ab763 fender still humming with no valves

[andrewking]

can still play audio but is distorted but in amongst the distortion you can hear whether the tremolo is on or off and the reverb is on or off in amongst the distorted signal. There is a hum with all inputs unplugged and all the valves removed. i have both the 340vac going to diodes at the main transformer and the 460v is present to the standby switch until i start plugging valves in and then it lowers to about 385v. there is 6.3v for filaments, had replaced all the caps and valves ( biased the mains ) about 8 years ago and it has had very little use. help!!

[Audiorepair]

If there is a hum (through the speaker) with all the valves removed, then it is only really possible the Output Transformer is faulty.
Though this symptom actually seems unlikely.

The 2 halves of the primary winding should measure roughly similar dc resistance  (up to 40% or so difference can be typical in some amps though)
And there should not be any resistance measurement at all between any of the primary terminals and ground.
Or any measurement between primary and secondary windings once they have all been isolated from ground.



Edit: Unless by "all the valves" you actually mean "all the pre-amp valves".

[andrewking]

Wow a reply, thanks. Will get onto that when I get back. In the process of exploration these two resistors are a bit ugly looking. Will check what they’re meant to be in the schematic when I get back.
And yes ALL the valves removed I still get a hum unless I turn off the standby switch.

[andy3055]

Do you get a hum if you ground all the inputs?

[andrewking]

Haven’t checked but there’s no leads plugged in and all the valves are removed

[srb1954]

Wow a reply, thanks. Will get onto that when I get back. In the process of exploration these two resistors are a bit ugly looking. Will check what they’re meant to be in the schematic when I get back.
And yes ALL the valves removed I still get a hum unless I turn off the standby switch.

Those resistors look as through they have running too close to their maximum rated power for some time. I would replace them with double the power rating to have some safety margin and a longer life.

Also, some of that soldering looks pretty bad. The bottom lead of the centre cap might be even be open circuit!

[floobydust]

What model is this? 763 apparently refers to July 1963 and could be Deluxe, Deluxe Reverb, Twin Reverb, Super Reverb, Band-Master, Showman, Pro, Vibrolux, Vibroverb and Tremolux. https://robrobinette.com/AB763_Model_Differences.htm

It's not a normal repair because the last resistor (far right) 10k feeds the pre-amp tubes which have 100k plate resistors, so there's no way you can draw enough current to smoke that 10k. If the 22uF 500V cap shorted it could do that. So I'm a little suspicious when I see burnt parts that can't get burnt by the normal circuit faults meaning Mr. Mod might have been in here and messed up bad.

[andrewking]

Hi, thanks. Well spotted, I unsoldered them to measure out of circuit😂

[Audiorepair]

Those caps are F&T replacements for the originals, so at least this amp has been repaired in the not very distant past with good parts.

[andrewking]

Yeah that was me. I went through and replaced everything capwise. Much to the disappointment of purists I guess but I’ve been repairing electronics since just after valves were a thing and electrolytics do have a lifespan and I wanted something reliable. I’m  a little out of my depth with valves as I’ve really only dealt with the television side of them and not audio.

[Audiorepair]

It is worth replacing the original Illinois caps in the dogbox, they're a bit shit and do fail.

[andrewking]

Yeah they’re all new

[AndyBeez]

My very quick reading of the schematic (and I might have missed a lot), there is a center tap on the primary side of the speaker's output transformer. This is fed with 420V HT via the rectifier; If GZ34 is pulled or DoA, there is no rectification - just AC on the transformer primary. I also note a 47R to ground on the speaker's secondary side to provide 'feedback'. Is this value true?

[Andrewausfa]

No offence but some of those solder joints look awful, are a couple of those components connected at all?  Secondly, and without sight of the circuit, is that main smoothing capacitor on the LHS of your photo the right way around?

[Audiorepair]

Secondly, and without sight of the circuit, is that main smoothing capacitor on the LHS of your photo the right way around?

From memory I'm pretty certain Fender do put one round the opposite way.

[andrewking]

As per previous they are unsoldered to measure resistance values. So yes, they look quite rubbish solder joints in the photo.

[andrewking]

Those caps have been in there for about eight years without any issues, so if they’re the wrong way around I’m sure it would have been a problem before now.
I’ve never used forums before and I’m pretty impressed with the feedback so far, thanks
Will get back home today to measure some of the suggestions given so far.

[Audiorepair]

The elephant in the room here is still how do you get a hum through the speakers, when there are no valves plugged in.

[Andrewausfa]

As per previous they are unsoldered to measure resistance values. So yes, they look quite rubbish solder joints in the photo.

  Aaah gotcha, sorry, didn't see that. You definitely shouldn't be hearing anything with no valves in place.

[floobydust]

The elephant in the room here is still how do you get a hum through the speakers, when there are no valves plugged in.

Many things aren't making sense - it was put into storage 8 years ago, main filter caps rated only 350V not 450V, pre-amp B+ resistors cooked etc.
I've heard tube amp buzz before the filaments had warmed up, on a Wurlitzer jukebox amp 5U4 and it hit the 6L6's with 9V on the filaments for fast warm up.
It was strange to hear out the loudspeaker for a few seconds. I never bothered to figure it out, how the O/P transformer got a little AC flux.

OP I'd look to see if the rivets shorted to ground as well, on that warped phenolic. If the caps were in storage for 8 years, they need reforming.

[Andy Watson]

I'm a little confused about the time-line here:

Those caps have been in there for about eight years without any issues,

Are you saying that the amp was working perfectly 8 years ago in its present configuration, or has some work/mod been carried out in between?

The elephant in the room here is still how do you get a hum through the speakers, when there are no valves plugged in.

Quite!

With ALL the vales removed there should be no HT and the standby switch should have no effect. The schematics that I find on-line all show a GZ34 rectifier valve - has this been replaced/shunted with a pair of silicon diodes? If this is the case, is it possible that one of the rectifiers has become open or non-functional? This might explain why the HT voltage appears to drop so much when there is a minimal load put on the power supply. Can we confirm that the hum is actually coming from the speaker and not from the power transformer?

Also, is it a hum or more of a buzz? 50Hz or 100Hz?

[andrewking]

Hi, timeline…. 8 years ago got this from the USA and decided for reliability to replace all the valves and caps
Have been using ( with minimal regular use ) up until a few weeks ago when I took it to play at a wedding and it was due for about 10mins and then started sounding a bit distorted and garbled. I turned off and plugged into the desk for the remainder of the evening.
In the quiet if go
W I’ve noticed the hum coming from it with all inputs unplugged, volumes down and valves removed

[andrewking]

Is this what you’re calling rectifiers? Have measured all these with an analogue meter and they test ok

[andrewking]

Thanks for replying, Not sure we’re on the same schematic. Ab763? Mine shows 460v to centre tap of primary speaker transformer and off secondary side there is a 820ohm resistor in the feedback loop and a 100ohm resistor to ground

[andrewking]

Thanks for help. I get 37ohms on one primary winding and 35 on the other. No connection between primary and secondary windings, only to ground on the secondary side as per schematic