Valve amp fault - nasty buzz

[[email protected]]

Hi,

My Fender guitar amp has developed a fault and I'm pretty good working
with electronics, safety (discharging caps etc) and soldering, but I
don't have the circuit knowledge to work out which components are
faulty. Could someone here offer any help please ?

The symptoms are that it's making a horrible buzz on the 'drive' &
'more drive' channels, as well as letting some sound through on the
clean channel when the volume is at zero.

Here's the full schematic.... http://www.fender.com/support/amp_schematics/pdfs/Hot_Rod_Deluxe_Schematic.pdf

And here's just the preamp section: http://studentweb.eku.edu/justin_holton/images/HRDxpreamp.gif

Here's a Wav file of the noise it's making...
http://www.fileden.com/files/2006/6/26/89819/Amp_Buzz.wav
Be careful when listening to it, it starts off very quietly then
gradually gets loud - so don't turn up your speakers because you can't
hear it very well when it starts !

The increase in oscillation frequency is caused by me turning the
DRIVE knob from 12 down to 0. Turning the BASS knob from 12 to 0 also
has a similar effect. Adjusting any of the other controls does not
cause a change in frequency - only in volume / tone of the buzz.

This is happening *only* on the Drive channel, and gets nastier with
the MORE DRIVE button pushed.

The recording was done with nothing plugged into the amp.

With the amp on the clean channel, you can hear a little output from
the speaker with the VOLUME set to zero - which it never used to do.
Apart from this anomaly, the amp behaves fine on the clean channel
and
I used it for practice this week.

I've tried swapping positions of the preamp valves and also a
different set of power amp valves - no difference. I've also tried
resoldering R78 & R79, which apparently have a tendency to develop dry
joints, and all the big electrolytic caps.

I'm more than happy to give test point readings to anyone who can
help... the schematic gives TP voltages so hopefully that might be of
use.

Anyone ? Many thanks in advance - I really can't afford to take this
anywhere to be repaired.

 

[[email protected]]

There is much more in the drive circuit than valves. There are a
couple FETs an op-amp, zener diodes, caps etc... All of which could
and probably are responsible for that motorboating.

I could track this down in 10 minutes or less with that schematic and
my oscilloscope but you my friend and fellow guitarist haven't a
snowball's chance in hell with this repair.

Well, thanks for that vote of confidence ! ...but I *will* fix
it. ..of that I am certain.
I've successfully repaired many things 'remotely' with the help of
kind folk like Sam Goldwasser.
This is only an amplifier after all.

Here's a web page with some of my gear;

http://s273.photobucket.com/albums/jj220/meatplow1/music/

Err...Ok..... Thanks.

 

[Arfa Daily]

There is much more in the drive circuit than valves. There are a
couple FETs an op-amp, zener diodes, caps etc... All of which could
and probably are responsible for that motorboating.

I could track this down in 10 minutes or less with that schematic and
my oscilloscope but you my friend and fellow guitarist haven't a
snowball's chance in hell with this repair.

Well, thanks for that vote of confidence ! ...but I *will* fix
it. ..of that I am certain.
I've successfully repaired many things 'remotely' with the help of
kind folk like Sam Goldwasser.
This is only an amplifier after all.

Here's a web page with some of my gear;

http://s273.photobucket.com/albums/jj220/meatplow1/music/

Err...Ok..... Thanks.

Sadly, I'm with Meat on this one. I fix these things for a living, and I had
one of the same model doing exactly the same thing a few months back. I
spent literally DAYS on it, and was no nearer when I had finished.
Eventually, as a result of some complicated negotiation between the owner of
the shop it came via, the Fender rep, and Fender themselves, their service
department agreed to take a look at it (normally, they will only take stuff
back that is in warranty, and this was out).

Some weeks later, it came back, and was apparently fixed, although I didn't
see it again myself to confirm this. We tried to find out what exactly the
problem was, but all we could get out of them was a vague-ism along the
lines of "it was a couple of resistors".

It seemed to me that the dirty channel is designed with altogether too much
gain in it, in the first place, made even worse when the 'More Drive' button
is engaged. Personally, I think that the HRD is one of the worst designs
that Fender have ever come up with, but others may think differently.

I wish you luck with it, and if you happen to find out what it is, or
luckily drop on the problem, please come back and tell us.

Arfa

 

[Arfa Daily]

[email protected] wrote in message ...


That buzz is the classic case of motorboating which is usually caused by
a
decoupling capacitor gone bad. The concept is that your amp begins to
oscillate due to feedback, and the feedback path is in the power supply.
The power supply connects all of the different stages of the amp together.
It should be DC, but if a decoupling cap goes bad, the power supply will
have AC. Any high gain stage can amplify that AC which puts more AC on the
power supply which in turn gets amplified, and you get the idea, it
oscillates.
I would suspect any of the filter caps in the power supply, especially the
16 volt supply, in section A4 of the schematic. Check that the 16 volts is
16 volts(plus and minus 16 volts). It would be common for one of those
zeners to go bad or one of the 470 ohm 5 watt resistors to go bad which
would destroy the voltage regulation. The 16 volt supply powers the
opamps.
Test or replace all four of the caps in the 16 volt supply.
You also have four caps in the high voltage section right above the 16
volt
power supply. The later sections such as the 345 volt B+ would be used to
power the preamp tubes. If those caps open up, you probably would get
motorboating too.
bg

