Hot Rod Deluxe Oscillation.

[brilliantblue]

Hello and thanks for reading.

1999 Hot Rod Deluxe.

I fixed and a different problem and verified functionality with testing (replacing switching jfets for More Drive feature) and was about to button this amp up for a friend, when I decided to "improve" an area of concern.
The circuit board also had the notorious 16v +/- supply dropper resistor blackening, and the 330 ohm resistors were tight to the board, not helping things. I determined to help avert future problems there by lifting the resistors, but found that a previous tech had apparently used some of their lead-length to jump parts of the trace that had been lost. I opted to use wire, and the specific points where I used them was from R78 (330ohm) to R91 (10k), and from R79 (330ohm) to the neg lead of C42.

Now, unfortunately, I have oscillation in the Drive/More Drive Channels. Do any of you see where these jumps would cause it? I believe I have followed both the trace and schematic effectively.

The tone is low midrange, near a G#, second octave on a guitar.

Pulling v1 does not stop it, pulling v2 does.

The note of the oscillation drops several cents when turning the treble knob to one side.

No oscillation in the clean channel, not even apparent "bleed" when cranked.

I've found a couple of other threads elsewhere online recommending reflowing solder joints and testing power caps with a known good power cap - I tested all power caps and reflowed all joints in the preamp section. I also swapped around the preamp and PI tubes, but with no change. I found one thread with this exact problem as mine but the last post was "take it to a good service person".

I recognize that something may have been loosened during the numerous times moving the circuitboard out from "home" position and flipping it for soldering/testing etc, but I've not been able to find the culprit. I reflowed all solders on the tube connection multi-lead cables as well, and verified continuity from on both ends.

I have an oscilloscope that I plan to get out later this evening, but I may miss some areas of concern - my prior experience is with more simple circuits. I'd appreciate any suggestions in tracking this bugger down!

Thank you any help.

Jeff

[Mick Bailey]

Are you getting +/- 16v on the opamps? Also, have you checked for solder or flux bridges? My thoughts are that if these are ok then you're looking for something else that's been disturbed as these are DC voltages and don't lie directly in the audio path.

Sometimes pulling back the main PCB to work on it disturbs the orientation of the ribbon cables. In certain amps these need pulling outwards to remove the kink and this can stop oscillation.

I would also swap over V1 and V2 to see if the fault condition changes. Feedback in these amps is mainly due to a faulty preamp tube and it could be pure coincidence that this has started to happen after you'd worked on the amp.

EDIT: Ah! I see this problem has already been posted. The man who chases two rabbits catches neither.....

[g1]

Like Mick said, the dress of the ribbon cables can lead to oscillation in these type amps, it's quite critical. Also any test leads etc. routed near the ribbon cables, or even not having the rear panel with it's shielding mounted can contribute.

[stokes]

Merely clipping a good filtr cap across a bad one doesnt always "cure" a problem.Think about it,you have a shorted preamp stage filter ,even if you clip a good one across it,the bad one is still shorted.Not saying you didnt move something and cause the oscilation,or that I disagree with Mick or g-gone,,good points,I'm just offering another,but I've seen way too many of these Deluxe,Devilles and Blues Jr's with an oscillation and all of them were a bad filter cap.I think Fender got an awful big load of bad caps.I would suggest changing the preamp filter associated with the dirty channel,you say its not there in the clean channel,dont just clip in a good one,remove and change it.If that cures the problem,go ahead and change the rest of them craps..er,caps.

[Randall]

FWIW, the Fender authorized shop here that I bench part time at will call Fender Support occasionally, and is often directed to the 22uF 500V filter caps in these newer type of amps.

[brilliantblue]

Thanks all, it turns out this problem is truly related to the jfets - they worked for awhile and then shorted, causing the oscillation. They worked long enough that I thought that issue was cured, but apparently not. Perhaps there's another problem in the low voltage supply. Thanks again for your help!

[Mick Bailey]

Merely clipping a good filtr cap across a bad one doesnt always "cure" a problem.Think about it,you have a shorted preamp stage filter ,even if you clip a good one across it,the bad one is still shorted.Not saying you didnt move something and cause the oscilation,or that I disagree with Mick or g-gone,,good points,I'm just offering another,but I've seen way too many of these Deluxe,Devilles and Blues Jr's with an oscillation and all of them were a bad filter cap.I think Fender got an awful big load of bad caps.I would suggest changing the preamp filter associated with the dirty channel,you say its not there in the clean channel,dont just clip in a good one,remove and change it.If that cures the problem,go ahead and change the rest of them craps..er,caps.

