the amp have solid state shit in it? If so, i would try to bypass that and see if i could get output by inserting the signal straight into the PI. If there are opamps in it, they will likely be in reverb circuits and/or effects loops. I would probably use some alligator clip leads to plug straight in to the PI input. Keep the input signal lowish, because it might get realfuckingloud if the power section is actually functional.

Basically, there are many many things that could be fucked up. Start with the quick/easy ones and work your way up.

Who checked the trannies okay? And if they could do that checkout reliably, why couldn't they fix the thing? What was the story there?

The story is that the guy sent the amp to a tech to get a quote and the tech (not being specialized in tube equipment) was going to charge him $300+ so he decided to sell it instead. Tech said the trannys where good and the problem was in the effects loop. Not really trusting the tech, I decided I was going to check it out myself.

my first post has the schematics... I edited to make it more obvious

edit: One of the fuse holders is missing the cover and obviously the new fuse holder that the guy gave me doesn't fit. No car = I am unable to go to the store and buy a new smaller one.

I DLed that yesterday, but it appears to be a user manual? I don't see the schematic in it?

Anyhoo. If the problem is in the FX loop, then it might be an issue with the solid state bits. I would guess that that era laney would have an opamp or two in the fx loop and then another one to four of them in the reverb.

As ever, start with a careful visual inspection, looking first for obvious damage or things that just don't look like they should be touching eachother.

I would then make sure the fx loop jacks were cleaned up. Check if you can plug a signal into the return and have it work correctly. If so you can know that it is the circuit before that point.

If the opamps/SS bits need work then is can be a little bit of a pain, more like working on a pedal than an amp. If the op amps aren't socketed, you will probably need some desoldering braid. Also a smaller iron tip than you would generally use for building a turretboard amp.

Does the amp have relay switching? If so does it appear to function?

Does the amp turn on, but not pass sound, or does it not light up at all?

whoops... meant to post this link. I can't turn the amp on as I need a new fuse holder.

http://www.free-service-manuals.com/Aud ... ms_L36920/

Ah yes, that is much more like it.

What is the issue with the fuse holder? Is it busted up, or just missing?

The part that holds the fuse is missing... so its just a fuse holder with no top. The guy had bought a new fuse holder but a different kind which doesn't fit(too large).

ok.... tubes in!

Clean channel is working perfectly(which I guess it means the power amp works fine too)!!!

Drive sounds splatty, thin, with very little volume. It reacts more normally with the drive engaged but still has a thin sound with little volume.

edit: swapped out the tubes one by one.... still splaty... but I did notice that it will momentary be loud and instantaneously drop to thin and splatty... maybe a capacitor?

Here's what the "How To Service Your Own Tube Amp" says.

Intermittent Distortion: Loose connection

Distortion at all Levels: Open cathode or plate resistor.

Check for abnormally high voltage on cathode pins means open cathode resistor.

High voltage on both sides of a decoupling cap between tubes means shorted capacitor.

Low voltage on plate pin means open plate resistor.

There's some info for you. Hope this helps. Not fix that tube amp so you can annoy your neighbors!

I'd check the plate and cathode voltage on the drive toobs as a first test.

Plate voltages for the first tube are low:

V1A @ 168v (supposed to be ~206v)
V1B @ 171v (supposed to be ~211v)

Second tube seems ok
V2A @ 251v (~253v)
V2B @ 153v (~168v)

I didn't measure V5A because the schematic doesn't label its voltage.

All cathode voltages are normal.

Okay, can you probe the voltage of the hot side of C8 and C5? Also measure the drop drop across R6.
Could be bad decoupling/filtering. Its got to be a fair few years old I'd guess and electros do go bad.

I suppose for completeness check C11 and R14 too although V21 is getting the right juice so they're most likely fine.

How do I measure the drop across R6? Measure the current and Ri=V? Or is it as easy as measuring the voltage across the resistor?

C5 = 245v
C8 = 252v
C11 = 290v

Just measure across the resistor. Should be 10V from looking at the voltages of C5 and C8.
Certainly the voltage coming off those caps looks sensible enough.

Other thoughts from the top of my head:
Both R5 and R7 are bad - seems unlikely that both would fail open and balls up the plate load though.
C4/C6 anode bypass caps are bad and are leaking DC to ground - again unlikely that both are bad, could try removing them to see if that helps although the amp will be crazy bright! This would have borked the bias I would have thought though.

The input/R2 could be bad but thats pretty unlikely.

After this it might be ghetto signal tracing time. Stick a sine wave into the input (400Hz is fine, I use my HTC with favourite frequencies which produces a decent enough sine although its designed to drive 32R headphones not an amp) and then switch to the clean channel. This should still put signal into the lead preamp but disconnect it from the PI. Turn the clean volume to 0 so its not passing anything to the PI. Get a well insulated crocodile clip and put one end on the input of C16 which is the input cap to the PI. Take the other end and stick it on following:

output of C2
output of c6
output of c9
input to tone stack
output of R20

You'll need to run the gain pretty low to stop the sine wave from getting too distorted. This should help at least work out which stage is playing up. It looks like the V1A/B but its worth doing if you're stuck.

This is far easier with an o-scope but thats somewhat a luxury in DIY amp teching. I only got one because work were throwing them out.

I could probably use my laptop/interface to produce sine wave I also found some info on some of the problems I'm having, and they point to the coupling caps, so I might as well buy some caps just in case.

So I'm going to need:
2x 100k resistors (R5+R7)
1x 47pf ceramic/silver mic (C4)
2x 0.022uf orange drop to keep with the theme (C6,C9)
1x 0.0022uf coupling cap (C2+C3, they in a line right next to each other so I swap them out with a single cap)

Any other component I should get that wouldn't be a bad idea?

On a positive note, the amp drains its filter caps pretty quickly. I'm not taking any chances but it does bring some peace of mind. I'm going to recheck the voltages just in case I fucked up one of the reading

I guess a 2n2 would be fine. Not sure why they use 2 4n7's as the value isnt exactly mission critical.
Do the drive or gain pots make a lot of noise when they are turned? That would be an indicator of shitty pot or DC on the pots.

Are there relays in this circuit? If yes, do you have to push a button to switch from clean channel to dirty channel? If possible bypass the relays on the dirty channel and see if that fixes the problem. You could also use the old method of troubleshooting tube circuits. Start from the output and move forward to the input. Touching the grid with a probe will produce a pop. The pops get louder when you move toward the input stages. No pop or weak pop indicates a problem with that stage of the circuit.

Ok I redid all the reading and either i fucked something up(probably) or my amp is fucking with me:

B+ goes from 350 - (r19) - 305 - (r14) - 280 - (r6) - 268 (page 6)

V1A
P: 186v
C: 1.9v

V1B
P: 189v
C: 2.0v

V2A
P: 231v
C: 2.8v

V2B
P: 161v
C: 1.12v

V5A
P/B+: 305v
C: 171v

everything is lowish but within reason right? When my sis wakes up I'm doing the em666 test.