Hello,
I am recapping a Revox B750 that I bought as junk. I’ve never used the amp before.
I have changed all the tantalum and Frakos, output transistors and few things here and there.
Today I switched the amp on. It didn’t pass the smoke test. I’ve noticed immediately smoke from, I believe, the power supply. I switched off the amp and can’t see any physical damage. The regulator are the originals, they seemed to test fine.
Now 2 things to mention, I’ve switched on the amp with only one power board plugged. The reason for that is that I wanted to set the bias separately. Could it be the reason?
The other thing is I followed advice on a mod on purpose to reduce noise.
I am sure the advice is safe, but I am unsure I have done it correctly. Thus I believe the issue could come from there. Here is the mod and 2 pictures of the power supply board. Also a picture of the schematic and what I basically have done with the diode.
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2. Up output capacitance for all 4 regs significantly - to like 220-470 µF if you can fit those.
3. Add the customary protection diode from output to input (and reversed for the 79xx) - 1N4002 or something. This is just so that the cap from step 2 can discharge upon poweroff without damaging the reg. »
I’m very green when it comes to this, first recap for me. Any advice welcome. Thank you
I am recapping a Revox B750 that I bought as junk. I’ve never used the amp before.
I have changed all the tantalum and Frakos, output transistors and few things here and there.
Today I switched the amp on. It didn’t pass the smoke test. I’ve noticed immediately smoke from, I believe, the power supply. I switched off the amp and can’t see any physical damage. The regulator are the originals, they seemed to test fine.
Now 2 things to mention, I’ve switched on the amp with only one power board plugged. The reason for that is that I wanted to set the bias separately. Could it be the reason?
The other thing is I followed advice on a mod on purpose to reduce noise.
I am sure the advice is safe, but I am unsure I have done it correctly. Thus I believe the issue could come from there. Here is the mod and 2 pictures of the power supply board. Also a picture of the schematic and what I basically have done with the diode.
«
2. Up output capacitance for all 4 regs significantly - to like 220-470 µF if you can fit those.
3. Add the customary protection diode from output to input (and reversed for the 79xx) - 1N4002 or something. This is just so that the cap from step 2 can discharge upon poweroff without damaging the reg. »
I’m very green when it comes to this, first recap for me. Any advice welcome. Thank you
The amplifier should have been tested before any repairs were done.
Are you certain the diodes and other parts are connected in the correct polarities?
Are you certain the diodes and other parts are connected in the correct polarities?
Yes, I know. I've powered it up and don't remember the smoke before. Also checked all traces and they seems good.
Regarding the polarities, I doubled and triple checked, but yes it seems it is connected correctly to my understanding of the mod which is my annotation on the schematic above. Now I can just revert back to the original capacitors and remove the diode, to see if that was the issue. It's a quick job.
Just to rule this out, can it be caused by the fact that I did not connect the 2nd power board?
Regarding the polarities, I doubled and triple checked, but yes it seems it is connected correctly to my understanding of the mod which is my annotation on the schematic above. Now I can just revert back to the original capacitors and remove the diode, to see if that was the issue. It's a quick job.
Just to rule this out, can it be caused by the fact that I did not connect the 2nd power board?
Most power supplies should work under no load, although the voltage will rise.
A drawing of the board is not a schematic, and conveys little information.
A drawing of the board is not a schematic, and conveys little information.
When you find an antique car in a barn, before you make an attempt to start and drive away, you check for water in the carter and cylinders, change the oil, change the coolant, change brake fluid and bleed the brake system, put in a battery and maybe change the tires.
An amplifier is not a car. When restoring an amplifier to a working condition, first you diagnose what is wrong, which components fail, and make a decision which components to replace and make a careful selection of components used for replacement. Definitely not the final transistors and a few components here and there.
Unlike a car where everyone has followed the Saturday Mechanic's School of Automotive Engineering, you should have a basic understanding on electronics and at least a few year of experience.
And if, despite of all that, it still goes wrong we are gladly willing to try and help. Provided you post a full schematic and an accurate description of which components you replaced, and with which new components. Posting a picture of a PCB tells exactly nothing except maybe let us understand roughly when the amplifier was manufactured.
I think you are all very right here. I am willing to learn, but I understand my limitation. Believe it or not, I can work on a car and it is indeed more straightforward that this, so I understand the analogy.
To go back on the subject, I think I have found where the smoke was coming from... it was a solder bridge on a rectifier 🙄I does not seems to smoke anymore now. Although I find the transformer a bit loud, sort of hum, not sure if that's normal.
I will do my research a bit more, see if I can get that amp running. If it gets too complicated for me ( and believe me I know my limits, and I don't like playing with electricity), I'll send it to a technician.
To go back on the subject, I think I have found where the smoke was coming from... it was a solder bridge on a rectifier 🙄I does not seems to smoke anymore now. Although I find the transformer a bit loud, sort of hum, not sure if that's normal.
I will do my research a bit more, see if I can get that amp running. If it gets too complicated for me ( and believe me I know my limits, and I don't like playing with electricity), I'll send it to a technician.