Solid State Guitar Amp Forum | DIY Guitar Amplifiers

Hi Everyone,
I am new here and I am having a problems with a friend's amp.
I am an electronic technologist by trade and have been repairing all things electronic for many years.
This one has me a little stumped.

They were playing one night, then suddenly this problem occurred.
I notice 1st thing that the 3300uF was loose and defective - replaced it - no change.

I have very low volume level on normal and even lower on override.
I tried Roly's suggestions:
patched send and return = no change
injected music into return = good proper volume overall
injected music into reverb out = good
all voltages = good
have swapped RC4558P's = no change

Somewhere in the preamp area?
Anyone had problems with the TL604CP?
The "Treble/Pull Ch. Select" does select the channels.

I do have an audio probe and can follow the music through the circuit, is there somewhere I can really concentrate on?

I have attached the schematic & User Manual

Any help would be greatly appreciated.

Thanks,
Dan

  There is no schematic in the attachment.

So the power amp works, and you have isolated the problem to the preamp.  Good so far.

Has anyone eve had a bad TL604?  Yes.  There is no cocmponent in any amp that someone has not seen a failure on.  Specifically the 604 is used as a switch or signal router, and TI stopped making them long ago, and they are generally not available.  There are ways around that, but let us not get ahead of ourselves.  Plus Peavey makes a little thing you can solder in place of a TL604 to take its place.

You have a signal tracer?  Good, once you get past the first stage, the level doesn;t shift a lot, so go along the signal path looking for where the signal level drops a lot.

When I have such a situation, I often start by just going down the board loking at output pins on all the op amps, to see if any have unwanted DC offsets.  Takes a few seconds, and if you find one, that is a good place to start.

Hi,

Sorry about the schematic, I have now attached it.
Thanks for the replies, I will get at it soon.

Dan

Agree and add.

Both channels have muting FETs blocking their signal path, either discrete or encapsulated (TL604).

So maybe gain stages and switchers are fine, but somehow the latter are missing their turn-on voltages ..... = silence .

So as Enzo suggests, best would be to inject audio at the input and trace it along the path to see where it stops.
Second best is to kludge a signal tracer and do the same job.

As a side plan, check voltage at the preamp.
We expect around 24V DC where indicated and around half that at FET drains.
Meaning the preamp ones.

The switcher Fets will turn on and off following what's on RC4558 pin7 ,
Do Leds indicate switching condition?

Hi,

I have double checked the voltages and the +24, -12 & +12 are all there.

I have traced a good audio signal to Pin 7 going to reverb, then nothing on pin 3 from reverb.

A bad reverb?  Neither side has open resistance. It is 9EB2A1B Sound Enhancements Inc.

Output = 240 ohms
Input = 60 ohms

Will it harm the circuit to feed through/bypass the tank?

The output and input resistance seem to be good. However, very strange to see pin 3 on the 4558 chip return nothing. Can we blame the reverb unit? I don't think so yet. I am not the best person on this site to make any assumptions, but my guess is something is lost in translation. I hate shotgunning the parts and I know everyone with greater experience would say the same. However, with the readings you have from the reverb, as far as resistance, it would be guess to swap out a known good 4558 chip in this instance to rule out a bad opamp.

Edit: I guess I could have read that you did that already whoops!
Still jump back to JM's post "The switcher Fets will turn on and off following what's on RC4558 pin7 ,
Do Leds indicate switching condition?"

You also typed "Somewhere in the preamp area?
Anyone had problems with the TL604CP?
The "Treble/Pull Ch. Select" does select the channels."

Does that answer his question here? What do you exactly mean by that statement?

Still not sure where the problem is... Possibly the RCA jack needs to be looked at on the output side of the reverb unit. Assuming you ran signal directly into pin 3 to test the op amp...??? Since now your wondering about jumping the signal across the reverb tank, right? So maybe it just a bad RCA cable connecting from the output of the tank to the return circuit.

Dear DckTech, please do not jump all over the place, solve one problem at a time.

*What* is your actual problem?:

QuoteI have very low volume level on normal and even lower on override.

or
"no reverb" ?

