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Old 1st Mar 2017, 1:20 am   #201
Techman
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Default Re: Restore a Beau Decca

A quick look at your resistor readings and comparison with their function in the circuit doesn't indicate any particular problems. R45 and R46 would be expected to read around half their value as there's other circuitry in parallel with them. You'd have to disconnect them to get a true reading, but as they're both reading the same in circuit, I think they're probably ok, but if you could do as PJL requested and just re-confirm the voltage across R47 and R48 just so we know that nothing has changed in this area.

The capacitor 'can' that had originally become nearly too hot to touch was obviously very badly electrically leaky and this would have been dragging the HT voltage down to a lower level. Now you've replaced it, the HT voltage will be considerably higher than it was before, so this may well have caused another fault to show itself, which could be responsible for the hum and distortion. It's also possible that you may have disturbed something, this is why it's now very important you let us see those voltage readings as in post #197. When you've given us these readings, we could look at any significant difference the increased values of R27 and R33 are making, also whether their's any problem with the resistor R39 that you couldn't check.

Could you confirm that all the old original smoothing capacitors that have been replaced with new components, are electrically isolated with their positive connections disconnected and it's just the new replacements that are now connected in the circuit?

And note down ALL those voltages for members to look at - in your own time, there's no need to rush. You're certainly learning a lot about this type of amplifier and its power supplies, you'll be able to 'walk' the next one you do!
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Old 1st Mar 2017, 1:20 pm   #202
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Default Re: Restore a Beau Decca

Decent quality modern electrolytic capacitors will have their ripple current ratings in the manufacturer's datasheet. When I suggested using film type capacitors for the reservoir section I had looked at some of the high voltage low value electrolytics and the ripple current ratings looked a bit marginal at 100Hz. Sometimes being physically smaller isn't necessarily helpful in vintage replacement applications.
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Old 6th Mar 2017, 12:05 am   #203
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Default Re: Restore a Beau Decca

Thanks Techman and Herald1360. I've completed a substitution exercise on C47/8. With the original twin can, the audio sounds clear and not distorted, but the cap gets hot. Substituting C47 with new 22microF capacitor and leaving original C48 results in the twin cap getting very hot. With C47 on original cap and new 10microF cap replacing c48, the twin cap gets warm but not hot (after 60mins). Also, I pervieve that the audio is less distorted than with using both new caps, but will need to do more checks to confirm this, as setting of gram/brilliance knob makes big difference also.

The loud hum has now gone away, the polarity of the speaker connector appears to affect hum level.

Also, just to confirm that when substituting C47/8 i did disconnect the original positive connections of the twin cap can, although I did leave the negative connected to the twin cap can negative.

I re-checked the voltages of R47/8: V (R47)= 1.4v and V(R48)= 1.82v

I've also done some of the voltage checks suggested in post no. 197- see below, will complete these tomorrow hopefully.

1) The hot end of the new reservoir set of four capacitors. (407V)
2) The positive end of C48. (369V)
3) The positive end of C47. (338V)
4) The positive end of C43. (149V)
5) The positive end of C38. (166V)
6) I would also like to see the anode voltages of the three preceding valves. (not done yet)
7) Also, the voltage on the screen grids of the two pentodes, ask if not sure. (not done yet)
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Old 6th Mar 2017, 10:15 am   #204
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Default Re: Restore a Beau Decca

Were these voltage readings taken without the lamp limiter and with the new capacitors fitted?
Were they taken after the amplifier had been running for at least 5 minutes?
If that is true then:
1. The output valves are running well within the safe operating power
2. The HT voltages are a little low but this is good whilst we continue to fault find
3. R26 and R34 are both passing too much current but we need the anode voltage measurements to identify the cause

Don't worry about hum and distortion as there can be many reasons for this. We can look at these problems when we have replaced the faulty components and the valves are all operating correctly. We need the anode and screen voltages for this.

Personally, I would not use capacitors of unknown specification in C47/C48 position, however, if the new capacitors are not running warm then they will be OK to get the set going.
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Old 6th Mar 2017, 4:24 pm   #205
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Default Re: Restore a Beau Decca

The test you made by re-fitting C47 and C48 have basically proved what I already said that the bad leakage in one of these capacitors was dragging the HT down to such a level that it was masking another fault that showed up when the new capacitors were fitted and the HT level rose to a more normal level. C48 is obviously very badly electrically leaky and C47, although not as bad, is also slightly leaky. I would NOT have advised you to do this substitution back to the original capacitors, as when they’re as bad as this, they can blow the ends out which can make an awful mess, or worse! Anyway, you were lucky and got away with it, and proved a point.

The next thing you must now do is easy. The voltages at 4) and 5) tell me that there’s a pretty good chance that C43 and C38 have 'had it', so you need to disconnect the positive end of each from the circuit and fit replacements from your stock and then measure those two voltages again.

You’ve been rather unlucky in that it seems that most of the smoothing / decoupling electrolytics in this amplifier are U/S. I can count on one hand the smoothers I’ve had to replace over the last few years – and you should see some of the ‘relics’ I’ve worked on that have been dragged out of barns, sheds and lofts. I did have a faulty reservoir in a tape recorder a few weeks ago, but that’s quite unusual. In your case it could be as has been commented on earlier, that they’re all a bit crammed in under the chassis, so have probably had a hard time in an unventilated area for a very long time when the gram was in regular use.

I don’t have the full service information in front of me, so could you have a look at your copy and see if it quotes the HT voltage anywhere, as this would be very helpful to know for comparison with your measured voltage.

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Old 7th Mar 2017, 8:48 am   #206
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Default Re: Restore a Beau Decca

Thank you Techman and PJL. Yes it is a bit unfortunate that most of the electrolytics seem to need replacing- the amp is starting to look a bit 'non-original'. However I'm very happy with our progress- perhaps I can look out for 'New Old Stock' caps once its all up and running smoothly- especially for C47/48.

