Fender Hotrod Deluxe Circuit Board Buzzing

[VGuitarist]

Hello,

I recently noticed a moderately loud buzz (switching noise?) coming from my 2002 Fender Hotrod Deluxe tube amp. It appears the buzzing is centered around the rectifier diodes D6-D9 (not transformer noise). Per the schematic, there is not much in this section -- four rectifier diodes and non-polarized capacitors. The large filter caps are further to the left (none of them are leaking or bulging). The amp works fine and the audio is clean, but the buzz is driving me crazy. Does anyone have any idea what is causing this?


I recorded the buzz and posted it here: https://soundcloud.com/jeffk28277/amp-buzz

Thanks in advance.

-VGuitarist

[TomCarlos]

VG -

Interesting sound.... thanks for posting the audio file.

Have you tried to probe with a chop stick to see if anything triggers the sound (higher, lower)? What about the bottom side of the PC board? Do all the solder joints look ok?

See if you can make a stethoscope using a plastic tube - in an effort to zoom in on the potential problem.

And you are sure this is a mechanical buzz, always there, regardless of a guitar plugged in, regardless of the volume level?

BTW - Your first post? Welcome to this forum.

Tom

[Enzo]

Does changing channels make any difference?

I have encountered buzzing relays in the Fender circuits. Put your finger on the top of each relay to see if one feels like it is buzzing. I would go through the test points in the footswitching circuits, as they control all the relays. Seems to me an LED in the circuit was causing a wrong voltage drop.

[TomCarlos]

VG:

One other thing.... and this may or may not lead to a solution for your specific problem... let's solve your problem first then address "extras" later. Don't go replacing everything suggested (below) until we find the cause of the buzzing (and fix that). Do a google search on "fender hot rod deluxe buzzing problems". You will find other posts (other blogs and YouTube) describing something similar to what you are describing; some of those posts are here on MEF. These posts (in addition to what Enzo suggests) could help you check for problem areas.

I worked on a HRD amp once. It was suggested that I change out the plate resistors on the preamp tubes (R4, R11, R16, R22, R57, R58) from Carbon to Metal @ 1/2 watt. (In fact, two of my resistors were way off value- so it was a good suggestion). These plate resistors tend to fail. Also there are two resistors (R78, R79) that heat up and burn the PC board. Your test points on these are +48 volts and -48 volts. I changed those resistors and (slightly) lighted them off the PC board in order to help dissipate the heat generated by each. Check out this video to see what I am referring to: Two Main Problems in the Fender Hot Rod Deluxe.

Tom

[52 Bill]

I think Enzo is on the right track, there was a very early tech bulletin regarding relay chatter. It required a resistor change in the switching circuit. If you can't find the info on-line, I'll look and see if we still have a copy in the shop.

[TomCarlos]

Bill -

I found this thread on MEF - Fender Hotrod Deluxe relay chatter...

See entry #21.

[TimmyP1955]

Bill -

I found this thread on MEF - Fender Hotrod Deluxe relay chatter...

See entry #21.

There is no entry 21 :-)

[TomCarlos]

Make sure you are viewing threads in the "Hybrid" mode. Please revisit that thread: https://music-electronics-forum.com/showthread.php?t=46358

Dangling Fury wrote this:

"I actually bought 10 watt resistors... And I did mount them off the board... Well I finally got this dude repaired....Fighting mysrlf all the way... so I replaced Q4 today... that made the amp get stuck in the drive channel... Racked my brain as to what it could be... Then I changed R78 and R79 to 470 ohm 10 watt resistors...still stuck....Then I looked at Q4... NPN dummy! I put a PNP there by mistake so I switched it to an NPN... still stuck... Apparently the trace on the +16 volt line was just barely not touching the pad after I replaced them.... so.... finally.... After all of that, Im thinking the trace on thre +/- 16 volt supply was very brittle and intermittent on and off caused the original problem.... Q4, Q3, and U3 being bad is what caused my grief... The brittle traces probably caused it all... I remedied that... lol! Thanks again everybody! This forum is the best!"

Thanks!

Tom

[g1]

Here's the direct link. It shows as post #20, no idea why hybrid mode claims there are 21 posts.
https://music-electronics-forum.com/...l=1#post492304
(if you right click on the post number, and 'copy link location', you can link directly to a specific post)

[SoulFetish]

I'm not sure how loud that noise actually is. The recording is very clear though. My question is, can you hear the noise with the back panel on it and the amp assembled?
To me it sounds like buzzing ceramic snubber caps they bypass the rectifier diodes with. Every Hot Rod I've serviced (that I can remember) will do this. It is audible may be worse in some cases over other. If you don't hear it with the back panel on and amp powered on, out of standby, then don't worry about it. That's normal for these amps, as annoying as it is. But if this was loud enough that this is why you removed the back panel, and wasn't there before, then you probably have an bigger problem.
Is the sound coupling into the audio so you hear it through the speaker?

[VGuitarist]

Thank you for your replies. I have the amp apart and have inspected the solder joints using a large magnifying bench light. No cracked or burned pads/traces...they are all as shiny as when originally wave-soldered. Note: the sound is audible even with the back panel secured (seems to come through the top control panel). Is there a way to attach a couple pictures? I'd like to show you where the sound seems to be coming from on the circuit board.

-VGuitarist

[Enzo]

HAve you done my finger on a relay test? CAREFULLY, ther is high voltage present.

We need to determine if it is or is not relay chatter. It is real doubtful the diodes themselves are making any noise.

You can roll up a magazine, or use a cardboard tube from paper towels. Hold it to your ear and use it as a stethoscope to listen closely to various areas. You can hover right over your diodes to see if they are buzzing. The sound in that chassis will bounce around. Your ear tube tool will help pinpoint the source.

[TomCarlos]

VG - Read the info (attachments) on this page https://music-electronics-forum.com/...b3_attachments.

When you Post or Reply, make sure you select "Go Advanced". That will get you to an icon that looks like a Paper Clip or if you look below, you will see the "Manage Attachments" options.

[SoulFetish]

It is real doubtful the diodes themselves are making any noise.

It isn't the diodes that make the audible noise I was referring to. It's the ceramic caps they use as snubbers across the diodes. It's a acoustic noise they generate, and doesn't couple into the audio signal. But it can be loud enough with the back panel off, that it might sound like there a problem if you weren't familiar with the amp.
I thought there was a problem the first time I heard it, but I realized that it's normal operation for Hot Rods.
Similarly, I remember connecting an output transformer to a dummy load while driving a 1kHz signal through it for the first time. No one told me that OTs will audibly ring under these conditions, so the first time it happened, I immediately shut the amp off and removed the input signal thinking I just f'ed up my transformer.
In any case, if the OP is hearing the noise through the top of the amp with the back on, it almost certainly is not the snubber caps. I'm going to double back and read over the thread and listen to the clip again.

[VGuitarist]

You can roll up a magazine, or use a cardboard tube from paper towels. Hold it to your ear and use it as a stethoscope to listen closely to various areas...Your ear tube tool will help pinpoint the source.

Enzo, great idea on the tube! The sound seems to be slightly to the right of the diodes -- although, still tough to tell. I'm now focused in the area around the diodes, C31 (47uF/500V), and C33 (22uF/500v). See attachments.

Again, none of the caps are leaking or bulging.