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Frontman 212 reverb question

Started by gbono, June 07, 2016, 08:29:42 PM

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gbono

See the section of the schematic from a Fender Frontman 212. What is the purpose of D15? It's back biased by a -16V supply.

Enzo

Offhand, I'd say it was there to prevent any inductive kickback from the transducer coil in the pan.

gbono

Odd, I've never seen a flyback diode used on a reverb transducer? This amp's reverb isn't working but the driver OPAMP has +-15V rails and pin 1 of U4 has a nice strong output along with a small signal at pin 7, the reverb tank has appropriate DC resistance at the input and output and all cables check out. I can hear a faint "ringing" when I tap the pan but the signal is very weak.

Enzo

make sure the shield side of the drive RCA jack on the pan is NOT grounded to the frame or chassis.  Pull the return cable out of the OUTPUT jack on the pan and touch the tip of the plug with a finger.  Does that make loud hum out the speaker.  Assuming reverb control is up midway.

The entire recovery stage is just U4b and a few small parts.

gbono

#4
Strange problem here - I couldn't find an issue with the circuit and finally subed in another pan and now the reverb works. Trouble is the "bad" pan has continuity on the input and output coils - they measure 170 and 58 ohms. What else to check? I tried a third pan from an Orange Crush and it works fine too. The cables to the pan's in/out off the PCB were used on all three pan "tests".  :grr :grr :grr

phatt

#5
Obviously that tank has an issue.
As Enzo noted ,, the circuit requires the "Drive RCA socket" to be isolated from the Tank Chassis.
(That connection at the tank can short out causing problems with current drive reverb) 

If the PU end of circuit works,, i.e. tap the hot tip of RCA plug and Amp buzzes then it's the tank end RCA socket.
It maybe an intermittent issue caused by corrosion of the contact area.
Or,,,
I've had hair strands of wire dead short parts of circuits and they are a pain to find.

So you need to look very closely and do continuity checks through every part of the circuit path to track down bad connections.
Phil.