Hot Rod 3 Bright Switch squeels

[beezerboy]

my friend has a Hotrod Deluxe III and everything works like new except switching in the Bright switch makes a squeel (like a shorted input). all the tubes have been replaced with new but it still does it. what to check next/first? tnx

[xtian]

Does it squeal continuously, or just chirp when switched?

The bright cap may be microphonic. You can test this. When bright switch is engaged, tap the bright cap with a chopstick and listen.

Or you may just be getting high Q oscillation. Does the Volume control on amp make a difference? Does it happen with No Guitar plugged in.

[52 Bill]

What he said. Can you change the squeal with the tone of volume controls?

The bright switch on these amps does not bypass the volume control like most other Fender amps. It adds a cathode bypass cap to the second tube stage.

My first thought would be a bad filter cap in the preamp supply.

[xtian]

The bright switch on these amps does not bypass the volume control like most other Fender amps.

Oh. See? He actually bothered to look at the schematic.

[beezerboy]

Volume pot makes no difference. as soon as you start to engage the Bright switch the amp makes that harsh, white noise sound like when you touch the end of the guitar cord with your hand. noise comes as soon as contact is broken, does not have to fully engage the Bright position, and noise is present when fully engaged as well.

thanks lads

[beezerboy]

well.... now it isn't the Bright channel, its the "Normal" that makes the noise. wtf. I took the chassis out of the cab. cleaned and re-soldered bunch of stuff that looked sketchy, pulled C4 & tested it (not shorted, not open), put it back together & same noise. all I can think is the switch was sticking when installed in the fascia panel making me think that was Bright that was the problem. so..... what? C8 next? maybe R43 & it's ground. too late tonight. would also like to hear suggestions for other possibilities

[Jazz P Bass]

I would start by pulling preamp tubes one at a time.

See which one kills the noise.

[beezerboy]

pulling V2 kills the noise. now its back to screechin on Bright, and Normal sounds good. both channels worked as they should for about 2 minutes, then Bright went back to being a problem child. at a loss

[stokes]

What 52Bill said,preamp cap.Seen the exact symptom in more than one of these "Hot Rods",Deluxe,Deville.The cap that decouples V2 is the obvious culprit,but get all those crap filters out of there.

[beezerboy]

I;m a dummy.... give me numbers, I tested C4 and C8 on a Z meter & they met spec. you are talkin about what... C39, 42? 54, 55??? all? thanks & bear with, T

[Jazz P Bass]

Not to sure why no one has posted the schematic.

The preamp bypass cap on the HR is C36.

[beezerboy]

my diagram shows C36 in parallel to the 350V power supply, on the "Z" output. that what you are thinking?

[Enzo]

I thought we were discussing a Hot Rod Deluxe III. C36 in that is on the X node and is 345v. Z node is 430v. X node powers the preamp. Z node is the power tube screen grid supply.


Yes, the idea is to check the cap that serves the preamp circuits, X on my HR schematics.

[Jazz P Bass]

my diagram shows C36 in parallel to the 350V power supply, on the "Z" output. that what you are thinking?

Sheesh.
How are we supposed to help if you are the only one with a schematic?

O/k.
I am at my files.

HotRod_DeluxeIII_schematic_Rev-B.zip

[beezerboy]

OK.... replaced C33, C35, C36. no change. continuous loud screech/buzz whenever Bright is selected no matter what channel (Normal, Drive, More Drive) to me that means K1, Q1, and Q2 and that circuitry are probably not part of the problem (but I could be wrong). the reverb, tone pots and presence setting do not reduce the noise but tone pots change the tone. the volume pot will lower the noise but it is still pretty loud with the Vol all the way down. noise goes away with V 2 out. all the tubes are new, I have switched out V2 with several new tubes that all check good. I have re-soldered the tube socket & the ribbon connector terminals. the first things I did was check what seemed like the obvious... C4 (used a Z-meter), the switch contacts, R17, R43 and the ground. all OK. whats next? and thanks guys

edit: this noise is with nothing plugged into the input jacks

I can plug in and play in the normal (not Bright) mode & its fine on all 3 drive settings. tone pots, etc work as they should. switching to Bright makes the same noise as above