Crate g60-gt no output from distortion and wimpy reverb

[Duojet]

So I picked up a crate g60 as part of a CL deal, looking for a good SS amp to serve as a small PA, gig amp and lesson studio knock around.

At first it hummed like crazy, but that was fixed by simply removing the power stage and cleaning the chassis a bit (it's crazy rusted with bug bits inside).

The reverb is wimpy and the distortion channel lights up, but has no output.
The most sophisticated diagnostic gear I have is an analog meter. Should I even mess with this thing, or toss it back into the CL pit?

[Enzo]

Got FX loop jacks?  Plug a spare cord from send to return, any help?   Plug the guitar into the FX return, get sound?

Are the cables plugged into the reverb pan at the correct ends?


Got a schematic?

[Duojet]

Oh, I didn't even consider the fx loop. I assumed that it bypassed the tone stack, and somehow that would be the issue, but now that you put it like that, I'm not sure how I got to that conclusion.

I haven't tried swapping the plugs on the reverb, because I'm not used to working with higher voltages (mostly pedals and pickups) and I didn't know if that would fry something. I guess i've got not much to lose at this point.

I do have a schematic, I got it searching for the owner's manual. It's on a different computer, so I'll have to follow up on that. I'll try those ideas as soon as I can turn up the amp without pissing anyone off

[Duojet]

Ok, so I swapped the reverb bag cables - got a crazy hum - swapped them back - it started working?!? how is that even possible? I guess I won't ask any more silly questions.

I put a dummy load on the fx loop, no change. plugged the gtr straight in to line in, and got a lower output clean sound. I checked the schematic. It looks like the loop ties straight into the power stage.

so, to sum up
Clean channel - check
'Verb - check
Loop - check
Dirt channel - lights, no sound.

as promised, I'm attaching a schematic

[phatt]

I'd be looking at Q1 or Q3 as these switch the audio signal on and off for drive channel.
Others here are better qualified as to how to test them.

Reverb issue is likely just a dirty (corroded) plug/socket.
By reinserting the plugs wipes the metal surface clean again and hence a magical fix.
Phil.

[Duojet]

Well, from the you-told-me-so dept.

I put a - probe on the middle leg of all the transistors on the preamp board, and then touched the + to each outer leg. There was super high resistance on one side, and almost none on the other. Until.... I got to one in between the two channels and right next to the switch on the front panel. The needle didn't even twitch. Double checked it, same result. I didn't have time to turn over the PCB, but my money says it's probably Q3, given the location on the schematic.

Now I just need to see if my local supplier (ACK supply) has one 2N5638 on tap. All the datasheets say that it's "end of life" and "limited supply" but that's what they say about the MPSA13s I use for every fuzz pedal build.

[Duojet]

Cue the trombone and muted trumpet music. So I swapped out Q3, it tests fine now, but no joy. Dirt channel still lights up, still makes no sound, clean channel works 100%, verb is working as well, fx loop has no, well, effect.

I'd still love to fix it just on general principle, but I gave up and bought a Silver Stripe Peavey Studio Pro 112 on CL. Paid $50 for it, so I don't feel bad about it.

If anyone has any more (cheap) ideas, I'm keen to try them. When it comes to debugging PCBs with more than 10 components, I'm not an expert, so I'm done otherwise.

On a related note, if anyone in the Atlanta area wants a pet project with a U.K. made celestion in it, let me know. 

[Roly]

Gidday from Awstralia.

Dirt channel still lights up, still makes no sound, clean channel works 100%, verb is working as well,

Okay, if we look at the circuit we can see that quite a bit of your amp is now working properly, particularly the rather nasty bit, the output stage.  You are actually well ahead.

The Dirt channel is the bit between R6 and the "Level" control (and maybe the control circuit from the footswitch socket to Q6).

Please confirm (1) that the normal channel works okay until you select the dirt channel, then the amp goes silent, that is that the clean channel is being switched on and off.  It's a bit hard to read on the circuit but I think the channel switching line (A) is test point TP9(?).  (2) This should be going from near +16v to near -16V when the channels are switched.

The Dirt clipper diodes D6 and D7 are shown as LED's, so when Dirt is selected (3) these should light up in response to a signal being fed through.  If they are lighting in sympathy with an input signal then your fault is after there, if not lighting then it's before there.

The Dirt channel is switched by the FET's Q1, Q2 and Q3.  When Dirt is de-selected, that is Clean is selected, these three FET's should be close to a short, tens to hundreds of ohms only.

When Dirt is selected the (A) line, TP9(?) should be near -16 volts, and these FET's should be off.  This means that if you measure the resistance across each you should get the value of the residual resistance.

(4) In the case of Q1 this is R56(?) 22k (red,red,orange).  For Q2 it will be R67(?) 220k (red,red,yellow).  For Q3 it will be the value of the Level pot, 50k.  If any of these show a much lower resistance then it means that the associated FET isn't switching off and is faulty.  These might be more easily measured by finding the resistances and measuring across each for very low (Clean), or marked value (Dirt).

