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Old 13th Feb 2017, 6:52 pm   #141
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Default Re: Restore a Beau Decca

Quote:
Originally Posted by Herald1360 View Post
Don't forget that the midpoint needs to be insulated from chassis if using metal can types!
Yes, good point. It would also be well worth checking those balancing resistors R50 & R51 are still within reasonable range of their 100K value.
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Old 13th Feb 2017, 7:00 pm   #142
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Default Re: Restore a Beau Decca

And I'll add to the above in this separate post - I think it would be wise not to individually 'tack' separate capacitors across just one of these that are in series at a time for testing (now I've studied the circuit). You should do the test (if you're going to do it) on the others in the circuit first and think about the reservoir last, if you still haven't identified the hum, which I'm definitely now thinking is going to be caused by one or both of the ones you've identified in your picture.
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Old 13th Feb 2017, 10:57 pm   #143
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Default Re: Restore a Beau Decca

Thank you. I have kept the lamp limiter in place continually so far.

I will check the 8mf capacitors first and leave the reservoir until these checks are complete. I'm away for a couple of days so will do this on my return. I have a few of these 10mf caps that I purchased from justradios which i plan to use - at least for checking.
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Old 14th Feb 2017, 1:35 pm   #144
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Default Re: Restore a Beau Decca

You can expect hum when running on a lamp limiter.

You need to get it on full power before you can determine if you still have problems. I will leave Techman to give detailed advice:
1. Replace any remaining capacitors.
2. Measure the resistance of R45 and R46 which should be 22 ohms, and the common cathode bypass resistor R44 which should be 600. If R44 is less than 600 ohms then it would be best replaced.
3. Check output stage current by measuring common cathode bypass voltage (I=V/R). It should be 52V according to ERT but much less with the limiter.
4. Take out the lamp limiter.
5. Monitor HT on first switch on. It should rise to 400+V then fall back to around 375V.
6. Check common cathode bypass volts again to confirm current is still to specification. It should now be 52V but switch off if above 55V.
7. Measure voltage across R45 and R46 which should be slightly less than 1V. Now calculate the anode current from the resistance measurements taken above (I=V/R). It is important that the valves are fairly well matched when using common cathode bias.

Not sure if you have checked the resistance values but there is a very good chance that many will have changed value and would be best replaced. The first stage anode resistor, R27, would be better as 2W metal film as it will introduce less noise.

The ERT indicates the PX4 anode dissipation = (365-52-1)*.043 = 13.4W. The spec for the later PX4's was 15W.
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Old 16th Feb 2017, 11:59 pm   #145
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Default Re: Restore a Beau Decca

Thanks Techman, Herald1360 and PLJ. Lamp limiter is still in place. I will do all the checks suggested, here is the story so far...
I have connected the 10mf capacitor in parallel with the 2 caps identified in the earlier photo (C43 and C38)- but no change to hum level.

Will check R45 and R46 tomorrow.

Checked R50 and R51. For R50, multi-meter on 200K range steadied at around 40K and for R51 at around 100k. I think both should be 100K. When powered up with limiter, there is 48V across R50 and 170V across R51.

Also I checked Voltage between Live and neutral on the mains plug- only around 9 ohms according to the meter.
I would prefer to only replace components that need replacing. Will do more checks tomorrow.
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Old 17th Feb 2017, 12:39 am   #146
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Default Re: Restore a Beau Decca

It's possible that the lower reading that you got of 40K across R50 may be indicating some leakage in C49, the capacitor which it's across. You'd need to disconnect one end of it to isolate it for a definitive reading.

You'll get a low ohms reading across the mains input to the transformer as this is the DC reading, which is different from the impedance at AC.

It would be interesting to measure the actual AC voltage across the input to the transformer with the lamp in line, although as has been said, it's tempting to remove the lamp and apply full mains with caution. With regards to the hum with the lamp in line, this is a good point. However, it just depends on how much hum. I've just conducted an experiment here on a similar twin PX4 type amplifier on reduced voltage. At 150 volts input the amplifier worked well with clear music and no hum, but obviously with very much reduced output power. Between 180 and 200 volts some hum was evident. Between 200 and 240 volts the hum went back down to normal level of nearly none at all. I also conducted the test in reverse with the same results of the hum, such as it was, peaking between 200 and 180 volts to the input of the mains transformer - I was using a Variac for continuous voltage variation.

So the hum may be misleading depending on voltage input, or you may have leaky reservoir/smoothing capacitors. You may now need to apply full mains power to make definitive tests. It may be wise to check resistor R50 out of circuit. Please be careful when measuring mains voltage, even the other side of the lamp limiter, and also make sure you've always discharged those capacitors before connecting/disconnecting them, or from them.
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Old 17th Feb 2017, 9:18 am   #147
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Default Re: Restore a Beau Decca

To describe the hum...

If you take 1 to be very quiet and 10 to be very loud, and a moderate listening volume to be level 4, then with the deck playing music and the volume level set lowish, the hum level and the music level are about the same level and combined are a level 4. If you disconnect the deck the hum gets much louder- about a level 6.

I've now disconnected and checked R50 and it appears to be faulty. The meter shows infinite even at the 2000K and 20M scales.
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Old 17th Feb 2017, 11:41 am   #148
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Default Re: Restore a Beau Decca

You will also get hum if the output valves are not reasonably well matched. Push pull amplifiers rely on the cancellation of HT ripple in the output transformer. Measure the voltages across R45 and R46 to confirm the PX4's are passing roughly the same current.
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Old 17th Feb 2017, 12:31 pm   #149
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Default Re: Restore a Beau Decca

I would replace R50 and R51 with 2W types. These are available from Maplin. You might want to check other resistor values (up to 20% out is OK).

