Curious common mode fault on Philips PM5518

[Apfelmus]

Hi all,

I have a curious fault, where two identical circuits have developed the same symptoms, although they do not share any component!

Its a pattern generator where I noticed a problem with the sound output - the generator gave only a very faint distorted signal on the TV (and it was not the TVs fault). The video part of the pattern generator works perfectly.

The PM5518 has two internal audio frequencies, 1 and 3kHz. There is one AF-generator circuit for each frequency. Since the symptoms occur for both frequencies I suspected the modulator or some part of the circuit where both signals come along. Then I discovered that the sound is perfect when applying an external AF-generator - so the fault cannot be after the MUX circuit (IC 305)!

Now both generators deliver about the same distorted low signal, although they do not share anything in common except power supply! Of course I checked power supplies, but could not find anything wrong.

Attached are some scope pictures taken from the 3 kHz generator (the one on the top of the drawing). The first is on pin 7 of CA3086. The second behind the capacitor 507, and the third at pin 1 of IC 305 (behind R696). The signals after C507 are of much lower amplitude, I wonder why.

The waveforms on the 1kHz generator are essentially identical!

Any thoughts? Thanks already.

Cheers
Martin

[Apfelmus]

To add some information, attached pictures of the unit 7 with the board-layout from the service documentation.

The two audio-generators are on the top left with near to identical layout.

The unit is not easy to service. ICs are soldered, and the board is rather inaccessible when plugged in. I do not have the extender-board, what I did is extract the neighbouring boards as they are not required for board 7 to work (unit 6 is video, and unit 8 is the sound modulator).

cheers
Martin

[REDGUUZ]

Martin,

If you (temporarily) detach C507 and C518 and observe the signal(s) OUT ( 1KHz and 3 KHz), are they then still distorted at the outout of C507/C518?

If not, I still would suspect IC 305, the mixer.

Proof of the pudding: Inject a 1 KHz signal at R 695 (C507 and C 518 detached).
However, if everything is now OK, I would recheck the 1 / 3 KHz generators.(see below).


If the signals out of C507 and C518 are still distorted (after decoupling C507 and 518),
I still would suspect +/- 12 V powersupplies (magnitude +/- 12V , and noise or ripple due to defect in power supply).

Just my 2 $C, I'm only an amateur!

Regards,

Martin

[Apfelmus]

Originally Posted by REDGUUZ

If you (temporarily) detach C507 and C518...

Thanks Martin, that should be a good strategy to isolate C305 from the generators, to be sure if C305 has any influence.

C305 is not mixer but a multiplexer. It correctly routes through all the signals from the entry to the exit, so far. Meaning the waveform is the same on both sides.

You'll hear from me.

cheers
Martin

[cmjones01]

It could be that the mux is faulty, but also likely is that both C507 and C518 have dried up. It looks like they're radial electrolytics which, having sat inside a row of PCBs getting hot for years, may have reached the end of their lives.

I had exactly this problem on an arcade game PCB (Mr. Do!, I think it was) which used a dozen or so little electrolytics to couple various audio circuits and filters together. It turned out that they were nearly all faulty!

Chris

[Apfelmus]

Originally Posted by cmjones01

It could be that the mux is faulty, but also likely is that both C507 and C518 have dried up...

Hi Chris,

guess what I found out yesterday... both C's have little to no capacity left!! The other electrolytics seem to be alright, more or less, but well... looks like I have some work to do.

I've already heard that these red Roedersteins tend to leak. In the Stabilock I have hundreds of black whatever brand, at least two had developed shorts. These blue Philips caps looked OK to me, I didn't find visual signs of leakage.

But sometimes caps seem to last forever. I have a 1975 colour TV from Telefunken, It sat on the shelf for 30+ years and still functions near perfectly. For sure they did not use military grade Caps.

Cheers
Martin

[cmjones01]

I'm glad to hear you found the fault! I find that most failed electrolytics, especially good quality ones, don't show any external signs of problems, but lose capacitance or gain series resistance to the point that they're useless. Philips ones are definitely among the best of the bunch from the 1970s and 1980s, but the radial ones don't seem to be as long-lived as the axial ones.

Chris

[Apfelmus]

Hi all, Chris,

the generator's waveform ist still the same when uncoupled from the MUX. Not surprising, given all the other electrolytics which are bad...

I've never thought that electrolytics can fail so evenly! All the 1uF are down to 300n, the 10uF are down to 6uF, the 100uF are down to 90...95uF. Only three caps were worse than that, down to 30n.

Looks like I have to buy a stock of radial electrolytics. What would you suggest buying? What do I have to look for? My local dealer (Conrad Electronic) has 5 brands for a 1uF 63V: Vishay, Würth, Teapo, Jackon and Hitano. Hard to tell the difference, except for their colour... Teapo seems cheapest, Jackon most expensive.

Cheers
Martin

[Chris55000]

Hi!

You should be ok with Würth - they're a good-quality Continental-brand electrolytic I've used in industrial repairs in my employment that aren't outrageously expensive, and e.s.r. wise probably comparable to the ones originally used by Philips when these generators were in production!

Chris Williams

[Apfelmus]

Thanks for the Info.

There is a myriad of electrolytic C's, with a myriad of acronyms surrounding them on the distributors websites...

I found an interesting article (linked by Wikipedia) about capacitor development:
http://ieeexplore.ieee.org/document/...1/?reload=true

cheers
Martin

[Apfelmus]

Chris,

today I replaced the 7-segment-LED arrangement. Works, great.

Now for the capacitors:
I made an inventory. I also doublechecked with the parts list of Philips. Most of the capacitors there have a percentage figure (i.e. 10%, 50%) in addition to capacity and voltage. Do you know what that means? Its explained nowhere.

Attached an example page. Of course I can go ahead without this information, but out of curiousity I'd like to know. Has it something to do with components size?

cheers
Martin

[cmjones01]

That percentage looks like the tolerance - the amount by which the value of the capacitor is allowed to vary. For electrolytics, don't worry about it. They are not precision devices, and are generally chosen to be roughly the right value, fit in the available space, and be cheap enough. There is no point in searching for exactly the right performance characteristics for electrolytics in a piece of equipment like this. Designs of the 1970s and 1980s were much gentler on their electrolytic capacitors than a lot of newer equipment is, because the use of high-frequency switching power supplies was much rarer. As long as the voltage rating of your replacements is high enough and their capacitance is between about half and double what the original was, nothing else really matters.

As for the brands you list, Vishay and Würth will both be good quality components that should last for 20 years or more in an application like this.

Chris

[Apfelmus]

Its done! I recapped electrolytics all over the unit.

Measuring the old ones gave a clear message: Of those 47uF and smaller electrolytics ALL have at most 30% of capacity left. Half on them were a complete failure (less than 100nF). No shorts, BTW.

I wonder how that unit could still deliver a decent test image on my TV-receiver!

The only capacitors I left were the 100uF and higher ones. Some of them were still OK, others down to 80-90uF.

The audio generators came back to life, delivering a correct sinewave at approximately the correct frequency.

BUT - the 1kHz generator, after some hours of testing, failed again!

It turned out that one of the replaced C's had a short! But the short remained even with the component unsoldered. Turned out it was a short that developed between the solder-pads on the component side (i.e. under the capacitor). It was invisible to the eye, even with the capacitor unmounted and using magnifiying glasses I could not see anything. But the short vanished as soon as I scratched the board between the solder pins.

Now everything works fine...

Cheers, and many thanks to all who helped me findent the culprits!
Martin

[cmjones01]

I'm glad to hear it's working now. Congratulations on a repair well done.

Chris