Vintage Capacitor replacements - Help please.

[Banksy76]

Even though I know very little about electronics (but slowly learning), with a little research and some advice attained I have just successfully re-capped a few vintage units in my home recording studio.

My next repair/restoration project is my vintage Pioneer SR101 valve reverberation amplifier which I guess is also long overdue for its capacitors to be replaced. I believe that this unit was produced in the 60's, possible earlier 70's and is basically a stereo spring reverb unit that was designed to be incorporated into a domestic home hi-fi set up to add addition ambience to playback material.

The last few pieces of equipment that I replaced the electrolytic capacitors in proved fairly straight forward once I was schooled in the basics and knew what I was looking for and it was simply a case of sourcing replacement capacitors of the same capacitance rating and Voltage and then soldering them in place.

However, what I have found within the Pioneer SR101 seems to be a whole different beast entirely and I was hoping that some of you guys here could help me out. These were the ratings of the capacitors that I found within the unit:

4.7uF 160V
3.3uF 160V
0.05uF 400W
0.01uF 400W
0.005(K) 400WV

I offer my apologies in advance for what will probably seem for you guys some silly questions but please bear with me as I am new to this...

So far i have only ever encountered electrolytic capacitors with their uF ratings and Voltage displayed so could someone explain to me why in this case that in addition to the standard 'V' rating we also find 'W' and 'WV'?

Also what is the meaning of the 'K' rating instead of the 'uF' in the last capacitor in my list?

Many thanks in advance for schooling this beginner.

[ronbryan]

"K" is 10% capacitance tolerance. Similarly "J" is 5% and "M" is 20%.

W and WV refer to the working voltage of the capacitor.

Ron

[Banksy76]

Originally Posted by ronbryan

"K" is 10% capacitance tolerance. Similarly "J" is 5% and "M" is 20%.

W and WV refer to the working voltage of the capacitor.

Ron

Thanks Ron.

So just to clarify, the replacements that I'm looking for are simply:

4.7uF 160V
3.3uF 160V
0.05uF 400V
0.01uF 400V
0.005uF 400V

Is this correct?

[ppppenguin]

4.7uF 160V
3.3uF 160V
For these 2 we really need to know whether they are electrolytic or not. Is one end clearly marked + or -? A photo would help, or at least the sizes. These are quite high values for polyester etc but quite low for electrolytic.

0.05uF 400V Use 0.047uf or 47nf. Minimum 400V
0.01uF 400V Use 0.01uf or 10nf. Minimum 400V
0.005uF 400V Use 0.0047uf or 4.7nf. Minimum 400V
Many different physical types available, all will work OK. Polyester will be cheapest and entirely adequate.

If the existing capcitors are some kind of plastic dielectric it's 99% likely they will be OK and no need to change them. Again photos would help.

[Banksy76]

Ok, I shall get some photos taken later and post them on here. Many thanks for for your advice and assistance.

[Banksy76]

Here are some photos detailing the individual capacitors:

[Banksy76]

And here are some pics of the 2 larger capacitors:

These last 2 photos show the larger 22uF 160V capacitor and the big ol' 40uF 350V cap. Would you recommend replacing these also?

Any advice/assistance/info regarding the 're-capping' of this unit would be greatly appreciated.

[geofy]

Mods, any chance of thumbnailing this (and any of this type of) thread please

Banksy76. First what are the symptoms of this amplifier, if it works then there is no need to randomly change every capacitor, the smaller ones especially. If the amplifier does not work properly then it needs investigating first. If there is bad mains hum this can be caused by the larger caps going faulty. But Japanese equipment in general is very reliable and a lot of the capacitors will have nothing wrong and you will be doing unnecessary work with the risk of damaging other components.

