Reissue deluxe reverb hum/ lessons relearned!

[Stevem]

I had one of these come in and I am not ashamed to say that it took me a near a full hour worth of time to find the root cause.
The issue was hum at a good level out of the speaker.

This hum was there with the PI tube yanked out, with the output tubes yanked out and with all 3 yanked out!

The only thing that killed the hum was unhooking the OTs center tap.

I though for a while that one output tube socket may have arced over and was now bleeding heater voltage to the OT, but removing the brown and blue plate leads from the socket proved that to not be the case.

I remove the feed back wire with no change to the problem.

I even removed the OT completely from the amp and as long as the OT had its center tap hooked up the issue persisted!

I even thought for a while that millvolts of V+ voltage might be bleeding from the primary to the secondary side , but that was not the case.

While had the OT out I confirmed it's not the issue by jumping it into another amp.

Anyone want to guess what the root cause of the issue was?

PS, and by the way and for the record I started off the repair by replacing 2 bad 22 UF 500 volt filters which went fine.

[Roe]

that's a strange case indeed

[Stevem]

Yes, it had me near ripping out what's left of the hair on my head, lol!

[sonicmojo]

Guess 1 - rectifier tube
Guess 2 - Standby switch issue

[Stevem]

Not either of those two.

[sluckey]

I even removed the OT completely from the amp and as long as the OT had its center tap hooked up the issue persisted!

Does this mean that even the secondary wires were disconnected? ie, the only wire connected was the primary CT?

[Stevem]

No I had to still have the speaker hooked to the OT, but the speaker jacks where unbolted from the chassis and with them off the chassis the hum was not as strong, but it was still there.
Regrounding the jacks to the chassis by means of a clip lead brought the hum level back up to the same as when everything was bolted in the amp.

[sonicmojo]

Bad pilot light holder slightly grounding....

[Stevem]

No, not that either.
The issue was that Fender in these reissue amps decided to make the first filter node a stack.

The first filter is a 220uf@ 100 volts, yes 100 volts, but they strap a big ohmage resistor across it.

The bottom filter of the stack is a 47uf @500 volts with a resistor across it and was bad although it did not look bad.

The total UF value of both filters when they are good works out to like 38 UF when the original amps used the normal single 20 uf.

I hope my pain of last night helps out others with a issue like this.

[xtian]

for the record I started off the repair by replacing 2 bad 22 UF 500 volt filters

Well, you really squirted us off in the wrong direction with this little half-fact.

[sonicmojo]

Yes, not a big fan of reissues. I’ve got a Twin Reberb for repair that has a low frequency reberb hum when reverb is turned up and about 3/4 reverb coming through from the normal channel. Only gets vibrato for the reverb on the normal channel. Probably a similar weird extra component thing. I’ll post a new thread on it if I can’t figure it out.

[Reeltarded]

I hate any Fender amp made after 1980 to work on. I am not anal enough to hate the classics blackface and before with the rat's nests, but once they go II models and then red knobs.. I'd rather puncture an eye just to feel good.

[martin manning]

In this DRRI example a stacked arrangement is used for the reservoir to get high voltage capability at low cost. This is just like some of the silver face amps, except some clever person figured out that a few more nickels could be saved by using unequal values. Other than that, it's just like any other stacked filter.

[Stevem]

Sonimojo, your twin there sounds like someone was in there and did the mod to get reverb on the normal channel the wrong way!

On my amp I posted about here what steered me wrong was the that the AC ripple I measured on that first stacked filter node was really not higher then what you find in a well filtering good working section, that being .182 mv AC.

[martin manning]

On my amp I posted about here what steered me wrong was the that the AC ripple I measured on that first stacked filter node was really not higher then what you find in a well filtering good working section, that being .182 mv AC.

That seems abnormally low... With good caps I'd expect something near 10V ripple on either cap's positive lead.