Decca SRG899 filter cap gone bad?

[al_kaholik]

Hi all,

There has been lots of loud hum on the left channel of my Decca. Having now loosened off the chassis (but not having all the wires out yet) and identified the left channel of the amp, I am thinking this is the culprit, centre with the curled up casing.

I welcome the views of others and any recommendations on a replacement (10uf 50VDC). Would these be acceptable? https://www.ebay.co.uk/itm/5-X-Panas...AAAOSwTPlaluvL and if I am replacing one side, is it worth changing the other at the same time?

Thanks in advance,

Al

[Refugee]

I can see another one of those caps that looks a bit powdery around the rivet.
There might be some value in replacing the two light green ones as new ones will be slightly smaller so that you can gain the space to use modern ones to replace the grey ones.
The replacements for the grey ones will go in slightly offset towards the negative terminal of the old ones.

[al_kaholik]

Thank you - It may be much easier and give me peace of mind if I replace all six of the upright ones. Presumably there is also value is replacing the horizontal ones (one of the small green ones is pictured) at the same time?

[DMcMahon]

Yes if you mean the light green electrolytic, the dark green (bottom left corner) non electrolytic capacitor is probably fine.

David

[al_kaholik]

Perfect, yes the light green ones - it hadn't clicked that there were more in shot than that.

Panasonics ordered for the larger ones however had to go with a no-name to get the axials 47uf 33v. I'm not sure it'll make much difference either way.

Thank you again.

[Refugee]

I would have gone for radials as those axial ones have one lead bent over to make them fit radial positions on the board.

[Uncle Bulgaria]

You could consult this helpful article on capacitor replacement from this parish: https://www.vintage-radio.com/repair...apacitors.html

As mentioned there, the capacity to deal with ripple current is more important than ESR in non switch-mode power supplies. I understand a specifically low ESR type, as ebay-linked in your post, used indiscriminately in old circuits can load the rectifier too much and cause problems not envisaged by the designers of the original equipment, when 20% capacitance accuracy was pretty admirable.

I second Refugee's use of the radials there. Presumably they didn't have anything else to hand so had to bend an axial over to make the connection. Sometimes new axials don't have long enough legs to replace that sort of connection.

[al_kaholik]

Originally Posted by Refugee

I would have gone for radials as those axial ones have one lead bent over to make them fit radial positions on the board.

Again, perhaps I haven't been as clear as I could. I've gone for radials to replace the upright capacitors and radials to replace those which are horizontal in the picture. I'm reassured by your comment I've done the right thing.

Originally Posted by Uncle Bulgaria

You could consult this helpful article on capacitor replacement from this parish: https://www.vintage-radio.com/repair...apacitors.html

As mentioned there, the capacity to deal with ripple current is more important than ESR in non switch-mode power supplies. I understand a specifically low ESR type, as ebay-linked in your post, used indiscriminately in old circuits can load the rectifier too much and cause problems not envisaged by the designers of the original equipment, when 20% capacitance accuracy was pretty admirable.

I second Refugee's use of the radials there. Presumably they didn't have anything else to hand so had to bend an axial over to make the connection. Sometimes new axials don't have long enough legs to replace that sort of connection.

Thank you for the link, I have read though it now. As a result I've done the research on the ripple current on the capacitors I've ordered with the values as follows. Do these look reasonable for what is required?

Cap / ripple current

10uf 50v = 250mA
100uf 50v = 870mA
330uf 25v = 950mA
47uf 16v = 125mA

[Uncle Bulgaria]

Have you got a circuit diagram? I'm by no means an expert like some of the electronics whizzes on here, who might be able to recognise your piece of equipment from the photo. However, I can tell that at those values this isn't a valve rectifier circuit! If someone else doesn't turn up I can educate myself by investigating with a diagram.

[al_kaholik]

Afternoon, I don't - its the audio amplifier for a Decca SRG899 - from what I read it is covered in the Decca 613 service sheet which I haven't got my hands on yet.

The power circuit seems to work fine, everything powers and works well which is good!

[al_kaholik]

So a little update, I've replaced all the caps on the audio board as pictured and there is no change to the audio. There is still a loud hum on the left channel even without a source playing. I have switched the speakers over to rule this out.

There is a single large cap for each channel (pictured blue) that lives off the board but is attached, 1250uf 25v. Perhaps this is the culprit? There is no bulging or deformation visible however.
As for the power stage, there are three caps here a large '2000 + 2000 uf' 35v unit (pictured orange) and two smaller ones (pictured red - nut loose as I've removed it). One of which is 1250uf 16v (physically looks fine) and the other looks the same, size wise and from what I can make out of the text, is certainly 16v but I can't reasonably remove it at this time without unsoldering other components.

On the controls board (on&off/volume/tone/balance) there is a single 400uf 25v unit, which again looks physically fine.