Hi,

A bit of a recap:

Problem = low level output, even lower (pretty much non existant) when using overdrive.
Yes the dual colour LED changes, green=normal  red=overdrive
All voltages are present +12, -12 & +24

I inject audio into return = good quality sound.
I inject audio into reverb out = good output sound

I have a simple audio probe connected to a mini speaker to follow audio.

When injecting audio in main input: (using normal channel)
I get audio at:
IC - RC4558P (B) - pin 7 (no audio on any other pins)
IC - RC4558P (A) - pin 1 (no audio on any other pins)

When I perform the above in Overdrive - signal is gone, very low.

I can follow the audio from the beginning, through some of the FETs, then it just gets kind of hard to trace.

What I meant by:
You also typed "Somewhere in the preamp area?
Anyone had problems with the TL604CP?
The "Treble/Pull Ch. Select" does select the channels."
I realize now that at first glance i thought it actually switched the channels - wrong!


There are no audio signals on any pins of the TL604CP.


I see the left most part of the schematic is missing, can't be much, just the input jack?


I measured the voltages at some of the FETS, yes 24 volts where indicated, but pretty much around 2 volts on the other side of them.


I hope this offers some more insight, need a few more eyes looking at this.


Thanks,
Dan

For clarity I extracted the preamp schematic from the PDF and started labelling *every* part.

Did a few, you do the rest.

Used ancient Paint Shop Pro, you use what you have.

Select a similar size letter, paint it red for clarity, stick it real close to the part it names so as to avoid confusion, check that you missed none and repost.

Then we continue troubleshooting, because I see we have a little communication problem here.

Label resistors as "R", Capacitors as "C", any transistor as "Q", jacks as "Jx", switches as "Sw", pots as "P", ICs or TL604 as "IC" and so on, plus the corresponding identifying number, of course.

We'll save a lot of time ;)

EDIT: OF COURSE I forgot to attach ;)
Oh well, where did I leave the memory pills?  :lmao:

Hi,
I have attached a schematic with component identifiers in red & main board layout.

A bit of a recap:

Problem = low level output, even lower (pretty much non existant) when using overdrive.
Yes the dual colour LED changes, green=normal  red=overdrive
All voltages are present +12, -12 & +24

I inject audio into return = good quality sound.
I inject audio into reverb out = good output sound

I have a simple audio probe connected to a mini speaker to follow audio.

When injecting audio in main input: (using normal channel)

I can follow the audio from the beginning, through Q1, Q2 then nothing at R5?
I am not sure what Q3 & Q4 are supposed to be doing?

IC2 - RC4558P (B) - Audio on pin 7 (no audio on any other pins)
IC1 - RC4558P (A) - no audio on any pins
When I perform the above in Overdrive - signal is gone, very low.

The "Treble/Pull Ch. Select" does select the channels."

There are no audio signals on any pins of IC3 the TL604CP.

I measured the voltages at some of the FETS, yes 24 volts where indicated, but pretty much around 2 volts on the other side of them.

I HAVE:
swapped IC1 & IC2 = no change
replaced Q3,Q4 & Q5 = no change
plugged in a foot switch = no change
tried new RCA leads for reverb in & out = no change

I am hoping with the aid of the attachments, that someone will be able to see what I am missing.
I appreciate all of those who have taken the time to help solve this problem.

Thanks,
Dan

Hi @DckTech.

Quote from: DckTechI can follow the audio from the beginning, through Q1, Q2 then nothing at R5?
I am not sure what Q3 & Q4 are supposed to be doing?

Q3 and Q4 (along with IC3) form the channel switching.

When Q3 is on and Q4 is off the signal via Q1 and Q2 should be at R5.  That it isn't suggests that either Q3 isn't turning on, or that Q4 isn't turning off, or both.  All these FET's are the same and N-channel, so for signal to get past Q3 and Q4 the gate of Q3 will need to be close to ground (on), and the gate of Q4 at some largish negative voltage (off).

