It is common for resistors to have bumps and paint voids, little bubbles that have burst and set into a crater-like hollow.  Nothing to worry about I suspect - post close-up.

Ten ohms, in the wiper of the treble pot - you sure it's only ten?

That it sounds fine through the 'phones suggests you may have a speaker problem.

Have you tried running the amp into a different speaker cab?

Similarly, try running another (hopefully known good) guitar amp into your speaker (gently) and see how they compare.

And no, a valve amp must never be driven into an open circuit and must always have a suitable load; transistor amps must never be shorted but are fine open circuit.

These are common signs of a dirty connection somewhere in the signal path.  A common cause is dirt in the bridging contacts of the Fx Loop sockets.

By jumpering them with a lead you take these contacts out of circuit, replacing them with the lead.  This often effects a "cure", however the fault is still there and these contacts need to be cleaned.

Other causes are dirty input and output sockets, dirty or damaged pot tracks, and less commonly any plug and socket connections internally.


PS a some clear pix of the front, back, and the underchassis or internals would be a help considering the circuit is not yet available.

Undo what you just did - go back one whole step to the situation where it was disassembled and working; can you revert to that previous situation?  With what result?

Okay, so if it's blowing fuses, once you get your limiting lamp in series you will find that it lights brightly because the amp is shorting.

With limited power on the amp the thing then is to try and isolate what, exactly, is shorting.

You also need to disconnect the loudspeaker, and a simple way to do this in this particular amp is to plug a blank plug, or lead with nothing connected to the other end, into the headphone socket.  However you also will need to get inside the chassis to take voltage measurements so it may be easier to disconnect it altogether, however.

With the limiting lamp all the voltages will be very low compared to their normal values, but what we are looking for is some indication of what is shorting to what.

You will need to locate the main electrolytic caps C34 and C35, 4700uF/63V.  Using the chassis as the reference we hope to see a few volts positive on the +ve end of C34, and a few volts negative on the -ve end of C35.

We also need to find the voltage on the output line, the ungrounded speaker connection.  What is of particular interest here is if the output line is the same as the +ve supply, the -ve supply, or somewhere in between. (D12 and D13 are between these three points, +ve, -ve, and output).

If the output is +ve this suggest you have one or more shorted transistors on the +ve side, Q11/12/13, if on the -ve side Q5/8/9.

If there isn't much voltage at all it is possible there are shorted transistors both top and bottom, and the bridge rectifier D9 may also be shorted as a result.

The next step is to remove any suspect transistors from the circuit, carefully noting/marking where each came from, and testing out of circuit.  While Q13/14 and Q8/9 are out of circuit you can also power up for a few seconds and see if the short has cleared (the lamp settles to a dim glow or less).  This will clear the bridge rectifier D9 (and you should get tens of volts on both +ve and -ve supply rails), but if the short remains then D9 has most likely failed as well and will require replacement.  You can do this and retest before replacing any of the transistors.

Anyhow, using the limiting lamp, take some measurements and report back.

Quote from: oleskool on July 21, 2012, 12:04:21 AM
Oh well the line fuse went as soon as I hit the power. will get another tomorrow to see if it happens again.

Don't do that again - it will, and you will only cause more damage.

Do this;

Quote from: Roly
You will also need to knock up a limiting lamp, as per;

http://www.ssguitar.com/index.php?topic=2093.0

The diode in the ground leg is often simply to lift the output voltage a little bit (0.6-0.7V), just be sure it isn't a zener to lift it more than a bit.

To diagnose the shorting output stage you will need a light bulb limiter so you can get some power for measurements without blowing fuses or doing damage;

http://www.ssguitar.com/index.php?topic=2093.0

Well since you have to go inside anyway you could always do both, add a Pre Out and fix the main amp, even if only as a fallback spare Plan-B.

 :tu:

A clear majority of the repairs I do originate from poor connections, sometimes dirt or other contamination, sometime poor contact pressure as in the valve/tube amp currently on the bench where one of the output valves wasn't working simply because it wasn't actually making connection.

