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Old 16th Oct 2021, 1:10 pm   #1
ITAM805
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Default Oscillating Fender

Got this Fender Blues Junior 3 on the bench with what sounded like a case of bad HT caps, but I replaced the four 'IC' brand HT caps, a well known weakness in Fender amps but to no avail

Scoping the amp I found it was oscillating way up in RF spectrum, this I traced to the phase splitter. I tried redressing the ribbon cable from the main board to the valve strip, but this made zero difference, the oscillation was unaffected on the scope trace as I bent the cable in various positions? Looking through my notes I found that this can sometimes be cured by adding a 100pF across the anode resistors (R29/30), usually a reliable fix for obstinate oscillating BJ's, but not this time?

I have tried another pair of EL84's, ditto with the 12AX7 phase splitter. I tested the 4 HT rectifier diodes but they appear OK. Also I re-flowed the ribbon cable entry points to the valve board and also the valve bases, but the hum/oscillation persists.

I've now run out of ideas?
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Old 16th Oct 2021, 6:24 pm   #2
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Default Re: Oscillating Fender

Years ago, a local tech telephoned me to have a moan about a Fender sent into him for repair by another shop, it was oscillating.

I asked him what the original fault was? He did not know, but said the output TX had been replaced.

Immediately, I asked if the feedback tap was correctly wired; was it giving positive feedback? He called back ten minutes later to thank me for the diagnosis, the TX had been wired wrongly.
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Old 16th Oct 2021, 7:42 pm   #3
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Default Re: Oscillating Fender

What frequency?
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Old 16th Oct 2021, 8:08 pm   #4
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Default Re: Oscillating Fender

Thanks Kevin, dont know if this has had a new o/p tx fitted, however I shall unsolder the NFB wire and see if it stops oscillating?

Unsure of the frequency DA, my scope is an old steam driven D1011 with no such readout, that said I did try it with a little DSO150 scope which has a limited bandwidth, but it read as 250kHz. What's the significance of the frequency?
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Old 17th Oct 2021, 10:46 am   #5
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Default Re: Oscillating Fender

Might just be "FSD" for the DSO150 whose bandwidth is only 200kHz.


You can get an idea of the frequency from your D1011- use it to time the interval t between peaks of the oscillation waveform and calculate f=1/t.


At least the Telequipment won't alias......
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Old 17th Oct 2021, 11:30 am   #6
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Default Re: Oscillating Fender

What frequency it's oscillating at dictates what R/C filter values you need to stop it. We have to reduce gain @ x frequency. Try a 1k & 2200 - 4700p in series across anode resistors or cathode tail resistor. Just tack email on & test. R/C filter
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Old 17th Oct 2021, 12:14 pm   #7
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Default Re: Oscillating Fender

Thanks Chris, I never sussed out how to read time constant thingy's, I'm mathematically illiterate, combined with being a lazy type . The trace is set to 5µs and I count 4 graticules between peaks if that makes sense?

Thanks Andy, I get what you mean now. My worry is that there's something wrong rather than just instability, a fairly common problem with certain Fender models that use the semi rigid ribbon cables, and I might cover that up? On the other hand, a fix is a fix!

edit: I just tried a 2n2+1k across R30 as suggested and the oscillation has stopped, but it seems to have sucked the treble down as well, more experiments needed

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Old 17th Oct 2021, 1:03 pm   #8
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Default Re: Oscillating Fender

Something that can happen with old fender amplifiers (especially if they have been exposed to damp conditions) is that the black fibreboard used to layout the components on becomes slightly conductive. This can give rise to all sorts of weird effects that are almost impossible to pin down.
I have had this happen on a couple of amps over the years and my solution has been to rebuild the circuit onto a new fibreboard base. These are available from specialist valve amp repair places.

Here's one:
https://www.mojotone.com/amp-shop/pa...-boards?page=1

Steve.
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Old 17th Oct 2021, 1:48 pm   #9
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Default Re: Oscillating Fender

Quote:
Originally Posted by ITAM805 View Post
Thanks Chris, I never sussed out how to read time constant thingy's, I'm mathematically illiterate, combined with being a lazy type . The trace is set to 5µs and I count 4 graticules between peaks if that makes sense?