That is a pretty fair description of the problems that would normally be
associated with an unstable amp, but assuming that this example is the same
as the one of the same model that I had exhibiting *identical* symptoms,
then a power supply filtering / rail decoupling issue such as a bad cap, is
not what is causing it. It certainly wasn't on the one that came across my
bench. The rails remained very nearly as clean when the problem was
occuring, as when it wasn't. Bear in mind also, that the problem only
affects the dirty channel. You can drive the clean channel as hard as you
like, with any ancilliary control settings, without the amp exhibiting a
problem, which 99% knocks out the power supply, as a cause. As I said in my
earlier post, it eventually went back to Fender's own service department,
but the only comment that they made was that it was "a couple of resistors",
which probably means that they couldn't find the problem in any reasonable
period of time either, so just jammed a replacement board in it, and came up
with the 'couple of resistors' story ...

Arfa

 

[neon]

IF it is motorboating to me it means a decoupling capacitor has given up probably on a tube cathode. c1 i suspect. to isolate short from the beginning the input and see it goes away. then repeat going deeper into the amp. to isolate the stage . it coul'd be power supply cap one of them give up the gost with power off inspect to make sure you have no bulgy capsand with a meter check the ac on the dc component.

 

[[email protected]]

Well, I managed it - despite all the naysayers. It was one of the
22uF caps - C33, C35 or C36. I replaced all three and the problem is
gone !

Many thanks to all who tried to help, and ner-ner-ner-ner-ner to all
who doubted my abilities as a mere mortal. ;-)

 

[Jamie]

Good for you. I like a happy ending.

Oh yeah?
Been to a chinese restaurant lately ?



http://webpages.charter.net/jamie_5"

 

[Arfa Daily]

Good for you. I like a happy ending.

Likewise. It's written down now, so if I get another, I'll try those caps.
It's got that 'feel' about it, that it might become a stock fault.

Arfa

 

[Arfa Daily]

Likewise. It's written down now, so if I get another, I'll try those caps.
It's got that 'feel' about it, that it might become a stock fault.

Arfa

Additional note.

I have had a look at the schematic now to check what those caps were, and I
was surprised to find that they were the HT filter caps. Although I didn't
actually change the ones in the example that I had with the same problem, I
did give them exhaustive checks for ESR and value, and they checked out just
fine. The belief that they were indeed OK, was borne out by the fact that
the rails at 'x', 'y', and 'z' were all fundamentally clean, and became only
very little worse, when the amp was unstable. It's also hard to see how a
problem with filtering on these rails, which feed all the tube HT supplies,
would affect only the dirty channel. Given the fact that these caps are
relatively small in value in the first place, I guess that it's possible
that only a small reduction in performance of one that filters the rails to
the preamp tubes, might be enough to cause a stage to become unstable, when
it's working harder under 'drive' (dirty) conditions as opposed to when it's
switched to 'clean'. If this is indeed the case, then it's a pretty lousy
bit of design work on Fender's part, that a stage could go unstable with so
little provocation.

Just as a matter of interest, did you manage to actually fault-find the
problem to these caps, and if so how, or was it a case of gut instinct and
shotgun replacement ? Always interesting to know how others arrive a
solutions to odd problems.

Arfa

 

[Jamie]

I ordered carryout the other day. Hot and Sour soup and Hunan Beef
with a pork eggroll.

Didn't ask for the happy ending ?


http://webpages.charter.net/jamie_5"

 

[N_Cook]

Additional note.

I have had a look at the schematic now to check what those caps were, and I
was surprised to find that they were the HT filter caps. Although I didn't
actually change the ones in the example that I had with the same problem, I
did give them exhaustive checks for ESR and value, and they checked out just
fine. The belief that they were indeed OK, was borne out by the fact that
the rails at 'x', 'y', and 'z' were all fundamentally clean, and became only
very little worse, when the amp was unstable. It's also hard to see how a
problem with filtering on these rails, which feed all the tube HT supplies,
would affect only the dirty channel. Given the fact that these caps are
relatively small in value in the first place, I guess that it's possible
that only a small reduction in performance of one that filters the rails to
the preamp tubes, might be enough to cause a stage to become unstable, when
it's working harder under 'drive' (dirty) conditions as opposed to when it's
switched to 'clean'. If this is indeed the case, then it's a pretty lousy
bit of design work on Fender's part, that a stage could go unstable with so
little provocation.

Just as a matter of interest, did you manage to actually fault-find the
problem to these caps, and if so how, or was it a case of gut instinct and
shotgun replacement ? Always interesting to know how others arrive a
solutions to odd problems.

Arfa

Of course with this sort of problem it could be just fiddling inside
disturbed something , even just wiring looms and the onferlying problem is
still lurking there.
Unless something definitely found wrong with one of those caps

 

[Arfa Daily]

<snip>

We are relying on the OP being honest here. Personally I would
question this based solely upon the neener neener reply. I'm a firm
believer in "if the shoe doesn't fit" in these cases and would have
expected to find the problem in something other than the mains
smoothers if upon nothing else than my 30+ years repairing guitar
amps.

Likewise, based on a similar 30+ years of repairing said equipment.

Arfa