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

[mgrisoli]

Folks,

I came across this Studer A827.
It seems all original.
It all seems to work but the 15v stabilizer board is making a buzzing sound.
I found a capacitor that has no capacitance left, replaced it and now the board seems to be oscillating.
I am posting to see if someone here have experience with that type of tape recorder and could point me out in the right direction.

 

[Khron]

What is the new capacitor (brand, series, value)?

 

[Ike Zimbel]

There is very likely more than one electrolytic capacitor that has failed. It's a known issue with these machines, especially the "Frako" branded caps.

 

[mgrisoli]

Sorry folks, my regular job got in the way of my passion for a few days...


Here is the PCB layout


and the schematics

What is the new capacitor (brand, series, value)?

Here are my replacements:
C26 x1 Philips 118 LL 1000uF 40v -> Nichicon 1200uF 50v ESR 22mOhms UHE series
C20-C24 x4 Philips 118 LL 2200uF 16v -> Panasonic 2200uF 16v ESR 18mOhms EEU-FS series
C6 x1 United Chemicon?? 470uF 16v -> Nichicon 470uF 50v ESR 60mOhms UPW series (perhaps this ESR is too high?)
C6 is the capacitor that failed. I cant identify the manufacturer.

A resistor that seems to be R54 is discolored and has some overheat marks around it.


It seems these Philips were axial caps, but they cut one of the leads and put an aluminum jacket on them.
I am deducting that the positive is the lead in the middle and the three pins in the jacket are negative.


@Ike Zimbel I dont think there are Frako caps on this board.

The original problem could as well be in the PSU, however the owner has two of these tape records and when he swaps the stabilizer board between them this unit seems to work fine or at least no audible buzz.

 

[sodderboy]

I’ve seen those boards burn beyond repair from bad caps. You should check the work so far and make sure that the cap leads connect to the circuit on both sides of the pcb- very important. Sometimes removing the old caps can remove thru-plating on holes and break important circuit connections. Crazy boards those.
The loads can also cause oscillation, and that has been isolated by someone else, but verify.
Mike

 

[Lampie519]

The transformers are vibrating mechanically on the boards and can therefor loosen a solder joint or 2. Have you checked this?
It looks like a resistor is arcing on this board... in any case this resistor has changed color because of overheating.
I just noticed that this has been pointed out already...

 

[waltzingbear]

These are very tricky to troubleshoot. That is not arcing, but collection of dirt from higher voltage differential. On some of these boards you need to connect all three ground holes together and especially pay attention to Via lifting.
Putting lower ESR caps in here is a problem. It often causes oscillation, Especially C6.

 

[mgrisoli]

These are very tricky to troubleshoot. That is not arcing, but collection of dirt from higher voltage differential. On some of these boards you need to connect all three ground holes together and especially pay attention to Via lifting.
Putting lower ESR caps in here is a problem. It often causes oscillation, Especially C6.

I guess I am going to need to try a few different caps to find the best one for C6.
By connecting the three ground holes I presume you mean the three holes where negative leads of the Philips caps were, correct?
Today I disassembled and reassembled everything with special attention to the connections in the upper side of the board.
I have replaced the discolored resistor as well.
It is not oscillating anymore, but the original problem of the buzzing noise is back, it sounds like a flash charging up to be used in photography.

 

[sodderboy]

As far as sound, they had a tiny sizzle like any Switcher up close, but never a whine like a flash charging.
PCB caps are not screw in lightbulbs, you don’t solder in caps and then change them out for this one and then that one without creating more issues than when you started. I would really check the work done already, and look all around for any solder splashes etc. Radials are the only replacement, don’t worry about esr, just get the correct F and up the voltage for a good fit for the leads. I don’t like them off the board.
And all replacement connections have to be checked for continuity to the next component.
Check it with the outputs disconnected first.
All the caps go bad in those machines, so like I said the load may be the real problem- audio cards? transport cards? Dried electros everywhere.
I really miss the Panasonic HFS series, 105 degC, rad or ax, they worked everywhere, still do.
Mike

 

[waltzingbear]

In this circuit esr has been a factor in cap selection. I suspect because it’s in the gain loop of the regulation.

 

[mgrisoli]

Based on previous posts about this board, people seem particularly concerned about the caps ESR. I couldn't quite figure if they were looking for lower ESR or higher ESR. Nor what their reference was, since nobody really posted the ESR of the original caps.

I made a few mistakes; one is that I was looking at the wrong version of the board in the SM. The diagram and layout I was looking at is not really for the board version I am working on. I have the 1.820.873.83 which is a few pages ahead of the one I snipped up there. No harm done because of that but the resistor that is discolored is really 150ohms, not 120ohms.

@sodderboy I retraced my steps and redid what I have already done and fixed my mistakes. Not sure when I may have compared caps with light bulbs? Fact is that electrolytic capacitors have a shelf life of about 15 years in general. It is reasonable to assume they need to be replaced. Find the appropriate replacement is really the key here since not too much reference seems to be available. Thanks for pointing out the double-sided PCB, the execution needs to be careful to not modify the circuit by accident.

@Ike Zimbel what does a Frako capacitor look like? Did they make just electrolytics or film? Are they identified as Frako?

@jtvrdy I see that you've gone through similar issues some years ago, do you have any insights to share?

And guys, yes, I will be testing with a dummy load.

 

[zamproject]

what does a Frako capacitor look like? Did they make just electrolytics or film? Are they identified as Frako?

Frako is a (German ?) brand, the lytic ones usually look goldish, with red print for brand, value, rating etc ...

