Budda Superdrive 80 repair woes

[iampoor1]

Oh boy, this one has been painful!

I have been working on a budda superdrive 80, and had it back in the shop multiple times now.

The issue is very strange. The amp will work normally, but at random, the amp will start buzzing, the eq will stop working, and when the master volume is at 0, sound will still be produced. Turning up the master volume control slowly will not change the volume, until BAM the amp kicks back into gear at a deafening volume. I have checked (and changed) the tubes, resoldered every joint, and inspected/tested all parts for cracks and stress but noooope, not even close.  The weird thing is, when I test the B+ or preamp  voltages with my multimeter, the issue instantly goes away. Any ideas? The build quality of this amp doesnt inspire confidence, and its very difficult to work on. I will be reporting back with more information soon, just very annoyed at this amp. I have probably spent 15 hours on this thing. Booo!

Here is a schematic with some obvious errors, but its pretty close.

 

[CJ]

can you throw down with some voltages on that schemo?

 

[psych60s]

Do you think it is a heat-related issue?

 

[Tubetec]

Ive had a budda in for repair before ,its a while back now ,so I cant remember the fault , I wasnt all that impressed with the build quality either . Phase invertor stage looks odd to me ,its different from anything Ive seen before .
Maybe check for a damaged wafer in your master volume pot , possibly caused by impact to the shaft ,thats the only thing that springs to mind from your description ,as far as the problem dissappearing when you hook up the meter ,that seems odd too ,maybe its just coincidental. Part of the fun of the repair game I suppose ,a real headscratcher comes along every now and again .Best bet is dont get bogged down on it ,move on to another job ,and come back to it another day with fresh eyes and ears .

 

[Tubetec]

Might also be worth looking at the shorting jacks around the area of the effects loop ,they can often be a source of troubles ,normally a shot of cleaner and  plugging in and out a jack plug a few times refreshes the contacts .You could of course just use a patch lead to bridge the effects loop also ,that way you can definatively rule it in or out as the source of the problem.

 

[desun]

Have you checked the contacts of the FX return socket? It's after the MV, so could be a candidate-- especially if you've already gone through the joints. Edit: Already suggested above.

Edit2: Socket tension? Maybe if you're probing voltages on the sockets themselves, that's enough to make the connection between the pin and the socket good?

 

[iampoor1]

can you throw down with some voltages on that schemo?

Yes I will. Will report back shortly

Do you think it is a heat-related issue?

No, its completely random. I had the amp stable for a week, now it fails within 5-10 minutes of startup. Before it would fail right at power on. This amp had dripping solder and bad joints everywhere at first, so I though the source of the problem was pretty obvious, boy was I wrong!

Ive had a budda in for repair before ,its a while back now ,so I cant remember the fault , I wasnt all that impressed with the build quality either . Phase invertor stage looks odd to me ,its different from anything Ive seen before .
Maybe check for a damaged wafer in your master volume pot , possibly caused by impact to the shaft ,thats the only thing that springs to mind from your description ,as far as the problem dissappearing when you hook up the meter ,that seems odd too ,maybe its just coincidental. Part of the fun of the repair game I suppose ,a real headscratcher comes along every now and again .Best bet is dont get bogged down on it ,move on to another job ,and come back to it another day with fresh eyes and ears .

Yes, this is honestly the nastiest built production amp I have ever seen. Hot glue on threads, solder blobs left in the amp, burnt flux residue on all the wires, no way to remove the main PCB without desoldering a gazillion wires, super strange PCB layout, and worst of all, CHEAP jacks. Okay now Im just ranting, but this amp is seriously nasty A peavy valveking has better build quality for crying out loud lol

I dont think the meter thing is a fluke (no pun intended) because it repeatably fixes the problem. If I probe any of the plates, or B+ voltages, the problem is temporarily fixed. Super weird! I took a video showing sort of whats going on

Oh its been fun, but now its just frustrating. I have had this one and not been able to fix it for a few months now. 

Might also be worth looking at the shorting jacks around the area of the effects loop ,they can often be a source of troubles ,normally a shot of cleaner and  plugging in and out a jack plug a few times refreshes the contacts .You could of course just use a patch lead to bridge the effects loop also ,that way you can definatively rule it in or out as the source of the problem.

