120hz hum Fender 65 Twin Reverb Reissue - it was dropped and poorly manipulated

[spunko]

Hello friends. This twin came with a bent chassis, it fell facing forward and broke the knobs from bass, reverb, speed and intensity. Lucky, the pcb and all pots are ok.

The owner said it works ok and it only needs a clean up  yeah right!. I first checked the fuse, and the previous tech had soldered a piece of wire to the blown fuse. Why do people do this!!!
I replaced the fuse and using a bulb limiter I tested the amp as it came, just for a couple of minutes, and it does a loud POP everytime I switch from standby to working condition, also there is a noticeable 120hz hum. But everything works, all pots, switches, reverb, tremolo.

I took the chasis out of the cabinet and this is what I have found, tested and replaced:

- One 6L6 (V7) was overheated and dark, the rest seemed ok. - I have a new matched quad for testing.
- Main IC caps were all leaking gunk (C30, C31, C32, C33, C34). Previous tech had cut the leads from the IC caps, J hook them and soldered back. Only one 22uf/500v C32 was replaced using a low quality brand "Jackel" rated at 450v but that thing was tiny, also radial and was bulge on top. I replaced them with F&Ts and MODs.
- R67 (1k) was overheated and darker, it read ok but I replaced it anyway.
- The trace from CP2 to the thermistor TH1 has been rebuilt using heavy gauge wire. The original trace is not present, it seems it burned bad.
- All B+ diodes and the bias diode have been cut on one lead by the previous tech and soldered back (CR1, CR2, CR3, CR4, CR5). I don't have spares, will buy this week.
- The hum balance pot was resoldered and the wires were melted from the iron, so I took the pot out to clean and it tests good.
- 470 ohm and 1.5K screens and grid control resistors on the power tube sockets looks and test good.
- I took out the controls PCB and straightened the front of the chassis the best I could.

After re-cheking twice all the work done so far, I powered the amp and it keeps POPing loud everytime I switch from standby to working condition, and the 120hz hum is still present, only quieter.
I replaced the balance pot for a pair of 100 ohm resistors, even tho the pot reads good, with the same result so I put it back. Long time ago I had a twin with bad balance pot, so I had to give it a try.
If I take out v6 the hum lowers drastically and I can only hear it if I put my ear next to the speakers. If I put a new 12AT7 in V6 it gives the same result.

I thought 120hz hum comes from B+, and seeing those leaking caps it made sense, but re-caping didn't fix the pop and hum completely.
I have yet to replace the diodes as I don't have spares at hand, but could those diodes be the source for the pop and hum?
Considering taking out V6 lowers the hum, but replace it won't, could it be that the new C32 cap is also bad?

I can easily buy new diodes this week, but I don't have anymore caps, those were my last, I have to order overseas and it will take not less than 3 weeks to get them.

[mresistor]

Don't know if I missed it..   Did you check the filter caps in the bias supply? 100uf/100v   22uf/63v (that's odd)

[stratomaster]

Time to start measuring.  Look for AC voltage at each filter node. This will clue you in to whether you still have a filtering problem or is the source of the hum is elsewhere. 

There's a ground trace on the controls PCB that could have been damaged when the pots impacted (or damaged during the replacement of the pots). It's worth carefully inspecting that board.

[Willabe]

Might seem like a dumb question, but do you have the volume control all the way down when you flip the standby to play or off.

If not all the way down, the speakers will thump and pop.

[spunko]

Don't know if I missed it..   Did you check the filter caps in the bias supply? 100uf/100v   22uf/63v (that's odd)

I have to change those, but I'm not sure if bad bias cap could induce 120hz hum.

Might seem like a dumb question, but do you have the volume control all the way down when you flip the standby to play or off.

If not all the way down, the speakers will thump and pop.

All controls at 0, also there are no preamp tubes in the amp. Only power tubes and PI

Time to start measuring.  Look for AC voltage at each filter node. This will clue you in to whether you still have a filtering problem or is the source of the hum is elsewhere. 

There's a ground trace on the controls PCB that could have been damaged when the pots impacted (or damaged during the replacement of the pots). It's worth carefully inspecting that board.

I have checked the controls PCB, no broken trace or solder.
I'm seeing 2.6 VAC with my meter between the caps in series C30 - C31, and also 2.6 VAC on C32. Reading across C33 and C34 gives 0.012 VAC. No preamp tubes installed, only PI and power tubes.