The large can electrolytic capacitors are generally very reliable. It is very easy to assume hum means they have failed but it is rarely the case. If you do need to replace them you need to get high ripple current versions (F&T make them).
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Old 17th Feb 2017, 1:07 pm   #150
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Default Re: Restore a Beau Decca

Thanks both. I have now checked resistance of R46 and R45
R46 shows 11.3 ohms (in situ) and R45 shows 11.2 ohms (in situ). SHould be both 22 I think!

However now there is no music at all coming through the speakers- still some hum though, although a little quieter and volume control does not affect it. Maybe the o/c R50 has caused this?

I did check voltage across R45 and R56 but this is irrelevant I guess until we get sound back. Current readings (powered with limiter in place still) are:
both around 0.3Volts.

I have ordered 2 2W 100K ohm resistors to replace R50 and 51- should arrive in a day or so.
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Old 17th Feb 2017, 1:56 pm   #151
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Default Re: Restore a Beau Decca

If you look at the circuit, you will see that the resistors are joined at one end and are connected through the low resistance valve filaments at the other. With the PX4's in circuit, an in-situ measurement will see the other resistor and the filaments in parallel hence why you measure 11 ohms.

I was an idiot and didn't look at the circuit correctly and this measurement won't work!

The best option for checking the anode currents is to measure the voltage across R47 and R48. BUT, this is at very high and lethal voltage and without R50 and R51, the capacitors will retain the high voltage for a long time after switch off.
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Old 17th Feb 2017, 5:44 pm   #152
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Default Re: Restore a Beau Decca

Have checked R47 and R48 now:
R47 shows 48.1 ohms (in situ) and R48 shows 42.6 ohms
However when powered (with lamp limiter) the voltage readings are different between the two unfortunately. R47 has 0.4V across it, and R48 has 1.0V.

The audio is back- was just a loose connection on the 2 pin output connector. Still same level of hum.

Also I tried removing the PX4 valves and rechecking R45 and R46, but still get about 11.2ohms for each.
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Old 17th Feb 2017, 6:06 pm   #153
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Default Re: Restore a Beau Decca

Have also checked R44 (pictured). Although it looks a bit worse for wear it does read 0.6 on the 2K scale- which I guess is 600ohms.
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Old 17th Feb 2017, 10:26 pm   #154
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Default Re: Restore a Beau Decca

Ignore R45 and R46, they will be fine.

One PX4 is taking 23mA, the other only 8mA. What we need to do now is check the grid circuit to see if that is the cause.

With the power off measure the resistance of the PX4 grids to earth. They should both read around 101K. You did replace C45 amd C46?

If the resistance is OK, then switch on and measure the PX4 grid volts from chassis. This should be near zero.

If it is near zero then swap the PX4's and very carefully (dangerous voltages!) measure the voltage on R47 and R48 again and see if the higher voltage has swapped over. Also measure the voltage across R44 so we know the bias condition.
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Old 18th Feb 2017, 2:52 am   #155
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Default Re: Restore a Beau Decca

I don't understand what's now different.

Back a page on this thread and the voltages across those anode resistors had already been measured and were identical at 1 volt across each.
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Old 18th Feb 2017, 11:21 am   #156
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Default Re: Restore a Beau Decca

Those readings were taken from chassis so only approximate but the latest were taken across the resistors. If we can get the output stage running at safe operating conditions, full power can be applied. Swapping the valves over will confirm if one is weak but it seems worthwhile checking the grid circuit is good first.

If one is weak, it might be worth running the set without the good one in place and monitoring the anode current as sometimes valves self-heal. If that doesn't work we could temporarily alter the bias conditions of the weak valve using a potential divider and smoother off the common cathode bias resistor.
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Old 18th Feb 2017, 5:01 pm   #157
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Default Re: Restore a Beau Decca

Thanks PJL and Techman. As we've done lots of reading over the last few days I thought it might make things clearer if I did them all again and annotated them on a circuit diagram- see attached for what I found.

Taking a voltage reading across R47 and R48 gave:
R47 (48.6 ohms measured) = 0.14V
R48 (42.4 ohms measured) = 0.41V

The readings are slightly different from those I took previously of 0.4V and 1.0v respectively.

When I swap the valves
R47 (48.6 ohms measured) = 0.91V
R48 (42.4 ohms measured) = 0.21V
Unfortunately it does look like the low current follows the valve.

The only difference in the circuit as far as I know is the R50 is definitely o/c as wire is cut (may have been o/c anyway before- certainly is now).

Just wondering if we should go ahead with techmans suggestion and take out the limiter- is there any chance that the valve difference could just be due to the long period of no use/ low voltage use?
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Old 18th Feb 2017, 8:26 pm   #158
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Default Re: Restore a Beau Decca

It is a bit odd that the first set of readings are half the second. Maybe the valves were not fully warmed up?

Did you check the grid voltages?

I have checked with the datasheet and the 'good' one if anything is a bit high but the poor one is definitely low.

If we are playing safe then put another lamp in parallel with your existing one to get the voltage a bit closer to normal working conditions then check the voltages on R47/R48 again. Anything below 52mA or 2.5V is safe.
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Old 18th Feb 2017, 9:19 pm   #159
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Default Re: Restore a Beau Decca

Just tried it without the lamp limiter....

Vanode 254v for both Valves

R47 is 1.6V
R48 is 1.0V
left it on for about 60 secs and checked again- same readings.
Hum does seem a bit lower, but is still prominent.

Just thought, as the V9 and V10 have combined cathode/heaters- would fitting replacement mains caps C51 and 52 help?
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Old 18th Feb 2017, 9:34 pm   #160
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Default Re: Restore a Beau Decca

Left it playing for about 5 mins. Noticed that C50 (the one with the o/c resistor) gets hot- almost too hot to touch.
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