ab763 fender still humming with no valves - Page 1
Forum section: electronics / repair

author: andrewking
author_rank: Contributor
timestamp: December 13, 2022, 08:50:01 pm
content: 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!!

author: Audiorepair
author_rank: Frequent Contributor
timestamp: December 13, 2022, 09:07:56 pm
content: 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".

author: andrewking
author_rank: Contributor
timestamp: December 14, 2022, 03:27:05 am
content: 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.

author: andy3055
author_rank: Super Contributor
timestamp: December 14, 2022, 03:52:51 am
content: Do you get a hum if you ground all the inputs?

author: andrewking
author_rank: Contributor
timestamp: December 14, 2022, 04:16:34 am
content: Haven’t checked but there’s no leads plugged in and all the valves are removed

author: srb1954
author_rank: Super Contributor
timestamp: December 14, 2022, 05:02:37 am
content: Quote from: andrewking on December 14, 2022, 03:27:05 amThose 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!

author: floobydust
author_rank: Super Contributor
timestamp: December 14, 2022, 05:50:36 am
content: 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.htmIt'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.

author: andrewking
author_rank: Contributor
timestamp: December 14, 2022, 07:15:15 am
content: Hi, thanks. Well spotted, I unsoldered them to measure out of circuit😂

author: Audiorepair
author_rank: Frequent Contributor
timestamp: December 14, 2022, 07:52:51 am
content: 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.

author: andrewking
author_rank: Contributor
timestamp: December 14, 2022, 08:11:16 am
content: 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.

author: Audiorepair
author_rank: Frequent Contributor
timestamp: December 14, 2022, 10:07:06 am
content: It is worth replacing the original Illinois caps in the dogbox, they're a bit shit and do fail.

author: andrewking
author_rank: Contributor
timestamp: December 14, 2022, 11:58:40 am
content: Yeah they’re all new

author: AndyBeez
author_rank: Frequent Contributor
timestamp: December 14, 2022, 12:25:02 pm
content: 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?

author: Andrewausfa
author_rank: Contributor
timestamp: December 14, 2022, 01:05:48 pm
content: 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?

author: Audiorepair
author_rank: Frequent Contributor
timestamp: December 14, 2022, 01:07:34 pm
content: Quote from: Andrewausfa on December 14, 2022, 01:05:48 pmFrom memory I'm pretty certain Fender do put one round the opposite way.

author: andrewking
author_rank: Contributor
timestamp: December 14, 2022, 08:14:04 pm
content: As per previous they are unsoldered to measure resistance values. So yes, they look quite rubbish solder joints in the photo.

author: andrewking
author_rank: Contributor
timestamp: December 14, 2022, 08:17:01 pm
content: 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, thanksWill get back home today to measure some of the suggestions given so far.

author: Audiorepair
author_rank: Frequent Contributor
timestamp: December 14, 2022, 09:01:29 pm
content: The elephant in the room here is still how do you get a hum through the speakers, when there are no valves plugged in.

author: Andrewausfa
author_rank: Contributor
timestamp: December 14, 2022, 09:59:57 pm
content: Quote from: andrewking on December 14, 2022, 08:14:04 pmAaah gotcha, sorry, didn't see that. You definitely shouldn't be hearing anything with no valves in place.

author: floobydust
author_rank: Super Contributor
timestamp: December 14, 2022, 11:30:01 pm
content: Quote from: Audiorepair on December 14, 2022, 09:01:29 pmMany 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.

author: Andy Watson
author_rank: Super Contributor
timestamp: December 15, 2022, 12:11:53 am
content: I'm a little confused about the time-line here:Quote from: andrewking on December 14, 2022, 08:17:01 pmAre 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?Quote from: Audiorepair on December 14, 2022, 09:01:29 pmQuite!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?

author: andrewking
author_rank: Contributor
timestamp: December 15, 2022, 02:24:55 am
content: Hi, timeline…. 8 years ago got this from the USA and decided for reliability to replace all the valves and capsHave 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 goW I’ve noticed the hum coming from it with all inputs unplugged, volumes down and valves removed

author: andrewking
author_rank: Contributor
timestamp: December 15, 2022, 02:27:06 am
content: Is this what you’re calling rectifiers? Have measured all these with an analogue meter and they test ok

author: andrewking
author_rank: Contributor
timestamp: December 15, 2022, 03:04:58 am
content: 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

author: andrewking
author_rank: Contributor
timestamp: December 15, 2022, 03:20:00 am
content: 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

author: andrewking
author_rank: Contributor
timestamp: December 15, 2022, 03:27:04 am
content: Definitely buzz in the speakers not a transformer buzz. Maybe not 50hz but higher frequency. Hard to tell with a buzzing noiseThe components under the “dog house” what values are they meant to be. I’m having trouble locating this stuff on the schematic.

author: andrewking
author_rank: Contributor
timestamp: December 15, 2022, 03:43:03 am
content: Ahh Andy, I just looked on the web and I think I found the schematic you’ve been looking at is a different amp. Mine is a twin reverb. No gz34.

author: floobydust
author_rank: Super Contributor
timestamp: December 15, 2022, 05:13:49 am
content: This model is an export for 240VAC power? or are you using a 240-120V transformer to power it? The power transformer TR1 p/n I believe would be125P34AX(022764) 1964-mid 1970's or 013830 for 70-80's 135W models (non USA).Grasping at a few straws, the hum could be caused by a carbon-tracked tube socket. If the filament pin 2/7 shorted to plate? For the hum to be making it to the speaker through the feedback resistors, that would be several parts shorted, not likely. Unless there is a short-circuit in the wiring or phenolic plate rivets.I would go after the fault causing the last burnt resistor feeding "D" the pre-amp section. That is a strange failure... There would have been a lot of smoke and stink when those two resistors roasted.I'm wrong about this model, the two main filter caps are in series, so 2x100uF 350V for giving 50uF 700V which is ok.Careful- don't run it then switch stand-by switch off, with no valves there is nothing to discharge the three large electrolytics "B","C","D" and a terrible shock hazard. Check DCV with a multimeter on the electrolytics before going in.

author: andrewking
author_rank: Contributor
timestamp: December 15, 2022, 08:30:13 am
content: FIXEDHi all. Firstly, I am humbled by the fact people all over the planet have taken the time to constructively share their knowledge in such a positive way.I started my electronics apprenticeship in 1986 and I have never seen a resistor go low in value. As per the diagram a filter cap was open circuit and the two cooked resistors went low in value and hence drew more current and got all toasty. I have never seen this. It’s a bit like saying my tire exploded because it pumped itself up too much…..doesn’t happen!The proof of concept is dodgy as all shit but the correct value and shaped parts are on the way.Thankyou again!

author: wasedadoc
author_rank: Super Contributor
timestamp: December 15, 2022, 02:07:40 pm
content: Still no confirmation that the hum is from the speaker cone, not from the mains transformer.

author: andrewking
author_rank: Contributor
timestamp: December 15, 2022, 07:46:35 pm
content: Quote from: andrewking on December 15, 2022, 03:27:04 am