I've noticed that only one original diode 1N4006 is in place, the other 3 are 1N4007 for B+. All diodes, including the bias diode, have been tack soldered to the existing leads. I took all that crap out and soldered new 1N4007 diodes in place, but with the same result and the 120hz hum and POP is still present, nothing changed.

I once had a PRS Sonzera with blown diodes, they were 1N4006 and I replaced them with 1N4007 and I remenber the amp got noise, but can't recall if it was 120hz hum, this was two years ago. I replaced the diodes again with the same result, so I ordered a pack of (5) 1N4006 overseas, replaced them and the amp was quiet again. My bad I didn't ordered at least 4 more packs. Unfortunatelly you only find 1N4007 in my country, no other variant.

The schematic shows 1N4006 800v - 1A. Could it be that I need that specific variant of diode? Like it happened to me with the PRS Sonzera?
The only difference between the two seems to be the reverse peak voltage, 1N4006 at 800V and 1N4007 at 1000V.

[SEL49]

... I'm not sure if bad bias cap could induce 120hz hum.

The TRRI uses a half wave rectifier so any AC ripple on the bias dcv would be 60Hz.

[Willabe]

Don't know if I missed it..   Did you check the filter caps in the bias supply? 100uf/100v   22uf/63v (that's odd)

I have to change those, but I'm not sure if bad bias cap could induce 120hz hum.

Yes, a bad bias cap can inject noise. Because the -bias is applied to the power tubes control grid and a bad bias cap lets ripple in. That causes noise.

As far as diodes, there should be no difference in noise using a 1N4006 or a 1N4007. As long as there good diodes. But you want the higher PIV rating with the 1N4007. 

A lot of guys have switched to using ultra fast diodes, UF1N4007, for less noise. Their about the same price now, so why not use them? Buy a 100 and your set.     

[Willabe]

I'd go with 100uF/100v for both caps for the bias. I wouldn't put in a new 22uF/63v. The higher voltage rating gives it some voltage head room for turn on surge stress. And the 100uF is a little better than the 22uF for stability and noise.

But don't go higher than 100uF, the higher that uF rating the longer it takes for the cap to charge up. It won't give the full designed -dcv bias until those bias caps are fully charged up.

You want as close to the full -bias to be there for the tube when the tube warms up and starts to draw current, so it doesn't run away and stress the tube every time you start up/turn on the amp.

[spunko]

I'm still trying to figure out what the problem is.

I have found two power tube sockets loose, those were making noise when I touched the tubes. I re-tightened the sockets and no more strange crackling noises, but the 120hz is still there.

I manage to find some 1N5399 that look sturdy compared to the 1n4007 that are tiny (the ones that you find in my country). I replaced all the diodes including bias (and bias caps) because they were all tack soldered anyways, but the problem persists.

I remembered that I have some new generic axial caps saved in the drawer (branded "C"), so using some aligator wires I put them in series with the new ones in the dog house, one by one and the 120hz dropped just a little bit, but it is still present and also the POP is still strong (that didn't change). So two extra 220uf/350V to lower the noise a tiny bit. Adding a third one in there don't do anything (100uf/350). Also adding the rest of the caps in series (22uf/500V) don't do anything.

I find hard to believe that the new F&Ts are bad, because even with 4 new caps in there, the 120hz noise is still present. Right now I have a HR Deluxe with just one bad 47uf/500v IC cap, the moment I put a new one in series the amps gets dead quiet.

Could it be that the power transformer got bad, even tho it works? am I chasing rabbits? I don't have another TRRi to compare.

Here are some shots of how I found the amp before doing any work.

[stratomaster]

The work around TH1 looks like trash. And the thermistor looks like it might've been reused.  For testing purposes you can go direct from the power cable in to the transformer and bypass all of that mess on the PCB. See if that alleviates the standby pop.

If that ends up being the culprit and you want a thermistor (not an unreasonable desire) you can add a terminal strip or you may be able to use the unused C29 part of the board. 

Elevating the thermistor will go a long way to keeping it healthy. I often find questionable solder joints on Twin Reverb Reissues on the thermistor. This leads to arcing, intermittency, etc.

[spunko]

The work around TH1 looks like trash. And the thermistor looks like it might've been reused.  For testing purposes you can go direct from the power cable in to the transformer and bypass all of that mess on the PCB. See if that alleviates the standby pop.

If that ends up being the culprit and you want a thermistor (not an unreasonable desire) you can add a terminal strip or you may be able to use the unused C29 part of the board. 

Elevating the thermistor will go a long way to keeping it healthy. I often find questionable solder joints on Twin Reverb Reissues on the thermistor. This leads to arcing, intermittency, etc.

