Hot Rod Deluxe fault - any techs here ?

[aneng]

Hi,

My HRD has developed a fault and I'm pretty good 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_sc..._Schematic.pdf

And here's just the preamp section: HRDxpreamp.gif

Here's a Wav file of the noise it's making... 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.

[rudutch]

the seach function is your friend, there are many postings

fwiw - PCB mounted tube sockets and power resistors seem to be the normal trouble areas. IF you have not swapped preamp tubes, then I would start there

power resistor issues look like this:

[NTBluesGuitar]

Have you taken a visual inspection to see if anything stands out to you?

The most common problem in these amps are cold solder joints. Since you say you started with the 5W resistor joints, I would check the joints on V2 (and all the tube socket pin joints for that matter) next.

The motorboat/motorcycle oscillations can have several causes. Bad filter caps, improper lead dress, improper grounding, bad filtering (inadequate de-coupling), or a conductive component board. In the Hot Rod's case, there are some solid state components in the Drive/More Drive channels to add to the mix.

How old is the amp?

[aneng]

The amp is only 18 months old - but because I'm in the UK, it's out of warranty (only 12mths here). Besides, I returned it under warranty once before, and it took six weeks to come back. I can probably repair it myself in far less time - hopefully !

I'll resolder the valve sockets, but if it were those wouldn't the problem also show up on the clean channel ?

I can't see anything obvious visually - apart from what looks like a minor seeping on the +ve end of C33 here... (I scraped half the deposits away).

[mageerc]

That cap is toast!

[NTBluesGuitar]

A bad cap and the amp is only 18 months old? Hmmm...an easy fix, but you'd void your warranty doing it yourself.

[ziess]

Warranty in the uk is 5 years just like everywhere else.

[steve25]

that cap is not bad, its just silicone glue use as a anti vibration compund! I do that all the time my self on amps that I know will be bouncing around in use on the road 4 or more days a week.

[mageerc]

that cap is not bad, its just silicone glue use as a anti vibration compund! I do that all the time my self on amps that I know will be bouncing around in use on the road 4 or more days a week.

I have to disagree with you on that cap... I'm not talking about the clear silicone under the cap, rather it's the electrolyte that has clearly leaked out of the cap. He even said that he'd scraped half of it away.

[NTBluesGuitar]

Yep, that dust that resulted from the scraping is NOT silicone.

It even looks like flux to me, but not silicone.

It's under warranty, so I'd take it back to the store (if you can), and get an exchange.