Thanks Andy, I get what you mean now. My worry is that there's something wrong rather than just instability, a fairly common problem with certain Fender models that use the semi rigid ribbon cables, and I might cover that up? On the other hand, a fix is a fix!

edit: I just tried a 2n2+1k across R30 as suggested and the oscillation has stopped, but it seems to have sucked the treble down as well, more experiments needed

4 times 5uS intervals between peaks is 20uS between peaks, so frequency is 1/20uS = 50kHz


In this case, time constants don't come into it, they're either R*C or L/R.


This case is just the basic relationship for a repetitive waveform where frequency(f) = 1/period(t) and vice versa (t=1/f)


HTH
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Old 17th Oct 2021, 2:45 pm   #10
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Default Re: Oscillating Fender

Thanks Steve, this has the cream board but agree about the black board problems though, eeeekkk!

Chris, I'll get it one day ...

I settled on 470pF+1k across R30 which has stopped the osc without affecting the top end, thanks Andy What's the resonant frequency of that?

Still the got hum or rather buzz (100Hz) which I thought I'd found the cause of, an open 100R screen resistor (R35) but no it's still there, although the amp sounds better unsurprisingly. From experience I know eliminating hum can be a proper pain in the
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Old 17th Oct 2021, 4:26 pm   #11
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Default Re: Oscillating Fender

470pF + 1k doesn't have a resonant frequency as such, you'd need some inductance as well. It does have frequency dependent impedance and phase characteristics though. The series 1k dominates at high and the 470pF at low frequencies with the frequency of the transition from capacitive to resistive impedance determined by the relative values of R and C.

Just looking at the circuit and the interesting sprinkling of capacitors around the phase splitter circuit suggests that the designer(s) had some fun making it stable with the degree of feedback chosen. Stray tolerances in the output transformer may be a bit loose making the whole setup potentially twitchy!
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Old 17th Oct 2021, 9:30 pm   #12
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Default Re: Oscillating Fender

Thanks Chris

Well I found one of the main reasons why the amp is buzzing. One of the first things I did when I got the amp was a cursory check of the grid bias voltages, it was 11V on both valves, 10V is the spec so ball park. Today I got round to testing the anode currents, one valve was dissipating 14W, the other 21W, somewhat hot for an EL84 . I trimmed the bias network and got it down the best I can but the valves are now way out off balance, one testing about 70% emission despite supposedly being new!
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Old 17th Oct 2021, 11:17 pm   #13
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Default Re: Oscillating Fender

These are a particularly nasty little amp!!! I always rip out the super rigid flat cable and replace them with flexible wire. I have posted pics here before showing what happens. Basically you only get one "chance" at flipping the output board PCB over. Check plate resistors in the phase splitter, they are usually modern 1/4 watt and prone to failure. I replace them with 1 watt metal oxide as they still fit the PCB. I have had great success removing the whole output valve PCB and hard wiring directly to the valve base and PCB. When I do this I also change the EL84/6BQ5's with either 6V6 or 7C5. A VAST improvement in everything except output power which does go down a bit. The last "decent" Fenders were made in the early 60,s and did have the bit of cardboard tagstrip. I guess its dry enough in Au as I have never had a problem with it.

HUM ?? What hum? Most guitarists can't hear it, or say its a toob amp and all toob amps hum.


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Old 17th Oct 2021, 11:19 pm   #14
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Default Re: Oscillating Fender

I should have also mentioned, when I change the output bottles I remove the fixed bias and fit cathode bias, combined so both valves run together.

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Old 18th Oct 2021, 3:12 pm   #15
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Default Re: Oscillating Fender

Thanks Joe

Quote:
HUM ?? What hum? Most guitarists can't hear it
Actually, the owner, a guitarist, passed to me because he said it hums
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Old 20th Oct 2021, 3:37 pm   #16
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Default Re: Oscillating Fender

I agree,there may be another fault elsewhere causing the issue, but using a RC filter to stomp on the oscillation saves mucking about.

1k & 470p - 1/(6.26 x 1000 x 0.000000000470) = 338,799hz or 340khz ish which doesn't make sense. It depends where you've tacked it onto though, the anode R or cathode R will be influence things.

Using an ECC83 as a phase splitter to my mind is plain daft, Fender would have been better with a low gain LTP using an ECC82 preceded by more voltage gain stages perhaps. They were probably trying to save money.

Another way to stomp on the oscillation is to do the same thing from each OP valves anode to ground as well as popping a zobel network across the OPT secondary, a belt and braces approach.

Glad to hear it's sorted anyroad.

Andy.
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