Thanks, I was also thiking on bypassing the thermistor. But could that help with the 120hz hum? I'm going to try tomorrow morning.

A friend of mine owns four TRRI (2 well used and 2 new). I went to his place to check and all of them make that loud POP, even the new ones so I'm no worry about it anymore, but none of them have the 120hz hum like the one I have. I mean I could hear it in all four of his amps, but it was barely noticeable, I have to get close to the speakers to hear it. Not like mine which hums kinda loud.

He lent me the newest one and I got it home to compare with mine. I did test some basic stuff like swaping my tubes with his, same result and noise. I even went so far to test my amp with his IC caps just to get that out of my head, with the same result.
I also measured the AC across the caps from the new amp and it is the same as mine (2.6 VAC between caps in series C30 - C31, also 2.6 VAC on C32. Reading across C33 and C34 gives 0.012 VAC).

Maybe there is a groond loop somewhere? I'm going nuts 

[stratomaster]

Something catastrophic happened in yours enough to where the rectifier at the secondary and the neutral leg of the primaries of the PT were affected. Since you have two now, and they are on quick disconnects, you can very quickly swap them to see if there's damage to the PT.

Additionally, grounds are made through the front panel. If you don't have good contact with the pots and hardware you'll have noise.  With the damage to the front I can see the connection becoming compromised.  One idea is to clip an alligator cable to the front flange then go down the line of the pots clipping to the bracket that secures them to see if grounding any of them helps with the noise.  If so, then run a jumper from the nearest pot bracket. They'll accept solder.

[spunko]

Something catastrophic happened in yours enough to where the rectifier at the secondary and the neutral leg of the primaries of the PT were affected. Since you have two now, and they are on quick disconnects, you can very quickly swap them to see if there's damage to the PT.

Additionally, grounds are made through the front panel. If you don't have good contact with the pots and hardware you'll have noise.  With the damage to the front I can see the connection becoming compromised.  One idea is to clip an alligator cable to the front flange then go down the line of the pots clipping to the bracket that secures them to see if grounding any of them helps with the noise.  If so, then run a jumper from the nearest pot bracket. They'll accept solder.

I have swaped both power transformers and the 120hz hum persists. I installed the old PT in the new amp and it is dead quiet, so PT is good.

I'm currently working only on the power amp part. There are no pre amps tubes in the amp, only PI and 6L6 (x4).

Actually there is no controls PCB right now in the amp. No difference if I connect it to the amp or not, the hum is always there, so I took it out for easier work and free space.

Could the output transformer induce 120hz hum? I would preffer not to swap OTs 'cuz they put a lot of work since the wires are soldered, but if is necessary I will do it.

[Willabe]

Are all the tube heater wires hand wired?

Sometimes the heater traces on the PCB are located in the wrong place, to close to ac signal traces, and they cause the hum. 

[stratomaster]

Are all the tube heater wires hand wired?

Sometimes the heater traces on the PCB are located in the wrong place, to close to ac signal traces, and they cause the hum. 

I tend to run these direct to the socket and add 1/4 watt 47-56 ohm resistors to each leg of the hum balance pot.  This eliminates an unnecessary quick disconnect and jumper on the board, allows cleaner lead dress, and doubles the power handling of the pot which is prone to failure. 

I don't think these are at the root of the problem, but they are low hanging fruit for reliability improvements on these amps.

[spunko]

Sorry for not posting, I have been busy and I forget to share the good news.

It was the choke all this time 
I was suspicious but also doubtful. At the end I tested it with my multimeter and it read 1.2 ohm  then tested with a good choke from the other twin and dead quiet 
Ordered a new choke overseas, and 3 weeks later it arrived. I installed it today and the amp is ready.
I also moved the heater wires from the PCB to the octo socket, and replaced the hum balance pot with two 100 ohm 1/2 W resistors on the second octo socket (Psionic audio style)

[Willabe]

Ordered a new choke overseas, and 3 weeks later it arrived. I installed it today and the amp is ready.

I also moved the heater wires from the PCB to the octo socket, and replaced the hum balance pot with two 100 ohm 1/2 W resistors on the second octo socket (Psionic audio style)

Did you test the amp for humm after the new choke but before moving the heater wires?

[spunko]

Did you test the amp for humm after the new choke but before moving the heater wires?

Yes, when I replaced the bad choke with the good one from the other twin, all wires were in place with their quick connects, all original. Also when the new choke arrived, all with quick connects like factory.
Once the amp was good and no hum, no noise, I moved the heater wires and added the 100 ohm resistors.

Thanks for the help