[Lucien Nunes]

I agree with Geoff. There are two totally different kinds of capacitor here and they have different prognoses. The electrolytics (40, 22, 4.7, 3.3) can fail at random and might have deteriorated with disuse although these are of good make and may well be OK, especially if reformed (see elsewhere on the forum for instructions.) The paper-in-oil caps (0.05, 0.01, 0.005) are probably fine and at any rate can be checked - if they read good they are good. Indeed there are people out there who would probably prize used PIO caps like these, to fit them in place of cheap'n'nasty metallised films.

Shotgun recapping (changing every cap regardless of its condition or function in the circuit) is practised as a preventative measure with later hifi gear that has a large number of capacitors, some of only limited lifespan, that were pushed to their temperature and voltage limits and beyond. It's also sometimes sensible with older kit where the caps are simply too unreliable (e.g. waxies) to have a reasonable chance of further survival even if they work OK now. But it's not always necessary!

Last week I was working on a PA amplifier built in 1937 that has 16 capacitors, all screw-terminal paper-in-oil. All 16 are original, all work perfectly, there would be no point (and a lot of disadvantages) in changing them.

Lucien

[Banksy76]

The problem with the unit is that one channel has a really low output and poor frequency response which doesn't appear to be due to a faulty reverb tank. In addition to this the overall volume is really low resulting in a poor signal-to-noise ratio.

I recent purchased a more pro-end rack mount spring reverb unit for my home studio which suffered from the same symptoms; L&R output in-balance, Low output and poor frequency response. After replacing the electrolytic capacitors in the unit it is now working as it should and sounding fantastic. I then remembered about this Pioneer SR101 which has been sitting around waiting for a diagnoses and repair and it suddenly dawned on me that the issue seemed to be the same and that this too could also require its capacitors replacing in order to have the unit in full working order again.

So what do you guys think, do you agree that this problem could be due to bad caps and if so which ones do you think would most likely be the culprits?
Can I perform a reliable check of the caps with a simple multi-meter ?

Many thanks

[Tractorfan]

Hi,
By "channel" I assume this unit is stereo? If so, I would be inclined to check the various voltages are OK. You've got one channel that is giving full output so you can use that as a reference (if you haven't got the schematic). Also you could swap the valves from one channel to the other to see if they are the culprit (although unlikely).
Re-capping for the sake of it seems a bit pointless in my personal view.The fault may easily rest elsewhere. You say that the overall volume is down, and this could be due to low HT.
Hope this helps.
Cheers, Pete

[Sideband]

Originally Posted by Banksy76

Can I perform a reliable check of the caps with a simple multi-meter ?

Many thanks

Simple answer to this is no. A simple multimeter with a capacitance range may give a reasonable indication of the value of the non-electrolytic types but it's no good for measuring leakage with any degree of accuracy. For electrolytic types it's even worse as the value can be + or - 30% and it will give no indication of series resistance, which is the most common failure mode.

To measure capacitors properly, you need a bridge and an ESR meter.


SB

[Banksy76]

Hi Pete, thanks for the reply

Originally Posted by Tractorfan

By "channel" I assume this unit is stereo?

Yes, unit is stereo.

Originally Posted by Tractorfan

Hi,
Also you could swap the valves from one channel to the other to see if they are the culprit (although unlikely).

Have swapped the tubes over and these do not appear to be the problem.

Originally Posted by Tractorfan

Re-capping for the sake of it seems a bit pointless in my personal view.

This does seem to be the general consensus here on this forum so far which I was surprised to hear as I have been lead to believe that after 20/30 years electrolytic capacitors tend to need replacing.

Originally Posted by Tractorfan

Hi,
The fault may easily rest elsewhere. You say that the overall volume is down, and this could be due to low HT.
Hope this helps.

As i stated in my original post, I am very new to the world of capacitors, resistors etc but steady learning. What exactly do you mean by 'HT'?

[Banksy76]

Originally Posted by Sideband

Simple answer to this is no. A simple multimeter with a capacitance range may give a reasonable indication of the value of the non-electrolytic types but it's no good for measuring leakage with any degree of accuracy. For electrolytic types it's even worse as the value can be + or - 30% and it will give no indication of series resistance, which is the most common failure mode.