I will try to make some further measurements and replacements later today (all measurements in post 203 were done without lamp limiter after 30mins running). I couldn't find reference to all the HT voltages, but the data sheet includes this information about the valves.
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Old 7th Mar 2017, 1:52 pm   #207
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Default Re: Restore a Beau Decca

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Originally Posted by thejazzageuk View Post
I couldn't find reference to all the HT voltages, but the data sheet includes this information about the valves.
That's very useful information and indicates that the main HT line is a little low, but no need to worry about that for now. Those last two capacitors may be dragging it down very slightly, although due to the resistors in the circuit at that stage, the voltage drop is going to be very local to those two stages and may not have a great influence on the HT voltage nearer to the output stage and main smoothing reservoir. It may well be that the rectifier valve emission is a little bit down on what it should be, but I wouldn't worry too much about that just now.

I agree that it's looking a little bit unoriginal now, but once you've got it working, you can always go back and tidy things up should you want to, as you've not made any irreversible changes. I would advise caution with 'new old stock' components, but so long as you don't go paying over the odds for them, then you may get a lucky find.
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Old 7th Mar 2017, 3:27 pm   #208
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Default Re: Restore a Beau Decca

NOS electrolytics are a bit hazardous - their shelf life, never mind service life, is limited. Better non-original function than genuine smoke!
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Old 7th Mar 2017, 4:54 pm   #209
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Default Re: Restore a Beau Decca

Thanks Techman and Ted Kendall. Based on the voltage information, should I still go ahead and replace C43 and C38?
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Old 7th Mar 2017, 7:06 pm   #210
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Originally Posted by thejazzageuk View Post
Based on the voltage information, should I still go ahead and replace C43 and C38?
Absolutely, replace them one at a time and note the voltage difference as said.
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Old 8th Mar 2017, 8:32 am   #211
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Substitution completed. Now voltages are:
C38=347V
C43=348V
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Old 8th Mar 2017, 5:18 pm   #212
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We need the anode voltages of all the valves to check their condition.
I would also like to see the R47/R48 voltages once again as the HT voltage will have risen again.
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Old 10th Mar 2017, 8:41 am   #213
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Thanks Techman and PJL
Not fully sure of the best way of measuring the anode/grid voltages. The area under the valve sockets is full of components and pushing in a probe could easily short something. For the anode voltages of V7 and V7, i took a reading from R33 and R27 as these appears to connect directly to the anodes.
R33 is 349V at one side and 147V at the other- so anode V is 147?
R27 is 348V at one side and 155V at the other- so anode V is 147?
For the grids I planned to measure R28 and R35, but cant get the probe to contact without pushing some components out of the way, so will try to do this today and also to re-measure R47/R48.
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Old 10th Mar 2017, 9:52 am   #214
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Default Re: Restore a Beau Decca

Both of those are good. The anode voltage measurements in the service data are much lower because of the low resistance of the meter they used which is a common problem with old service data.

Because of this it would be better if you measured the voltage at both ends of R33 and R39 rather than the anode voltages of V7 and V8.

Then measure the anode voltages of each PX4 and the voltage across R47/R48.

When switched off and given time to cool down, measure the resistance of R38 and R39. This is the phase splitter and the values need to be reasonably matched or the input signals to the output valves will not be the same.
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Old 10th Mar 2017, 10:08 am   #215
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Default Re: Restore a Beau Decca

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The area under the valve sockets is full of components and pushing in a probe could easily short something.
Sleeve the business end of your probes, so that only the pointed tip is exposed. It'll then be exceedingly difficult to short anything to anything in a valved set.
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Old 10th Mar 2017, 9:13 pm   #216
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Default Re: Restore a Beau Decca

The leads for my (cheap far-eastern) multimeter failed the other week, and I bought a new set from Maplin. 4mm shrouded plugs to probles. Anyway they came with a pair of little caps to put on the ends of the probes. With the caps removed you get the normal long metal probe. With them fitted, just the tip is exposed. They would certainly reduce the risk of shorting.

So far I've found that quite useful. I wonder if you can get better-quality leads with the same feature.
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Old 13th Mar 2017, 8:10 pm   #217
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Here are the results of the tests suggested in post 214...

Because of this it would be better if you measured the voltage at both ends of R33 and R39 rather than the anode voltages of V7 and V8. RESULT: Voltages at either ends of R33 are 148V and 350V. R39 is not easily accessible.

Then measure the anode voltages of each PX4 and the voltage across R47/R48. RESULT: V9 anode reading is 366V and V10 anode reading is 364V

When switched off and given time to cool down, measure the resistance of R38 and R39: RESULT Difficult to probe without disturbing/ un-soldering components. The components under V8 are cramped, in the corner and obscured by the waxCap (C44?)- see photo.

V across R47=1.55V
V across R48=2.03V

The audio sounds good now, with relatively little hum on the basic 'gram' setting, although some hum is introduced with the 'brilliance'.
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Old 13th Mar 2017, 8:30 pm   #218
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Default Re: Restore a Beau Decca

C44 - I don't know how that one's escaped? That's an anode to control grid coupling capacitor (look at your circuit) and needs replacing immediately. You're nearly there with it.
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Old 14th Mar 2017, 12:06 am   #219
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V7 anode is 0.75mA which is perfect.
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Old 18th Mar 2017, 10:55 am   #220
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I've replaced C44, and now R38 and 39 are accessible and have also been measured. I've attached a diagram that shows all the resistors and voltages measured so far- plus I've tried to indicate the calculated current in the important areas. Would appreciate a check over to feedback if they look correct.
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