strange brown concert vibrato circuit help please

[nosaj]

If foil caps show increased capacitance, this typically indicates that humidity has entered the interior via micro-cracks. Consequently it's likely to find significant leakage.

Which in the state of Fl would be a very likely possibility.

nosaj

[nickb]

Let me have a stab at decoding how this works. The key principle here is that the gm of a triode and thus the voltage gain, varies with cathode current. The low frequency oscillator feeds the upper half of the 7025 on the right and also feeds an inverter, bottom left. The output from the inverter feeds the other half of the 7025. The output is taken as the sum of the outputs of these two triodes. The voltage swing on these two triodes will be in opposite phases so the sum will have no net DC change. The instrument signal is fed to just the lower one so that its output is amplitude modulated by the oscillator signal. The top triode gets just the very low frequencies of the input signal by virtue of the low pass filter formed by the 47k and the 5nF. This will reduce the amplitude modulation at low frequencies, presumably because it sounds better.

[Jazz P Bass]

A good read here on the circuit topology & function: https://music-electronics-forum.com/...ad.php?t=10295

[Randall]

Wow, that is over my head. But, changing out 6 of the worst out of spec caps in the trem circuit did the trick. It looks as though nearly all of these yellow Astrons had drifted north. If it were mine, I would have replaced them all with Mallory150s, but the customer wanted it to remain as stock as possible, so I stopped once I got the trem working. I'll never understand that way of thinking. For me, it is more valuable if it accurately represents and performs as it was originally designed and built, not with worn out and drifted parts just because they are original. Whatever, it's his amp.

[g1]

If he really loves the sound of it now, he may be worried about changing that (with new parts). Sometimes, the way it sounded when new is a disappointment to someone who loved the tone of their amp with parts drifted out of spec.
Otherwise, I agree.

[Randall]

He recently acquired the amp in non-working condition, so he has yet to even hear it. It had two disconnected at one end filter caps, 470,000 ohm screen resistors, wrong fuse cap, 6 amp fuse, and a frayed 2 prong AC cord. And sadly, one of his 1959 Jensen P10R bluebacks is buzzy.

[Justin Thomas]

Wow, that poor Concert! I'll stop whining about whoever changed literally every cap in my Concert...
Which, incidentally, belonged to an old dude in FL somewhere. I found his son on FaceBook.

Justin

[Randall]

This one just came here from Michigan I believe.

[Helmholtz]

In one of his books Zollner explains the 5G13 "vibrato" circuit:

The action of the LFO makes the signal path shift/alternate periodically between a high-pass and low-pass channel. The sum signal shows:

- frequency dependent amplitude modulation of the spectrum (harmonic content)
- up to 120° phase modulation
- up to +/- 10Hz frequency modulation (a little real vibrato)

[Randall]

Customer picked up the amp today, and he is beyond happy. Apparently, it has been in the family for a long time, it was his father's and hadn't worked for years. Someone hacked it long ago, and there it sat until his father passed. Now it is back, he is excited and happily paid me ~$300. My work here is done.

As always, thanks very much for your help fellows!

[Chuck H]

he is excited and happily paid me ~$300. My work here is done.

So that's $300 for the time you spent with a hot iron (undetermined) and basically zero $ for the time spent obsessing over the issue which included a customer attitude detrimental to the actual repair Easy money

[Randall]

Not exactly, the customer is a good guy, very reasonable and appreciative. The $300 included repairing a buggered octal socket, 7 filter caps, 5 cathode caps, 2 bias caps, 7 trem caps, a couple of 2 watt screen resistors, an AC cord, a fuse holder cap, a bias tune up, speaker buzz diagnosis, a cleaning, four correct Fender screws, and a fuse.

I think that is all.

[Leo_Gnardo]

All electrolytics, PS, bias and cathodes. AFAIK there are none in the trem circuit. There are only five on the main board, all cathodes.

Well there is a 2 uF in the common cathode circuit of the output section, see nickb's kandy-kolored schematic (I applaud!) in post 17. If that cap isn't working right, that's not good for the circuit. 2.2 uF close enough. Since you now have the amp working properly, this info is for anyone who might read this thread in the future. FWIW I replace that cap as a matter of course when working on this series of amps. "If it's still working, it's a miracle. And might quit sometime soon so might as well replace it. Cheap insurance" is what I'm thinkin'.

[pdf64]

The topic of harmonic vib, and mention of Roly Roper in a concurrent thread, reminds me to link to his very helpful explanation of the circuit on another forum https://www.guitargear.net.au/discus...?topic=30731.0

[Helmholtz]

Other than the Fender 5G13 circuit, the AC 30 vibrato uses 2 allpass filters, resulting in phase and frequency modulation but (almost) no AM.