If these check out then it starts to look like one of the op-amps has died, IC2A or IC1B, so (5) measuring the voltages at their outputs, TP2 and TP3 will also be helpful.

Please do these measurements and report what you find, and we can proceed from there.

Chin up; you have already made good progress and are getting close to finding the fault in the Dirt channel.

Also, don't worry too much about exact replacements, lots of common FET's and op-amps will do the job.

[Duojet]

First, thanks for all the ideas! I'm glad to have any help in this case.

I am so confused. So, there are no numbers to indicate which component I'm dealing with, so I'm having a hard time tracing circuits to find test point 9. And I do realize that not every board is completely numbered, but this one has nothing!I'm used to bending on cheap keyboards and pedals, where at least the caps and transistors are labeled.

But, I did test the clipping LEDs and noticed that one ( I think the second one in the circuit, it was closer to the clean channel ) stayed ON. And BRIGHT. The other one, dark as night, not even a flicker. I'm going to try and post pics.

[J M Fahey]

That suggests the Op Amp driving it is tilted fully towards one rail.
Or, as Enzo would say, one rail may be missing.
Check the proper pins.

[Roly]

there are no numbers to indicate which component I'm dealing with

Oh wonderful.  That's a great help isn't it?   

However your observation of the clipping LED's is a red hot clue, you're getting seriously close now.  :tu:

The LED's are connected to common on one end and to a 4k7(R16) at the other.  This then goes back to the output of IC1b, pin 7, which is also TP3 - locate and measure the voltage there, and it's Sydney to a brick it will be stuck at + or -15V (roughly).

Okay, go to the PDF, down to the circuit, (rotate so it's upright), bottom left is a drawing of the dual op-amps top view, says for IC1&2, the ones we are interested in, that the supplies should be -15 on pin 4 and +15 on pin 8.  Find and check the voltages on the IC pins (being careful not to slip with the meter probe and short two pins together - never helpful).

Are both supplies actually making it on to the IC pins?  If not you need to trace the supplies back to the zeners and find out what is wrong there, but since the amp works on the clean channel, and that requires IC1a to be working, it looks like it is either the IC1b op-amp itself, or a voltage is somehow finding its way to its input.  My money is on the IC itself.

Tell us what you find.

The 4558 isn't much chop as dual op-amps go and since they are mostly pin compatible there are a number of more modern substitutes such as the LM833 (but do check the datasheet for what you can easily get for pin compatibility).

[J M Fahey]

Dear duojet.
I suggest you get some *fine* point pen (not necessarily a marker, I'm thinking a drafting pen or similar), you follow the schematic and write at least a few part ID numbers on the component side of the board.
It helps, because worst case, if you have 2 connected resistors, only one of them mrked, you can easilky find which one is the other.
Better than nothing.
And definitely take your time to mark and identify Test Points.

[Duojet]

Thanks guys, I'm going to crack the amp back open as soon as I can, in the meantime, here are the pics of the nasty PCB. I would buy Deoxit, but I'm already in the hole on this one, so there's $13 I don't want to spend if it won't help.

[J M Fahey]

Don't know about this amp, but Deoxit cleans guitar, pedal and footswitch controls, so it's worth having anyway.

[Roly]

Yuk!  Deoxit or not - stiff brush like a toothbrush, and you can move up to a bronze brush or similar for the pot cases.  The copper side looks pretty good.  Once you get all that crud off the board you'll feel better about it.  Cleaning also gives you the chance to give it a close inspection all over and you may even get lucky and spot the problem, dry/corroded joint or component lead.  Get right in around the IC pins to remove any possibility of leakage caused by this crud getting moist.

Looking at your copper side pic, the area of interest is directly behind the pot with the orange shaft ("Gain"?).

Between the pot and the IC there are a couple of solder joints that look corroded to me (the component wires look quite dark).  The side of the IC closest to the pot just happens to be the op-amp that drives the clipping LED's (pin 1 is marked with a small dot which a track happens run through).  It is possible that these joints being faulty could be the cause of your problem.

Give the board a good clean, check these joints for any signs that they are less than firm, try resoldering them and see if that makes any difference.  If you can't get a nice clean and bright joint you may need to replace the two components concerned (see below).

The resistor R16/4k7 (yellow/violet/red/gold) that connects the output of IC1b pin 7 to the clipping LED's is the third component behind the pot with the black shaft.

R15/270k (red/violet/yellow/gold - 'tho it looks more like a 470k in the pic) and C11/100pF are directly behind the pot with the orange shaft, which should be P1/"Gain".

I use a red and blue felt tip pen to dot along the +ve and -ve supply rails on the copper side to help me get oriented.

If it were me I'd be sticking with this one because at this point it looks very recoverable with a bit of work and maybe a few cheap components.  :tu:

HTH; Keep us posted.