Quote from: DckTechthe dual colour LED changes, green=normal  red=overdrive

The channel selection circuitry is centred around IC1 and from the LED indication it would seem that IC1 is working properly, inverting the selection signal from the footswitch (or front panel switch).

What I would do is remove Q4 and see if that brings the normal channel alive.  If not I'd then try linking across Q3 between the volume pot wiper and C4, to R5.  Having thus eliminated the channel switching I would be surprised if the normal channel didn't then work.  That just leaves finding out why the channel switching FET's aren't switching.

I suspect that something has failed in IC3 which is loading down the switching circuit and preventing the other FET's from switching (despite the correct LED indication).

HTH

Hi Roly,
Thanks for the information and ideas, I will try your ideas it later tonight.
I should have mentioned that the foot switch I attached did not do anything, unlike the pull switch.
Sorry I did not mention that, does that help any?

Dan

Hi,

I have tried your suggestions Roly, I removed Q4 = no change.
I jumpered from the wiper of P1 to R5 = no change.

I am not sure what to try next. 
I was hoping that would bring the the amp to life.

This is one stubborn amp.

Thanks,
Dan

Quote from: DckTechI can follow the audio from the beginning, through Q1, Q2 then nothing at R5?

So to be specific, you removed Q4, then linked the volume pot wiper to R5 and you still didn't get any signal?

Okay, I think this is unlikely given the other symptoms, but this means that you are getting output from Q2 but it isn't getting past that volume pot.

So, with signal input (Q3 and Q4 out) and the volume pot turned up, do you get signal on its wiper?

Quote from: DckTechthe foot switch I attached did not do anything, unlike the pull switch.

The footswitch is in parallel with the panel switch "PNL" and will only be effective when the PNL switch is open - otherwise there is a connector/cable/switch problem with the footswitch, but seeing that the front panel switch PNL changes the LED colour it is at least working somewhat.

Measure the voltage on the output of IC1 (pin 7) and post the voltages in the two states (one +ve, the other -ve to ground).

Probe the wiper of the treble pot P3 (set mid position) or stick your finger on it - do you get any crackles or blurts?

Ditto for the ends of R5.

Hi,

I have finally had some time to check this amp out some more.

Roly:
To be more specific:
I removed Q3 & Q4 and jumpered the wiper of P1 to R5 = good sound @ Pin 7 IC2
(should there not be audio at Pin 5 IC2? = no audio)
I can hear the effects of Treble, Bass, Middle & Reverb on main speaker, although signal is low.

I have good audio at Reverb In and no audio at Reverb Out Pin 3 IC2.
Why no audio on my probe at this point, yet the audio is making it to main speaker?
No matter the position of the Reverb pot.

The voltages on IC1 Pin 7 (still have Q3 & Q4 out at this time) are:
Normal = -9.5v
O/D     = 0V

Probing wiper of P3 = loud burst & crackle
Probing ends of R5 = softer bursts & crackles

With regards to the Reverb, I have:
swapped the tank with a known good one = no change
made new set of RCA patch cables for In & Out = no change


Let me know what you think, or if you have any other ideas.

I feel we are getting close to figuring this out, thanks to all the help.

Thanks,
Dan

Quote from: Roly on May 22, 2013, 01:57:39 PMI suspect that something has failed in IC3 which is loading down the switching circuit and preventing the other FET's from switching (despite the correct LED indication).

After reading through this thread I get stuck on Roly's thought here... So if IC1 is working as the switching FET that leads over to IC3 TL604, hence leg 2 AD/DA is working on IC3. Then shouldn't there be signal that is routed through S1 and S2 through that IC? Which leads us through to Q6 and Q7 and that circuit for the distortion channel. It all leads us back after that to the R5 connection. I am not sure what I am alluding to here, but really just have questions. I want to understand this circuit better as others here can explain where I might getting off track with my questions.  8|

Quote from: DckTechI removed Q3 & Q4 and jumpered the wiper of P1 to R5 = good sound @ Pin 7 IC2
(should there not be audio at Pin 5 IC2? = no audio)

Audio at Pin 7 IC2 is real good.  Yes, there must be audio at pin 5 for there to be audio at pin 7 (it's coming out, so it must be going in, right?), but there is quite a lot of gain between those points (roughly R19/R18, 1meg/2k2 or 1000/2.2 ~=500 times).

