Hi,

Roly:

I connected an external speaker directly to the "reverb in" rca using aligator clips:

Reverb channel = normal = no audio, increased blend = louder hum + output audio down
Reverb channel = brilliant = no audio, increased blend = louder hum + output audio down

Transformer markings:
80-5056-1 (VOX Part#) = Reverb Transformer
--606-646-

Bad Reverb Transformer?

I wish there was a better schematic, I agree some values are hard to read.

Thanks,
Dan

Hi,

I have tried to back track and find the info on the original reverb tank.
It was stamped with a "1122" and other faint info on the underside "AO-23580-14"
It was bad - I could not even get an ohm reading.
I emailed Tubesandmore, did an exhaustive search on the net, after searching even more now it may look like the output impedance is too high.  I can't really find a straight answer anywhere about a proper replacement tank.

The "crash" test does give a loud noise in normal and brilliant.

Also noticed that a "HUM" gets louder as the music level goes down as the blend is increased - in a very linear fashion.

I have cleaned the reverb channel switch, and checked for bad wires.

I do not have a scope.

Thanks for all your input,
Dan

Hi,

Roly:
OK, I deserve that, no more "shotgunning". I got it.

There is no more buzzing - that part was a broken wire.

the original problem is:
Normal + Reverb = overall sound level drops, when reverb blend increases.
Brilliant * Reverb = overall sound level drops, when reverb blend increases.
OFF = normal operation, good sound.

Q201 & Q203 = (green/yellow dot) = 2N2925 - correct JHow.
Q202 = 2SC4793 or 2SC5171 (recommended replacements)
        = it is a TO39 pkg  (tin can with heat sink) - correct JHow.

Any ideas on this:
Q203 - (schematic shows cap on C to gnd actual cap was C to B)?

I have confirmed the reverb rca cables, they are correctly soldered to the circuit board.
Reverb "IN" comes from Q202 & transformer side & not grounded.
Reverb "OUT" goes to Q203 and is grounded.


Looking for suggestions, thanks for all your input.

Dan

Hi,
I check out a few things, thanks for the ideas.

phatt:
I checked the rca reverb connectors = good
According to the schematic reverb out is grounded = confirmed on amp
Buzzing sound was a pulled off wire = fixed now = no more buzzing
Just back to original problem now, glad that buzz is gone.

JHow:

I did find a broken wire - thanks
Could not see any missing grounds.
Did manage to measure some voltages in the reverb section:
Q201 - E=.82v, B=.80v, C=2.3v
Q202 - E=1.73v, B=2.3V, C=27.6v
Q203 - E=1.86v, B=2.85v, C3.5v (schematic shows cap on C to gnd actual cap was C to B)

I hope this info offers some insight to someone, the help is appreciated.
Thanks,
Dan

Hi,
I'm back with another problematic amp, at least to me.

It's a VOX Buckingham V1121, the original problem is:
Normal + Reverb = overall sound level drops, when reverb blend increases.
Brilliant * Reverb = overall sound level drops, when reverb blend increases.
OFF = normal operation, good sound.


I have:

     Replaced the original "1122" reverb tank with a "P-MOD-4AB3C1C" = best spec I could find to equal original.  The original tank had an open reading on one end and needed replacing.
     Replaced all of the electrolytic capacitors. (based on internet research)
     
Now:

Normal + Reverb = loud buzz/noise (can't even leave it on)
Brilliant + Reverb = overall sound level drops, when reverb blend increases. (same)
OFF = normal operation, good sound.

I have attached the schematic, parts list & documentation.

I am not sure where to start, the layout of this circuit is very tight, older and not labeled.
But, I believe someone here can help, based on my last problematic amp.

Any ideas would be greatly appreciated.

Thanks,
Dan

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

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

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

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 

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,

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

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

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 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,

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