Agreed;
I change lots of the 22uf grey Illinois caps. They often have a yellow crust appearing around the +ve end. They mainly go high ESR and cause all kinds of problems. Initially this will be a vague "doesn't sound quite right" but eventually ends up being a definite fault.

Clipping in a filter cap has some misleading results if; a) the original cap is either shorted or leaking DC, or b) the solder joint has gone bad.

[brilliantblue]

Thanks for the info Mick! and Stokes!

The original Illinois caps had already been replaced with Sprague Atoms. Thanks for the tip on checking caps. I knew it "clipping in parallel" was "quick and dirty", but thought a significant change in the oscillation frequency would've been a clue. I did use the original cap legs for the positive, but ran the neg to the chassis.

I pulled the "more drive" switching jfets out and the oscillation disappeared.

Agreed;
I change lots of the 22uf grey Illinois caps. They often have a yellow crust appearing around the +ve end. They mainly go high ESR and cause all kinds of problems. Initially this will be a vague "doesn't sound quite right" but eventually ends up being a definite fault.

Clipping in a filter cap has some misleading results if; a) the original cap is either shorted or leaking DC, or b) the solder joint has gone bad.

[Radmaximus]

Check your C36 cap to, just fixed one that just went wild when in Drive mode and the tone controls were turn up past about half way. That cap was bad and needed replaced, caused all kinds of havoc!

Radmaximus

[brilliantblue]

Well, Its just in MORE DRIVE mode now.

I'll let the other thread, related to the jfets, die, as they have been apparently effectively replaced and I do now have three modes: Normal, Drive, and More Drive.

However, that ugly oscillation now persists only in More Drive mode - none of the others.

One question: Does this amp require, or depend on for ground, the circuit board to connect to the control panel? If i have everything in place, the amp has the three modes. However, if the controls are unscrewed from the panel and the circuit board backs off a mm or so, the Drive mode defaults to More Drive, and the oscillation returns. I've tried using jumpers between the knob shafts and chassis to provide "ground", and between the pot frames and chassis as well.

Yes, I do have the green ground wire at the input attached to the chassis - but some other aspect of the circuit, apparently the low voltage or drive/more drive switch, appears to be relying on mechanical ground.

Also I'm wondering if I could have reversed something in the pt wires, or if that could even contribute to an oscillation. I marked them all CP4, CP2 etc when I first detached them to gain access to the solder points, but found that I had mis-marked one when putting them back - apparently I'd marked CP1 as CP6. Since I made that error, I recognize I could've made another, but would anything in that section contribute to oscillation if the amp passes signal in Normal and Drive modes? I mean, it sounds great for an HRD in those modes. Also, all AC wires (filament, b+, low voltage at least) are twisted pairs.

Thanks for your insights. I'll swap out C36 now because you've all suggested it. Hopefully that will cure the amp, but I have my doubts considering the good Drive performance. Hopefully I'll be surprised!

[Enzo]

You may now have three modes, but are all those TP voltages now pretty close? TP31-36.


You PMed me about buzzing relays. ( really it is better to keep all the discussion out here) You can sit your finger on top of a relay and feel it vibrating in those cases. And the sound will be coming from in the chassis.

[brilliantblue]

I confirmed swapping out C36 for another cap - still having the oscillation.
I looked at it with a spectrum analysis app - 150hz.

I'll double check all test voltages.

Can anyone confirm or deny whether the circuit board needs contact to the chassis for total functionality - any mechanical grounding in the design?

[Enzo]

Look at the jacks, the front surface that would press against the chassis wall. On SOME jacks there is a small little point on the front to make contact with the chassis. Do your jacks have that? Other jacks have metal bushings to make contact with the chassis, do yours? Sometimes with plastic jacks they put a sort of solder lug thing around the bushing and soldered to ground on the pc board. That lug around the snout of the jack makes contact with the chassis. Yours?


Out of the chassis, look at the pot brackets. They should all be "wired" together. On the schematic is a 10 ohm resistor by the input, that says to pot brackets. Verify continuity between ALL controls's brackets.

[52 Bill]

I confirmed swapping out C36 for another cap - still having the oscillation.
I looked at it with a spectrum analysis app - 150hz.

150hz, Is this right? That's more like a ground hum. Do any of the tone controls or volume controls affect the noise?

[Enzo]

I won't yet rule out the buzzing relay thing. it isn;t all that common, but it happens. And 150hz is more or less the sort of freq it would make.