Univox U65G Tone & Reverb Control Noises

[xplorations]

Hi All,

I like the sound of this old amp,
however there are two problems.

1. At the end of travel of two of the knobs:

When the Tone is switched all the way to the right,
AND/OR
When the Reverb is switched all the way to the left,
they will each make a LOUD unpleasant noise:
Much like a LOUD Distorted Trebly Fuzz.

The signal is obviously being distorted somewhere in the circuit.

It DOESN'T happen when I turn the knobs
and a guitar is not plugged in.

It's an old solid state Univox U65G.
I like the tremolo built into the amp.
And I like the tone of the amp.

If anyone has experience working on this amp,
or has helpful ideas, I would appreciate it.

I don't want to invest much money in this,
but am wondering if it is likely a simple fix.

There's also a hum I'd love to get rid of.
But can live with...

Analytical Thoughts Towards Fixing the Problem Are Welcome.

Thanks in Advance,

Vic
***@earthlink.net

[mykey]

Post by xplorations
Hi All,
I like the sound of this old amp,
however there are two problems.
When the Tone is switched all the way to the right,
AND/OR
When the Reverb is switched all the way to the left,
Much like a LOUD Distorted Trebly Fuzz.
The signal is obviously being distorted somewhere in the circuit.
It DOESN'T happen when I turn the knobs
and a guitar is not plugged in.
It's an old solid state Univox U65G.
I like the tremolo built into the amp.
And I like the tone of the amp.
If anyone has experience working on this amp,
or has helpful ideas, I would appreciate it.
I don't want to invest much money in this,
but am wondering if it is likely a simple fix.
There's also a hum I'd love to get rid of.
But can live with...
Analytical Thoughts Towards Fixing the Problem Are Welcome.
Thanks in Advance,
Vic

the amp is going into oscillation, you need to put a 3-8pf 1000V
silver mica cap between pins 1 and 2 of V1 tube. That usually
will stop oscillations in the preamp.
check for broken wires on the reverb tank, that's a common cause.
The hum caused by old electrolytic capacitors, old power tubes,
you need to change them all, rebias.
that should get it better
I'm not aware what kind of guitar, but humbucking pickups go
a long way in reducing those noises too. If you are using single
coils, expect more hum and noise, even oscillation.

[Dave Curtis]

On Sat, 9 Feb 2008 22:08:44 -0800 (PST), mykey

Post by mykey

Post by xplorations
Hi All,
I like the sound of this old amp,
however there are two problems.
When the Tone is switched all the way to the right,
AND/OR
When the Reverb is switched all the way to the left,
Much like a LOUD Distorted Trebly Fuzz.
The signal is obviously being distorted somewhere in the circuit.
It DOESN'T happen when I turn the knobs
and a guitar is not plugged in.
It's an old solid state Univox U65G.
I like the tremolo built into the amp.
And I like the tone of the amp.
If anyone has experience working on this amp,
or has helpful ideas, I would appreciate it.
I don't want to invest much money in this,
but am wondering if it is likely a simple fix.
There's also a hum I'd love to get rid of.
But can live with...
Analytical Thoughts Towards Fixing the Problem Are Welcome.
Thanks in Advance,
Vic

the amp is going into oscillation, you need to put a 3-8pf 1000V
silver mica cap between pins 1 and 2 of V1 tube. That usually
will stop oscillations in the preamp.
check for broken wires on the reverb tank, that's a common cause.
The hum caused by old electrolytic capacitors, old power tubes,
you need to change them all, rebias.
that should get it better

Exactly what part of "It's an old solid state Univox U65G" don't you
understand, mykey ?

If you're going to reply to posts, you might want to read them so you
don't look *quite* so ignorant.

To the OP: It might be as simple as cleaning the pots.

-DC

[t***@gmail.com]

There aren't any tubes in it and it's from an era when bias adjustment
in a SS amp was luxury. The only thing I can second from mykey's post
is changing those electrolytic capacitors. There's likely a boatload
of them (like in any SS amp from that era) so the task will not be
very pleasant. For starters I would change at least all of the supply
caps and the output coupling cap. The first ones will affect in
reducing amount ofd hum and at best cure all sorts of strange
problems. The latter is a must if the speakers are precious to you. If
you are unconfident with your PCB working skills leave the rest alone
for a while. Very likely the potentiometers will need some cleaning as
well.