In this situation you should also have some audio at pin 1 IC1 (or Fx Send), but again the level may be a bit low for your signal tracer.

Quote from: DckTechI have good audio at Reverb In and no audio at Reverb Out Pin 3 IC2.

This suggests that there may be an open somewhere through your reverb tank.  A better place to probe would be the output, pin 1 IC2, or just on the hot end of the Reverb level control, BUT at this point I would put your reverb problem to one side for later and concentrate on the channel switching which is ahead of that point, since a problem with the reverb tank doesn't deal with why there is no signal getting past IC3, and until you can be sure of the input to the reverb chain is working correctly it is better to concentrate on that first.

Before we do however you can try two tests; turn the Reverb control up to half way and "crash" the reverb springs.  This should produce a loud spring crash from the speaker.  No crash?  Then disconnect the reverb tank output to pin3 IC2 and try the crackle/blurt test at pin3 IC2 which will show if the recovery amp (pin 1,2,3 IC2) is working.  If it is check the tank coils for continuity, and look carefully where the tank wires go internally to it's sockets and see if one of them are broken off.

Quote from: DckTechWhy no audio on my probe at this point, yet the audio is making it to main speaker?
No matter the position of the Reverb pot.

Because the audio is going directly via C14 and not via IC2 and the Reverb control - IC2 and the reverb tank are a "sidechain".

Quote from: DckTechThe voltages on IC1 Pin 7 (still have Q3 & Q4 out at this time) are:
Normal = -9.5v
O/D     = 0V

Okay, this is good and indicates the audio switching control circuit is working correctly, but the audio switches it controls, Q3 and Q4 in the Normal path, and IC3 in the Overdrive path, are still suspect.

We always look for a single failure first, and what is still possible is that a failure in IC3 is preventing the line that feeds the Gate of Q4 from going -ve and turning off this shunt switch, and if it can't turn off, despite its mate Q3 turning on, it will still be sinking most of the signal to ground.

Quote from: DckTechProbing wiper of P3 = loud burst & crackle
Probing ends of R5 = softer bursts & crackles

These are very good signs.  The loud noises from the wiper of P3 tells us that what follows, IC1 (pins 1,2,3) and the main amp are apparently healthy.  We expect there to be less response from R5 because the tonestack (R16, C11,12,12, P3,4,5) is lossy and reduces the signal level quite a bit, so this is all good.

You can check that when pin 7 IC1 is around -9V that the Gate pad for Q4 is half that, around -4.5V.  This is required to switch Q4 off so it stops shorting the signal path in the Normal direction.  Similarly pin 2 of IC3, the TL604, should also be at -9V (and at the moment I'm suspecting IC3 has failed internally and you will find a more positive voltage here, maybe 0V but also maybe higher, and this is what is preventing Q4 from turning off).

If you restore Q3 and Q4, taking care to get their leads correctly oriented, and lift one end of R15 you should get switching of the Normal channel off and on (but by lifting R15 will have isolated IC3 and won't get switching of the Overdrive path).  If this works correctly then the finger is pointing at IC3.  This is a little difficult since TL604's aren't made any more, however there are small sub-assemblies available that will substitute.


Note that in the drawing I have (that you added component numbers to) the input of Q5 is shown going up to the ground rail under Jx1 - this cannot be right and it must actually be coming from the hot side of Jx1, i.e. at C1 (or it simply wouldn't work at all).

HTH


@DrGonz - the TL604 is an analogue switch which controls the path pin 7 to pins 4 and 5, doing the same job as Q3 and Q4.  Why they chose to do one channel with FET's and the other with an IC beats me, but basically they are identical in function while opposite in action.