Printed circuit fractures are very common with the modern practice of mounting external connectors directly on the printed circuit, often with no other support at all.

With computers the proportion of "bad contact" faults is even higher, I estimate around eight out of ten of the machines I deal with.

It's actually a pleasant change when I have to fire up the multimeter or CRO and actually "technish".  :cheesy:

"Psychedelic Sound" - ha, that's a good one!  I'll bet that's just the trem and their wishful thinking.

I'll also bet that it's no more complicated than a single PSN LFO driving a buffer transistor.  I've seen secondary indicators of the front panel but this is the first where they have been too cheap to use two globes.

Just to reinforce what JM said; all the wiring between the mains power inlet/lead, fuse, main power switch, to the power transformer, is deadly.

Normally these days this area is pretty well covered up, but you still come across amps where some of these points, such as the rear of the fuse for example, is exposed metal.  When I encounter this I put something over that area, such as heavy cardboard or a plastic folder to prevent accidental contact.

Switch off, unplug from the power socket, disconnect speaker etc, remove amp chassis from its case.

When we say "pull the power plug out" we don't mean just turn off the switch - we really mean PULL THE PLUG - okay?  This is a habit you must get in to - except when you are doing power on tests, the mains plug must be out of the socket and somewhere on the bench in front of you where you can easily see it.

Take the best camera you can find and two or three desk lamps ONO; pour light into the chassis from two or three different directions close up and take the sharpest pix you can, one overall, and say three more end/middle/end up as close as you can get while still staying sharply in focus (use "tulip" macro if available, and manually turn the flash *off*).

Post pix.  Then we will all have some common ground.

If the printed circuit board has component numbers marked on it, then try and find D9 which will be close to where the one black and two red wires from the transformer go to the PCB.  This is the main rectifier and appears to be a four-diodes-in-one-glob in this amp.  Somewhere nearby you should find electrolytic caps C32 and C33 (marked 470uF/16V).

You should also try and locate diodes D12 and D13.  These may be near where the speaker wires connect to the PCB (but they could be just about anywhere).

You will also need to knock up a limiting lamp, as per;

http://www.ssguitar.com/index.php?topic=2093.0

You will need to get the chassis up on a table or bench where it is securely propped up and won't fall over (or into your lap!)

Having got your amp secure on the bench, well lit, and the internal mains connections covered, and having located diodes D12 and D13, (and since you have already been running it without a limiting lamp), plug in, switch on, and measure and record the DC voltage across D12 and D13, measure and record the voltages across C32 and C33 - switch off and unplug.  Post voltages.

The first two are the main amp supply voltages aka "rails", the second two are the preamp supply voltages aka "rails".  "Rail" normally implies DC, so the mains feed side would not normally be called a "rail".

HTH

Hi Bob, welcome - and a very helpful intro with your background and test gear.

Quote from: bobhill
How come Fender puts B taper pots on their volume controls in the SS amps?

One opinion is that it makes the level leap off the stop, giving the impression of a "powerful" amplifier.  You won't be the first to swap in a log pot to get a better and more controllable spread on the volume control.


You say the tremolo on the Toby GTR "doesn't work" - could you be a bit more specific, i.e. does the LFO oscillate but you get no signal modulation, or is there no sign of the LFO actually oscillating?

These are almost always phase-shift oscillators with series caps and shunt resistances.  If the LFO transistor checks okay, suspect any emitter bypass electro fitted.

Check that it isn't one of those circuits that won't work unless a footswitch is plugged in.  Most default to "on", but some don't, and  if the footswitch uses a switch-jack check that the switch is operating correctly and hasn't got dirty contacts.

What method of modulation does it seem to use, a lamp and LDR, or a transistor in the emitter bypass of a preamp stage, or...?

If you seem to have LFOscillation but no modulation it is likely that the small pea lamp in the lamp/LDR "roach" has gone open circuit, and the fix is obvious.

