Randall Century 1000 II Low Output

[DckTech]

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

[g1]

  There is no schematic in the attachment.

[Enzo]

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.

[DckTech]

Hi,

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

Dan

[J M Fahey]

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?

[DckTech]

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?

[DrGonz78]

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.

[J M Fahey]

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

*What* is your actual problem?:

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

or
"no reverb" ?

[DckTech]

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

[J M Fahey]

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:

[DckTech]

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

[Roly]

Hi @DckTech.

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?

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).

the 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

[DckTech]

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

[DckTech]

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

[Roly]

I 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?

the 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.