Hi,
So I did some more troubleshooting ( Q3 & Q4 back in)

Roly:
Test #1 = reverb crash = yes I get the "crash"
Test #2 = IC2 pin3 = feeding audio in at this point works great = good sound

IC1 pin 7 = -9.5volts = good
Q4 gate = -4.5volts = good
IC3 pin 3 =  -.7 volts = bad (suspect IC3 is bad)

I unsoldered & lifted one side of R15, isolating IC3 = no change

I noticed lots of comments about the schematic, I emailed Randall directly and this is the best they had, i guess.  I know some areas look questionable.

I sent this link & schematic to a college professor at the local college, when I once attended, this was his response:

Hi Dan,

I had a look at your schematic and posts and I have the following comments/questions
-when in normal mode the audio signal is amplified by Q1, buffered by Q2 and then sent to IC2 for more amplification
-when in overdrive the audio is amped by Q5, Q6, buffed by Q7 and then sent to IC2 (schematic with designators is not the same as schematic without see connection left of C1)
Is there a component missing on the schematic? I see a node to the right of Q4 but nothing connected to it.
Can you trace the audio signal through the overdrive stage?
I see the last comment on the post has you checking Q3, Q4, etc... I am very interested in the outcome of that test and will wait for your results
Have you tried injecting a signal on the right of R5? How about injecting on wiper of P3 (same idea that is in the last post)
I have included the expected gate voltages for Q3, Q4 to help you out (I assume the opamps are powered with +/- 12V)
PNL switch closed (normal mode)
Q3 gate approx. -0.2V
Q4 gate approx. -6V
PNL switch open (overdrive)
Q3 gate approx. -12V
Q4 gate approx. 0V
I don't expect Q3 or Q4 to be a problem because it should then work fine in overdrive mode

Hope this helps you out

So, I also went through the above and found:
- as I mentioned above I think the schematic is a bit flawed ( node by Q4 )
- yes I can trace the signal thru the OD section (possible my audio probe is not that sensitive) may no hear the audio at some areas
Normal:
- Q3 gate = -.2v
- Q4 gate = -4.5v

OD:
Q3 gate = -12v
Q4 gate = 0v

So I started to inject the audio signal in various areas:

- right side of R5 = good all the way through
- wiper of P3 = good all the way through

Then I started injecting audio more & more to the left of the circuit, I got:
"good all the way through" everywhere, EXCEPT to the left of Q1 (gate) = no & Q5 (gate) = no
I'm not really sure if that is a good way to test - but I was really curious.

I replaced Q1,C5,
The input jack - a problem -really?

Lots to take in, let me know what you all think.
Much appreciated,
Dan

Hi,

Update,
I have replaced the input jack and for some reason now that has improved things a little. 
After doing some more accurate sound injecting and testing it appears i am getting about 1/2 the total output volume when I inject the audio anywhere in the pre amp stage. Normal & OD = same results.
When I inject sound into the return, I get full volume.
I guess not really much has changed.


Thanks for any help,
Dan 

Quote from: DckTechTest #1 = reverb crash = yes I get the "crash"

Okay, well that means that everything from the springline to the speaker is working.

Quote from: DckTech- as I mentioned above I think the schematic is a bit flawed ( node by Q4 )

It's a rule of nature that all circuits have errors (and it's Murphy's Law that the error will be right in the bit you are interested in  ::) ).

Quote from: DckTechThen I started injecting audio more & more to the left of the circuit, I got:
"good all the way through" everywhere, EXCEPT to the left of Q1 (gate) = no & Q5 (gate) = no

Ho-kay.  Now this implies that the two channel switching circuits are working okay.  You should be able to confirm this by the Normal "Volume" (only) working in one state, and the OD "Gain" (only) working in the other state.

The latter part could be for two different reasons; input sockets are normally arranged to short to ground if nothing is plugged in to them to silence the amp.  If you didn't have anything plugged into the input this might explain the lack of signal at that point.

It could also be that either (or both) Q1 or Q5 have failed, perhaps due to an excessive signal voltage being fed in - it happens, and either or both are damaged and now shunting the input signal.

Check the Drain and Source voltages on Q1 and Q5.  With a 24V supply I'd expect somewhere around half that on the Drain, and somewhere around a couple of volts on the Source.