The only other remaining cap is on the radio power board, again which looks physically fine

Further thoughts or advice welcome. Is replacing every cap in the unit a sensible thing to do or am I chasing nothing here? Also if anyone knows a source of the service manuals, or which sheets cover everything in a Decca 899 that would be great to know.

Al

[Herald1360]

There's far too much heatsink paste been used for those output transistors! Is that original?
Too much is as bad as none at all if it prevents intimate metal-metal contact between the device and the heatsink. Heatsink paste has a thermal conductivity poorer than direct metal-metal contact but better than air and is used only to fill in the microscopic voids present between two apparently flat surfaces.
The ideal amount is a thin layer which just shows an even coating over the interface surfaces.


This won't affect your hum issue, though, unless an output device has failed as a result!

[al_kaholik]

Thanks Herald - This is how I received the unit, as I had noticed, and perhaps mentioned in another thread that I didn't think this is factory. A few pictures elsewhere on the forum show a liberal application of paste too, so perhaps it is factory.

If I can get around the hum issue, I will clean and replace the compound - trying to change only one variable at a time at the moment.

[Uncle Bulgaria]

Have you got an oscilloscope? Tracing the output signals will show you where the problem lies. Personally, I don't agree with replacing capacitors willy-nilly. It's not fault-finding and can easily obscure the issue.

Without an oscilloscope, perhaps grounding the signal path will demonstrate where the hum is originating from? Trace it back (or start in the middle to see if it's before or after the point chosen, and then halve the dodgy half until you isolate the stage where the hum begins) and we can go from there.

[Refugee]

If there is reasonable sound on the channel that has hum it is likely to be the bias settings.
Have you got any instructions for setting the presets?

[al_kaholik]

@Uncle Bulgaria. I don't have a scope. Its a tool I can't justify based on the usage I'd get from it. This is a one-off audio project. I have a digital multi meter and that is about it for diagnostic tools.

Certainly audio isn't something I'm very experienced in, and only trying to find the best way forward. Given the state of the capacitors, and following advice here, I opted to replace all rather than only the one that I thought looked suspect - again I'm not an expert here.

Form further limited reading, I believe it to be 50Hz and only on the left channel. It is present from the moment the unit is turned on, irrespective of the source selected.

@Refugee. There is very little sound from the channel compared to the volume of the hum. It is there, but overshadowed by it completely. I have purchased the Decca 603 service sheet which covers the pre-amp section but not the power amp section (the pictured board) nor the power stages.

Searches often come up with nothing regarding Decca, specifically the later SRG models, this being an SRG899. As I understand the modules were standardised and used throughout different equipment however it isn't immediately obvious how I can determine that.

They are used in the Compact music centre models, but again the service sheets seem allusive.

Thank you all for the help so far.
Al

[Uncle Bulgaria]

I did have a look for any circuit diagrams earlier and couldn't find anything on my usual resources.

I've recently got to grips with oscilloscopes and think they're indispensable - I've had two free and one from a kind member on here for £3 the other week after the previous ones gave up the ghost, so they don't have to be bank-breaking. Space is another matter, although I believe there are now USB oscilloscope probes that couple with apps on 'phones and computers, which would be tiny.

Here though it sounds like mains pollution which should be relatively easy to diagnose. I'm not an expert by any means, unlike some of the professional electronic engineers on here, but I have fixed more than I've broken. Personally, I would ground the signal on the dodgy channel at the junction between the pre and power amps and see if the hum remains. If it does, it's originating in the power section. If so, then trace the audio path and sketch it out, back to the pre-amp stage you have on the service sheet. Find out what the pinouts of the amplifying transistors are and compare voltages between the channels. Use your multimeter to do this and note down the results. This should expose refugee's biasing problem if that's what it is.

[Refugee]

Is the heat sink getting warm?

[Techman]

I agree with Uncle Bulgaria, he virtually took the words out of my mouth. Just replacing caps without knowing why you're actually replacing them is a road to nothing and probably a road to disaster if you introduce other man made faults along the way - although I would agree that replacing the couple of crusty ones isn't a bad idea, even though they were probably still operating perfectly well as capacitors.

This isn't a mains smoothing fault as there's no hum on the other channel. I think it's probably a biasing fault to the output stage, perhaps one of the transistors or a resistor, You'll need to do some proper tests and learn how to test transistor junctions with a meter - loads of info on the net. You've got the benefit of having the working channel as a comparison for when you're taking your voltage readings, so no need for a circuit diagram for this, just the transistor data, again, downloadable from the net to give you the pinouts etc. Transistor amplifiers like this are notoriously difficult to fault find due to all the usual DC coupling arrangements with a fault in one stage affecting all the others, but as said, you've got the other channel to use as a comparison.

[Techman]

Originally Posted by Refugee

Is the heat sink getting warm?

Good point!

Compared to the other channel?