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Old 18th Feb 2017, 9:47 pm   #161
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Default Re: Restore a Beau Decca

You shouldn't operate the amplifier with either of those resistors out of circuit as these are 'balancing' resistors that form a potential divider to keep the voltage the same across both capacitors regardless of any slight differences that each of them may have. With one resistor missing or open circuit, you could potentially have the situation of too much voltage across one capacitor under certain circumstances, although I don't think that this is what's happened here. Having said that, the resistor was infinite value anyway, so would have been of little help in this situation. The clue is in the 40K ohms you measured across it when still in circuit. This 40K ohms was obviously the resistance of the actual capacitor, which is far too low and indicates a very leaky capacitor, hence the heating, you were lucky it didn't go off bang! I don't think that it's likely to recover now, so will definitely need replacing, along with the other one, as they have to match.
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Old 18th Feb 2017, 9:55 pm   #162
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Default Re: Restore a Beau Decca

Thanks techman. I will not use it until the replacement resistors arrive and have been fitted. To replace C50 and C51 are two of the caps in post #143 good for the job?
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Old 18th Feb 2017, 10:03 pm   #163
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Default Re: Restore a Beau Decca

As regards the valve mismatch, I have to admit that the results are a bit confusing. Badly out of match valves will certainly cause some hum, but I now think that some of the problem is with that failed reservoir capacitor - you've been a bit unlucky with that, as I said, Keep feeling them for any signs of heating up. I've found that when these particular valves are far enough out to be causing the non-cancellation of hum, it's usually down to the heater of one of them not working to full potential, but you've visually checked and shown a picture of the heaters glowing and they looked the same. I would advise checking them again, just remove the rectifier valve so that there's no HT to blow up that faulty capacitor. You could measure the heater current on each one, but that's getting a bit complicated, although it may come to that in the end.
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Old 18th Feb 2017, 10:08 pm   #164
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Default Re: Restore a Beau Decca

Quote:
Originally Posted by thejazzageuk View Post
To replace C50 and C51 are two of the caps in post #143 good for the job?
In this case, unfortunately not, as they are for the reservoir which is directly following the rectifier. They need to be high ripple rating and as high a temperature rating as you can find. Have a look round on the net, but I think PJL mentioned where to possibly obtain some in one of his posts.

Edit: PJL mentioned who made them, so you'll need to find a supplier.

Last edited by Techman; 18th Feb 2017 at 10:16 pm.
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Old 18th Feb 2017, 10:19 pm   #165
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Default Re: Restore a Beau Decca

Thanks again Techman. PJL please can you advise where I can source a couple of suitable capacitors for this job? As I mentioned I've ordered a couple of 2w metal film 100k ohm resistors for the balancing resistors.
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Old 18th Feb 2017, 11:03 pm   #166
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Default Re: Restore a Beau Decca

I've been thinking that it might be a good idea to do away with that ridiculous series reservoir capacitor set up with balancing resistors and convert it to just one capacitor with no resistors, which I'd probably try to do if it were mine - what do others think?

I like to keep things original and those big old 'dog-bone' resistors look fantastic but, unfortunately in this case, they have to be replaced if keeping the twin capacitors, or go altogether if converting to a single capacitor.
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Old 18th Feb 2017, 11:12 pm   #167
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Default Re: Restore a Beau Decca

Actually, we don't know what the total HT is at that point just after the rectifier. If it's too high then a single electrolytic with a high enough working voltage rating may not be available.
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Old 18th Feb 2017, 11:39 pm   #168
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Default Re: Restore a Beau Decca

I looked for at least 300mA ripple current but F&T don't do a suitable 8uF. As it is only 4uF/630V required I wonder if a 4x 1uF polyester capacitors would be OK?

Last edited by PJL; 18th Feb 2017 at 11:56 pm.
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Old 19th Feb 2017, 12:20 am   #169
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Default Re: Restore a Beau Decca

I can't think of any real reason why 4 of those paralleled up wouldn't replace the series setup that's there at the moment. Dynatron used those big old square block paper ones which lasted forever - in most cases!

Cricklewood do the 1uf at 630 volts:-
https://www.cricklewoodelectronics.com/CFH1U0H.html
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Old 19th Feb 2017, 3:30 pm   #170
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Default Re: Restore a Beau Decca

Thanks Techman and PJL. I think replacing the reservoir as you suggest is a good idea- I will order 4 of the ones indicated by Techman today.

I have removed the rectifier and checked out the heaters, initially they were very different, and the brighter one was a bit uneven with some elements brighter than others. However after a few cycles they seem much more even and a bit closer in brightness- you can still see one is brighter though. I checked the heater voltages- both are showing 3.8v.
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Old 19th Feb 2017, 9:56 pm   #171
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Default Re: Restore a Beau Decca

So is the valve with the dimmer heater the same valve that is passing the lower current?