Second: You likely need a schematic for it. It's not that it is
unfixable without one but most people who could help you will need one
if they even bother to start troubleshooting in Internet. Some techs
won't even work on old solid-state gear without a schematic because
they think it's a waste of time. You likely won't find the schematics
for this amp anywhere (trust me, you're not the first one who would be
searching) so you better just start tracing that out. Boards of Univox
amps from the era should be fairly transparent so this should be very
easy. It will take few hours but you can easily waste days in
requesting and searching schematics (and still fall short in luck). On
top of that, if you ever find one you never know if it's accurate and
you need to sketch the schematic anyway.

A schematic for Univox U65RN can be found here:

Loading Image...

It's from the same era and many features seem to be the same so you
can likely expect a great deal of similarity. These are simple
circuits but you often need to tackle with illogical board layout.
Finding transistor substitutes can be pain as well.

Unfortunately, I can't really comment on the problem. It may be
oscillation but if so the right fix is not to damp it into oblivion
(and at the same time ruin the high frequency response) but to fix the
cause that started it in the first place. I'm pretty sure the amp has
worked properly when new. I would focus on reverb recovery stage and
the stages surrounding the tone controls first since, judging by your
description, these seem to be the culprits. Anyway, renew the
capacitors first since they might as well be the cause of problem that
just reflects itself to certain stages. Poor supply filtering may
easily cause oscillation.

Besides dirty potentiometers and old, drying capacitors you have a
good chance of finding resistors and caps with drifted values, and in
rarer cases transistors that are dying and making strange noises.
Likely the transistors of the power amp are PNP germaniums so they may
be in bad condition. It wouldn't hurt to renew their connection to
heatsink, which means fitting in new insulators and fresh thermal
paste. Likely the heatsink is very pathetic as well (like in most SS
amps from the era). In fact, I'm almost sure that the transistors are
just attached to the chassis. You should see if it can improve the
heatsinking arrangement somehow, especially if your amp doesn't have a
thermal-tracking biasing scheme.

[Dave Curtis]

Post by t***@gmail.com
There aren't any tubes in it and it's from an era when bias adjustment
in a SS amp was luxury. The only thing I can second from mykey's post
is changing those electrolytic capacitors. There's likely a boatload
of them (like in any SS amp from that era) so the task will not be
very pleasant. For starters I would change at least all of the supply
caps and the output coupling cap. The first ones will affect in
reducing amount ofd hum and at best cure all sorts of strange
problems. The latter is a must if the speakers are precious to you. If
you are unconfident with your PCB working skills leave the rest alone
for a while. Very likely the potentiometers will need some cleaning as
well.
Second: You likely need a schematic for it. It's not that it is
unfixable without one but most people who could help you will need one
if they even bother to start troubleshooting in Internet. Some techs
won't even work on old solid-state gear without a schematic because
they think it's a waste of time. You likely won't find the schematics
for this amp anywhere (trust me, you're not the first one who would be
searching) so you better just start tracing that out. Boards of Univox
amps from the era should be fairly transparent so this should be very
easy. It will take few hours but you can easily waste days in
requesting and searching schematics (and still fall short in luck). On
top of that, if you ever find one you never know if it's accurate and
you need to sketch the schematic anyway.

Any Univoxes I've worked on didn't have very transparent circuit
boards. And be careful not to use too much heat or the traces/pads
*will* lift.

-DC

[t***@gmail.com]

Post by Dave Curtis
Any Univoxes I've worked on didn't have very transparent circuit
boards

I don't know about those with tubes but here's a pic of U65RN board:

Loading Image...

If it's anything like this then tracing should be extremely easy. I
second the advise about heat. Be careful with those big radial caps:
Getting them off the board is usually troublesome if you can't clip
the leads. Better heat their solder joints with a lot of power for a
short time than with a low power for an extented period of time.