I'm surprised that changing the input jack has made any difference, but it could be that you have some sort of short from another cause (outer of screened cable melted into inner insulation) that you have disturbed in the process of replacing the jack.

If the voltages around Q1 and Q5 look reasonable I'd be lifting the input ends of C1 and C6 and trying injecting a signal into each to see if the respective preamps work okay disconnected from the input wiring.

It's all a process of progressive elimination.

The main reason for the difference in sensitivity between the preamps and the Return socket will be the tonestack which is by its nature fairly lossy.

Hi,

Roly:

Just to confirm:
Normal "volume" = works
OD "gain" = works

I understand the grounding on the input jack - that makes sense.

Voltage readings:
Q1 & Q5 drain = 1.92 volts (I'm assuming not good)
Q1 & Q5 source = 1.70 volts

I measured R3 & R7 = 33k
I compared Q6 drain = 22volts (R22 = 47k not 33k as stated)
The =24 volts is actually 28 volts, a bit high but I'm sure it's within spec.

Just because I had another FET, I replaced Q1 = no change

I did remove C1 & C6, and injected audio into the gates = no change.
I went over all the connecting wire & made sure no wires were exposed or touching anything.

This is all a serious "process of progressive elimination".

Thanks again for your help, any thoughts?

Much appreciated,
Dan

Well I may have played the fool in regards to how the TL604 chip functioned in this amp. However, I think that Q5 is not getting the correct voltage there on the drain. I did look at the schematic and start to question Q5 and Q6. Q5 would be my bet for testing and/or replacing.

Going back a page.   As someone said, if you have signal coming OUT of an op amp, it must have gotten to its input.   But in general you cannot measure the signal right at the input pins because they are at "virtual ground".  Look that up.   

If the IC is in a socket, you can remove the IC and verify signal to the socket pins.  If there is a resistor leading to the input pin, you can go to the far end of the resistor and see what is there, and if the resistor is good itself, we can infer the signal passes on to the IC.

And not so sure the Q4 thing is a mistake.  There is a dot on the line, and one nearby to the right on the ground bus.  I suspect they removed a part and neglected to clean up the dots.

Quote from: EnzoAnd not so sure the Q4 thing is a mistake.  There is a dot on the line, and one nearby to the right on the ground bus.  I suspect they removed a part and neglected to clean up the dots.

Agreed, that's what I assumed.

Quote from: DckTechQ1 & Q5 drain = 1.92 volts (I'm assuming not good)

Correct, that is very "not good".

You answered my next question which would have been about the supply getting to the first stages...

Quote from: DckTechThe =24 volts is actually 28 volts, a bit high but I'm sure it's within spec.

...agreed, but now you confound me with...

Quote from: DckTechJust because I had another FET, I replaced Q1 = no change

That is not what I would expect, because 2V on Q1 Drain suggests that it's boofed.

At this point I'm still inclined to the view that both Q1 and Q5 have been damaged by excessive input signal at some point (since they are both unprotected and it is not unknown for an amp output to get connected to another amp input, either by accident or as an an experiment)

Quote from: DckTechI did remove C1 & C6, and injected audio into the gates = no change.

This is to be expected as the DC conditions are so wrong (both Q1 and Q5 saturated on).

STOP THE PRESSES: I've just done an LTSpice sim and discovered that with MPF102's these stages won't bias properly.  When I tried 2N5484's (as used by Randall in the RG80) bingo, the stage biassed correctly! (see attach)

Now, either Q1 and Q5 have to be 2N5484's, OR the values of the Source resistors have to be increased to correctly bias MPF102's.  The sim says that R2 and R8 need to be increased from 2k2 to 5k6. (or perhaps a couple of 10k trimpots so that they can be rebiased for whatever FET you can get, J201's etc).

Hi,

Thanks to everyone for your replies in trying to solve this problem.

I am using 2N5486 as replacement FET's in this amp, the local supplier did not have the 2N5484, but according to the attached spec, the 2N5486 should be fine.

I could try replacing Q5 again and maybe Q6 & Q7.  That would be all the FET's in the preamp.