To measure capacitors properly, you need a bridge and an ESR meter.


SB

This is what I thought. Thank you for confirming this for me.

[Lucien Nunes]

HT = 'High Tension' = high voltage DC supply typically 250-450V that feeds the anode circuits of the valves.

after 20/30 years electrolytic capacitors tend to need replacing

Some don't even last that long, others will do 50+. There are many factors behind this and an experienced bench tech will often have an accurate hunch about the likelihood of failure before even getting the covers off. You'll get a feel for them in time, if you develop skills at diagnosing faults rather than changing a bunch of parts in the hope that the fault was amongst them. That way you never get a chance to correlate cause and effect, although it is sometimes necessary with certain kinds of nuisance faults.

With a stereo unit that has a working channel, you have a built-in troubleshooting aid. Audio valves handle AC signals, but they have to have the right DC conditions to work. A faulty capacitor will often upset the DC conditions, which will be revealed by the valve electrode voltages being different between the channels. When faced with poor audio on something like this, that would be the first test I would do once the covers are off, before even going to fetch the manual off the shelf. We can probably help you find the fault in this way, regardless of what part is faulty.

Lucien

[Banksy76]

Maybe I would be better off getting someone qualified to undertake a test and repair on the unit, though this will probably end up costing more than what i paid for the unit itself!

Just in case I cannot manage to diagnose the problem myself, can anyone here recommend someone in the Essex area that would undertake this kind of repair?

[Lucien Nunes]

Hopefully you'll be able to pin the trouble down pretty quickly. A bit of poking around the with the meter should help you keep the cost and time down to repair it yourself. I dont think any of us would say outright 'don't change all the caps' as you might well clear the fault that way. Just that a dyed-in-the-wool TV or radio engineer would have a stab at finding the specific fault first, before perhaps taking that route if many of the components seemed below par.

If you want to take the plunge with troubleshooting, don't hesitate to post findings and voltage readings if you want a second opinion on what they indicate. If you have a schematic, and it has voltages marked on it, I'd certainly recommend spending a few minutes comparing them with the voltages in your unit.

Anyhow, if there's no hum (or very little) on the good channel then leave the large electrolytics (reservoir & smoother) alone. They can be dealt with later once the unit is working, if you want to renew them as a preventative measure.

Lucien

[Banksy76]

I have located a schematic for the SR101 which I have provided below but I have no idea where to start with trying to diagnose the problem. Can I really make sufficient progress with just a basic multimeter as I have already been informed here that achieving an accurate reading of the capacitors alone would require a bridge and an ESR meter?

http://i374.photobucket.com/albums/o...1/Picture1.png

[Station X]

Most faults on vintage radios and amplifiers can be diagnosed with nothing more than a multimeter.

What you must remember is the failure mode of capacitors. They become leaky, that is to say that when subjected to their working voltage they start to behave like resistors. Now if you modify a circuit by substituting capacitors with resistors you change the operation of the circuit and therefore the voltages at various points.

Take C1 in your diagram. If that leaks current will flow through it and that current must come through R7. More current through a resistor means a greater voltage drop across it, so the voltage on V1A anode will drop below the 65V shown. The same current will flow through R9 and that will put a positive voltage on the grid of V1B.

If you have a meter measure all the resistors, with power off of course. Any which are more than 20% off value should be replaced. Be careful though as some of the resistors are shunted, ie have another resistor in parallel and this will give a false reading.

ESR meters are only intended for testing Electrolytic Capacitors, not normal capcitors with paper or plastic dielectric. They have their place in testing the numerous electrolytics in modern equipment, but are not much use when it comes to vintage stuff.

[jjl]

Now you have the schematic, if you feel you are competent to do this, you can take voltage readings and compare your measurements against the voltages marked on the schematic. Faulty capacitors could be the cause of the problem but this is not absolutely certain. The voltage readings are very likely to pinpoint the real cause of the problem.

John