Studer A 827 +/- 15v 24v stabilizer problems

[mgrisoli]

Guys, thank you for all the insights, they are really helpful.

I got to a point where the board is no longer oscillating.
I can't quite explain why but I removed tested and soldered back all transistors and diodes and may have fixed a broken solder joint as consequence.
Then the aluminum block where the transistors are attached accidentally touched the chassis of the transport (a mesh shelf with rounded holes) and I got a spark, which I found interesting because I thought the whole chassis was grounded...and I thought the transistors collectors were insulated from that plate, but allowing that accident to happen is plain stupidity.
The fact is that the board is no longer supplying the voltages and there is a clicking noise coming from T2 which I believe to be just a symptom, not the cause.
The board 3A fuse is intact, I decided to take a walk and go back to it perhaps tomorrow.

I replaced all electrolytic caps, and three film caps. But, addressing your point I was afraid to up the voltage, as I use to do in my other audio gear, because this seems quite finicky and all this chatter about ESR made me second guess if some of my regular approaches would work for this equipment. I replaced C6 with a Nichicon that is currently on the board, but to rule out the ESR thing I ordered a Wurth with 260mOhms ESR just in case.

 

[mgrisoli]

Ok folks, no harm no fault, everything is fine...

Except that I am back to square one, and here is a little video of the buzz:
It seems L2 is the buzzing component.
I already revised the solder joints of L2 and redid all of them, also added some glue to the magnetos.
+-15v and 24v supplies are steady and the tape recorder seems to be WORKING.

P.S.: On my previous post... I disassembled and reassembled so many times the heatsink of this board that the last time I did not test if the components were all isolated and D1 was not... that made the aluminum block hot with -25v when it touched the metalic surface of the chassis it shorted. But good thing this is sturdy enough to live with my mistakes. Electronics are always a humbling experience.

 

[radardoug]

Have you measured the switching frequency and verified it is correct?

 

[MS-TAS]

If you have +-15 and +24VDC and the recorder works, that's great! You fixed it!
I agree with radardoug, measure the frequency at L2. See if that is what you are hearing. Also if you can compare frequency with a known good PSU, that will help too.
It may be the coil is simply resonating at the frequency of the oscillation that the power supply needs to function. If so, and it is not terribly loud, I'd call it done. Otherwise, get a new coil for L2.

 

[mgrisoli]

All right, so back to this Studer, after a long break...

Update:
The stabilizer board is working, it is providing the -+15v and the 24v.
The tape recorder seems to be working as well.
But the coil L2 squeals, and there is some other board that could not identify yeat that is squealing.
Just to remind you, it sounds like a photographic flash charging up. It does not stop and it is loud.

I measured L2 with the oscilloscope (pins 1 and 20) and this is what it shows:


I also measured P1 (Pin 7) which is supposed to give me 76kHz according to the diagram


But it is actually reading 112kHz with a wave as displayed below:


I have no access to another PSU to compare these results and I believe there is something wrong with this PWM circuit.
Since I am not familiar at all with what to expect here, I am seeking for some guidance.

 

[radardoug]

This could be your problem. It will probably work at the higher frequency, but the coil might not like it. You need to work out where the 76kHz comes from, and then see what controls the frequency.

 

[radardoug]

The scope is saying 149.27 which is suspiciously close to double the intended frequency. Also that is feeding a phase lock loop ic, the 14046.

 

[sodderboy]

Could mechanical dampening be of help? A blob of hot-melt glue?
What is it powering? Is the whine the same loaded or unloaded?
Mike

 

[radardoug]

That 76 k comes from the tape control microprocessor, so it should be very accurate. If not, look on that board.

 

[mgrisoli]

That 76 k comes from the tape control microprocessor, so it should be very accurate. If not, look on that board.

In the wiring list, this is the first board in the TD-C76k signal...
But the 76k looks more like an input to this board than an output.
I am still trying to find where this is generated...

 

[radardoug]

Did you not read the post above? It comes from the transport control microprocessor card.

 

[mgrisoli]

I did, but since I could not find an assembly with exact name match I kept going through the wiring list.
Thanks for insisting though, I think it is the third in the wirling list MP-Unit TD Control MCH (1.820.781.31), I see a crystal there and when I push the reset button the squealing stops or changes tone.
To troubleshoot this board will be a beast for me, I am quite illiterate on digital stuff.
I read thought that these eproms sometimes just need to be rewritten.

 

[radardoug]

One thing if you are working on this machine. These processor cards have a battery on them which leaks acid onto the board and creates havoc with them. Inspect your board carefully, and if you see signs of corrosion, you need to fix them. There is someone doing a rebuild of these boards, and that would be your best option of you are not too technical. If the tone changes when you push reset, thats a big clue!

 

[waltzingbear]

As there are multiple smps modules in the deck, there is a need to have them all synchronized at the same frequency to prevent heterodyning between them and producing audible birdies. Hence all supplies in the deck get a 76khz sync signal derived from the master oscillator. All clocking signals are derived from the master including the bias frequency.

Most of the supplies are free running until the sync signal is supplied, hence radardougs comment re shift in whine frequency