Yes I have tried it with a 1/4 in cable in the jacks and it appeared to be okay. Will try again as it has been awhile!

Have you checked the contacts of the FX return socket? It's after the MV, so could be a candidate-- especially if you've already gone through the joints. Edit: Already suggested above.

Edit2: Socket tension? Maybe if you're probing voltages on the sockets themselves, that's enough to make the connection between the pin and the socket good?

The sockets on the tubes (especially the power tubes) are all loose I have mechanically rotated the tubes as far as I could in the sockets while the amp is working and not working, and it hasnt triggered the failure mode.

 

[iampoor1]

https://youtu.be/DbcZTQ8iJaI

I hope this provides some clarity!

 

[fazer]

is there leakage from a cap that biases the next stage improper and then when you probe it zeros out the DC leakage.  Just a guess.

 

[iampoor1]

is there leakage from a cap that biases the next stage improper and then when you probe it zeros out the DC leakage.  Just a guess.

Maybe, the only caps with obvious DC leakage are the 22nf between the phase inverter and power tubes. Approximately 40mv and 75mv on the other sides of them. Hmm, not sure if that's acceptable or not.

I didn't notice any leakage on any other caps, but the issue constantly goes away if I probe the plate side of the 22nf coupling cap on the input stage. Things are just getting weirder!

 

[iampoor1]

can you throw down with some voltages on that schemo?

Voltages https://imgur.com/gallery/VVxyV

Plate, cathode, and b+ voltages

 

[Tubetec]

Ok I watched the vid , thats an odd one alright , Those electrolytics look cheap and nasty .
You could try taking an output from the fx send or feeding the return directly from the guitar ,might help figure out if the fault is pre or post .
Maybe find a way of applying presure to the circuit board so the fault is constantly showing ,then probe from the output tubes grids back through the stages ,the pop should get louder as you work back towards the input ,if you probe an earlier stage and it gets quieter than the stage after it ,you might have localised the trouble . If you can take the amp out in bright daylight and check for loose end caps on a resistor or maybe a coupling cap thats going open circuit due to a bad internal connection. If you can use presure to make the fault stay ,then poke each component with a chop stick ,if it comes back on your close to the source of the problem. Retensioning the tube socket contacts as mentioned by another contributor is also worth a try ,you might find one tube in particular that requires less force to seat in its socket , damage can easily occur to the tube socket pins if a tube is incorrectly inserted or has a bent pin ,make a close visual inspection of the sockets under good light. I usually use a notice board pin with a plastic back on it for this  purpose I insert it between the tube socket and the leaf contacts and very gently bend the contact closed from one side then the other

 

[fazer]

35 to 70 mv does not sound like the problem but hearing your video is like a bad solder joint.  Do you have scope and 10 to 1 probe?  Input tone and trace signal through for where it stops.

Also be very careful poking around in an amp with a guitar around your neck.  The string will ground one hand while the other hand can touch 450 volts.  You will get stuck to the amp while voltage goes across your heart.  I know this first hand.  Very dangerous.  Set guitar down and put one hand in your back pocket while other takes measurements. 

 

[Tubetec]

Good advice from phaser about not grounding one side of your body through the guitar while you probe with the other .
Third triode from the left at the top its a cathode follower ,360 volts at the anode ,200 on the grid which connects to the previous anode , and then zero volts across the 100k resistor at the cathode , you should be reading close enough to 200 volts at the cathode too , I think the cathode pin of that tube isnt making contact with the socket .

 

[iampoor1]

Ok I watched the vid , thats an odd one alright , Those electrolytics look cheap and nasty .
You could try taking an output from the fx send or feeding the return directly from the guitar ,might help figure out if the fault is pre or post .
Maybe find a way of applying presure to the circuit board so the fault is constantly showing ,then probe from the output tubes grids back through the stages ,the pop should get louder as you work back towards the input ,if you probe an earlier stage and it gets quieter than the stage after it ,you might have localised the trouble . If you can take the amp out in bright daylight and check for loose end caps on a resistor or maybe a coupling cap thats going open circuit due to a bad internal connection. If you can use presure to make the fault stay ,then poke each component with a chop stick ,if it comes back on your close to the source of the problem. Retensioning the tube socket contacts as mentioned by another contributor is also worth a try ,you might find one tube in particular that requires less force to seat in its socket , damage can easily occur to the tube socket pins if a tube is incorrectly inserted or has a bent pin ,make a close visual inspection of the sockets under good light. I usually use a notice board pin with a plastic back on it for this  purpose I insert it between the tube socket and the leaf contacts and very gently bend the contact closed from one side then the other

Yes....for being a 2500$ amp...ugh! I will stop whining, but a Peavy Valveking has nicer parts, thats sad!