I think we’re getting to the bottom of this. It’s sounding like a corrode/oxidised connection issue. Measuring the heater current for each valve would be a good idea, but cutting into the circuitry around those old valve bases needs to be avoided. Measuring the current with extended crock clip leads with the valves removed would be the way to go, but I don’t think we’ll worry about that just yet. What you should do is remove the valves, noting which is which and with your meter set on the lowest ohms range (200 ohms?) carefully and accurately measure the resistance of the heaters of each valve at the valve pins. Note and report these readings, also note the reading on your meter with the test leads shorted together and report this reading, as we’ll need to subtract this reading from the two readings of the heaters. At this sort of low resistance, the resistance of the meter and its leads etc. are important and depending on the meter could be around 0.3 to 0.4 ohms. I’m suspecting that the ‘dimmer’ valve will show a higher resistance. If it does, then we’ll go from there. If it doesn’t, then we need to look at the valve bases, as it was a little bit inconclusive as to whether the fault was properly following the valves when they were swapped over.

Anyway, there’s no need to worry too much just yet as I personally have been here before with these valve types and this sort of problem.
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Old 19th Feb 2017, 11:36 pm   #172
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Default Re: Restore a Beau Decca

As Techman says, the valve tests are far from conclusive as they have been running at low filament and HT volts. Get the new capacitors fitted then we can advise on the steps to get it off the lamp limiter. In the meantime cleaning all valve pins and sockets with servisol would be a useful exercise.
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Old 20th Feb 2017, 11:02 pm   #173
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Default Re: Restore a Beau Decca

Yes- we are certainly making progress, I've ordered some servisol, so as soon as this and the caps arrive I will clean up the valve contacts and make the measurements suggested by Techman. I'm away for a few days now but will do the checks as soon as I'm back.
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Old 21st Feb 2017, 8:38 pm   #174
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Default Re: Restore a Beau Decca

The new caps have arrived today and I have a bit of time to install them tomorrow hopefully. I would have liked to have left the original 2 caps and resistors in place and just disconnected them, but the new caps are quite big and I think they will install best by removing the existing ones- unfortunatley.

Now to be clear about installation.
For the existing caps, at one end (not pictured) the 2 resistors are shorted together and the +ve and -ve of the caps are shorted together. At the end pictured- the resistor and the -ve terminal of C50 are grounded (A). The resistor (B) goes to the coil L27, and the positive of C49 goes to the rectifier cathode pin (C).

So.. for the new ones, if I connect in parallel and connect -ve to ground and positive to (C) going to the rectifier cathode- is that correct. Does (B) just need to be removed/ isolated?. Also, not absolutely clear which of the new caps is +ve and -ve as there are no markings. Normally there should be an arrow I think.
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Old 21st Feb 2017, 9:42 pm   #175
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Default Re: Restore a Beau Decca

These caps are not polarised so it doesn't matter which way around they are fitted.

I am not too sure why you have an A, B and a C. I suspect B and C are connected together. Have you removed the bolt that ties the tag strip to chassis?

The new capacitors are connected in parallel. One end is connected to the chassis. The other end is connected to the junction of L27 and the rectifier filament. If you bought replacement 100K resistors, I strongly recommend you wire them in series across the new capacitors as this will help discharge them when the power is off as the new capacitors will hold a charge for a very long time.
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Old 21st Feb 2017, 11:45 pm   #176
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Default Re: Restore a Beau Decca

I think me and PJL will have to agree to disagree on this one, but there's no need to include any resistors across the new capacitors. There's a circuit through L27 and L26 through to C48 and C47 and then on through to the rest of the circuit. The original resistors were just to form a potential divider balancing arrangement because of the capacitors in series arrangement and not to act as any safety discharge device. If we're going to include resistors here, then we may as well insist on resistors across ALL capacitors in power supplies in every amplifier we come across.

It's a shame you can't leave the original capacitors in place for historical reference, but I suspect one is only held in place by its connections anyway, so would fall out once disconnected. You may as well keep them in a packet with the other removed components somewhere safely in the bottom of the cabinet for any future reference.
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Old 21st Feb 2017, 11:57 pm   #177
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Default Re: Restore a Beau Decca

Thanks PJL- I will wire these in tomorrow. I've attached a diagram of what I think you are describing, is this correct? (Or should there be just a single 100K resistor across the capacitors?)
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Old 22nd Feb 2017, 12:01 am   #178
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Default Re: Restore a Beau Decca

Have just seen Techmans post. I guess the resistors are 'optional' and could be included or left out depending on preference. If I leave them out is the rest of the circuit correct?
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Old 22nd Feb 2017, 12:03 am   #179
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Default Re: Restore a Beau Decca

That series arrangement is correct. It's necessary, too. With just one 100K across the capacitors, the dissipation could be nearly 4 watts. With two in series it's about 2W or 1W each, comfortably within their power ratings.

Edit (just seen last post):

The resistors are optional in the sense that leaving them out wouldn't affect operation but having bleeder resistors across high voltage high value reservoir capacitors is a useful improvement in safety since they ensre the capacitors always discharge relatively quickly after switch off.
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Old 22nd Feb 2017, 12:05 am   #180
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Default Re: Restore a Beau Decca

Yes, but we don't need the resistors now.
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