Roly:
Thanks for running the sim, so what it's saying is the voltages I'm getting are not correct, should be 7.5v on the drain. 

Not sure what the next move is?

Thanks,
Dan

Quote from: DckTechNot sure what the next move is?

Correctly DC bias both preamp stages.

1. Put in known good FET's at Q1, 2 and 5.
2. Confirm the Drain voltage at Q1 and Q5 is way low (saturated).
3. Tweek the value of the Source resistor so the Drain voltage is ~half supply.(*).
4. Profit!

(* I just repeated the sim with 2N5486 and found it needs 8k2 as the Source resistor)

The easy way is to lash in a 10k-50k pot as the Source bias resistor; set this for 1/2 supply on the Drain, remove and measure value and fit nearest value fixed resistor.  The same value should also work for the other signal path (i.e. Q1 and Q5).

The slightly longer method is to fit a 8k2 and measure the voltage at the Drain; if it's above 1/2 V supply reduce R, if below, increase R - i.e. tune the Drain voltage to roughly half the supply.  {If you want to be picky about it with Q1 and Q2, you can split the difference between the Drain voltage of Q1 and the Source voltage of Q2 so that they both have similar voltages to ground and supply, i.e. maximise signal headroom, but I don't think that will actually make any material difference.}

Take home lessons;

- any active device that is saturated or cut off has negligible gain.

- in any Class-A amplifier stage the idle Drain/Collector/Anode voltage needs to be about half the supply to get maximum available output voltage swing (and stage gain), that is, it must be biased into its linear operating region.

Well Roly - you are the MAN.

After my weekend of other duties, I finally had time to get back to the Randall.

I put in good FET's and replaced R2 & R8 with 8k2's, which brought the drain voltage to 12v.
And in quoting Roly "PROFIT".
The amp is working great, lots of volume.

Recap:
Looking back, I'm assuming there was a few FET failures.  Brought on by the "loose/broken" leg of the 3300uF in the supply.  It was probably arcing, spiking the voltage or???
That cap was the first thing I replaced.
So replacing the FET's with a substitute, may have caused my agony also.

I have lots of "take home lessons" from this forum - thanks to Roly & everyone who helped.

Thanks,
Dan

Great!  Pleasing result.   :dbtu:

Quote from: DckTechIt was probably arcing, spiking the voltage or???

I'll go with the "or???" and suspect that the loose cap was co-incidental. 

If you can find the two FET's that originally came out of Q1 and Q5 and test them I wouldn't be surprised to find they were both shorted Gate-to-Source.

Quote from: DckTechThey were playing one night, then suddenly this problem occurred.

Given that this is a true, accurate, and complete account of what happened(*) it's still possible that an excessive voltage spike found its way into the inputs.

(* and people often don't accurately relate what happened, mostly because they are lousy  observers, but sometimes because they now realise that connecting the output of one amp to the input of another was, perhaps, in retrospect, a pretty stupid thing to do.  The main problem is that in attempting to save face they don't tell you things you really need to know to fix it, or to prevent it happening again.  You have to quiz clients about what happened and listen very carefully, but you also have to take what you are told with a fairly large grain of salt.)


To repeat ('coz it's important) a linear stage must be biased somewhere between cutoff and saturation, between no conduction and full conduction, so the output has some "room to move" up and down in voltage.

If the Drain/Collector/Anode is either at the supply (no current) or ground (full current) then the gain of the stage will be nothing; i.e. a change in the input produces no change in the output.

So in general we are looking for the Drain of a FET/Collector of a BJT/Anode of a valve to rest somewhere around half the supply voltage so that it has roughly equal room to move up and down from the resting voltage, maximum "headroom".


After a bit of initial diagnosis we often test our conclusions by substituting a suspect component with another.  We normally make a couple of assumptions; that the substitute is in good working order, and if not identical is near enough - and sometimes we get trapped because these assumptions are invalid for one reason or another.  In this case we all assumed that the substitute FET you had used was "near enough", and as it always is, it isn't until someone gets suspicious and tests that assumption that we find out what is really going on.   :tu: