Recapping a tape machine (M15) / caps that don't go bad

My studio tape recorder (Telefunken M15) has some trouble staying adjusted. After some time (weeks, usually) the amplifier on a channel might be off a few db, also depending on the time the machine has been kept on (warming) it seems. So I guess it's time to recap this 30-40 years old machine.

Now I'm wondering which caps to replace. Until recently I was under the impression that the only caps seriously degrading over time are electrolytics and tantalums. But in one of my synths (Chroma Polaris) the polypropylene caps failed, too.

So would it be a good idea to replace the poly caps, too? Some of them look pretty aged at least. My reasoning is that if I take them out for measuring I can just as well replace them.

What about polystyrene? Never had any trouble with those, can they even go bad?

Ceramics seem to loose cacity at a declining rate over time, so they don't seem to be an issue.


Anything else worth considering?

Thanks!

I've never seen film capacitors dying of old age. Whar I've seen is they break up due to accumulated vibrations and stress. They look ok but you can see microscopic cracks in the coating and inside the electrode is not making contact anymore.
When this happens, the consequences are generally audible: used as coupling caps, the result will be a lack of bass, used in filters, the frequency will shift much higher. The only instance where it may not be really audible is when they are used for rail decoupling. Except when the circuit starts to oscillate 
As much as I consider recapping the lytics a good preventative measure, I would replace film caps on an individual basis.
Anyway, after recapping, you must recommission the unit, isnt'it?

abbey road d enfer said:

I've never seen film capacitors dying of old age. Whar I've seen is they break up due to accumulated vibrations and stress. They look ok but you can see microscopic cracks in the coating and inside the electrode is not making contact anymore.
When this happens, the consequences are generally audible: used as coupling caps, the result will be a lack of bass, used in filters, the frequency will shift much higher. The only instance where it may not be really audible is when they are used for rail decoupling. Except when the circuit starts to oscillate 
As much as I consider recapping the lytics a good preventative measure, I would replace film caps on an individual basis.
Anyway, after recapping, you must recommission the unit, isnt'it?

Click to expand...

Do you mean mechanical vibrations and stress from movement of the whole unit, or the caps vibrating while in use? I know of electrolytics vibrating audible, but thought what was going through low voltage film caps isn't enough for that. In the Polaris it seems the actual material of the caps is just deteriorating. It's a common problem and happens mostly after the synth just stood there unused. Those caps are used for decoupling the S/H circuit, so the tonal stability goes away when they fail.

I think I'll replace the flim caps anyway, since there are better ones with a smaller footprint availible now. Having the signal go through 20 or so polypropylene caps instead of metallized polyester should make an audible difference. I can also use lower tolerances, which should make for better tracking between the channels.

What do you mean by "recommission"? Recalibrate? I'll certainly have to do that after the recapping.

living sounds said:

abbey road d enfer said:

I've never seen film capacitors dying of old age. Whar I've seen is they break up due to accumulated vibrations and stress. They look ok but you can see microscopic cracks in the coating and inside the electrode is not making contact anymore.
When this happens, the consequences are generally audible: used as coupling caps, the result will be a lack of bass, used in filters, the frequency will shift much higher. The only instance where it may not be really audible is when they are used for rail decoupling. Except when the circuit starts to oscillate 
As much as I consider recapping the lytics a good preventative measure, I would replace film caps on an individual basis.
Anyway, after recapping, you must recommission the unit, isnt'it?

Click to expand...

Do you mean mechanical vibrations and stress from movement of the whole unit, or the caps vibrating while in use?

Click to expand...

Mechanical vibrations.

In the Polaris it seems the actual material of the caps is just deteriorating. It's a common problem and happens mostly after the synth just stood there unused.

Click to expand...

Never seen that happen. What brand are these caps?

What do you mean by "recommission"? Recalibrate? I'll certainly have to do that after the recapping.

Click to expand...

Yes, recalibrate and check everything does what it's supposed to do.

abbey road d enfer said:

Never seen that happen. What brand are these caps?

Click to expand...

I think it's Yageo. They're orange, lumpy, with a "Y" printed on. Here's a picture someone else made:
http://www.polkadotdictator.net/kss/polaris_4.jpg

They're not orange drops, I think.

abbey road d enfer said:

Yes, recalibrate and check everything does what it's supposed to do.

Click to expand...

Yes. Still, this will probably take less time than calibrating takes now, since the channels should track better after the recap with low tolerance caps. I've also got a second set of every module just in case.

living sounds said:

abbey road d enfer said:

Never seen that happen. What brand are these caps?

Click to expand...

I think it's Yageo. They're orange, lumpy, with a "Y" printed on. Here's a picture someone else made:
http://www.polkadotdictator.net/kss/polaris_4.jpg

They're not orange drops, I think.

Click to expand...

According to their website, they now make only lytics and MLCC's. Maybe because they've been burnt once...

abbey road d enfer said:

Yes, recalibrate and check everything does what it's supposed to do.

Click to expand...

Yes. Still, this will probably take less time than calibrating takes now, since the channels should track better after the recap with low tolerance caps.

Click to expand...

Yes, after all, replacing them is not such a big job once the PCB's are on the bench.

abbey road d enfer said:

Yes, after all, replacing them is not such a big job once the PCB's are on the bench.

Click to expand...

I've taken out a couple modules today and put a few new caps in. Really easy to work on, I love those old boards. Nice, big through hole layout and non-ROHS solder. Single layer layout, no problems with traces coming apart. Totally discrete, too. I modified my Moog pedal yesterday, that was a lot more complicated. With just the electrolytics and tants replaced the channel now stays stable, wheras the other one changed its output amplitude over the course of half an hour.

Now I'm wondering if I should take the machine apart and have a look at the PSU. Probably not.

Usually the PSU electrolytic caps and mechanical connections are the first place to start.
If it is not mechanical in the first place, drifting alignments are more of a connection (contact or solder) or pot problem than electrolytic/tantalum caps.  Caps are more of a "where'd that signal go?" or "wot no lo end?" situation.  The age indicates a cap change anyway, but you should check/clean/condition the card contacts and connector solder joints as well. 
Something is changing value to change the alignment.  Old caps usually have a static, slowly going more incorrect, value.
Mike

sodderboy said:

Usually the PSU electrolytic caps and mechanical connections are the first place to start.
If it is not mechanical in the first place, drifting alignments are more of a connection (contact or solder) or pot problem than electrolytic/tantalum caps.  Caps are more of a "where'd that signal go?" or "wot no lo end?" situation.  The age indicates a cap change anyway, but you should check/clean/condition the card contacts and connector solder joints as well.  
Something is changing value to change the alignment.  Old caps usually have a static, slowly going more incorrect, value.
Mike

Click to expand...

I've now replaced more caps and consistency seems to be improving. But checking the mechanical connections sounds like a good idea. The connectors of the plug in cards have black stains on them, maybe some oil or one of those contact sprays or similar was used. It doesn't go away with isopropyl alcohol, so I'll have to use steel wool for that.

My main reasoning to suspect caps for the drift was that it changes when the machine is on for a longer period of time, since those are the components that warm up the most by themselves and tend to age. Those old carbon comp resistors are supposed to drift quite a lot while warming up, too, but they don't age, do they? Surely in 1970 they were able to build a machine that stays within parameters when cold-started... After all, it's a high end broadcast device, and when used as a backup in emergency you cannot wait for it to get to stable temperatures. At least I hope so, it's too noisy to have it run all the time, and also a sucker for electricity (converting most of it into heat)...

Just the reseating of cards could be making the difference.  Something so stubborn can be causing it's own complex impedance.
Be careful with the steel wool because it sheds and shorts things.  Really careful.  Try some Caig R5 on the contacts, clean it off with the isopropyl, and apply D5 conditioner.  Works like a charm.
And are you using a flux remover after recapping?  If you do you will be better than 90% of "professional" techs.
Mike 

sodderboy said:

Just the reseating of cards could be making the difference.  Something so stubborn can be causing it's own complex impedance.
Be careful with the steel wool because it sheds and shorts things.  Really careful.  Try some Caig R5 on the contacts, clean it off with the isopropyl, and apply D5 conditioner.  Works like a charm.
And are you using a flux remover after recapping?  If you do you will be better than 90% of "professional" techs.
Mike 

Click to expand...

Yes, the steel wool is problematic. I tried it on one card and got the contacts clean, then blasted it with air to get the small fibers from the wool off. It's not SMT and a rather scattered layout, but the stuff might still cause trouble.  I'll see if I can get R5 or an equivalent over here. I often remove the flux with isopropyl, but not all the time. Also found some strange dried liquid residue on one type of card, looks like they poured it over some transistors and small ferrite core inductors. Probably just glue, it doesn't cover them sufficiently and they're not positioned close enough for temperature matching.

Thanks,
Gregor

Don´t do that "Stahlwolle" thing. It destroys the contact surface (too complicated to describe it in english, sorry, but there´s a lot of metallurgy going on with contacts)
One problem you have might be the open trimmers. If you don´t fear recalibrating everything then replace all open trimmers, they are a source of trouble.

OK, I won't use the steel wool anymore. The trimpots I could replace, but would need to find the schematics first to get the correct values. Schematics for the M15 are hard to get by it seems...

I have used erasers to clean edge contacts.  Is there an issue with that jensenmann?