Yes, I isolated the efx loop first, the problem is the paraphase inverter has tons of gain and this effects loop sounds atrocious with unbuffered signals, but it helped initially isolate the problem.

Oh believe me, I have chopsticked, whacked, pushed, and prodded every part over 15 times. The problem I was having is that no 1 area of the board or part was clearly failing, it was all over the place, and hard to tell with how it was failing. But I think I found it!

The cathode follower with 0 volts on the cathode....thats when the amp was working normally. The problem would usually go away if the amp was played at max volume, ie the equalizxer would start working again, and the nasty distortion would go away. The pins in the socket were fine, but the pad on the PCB with the socket appears to have a microfracture. I made a little solder blob over it, and bam 200 volts on the cathode. Im really glad you caught that. I have been looking at this amp for over 7 hours today (unpaid of course!) and didnt think anything of it DOH! SO far the amp appears to be stable...we will see if something else comes up.

35 to 70 mv does not sound like the problem but hearing your video is like a bad solder joint.  Do you have scope and 10 to 1 probe?  Input tone and trace signal through for where it stops.

Also be very careful poking around in an amp with a guitar around your neck.  The string will ground one hand while the other hand can touch 450 volts.  You will get stuck to the amp while voltage goes across your heart.  I know this first hand.  Very dangerous.  Set guitar down and put one hand in your back pocket while other takes measurements. 

Thought so, thats for confirming that!
I do have a scope, that needed to be repaired and of course, the person wants their amp back with a days notice. Ugh, its been one of those days!

Thank you for that tip. Thats something I honestly didnt think about, and I try to take safety seriously. I have just become complacent, and thats not safe. I have no desire to be shocked by 400+ volts again. Already happened to me once, thankfully just on one finger tho! 

 

[Tubetec]

Sounds like you might have your buddies amp sorted , I hope he at least brings you round a six pack of beer and a baggie of bud for all your trouble . Yeah theres always a DOH moment when you end up sorting out ball buster of an amp , I was going to suggest incense ,sitting contemplatively and a prayer to the little guy with the podgy belly for inspiration .Anyway your a little older but wiser than before now and you'll spot a funky CF circuit quicker next time around , all the best and let us know how things turn out in the end with it .

 

[fazer]

Good Job. 

 

[Kingston]

What was that electrical law again - by a member of our forum - 99% of the times when the problem doesn't turn out to be a mechanical failure - it will turn out to be a mechanical failure in the end.

 

[pvision]

Thank you for that tip. Thats something I honestly didnt think about, and I try to take safety seriously. I have just become complacent, and thats not safe. I have no desire to be shocked by 400+ volts again. Already happened to me once, thankfully just on one finger tho!

Anecdotally, many injuries caused by working on valve gear are caused whilst recoiling from the shock. If there's something to hit your head on, you will do. Might sound like a joke - but it's not

Nick Froome

 

[Tubetec]

Touch wood ,never got shocked from a powered valve unit ,once or twice I did get a tingle from undischarged PSU lytics ,and on one ocassion way way  back I accidently shorted across  live and neutral solder joints with my finger tip ,left two little burn marks and a slightly odd smell in the air . Tesla used do all kinds of crazy stuff with voltage way back ,of course he was always carefull that the input/output path wasnt across his heart  . Valid point about the convulsive effect of electric shock though.
Was a guy and his son in a place I used work years ago ,they did painting and other odd jobs around the place ,one day while painting the outside of the building they moved the scaffold they were working on too close to overhead high tension cables ,there was a spark ,the father was thrown clear ,the son caught the full force of it ,apparently the soles of his shoes were melted into the ground.
Always respect the power (of electricity).