Help with Mackie Big Knob Repair

[sonolink]

The diode test mode shows the voltage drop in millivolts, across whatever you're measuring. A healthy, plain silicon (ie. not Schottky) diode, forward-biased, should show anything between 500-700mV. The other way around, "OL" or out-of-state.

Thanks Khron

So I measured all the diodes and I think they're all gone! None reads OL when I put the positive probe of my multimeter on their cathode (and the negative on the anode hehe). D3, D4, D5, D6 and D7 read around 1.2v like that and D1, D2 and D8 read around 0.3v.
Luckily I've got a bunch of 4004s

I guess it's time to change them all right? If so, maybe it's a good idea to lift one leg and check if the transformer is ok, as suggested by RuudNL?

Cheers
Sono

 

[Khron]

Sounds like at least one, if not several, is gone. I'd start with removing the diodes altogether and double-check them out of circuit.

 

[RuudNL]

Yes, separate the transformer from the circuit (by unsoldering the diodes of the bridge rectifier) and try again if the fuse doesn't blow.

Next question: what is the reason that the diodes were destroyed?

 

[sonolink]

Ok, I'm going to check both transformer and diodes pulling them out.

Next question: what is the reason that the diodes were destroyed?

Excellent question
I have a bad feeling about the transformer TBH

I'll report back asap.

Thanks again for the assistance

 

[user 37518]

You should really have a bulb tester unless you want to keep on blowing fuses, granted the current must be very low so it might not even work properly , with the LM317/337 power supplies, if there is a bad cap usually the voltage is way off but there is an output voltage, measure the bridge rectifier (turn the supply off for this) and the safety diodes, if they measure ok then I would surely replace the regulators.

In my experience transformers rarely fail, but I could definitely be wrong. An easy way to test the transformer is to desolder it from the main PSU board and apply a wirewound resistor of realtively low value (it depends on the transformer VA rating) your schematic seems like it says that the output is 380mA? I cant see the detail clearly, add a resistor such that it draws that ammount of current, if the transformer gets really hot, it is the transformer... when it comes to simple power supplies such as this I am really fond of the "finger" method, touch the components, the hot one is the bad one...  Do so at your own risk.

 

[RuudNL]

I don't think the regulators could be the problem.
Even if all three pins would form one big short circuit, the current would be limited to ~10 mA. (3.3K//2.7K)

 

[user 37518]

I don't think the regulators could be the problem.
Even if all three pins would form one big short circuit, the current would be limited to ~10 mA. (3.3K//2.7K)

If the problem is not in the diodes, the only other suspects are the Transformer and the regulators, I am ASSuming that the resistors are ok.

 

[sonolink]

Ok, so I lifted one leg on each diode of the bridge rectifier


save screen hd

and now BOTH fuses blew......

The transformer is dead right?

Cheers
Sono

 

[peterc]

Sounds like you are spending a lot on fuses.....

https://www.youtube.com/results?search_query=dim+bulb+tester+

My steps would be:
1. Disconnect everything after the transformer. Power up. Measure AC volts. If fuses blow, bad transformer.
2. Connect bridge and filter caps, no regulator. If fuses blow, issue is in the bridge/caps.
3. Connect regulator, if fuses blow, bad regs.
4. Connect the rest of the cct, if fuses blow, issue is in the circuit.

I know this might be a bit tricky, you might have to isolate some sections by butting PCB traces, hopefully not. There has been great advice in this thread, but ideas go back and forth, and it is not very clear what is causing what in certain situations.

Hope this helps, Peter

 

[sonolink]

Thanks for joining in Peter

I do have a bulb tester but TBH I only have used it with tube amps

My steps would be:
1. Disconnect everything after the transformer. Power up. Measure AC volts. If fuses blow, bad transformer.

Lifting the diodes leg is like disconnecting everything after the transformer, isn't it? In that case I have to order a new one...

Cheers
Sono

 

[RuudNL]

Did you also disconnect the capacitors across the secundary windings?
(Because if a capacitor is shorted, the output of the transformer is shorted.)
If you remove (both) 22 nF capacitors and the fuse still blows, order a new transformer!

 

[sonolink]

Did you also disconnect the capacitors across the secundary windings?
(Because if a capacitor is shorted, the output of the transformer is shorted.)
If you remove (both) 22 nF capacitors and the fuse still blows, order a new transformer!

Transformer is guilty... luckily it's a cheap one
I might order new caps and regs too just in case they're damaged and I'll check the resistors too before firing it up again

Thanks a lot to everyone for the help
Cheers
Sono

 

[RuudNL]

It doesn't happen very often, but sometimes transformers develop an internal short circuit.
The transformer isn't very expensive, let's hope you will find one that fits directly on the PCB!

 

[sonolink]

I think this one is a direct replacement:
https://www.digikey.es/short/zr5mfc

 

[Whoops]

I built some years ago this simple setup circuit with a neon bulb to test transformer windings for internal shorts.
it's pretty easy to do and inexpensive and quite useful, I highly advise it:

https://www.premierguitar.com/articles/the-super-secret-transformer-tester-1

 

[Pip]

I'm pretty sure its the Transformer. I have repaired these before and sadly that transformer is really not up to whats being asked of it when it comes to heat dissipation. They fry out in that first generation Big Knob. The good news is it is a readily replaceable item.

 

[RuudNL]

Interesting, thanks! I am going to try it.
But it seems the transformer is dead...
(And as far as I can see, it is a potted one, so there is nothing that can be done to solve it.)

 

[sonolink]

Ladies and Gentlemen,

the transformer indeed is dead, deceased, passed on, this transformer is no more! He has ceased to be! He's expired and gone to meet his maker! He's a stiff! Bereft of life, he rests in peace! His metabolic processes are now history! He's off the twig, he's kicked the bucket, he's shuffled off his mortal coil, run down the curtain and joined the bleedin' choir invisible!!
THIS IS AN EX-TRANSFORMER!!


;D ;D ;D
Cheers
Sono

 

[JohnRoberts]

Ladies and Gentlemen,

the transformer indeed is dead, deceased, passed on, this transformer is no more! He has ceased to be! He's expired and gone to meet his maker! He's a stiff! Bereft of life, he rests in peace! His metabolic processes are now history! He's off the twig, he's kicked the bucket, he's shuffled off his mortal coil, run down the curtain and joined the bleedin' choir invisible!!
THIS IS AN EX-TRANSFORMER!!


;D ;D ;D
Cheers
Sono

Good job, sometimes it is the obvious suspect...

JR

 

[Khron]

* It

(sarcasm) How dare you assume it's gender, though? ;D

Ladies and Gentlemen,

the transformer indeed is dead, deceased, passed on, this transformer is no more! He has ceased to be! He's expired and gone to meet his maker! He's a stiff! Bereft of life, he rests in peace! His metabolic processes are now history! He's off the twig, he's kicked the bucket, he's shuffled off his mortal coil, run down the curtain and joined the bleedin' choir invisible!!
THIS IS AN EX-TRANSFORMER!!


;D ;D ;D
Cheers
Sono