[xplorations]

Thanks to all who read my post
and responded with appropriate information.

I now know more than I did.

The pots don't 'scratch'
which would be a far simple matter.

I will be pursuing this
and let you know of the outcome.

Appreciation for all the useful advice.

Peace,

Vic
***@earthlink.net

[J.P.]

On Sun, 10 Feb 2008 09:29:00 -0800, "xplorations"

Post by xplorations
Thanks to all who read my post
and responded with appropriate information.
I now know more than I did.
The pots don't 'scratch'
which would be a far simple matter.
I will be pursuing this
and let you know of the outcome.
Appreciation for all the useful advice.
Peace,
Vic

Cleaning pots accomplish more than relieving scratch.

[Dave Curtis]

Post by t***@gmail.com

Post by Dave Curtis
Any Univoxes I've worked on didn't have very transparent circuit
boards

http://i37.photobucket.com/albums/e54/tin_courier/IMG_0329.jpg
If it's anything like this then tracing should be extremely easy.

Yup, looks like the same fiberboard stuff I've seen on my U45b amps.
The later boards aren't that opaque.

Post by t***@gmail.com
Getting them off the board is usually troublesome if you can't clip
the leads. Better heat their solder joints with a lot of power for a
short time than with a low power for an extented period of time.

Exactly. Don't leave the iron on the board for more than a few
seconds.

[J.P.]

Post by t***@gmail.com
There aren't any tubes in it and it's from an era when bias adjustment
in a SS amp was luxury. The only thing I can second from mykey's post
is changing those electrolytic capacitors. There's likely a boatload
of them (like in any SS amp from that era) so the task will not be
very pleasant. For starters I would change at least all of the supply
caps and the output coupling cap. The first ones will affect in
reducing amount ofd hum and at best cure all sorts of strange
problems. The latter is a must if the speakers are precious to you. If
you are unconfident with your PCB working skills leave the rest alone
for a while. Very likely the potentiometers will need some cleaning as
well.
Second: You likely need a schematic for it. It's not that it is
unfixable without one but most people who could help you will need one
if they even bother to start troubleshooting in Internet. Some techs
won't even work on old solid-state gear without a schematic because
they think it's a waste of time. You likely won't find the schematics
for this amp anywhere (trust me, you're not the first one who would be
searching) so you better just start tracing that out. Boards of Univox
amps from the era should be fairly transparent so this should be very
easy. It will take few hours but you can easily waste days in
requesting and searching schematics (and still fall short in luck). On
top of that, if you ever find one you never know if it's accurate and
you need to sketch the schematic anyway.
http://www.univox.org/pics/schematics/u65rn.jpg
It's from the same era and many features seem to be the same so you
can likely expect a great deal of similarity. These are simple
circuits but you often need to tackle with illogical board layout.
Finding transistor substitutes can be pain as well.
Unfortunately, I can't really comment on the problem. It may be
oscillation but if so the right fix is not to damp it into oblivion
(and at the same time ruin the high frequency response) but to fix the
cause that started it in the first place. I'm pretty sure the amp has
worked properly when new. I would focus on reverb recovery stage and
the stages surrounding the tone controls first since, judging by your
description, these seem to be the culprits. Anyway, renew the
capacitors first since they might as well be the cause of problem that
just reflects itself to certain stages. Poor supply filtering may
easily cause oscillation.
Besides dirty potentiometers and old, drying capacitors you have a
good chance of finding resistors and caps with drifted values, and in
rarer cases transistors that are dying and making strange noises.
Likely the transistors of the power amp are PNP germaniums so they may
be in bad condition. It wouldn't hurt to renew their connection to
heatsink, which means fitting in new insulators and fresh thermal
paste. Likely the heatsink is very pathetic as well (like in most SS
amps from the era). In fact, I'm almost sure that the transistors are
just attached to the chassis. You should see if it can improve the
heatsinking arrangement somehow, especially if your amp doesn't have a
thermal-tracking biasing scheme.

On this schematic it has like 6.3v 3.3U ...what's that stand for?