Keep us in touch with how you go.

HTH

I'm very sorry to hear of your loss.


Given what you said about the cause of the headphone output not working, there is a good chance that this could be fixed simply with a bit of glue and splinting the broken connections with hookup wire.  A couple of hundred bucks for such a repair is a lot more than I would charge for such a simple and generally straightforward job.  {but that possibly explains why I'm not rich}

Where amps have an Fx Loop or Pre Out/Main In connections these are at Line level, and in general terms are around the main volume control area.

So when you want to mate the output of the preamp of one amp to the input of the main amp of another you need to effectively patch the volume control connections of one to the volume control connections of the other ('tho for simplicity you may be able to go in to the inputs of the second amp).

In your case you could add a Preamp Out socket to the rear of the amp which will allow access to the preamp-processed signals; but now that the matter is not so urgent we should also examine the possibility of getting the main amp going again.


I certainly share JM's feelings about the trend to amps with in-built fancy digital processors.  I regularly service amps that are up to fifty years old, and show no signs of giving up any time soon.  But when I look at some of the modern gear that also comes across my bench (such as a PA with digital reverb last week) I have to wonder how much of it will still be going even ten years after it was manufactured.  In many cases these fancy digital Fx units are effectively unrepairable when they leave the factory - if they die they are replaced, and once spare modules run out, forget it.

Quote from: sewage666
I probably broke a lot of rules here.

You sure did,  :trouble  and you are one lucky fella that you aren't surrounded by curling silicon smoke.

Quote from: Enzo
After all, if it didn't break down, that would mean the analogy exactly described the whatever, which instead of analogy, then becomes just a definition.

Quote from: sewage666
I had test points 6 and 7 in marked wrong. Oi vey. I was thinking about it on the way home from work, traced the circuit again, and found I'd been testing in the wrong place. I adjusted the variable resistor, and they're well within their test range now.

Phew! 

You should check;

a) that you are getting the same small voltage across each of the 0.33 ohm resistors R31 to R38.  Apply Ohms Law; you are looking for an idle current of around 50mA for each; somewhere between 10 and 20mV.  This will confirm all the main output transistors are hauling their weight.

b) that the voltage between point (E), the main output, and ground (i.e. across the speaker(s)) is no more than about 100mV.

If I were doing this repair I would also go to the trouble of putting a low resistance load across the output and gently lifting the drive until the protection (Q2 and Q3) started to work, to confirm that it is working, but in this case...

What I would like you to do however is to go over all the Test Points and at least confirm that the DC voltages are correct for the table.

Normally there would be a few other health checks we would do, but if it's now making a very loud noise I think we can be satisfied that it's going okay.

On one hand you deserve a gold star and Win! button for carrying off the repair of a pretty major amplifier.  When confronted with something like this I consider it to be a fairly serious piece of work, and I've been doing it for a long time now.

But it must be said that you got very lucky just plunging ahead, and if you think you can get away with this on a regular basis ... well ... sooner or later you are going to be spending a fair bit of money and quite a bit of time muttering under your breath that you will be more circumspect in future, as you replace every transistors in the output stage and all the diodes in the power supply.

I'm sure we've all done it at some stage, I know I have, and it tends to be a salutary lesson that makes you a lot more careful in future.

Anyway, very well done  :dbtu: ; I had no doubt that you would get there eventually.  It is very pleasing to hear an amp you have been working on come to life, so much so it can be a bit addictive and cause you to actually seek out more "lost causes".   

You have asked a thoughtful range of questions which means you want to know the "why?" behind the "what?" - stay curious, it will take you far.  :dbtu:

Any time.

I'm going to leave you to JM 'cos he already has the circuit for this amp, but in the meantime you can make up a limiting lamp as per;

http://www.ssguitar.com/index.php?topic=2093.0

...because you are going to need it.

Disconnect the speaker.  Do any components seems to look overheated, browned off (if so, post photo)?

Replace the fuse with one of the correct rating, but do not try to power the amp up again at this stage.