Studer used them extensively in all product from 70' an 80', later they mostly switch to philips lytic.

By experience (3 consoles here, and multiple other restored) those capacitor are all dead now, and need to be changed.
The main problem (to me) being they lost the electrolyte by the leg and bottom rubber sealing, most of the time you don't see it that soon wile the pcb is already corroded.

I had a nightmare cleaning hundreds pads at dozens pcb from a 369.
Hopefully input modules are single sided so the traces don't suffer directly from the acid, that was more tricky for the 8 bus, master and monitors sections.

 

[Ike Zimbel]

You should also test the film caps. I know I'm guilty of thinking of them as a "never fail" kind of part but they sometimes fail in some circuits (like microphones, as many here would know...) and if they are working hard in this circuit, they might be prone to failure. A colleague of mine works on a LOT of Studer machines. A while back we happened to be at the same studio when he was dropping off the electronics for an A-80 that he had re-capped, and I noticed that there were both electrolytics and film caps in the bag of used parts he dropped off.

 

[mgrisoli]

You should also test the film caps. I know I'm guilty of thinking of them as a "never fail" kind of part but they sometimes fail in some circuits (like microphones, as many here would know...) and if they are working hard in this circuit, they might be prone to failure. A colleague of mine works on a LOT of Studer machines. A while back we happened to be at the same studio when he was dropping off the electronics for an A-80 that he had re-capped, and I noticed that there were both electrolytics and film caps in the bag of used parts he dropped off.

Great call out. I just had an episode of a film cap failure on a power amp and I cussed up to the third generation of those who made me think they are eternal. Also noted in a picture from another post for this same board that film caps were replaced.

 

[waltzingbear]

Switched mode power supplies are animals of a different nature. In a linear psu moving to lower esr caps is useful and not often a problem. Any experience with an A80 psu is of no value in an A820 or A827 power supply troubleshooting.

Equivalent Series Resistance is the actual resistance of the capacitor, lower esr makes it more like an ideal cap. Caps have improved over the years , moving more towards the ideal cap. ESR also changes over frequency.

The Studer psu is a digital switching amplifier used to regulate the voltage output. Changes in the frequency chacteristics of the amp and side chain can easily throw it into oscillation

 

[Lampie519]

Also noted in a picture from another post for this same board that film caps were replaced

Maybe they also had a hard time finding an issue and replaced everything ….

 

[MS-TAS]

As noted above, this is a switching power supply. Switchers usually oscillate at high frequencies (well, at least above audio frequencies) pass current through the transformer T1, after which the high frequency AC is rectified to DC.
If you are hearing a buzz, it may be due to excessive load on the power supply. Especially if this board works on other decks.
What voltages are on +-15 and +24V outputs? If these are low, does removing the load get you to a well regulated output?

 

[mgrisoli]

As noted above, this is a switching power supply. Switchers usually oscillate at high frequencies (well, at least above audio frequencies) pass current through the transformer T1, after which the high frequency AC is rectified to DC.
If you are hearing a buzz, it may be due to excessive load on the power supply. Especially if this board works on other decks.
What voltages are on +-15 and +24V outputs? If these are low, does removing the load get you to a well regulated output?

Thank you for your reply. The board oscillates without load too.
This board does not work on other decks, but the owner said installed another stabilizer board on this deck and it did not buzz.
I can measure the +-15 volts output (without load) coming out of the board oscillating between 13v ~ 15v.
I will check the 24v output tomorrow.
I am confused, since SM says the PSU is supposed to send 30v...63v DC to this board, not AC.
I measured the PSU output and it is showing 49v RMS with almost no ripple.
I presume the stabilizer would convert any high frequency ripple into DC.
I am not sure if the voltage out of the PSU is correct though.
A label shows this PSU had its middle transformer was replaced in 2010.

 

[MS-TAS]

Sorry, my mistake, I see the input is DC. This is a DC to DC converter, which uses oscillation to create an AC signal which then is stepped up or down via the transformer, then smoothed by the rectifiers in the secondary.
IC1 is a high current, Pulse Width Modulator/Controller. It's job is to oscillate and adjust it's pulse width to provide a certain amount of energy which, in a manner controlled by the opto isolator in the upper right hand corner and IC3 which receives a high frequency (is it 76kHz) from TD-C76K.
Here's its data sheet https://www.st.com/content/ccc/reso...df/jcr:content/translations/en.CD00000966.pdf
There's gonna be "oscillation" all over this board - except at the +-15 and +24V. Looking at the 3842 pin 6, you should see a pulse stream near whatever frequency is noted by P4 in the lower left hand corner. As you increase the load on any output, the pulses will get wider.
If the pulses are lower frequency, as you indicate since you hear a hum, you'll have some honest to goodness troubleshooting to figure out where the issue is. It will not be for the faint of heart, that's for sure.
It would be helpful if you can tell us what oscillation you note on +-15VDC.
I do note that the big electrolytics are rated at 16V for the 15V power supplies, which means they could be sensitive to any over voltage conditions. Checking C20, 21, 23, 24 is a reasonalbe thing to do. REplacing them with 25VDC versions would be a good idea, so long as they fit.

 

[waltzingbear]

And don’t forget supplies like these like to see some ( light) load to work correctly.
All the caps you we’re looking at except for C6 are output filter caps and less critical than other caps in this circuit. Overvoltage replacement is always a good idea here.
Buzzing comes from mechanical movement of a part, dampen with a finger til you find it, then figure out why.

The A820 is a dc powered recorder, it can be run off a huge 48V battery, no ac power required.