Fender 68 Custom Twin Reverb LF feedback howl, HF feedback shriek

[nevetslab]

I have a brand new Fender 68 Custom Twin Reverb that has loud LF & HF feedback, which a client just sent over. I preset the tone controls mid-point, volumes fully CCW. Reverb off Silver Face unit. After powering up in Standby, I finally switched into Operate, and began bringing up the Custom Volume pot ('Normal' Ch on regular Twin). Immediately greeted with loud LF feedback! Turned the volume back to 0. Grabbed the Vintage Ch Volume pot ('Vibrato' on regular Twin), and here, I was treated to loud HF feedback! Turned that volume back to 0. It was sent here due to the HF shrieking, as they put it, and supposedly only happens with the Volume past 5, and the Treble control all the way up. It's worse than that.

I pulled the chassis, brought up a 65 Twin Reissue schematic, not having the 68 Custom schematic. Looking at the output stage, I see V7/V8 with BRN O/T primary lead, and V9/V10 with BLU O/T primary lead. Checking to see which Org wire is connected to the grids, as maybe the output driver wiring is reversed??

From R45, that lead is showing up at.......V9/V10 grids, and from R44, that lead is showing up at.....V7/V8. So much for that theory. Driver, output tubes and O/T appear to be wired correctly. Checking the feedback resistor R39....connected to the Output Jack Tip. Wire color there is BLK, and to the GND terminal....BLU-GRN. Rats...that agrees with the schematic.

Any ideas on this one?

65_Twin_Reverb_SvcMan.pdf

Looking at the Preamp Stage wiring, there is a blue wire connecting V1B plate & V2B plate together. The cathodes for V1B & V2B are connected together at R23 as normal. Why would the two plates be connected here?

[The Dude]

Has the OT been changed? Those transformer wire colors aren't always correct when you replace one. For that reason, I've learned never to trim the wires until I verify that it works. I've had quite a few where the primary wires were reversed. It may be worth trying a quick swap.

[pdf64]

Maybe c34 (preamp HT decoupler) is bad?

The 2nd stage plates of either channel are probably connected so that both channels have access to the fx.

[Enzo]

Pull one power tube, any change?

When you have your big osc, is it riding the B+ anywhere?

[nevetslab]

Thus far, it's not a power tube issue. This chassis was a good example of extremely poor lead dress, and how to couple heater field into the signal leads. The Screen resistors were installed with full lead length, standing way up in the air. The plate leads on the power tubes pairs were dressed with the grid and cathode leads. All the AC mains primary/secondary leads laid in nice and straight the full lenght to where they plug into the PCB's male fast-ons, using the locking insulated female fast-ons. Don't ya just love those connectors. You can nearly (or often) yank the male blade right out of the PCB they lock so well!

So, after a bit of cursing in trying to get the screen resistors out of the terminals and not burn all the adjacent heater leads with the wire having memory to remain in the damage path of the soldering iron....I got the harness wiring properly dressed, though I really knew it wouldn't have any change.....just wanted to undo that clap trap.

Before and after the cleanup in the chassis wiring, the results remain. This time, just so I'm not listening to the loud roar from the speakers, I have it connected to the dummy load. Now, with 4 ohm resistive, the self-oscillation won't occur, but, unloading it to 16 ohms, which is probably low relative to the speaker impedance where the LF & HF oscillation occurs (no signal applied...just turning up the volume on either channel, and using the bass and treble controls to set it into orbit). What I'm seeing on the output of the choke (feeding the screens from the doghouse below), there's loads of ripple/signal on the DC mains, though I haven't looked with the scope as to the waveform.

At idle, volume pots fully CCW, tone controls at mid-position, the hum on the output of the choke is relatively low, but.....it's not a steady level...it's wandering up and down....in the range of 10mV to over 100mV. So, that DOES point to the power supply caps.

So, I'm about to flip the chassis over and go open up the doghouse to see what's inside.

I haven't yet reversed the plate leads of the output xfmr. This IS, after all, a brand new 68 Custom Twin Reverb. I do have to wonder how this made its' way out of the factory. Or, is this perhaps just one of the few premature failures that render it unusable? More to come........

[g1]

Definitely worth trying the phase flip of the OT. If it cures it, you can figure out the 'why' later.

[nevetslab]

Found the problem! None of us guessed it. I would have flipped the O/T phase if this re-capping of the main power supply board didn't change anything. All looked brand-spankin' new in there, but de-soldered the caps anyway, slipped in a knife blade to break the bond of the RTV adhering them to the PCB, dropped new caps into place, glued 'em in same fashion, put the cover back on, and flipped the chassis back over and re-connected the amp. Powered it up, and, before I switched out of Stand-by, I noticed the short GRN GROUNDING wire lug from right behind the 'Normal' Ch input jacks, corner of the main PCB....the lug was cattywampus, locked the grounding screw from driving it home. I backed the screw up a turn, re-seated the grounding lug, then drove the screw home.

Switched out of S/B, cranked the volume pots, Treble & Bass controls wide open, nice and quiet. OK....back to S/B, loosened the grounding screw like it was, switched out of S/B, cranked the volume pots and had nice stable oscillation! Grounding Lug not securely fastened!! I'll be buggered!

BTW, pdf64 had the added plate coupling wire right.....joins both preamp channels to get Reverb & Tremolo on each channel.

[TimmyP1955]

Had a bad ground get me in a Twin 65RI once. Anything and everything crackled intermittently. I thought "gotta be a power supply issue" but could find nothing. Then I remembered that the grounds are part of the power supply. Found it.