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Old 20th Jun 2016, 10:26 am   #101
Edward Huggins
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Default Re: Restore a Beau Decca

Are both the PX4 made by Osram? Edward
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Old 20th Jun 2016, 3:14 pm   #102
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Default Re: Restore a Beau Decca

Marconi/Osram. I'm not sure, but one could be for the home market and the other for export, but which way round?
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Old 20th Jun 2016, 5:41 pm   #103
Edward Huggins
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Default Re: Restore a Beau Decca

Don't get this comment? If both PX4s were by, say OSRAM, they might have been a matched pair originally. Edward
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Old 20th Jun 2016, 7:12 pm   #104
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Default Re: Restore a Beau Decca

Quote:
Originally Posted by thejazzageuk View Post
I notice that the PX4 valves have an OSRAM label
I think it's clear from what the OP says that they're both Osram valves.

They may have been 'matched' originally if Decca had bothered about such thing in those days, but it won't matter too much in that radiogram now - in my opinion, anyway.
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Old 25th Jun 2016, 10:57 am   #105
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Default Re: Restore a Beau Decca

Thanks Techman. I'm always happy to follow advice- and in the past yours has been spot on. I started with some static checks, all with set unplugged.
ONE: At first the primary winding check revealed infinite resistance between L and N. However I shored out the OFF/ON switch and then got a reading of 8.8 ohms (a bit low?). Removing the switch short the reading was a couple of hundred ohms eventually going down to 47 ohms with switch on. I guess the initial high readings could be due to oxidizing of switch contacts?
TWO: I then tried to get the heater resistance- from the V8-triode phase splitter valve socket (L63) it read 0.4 ohms between pins 2 and 7 heater.
THREE: Finally- to check high voltage 2ndry winding, I removed V11 rectifier and measured 180 ohms between pins 4 and 6 .
Do these seem reasonable?
My next step was to replace a few (the minimum) of components, as like you suggest I am reluctant to cut it about more that necessary. The ones I looked at and thought of changing are shown in the attached photo and listed below, but would be very grateful if you can advise it this is sensible.
C39 -0.01 paper cap- coupling for V7. REPLACE?
C40/41 caps for Tone control. LEAVE?
C37 0.5 signal valve screen voltage cap. REPLACE?
C35 signal coupling cap 0.5 REPLACE?
C45 abd C46 coupling caps 0.25 for V9 and V10. REPLACE?
C51/52 mains filter caps. REPLACE?
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Old 25th Jun 2016, 11:02 am   #106
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Default Re: Restore a Beau Decca

Hi Edward Huggins- just to confirm they are both valves are labelled OSRAM (and actually both look identical)
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Old 26th Jun 2016, 9:22 pm   #107
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Default Re: Restore a Beau Decca

Thanks, they may have originally been a matched pair. What a cracking circuit! Would be grateful for an explanation as to the split O/P secondary winding - it does not look like that one is a dedicated feedback winding. Anyone? Edward
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Old 26th Jun 2016, 9:33 pm   #108
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Default Re: Restore a Beau Decca

Connections on that schematic aren't dotted so both secondaries are shown paralleled. I suspect the two secondaries are interleaved with the two primaries to keep things symmetrical.

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Old 27th Jun 2016, 10:58 am   #109
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Default Re: Restore a Beau Decca

Hi Edward, on a high quality amp like this the op trans will be wound in several sections to improve the coupling between pri and sec.

Ed
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Old 27th Jun 2016, 11:11 am   #110
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Ahh, the Transformer Man has spoken! Many thanks for all your responses. Edward
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Old 27th Jun 2016, 12:24 pm   #111
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Default Re: Restore a Beau Decca

I think the Pye BBH Black Box O/P transformers were wound like this too, weren't they? Ed will know...
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Old 28th Jun 2016, 12:58 am   #112
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Default Re: Restore a Beau Decca

I personally wouldn't be replacing any capacitors until I had some sort of power through it to confirm that the major components are actually working.

However, you must remove or clip one end of both C51 and C52 before doing anything else. These can be left out for the time being but if you replace them they'll need to be special 'X' types.

You should only ever replace capacitors before an initial power up if you're completely confident that you're NOT going to make ANY mistakes. If you do a cap change and then find it doesn't work, we'll have a 20 page thread trying to diagnose a fault that you probably caused yourself.

If you really want to replace a couple of capacitors before you start then you could do C45/C46. You'll have to go to nearest preferred value, probably 0.22 and 400 or 600 volt working.

I would advise you to power it up via a lamp limiter if you're worried - you could use an old table lamp in series with the live input with something like a 100 watt bulb in it, and then increase the bulb wattage to 150 then 200. If you haven't got those bulb wattages then use two table lamps in parallel with a 100 watt in one and a 60 watt then 100 watt in the other. However, It might be best if you were to first remove the rectifier valve and give it full mains and see if all the other valve heaters light up as they should. Removing the rectifier will remove any HT voltage from the circuits and only the heaters will be powered. Looking down from the top of the valves, look VERY closely at the heaters of the PX4 valves and make sure that ALL the heater runs are glowing. These valves will still work with some of the heater out, but if it's just one valve then you may get some hum - not to be confused with bad smoothing. The heaters should have a dull glow showing on all the runs - you'll see what I mean when you look.

So to sum up - remove mains filter capacitors, remove rectifier, power up full mains and check heaters. Switch off, replace rectifier and connect a speaker and power up via a 100 watt lamp in series with mains input, with a DVM set to highest DC voltage range (600 volts?) clipped between first smoother after the rectifier and chassis to monitor the HT. Leave to warm up until you see some HT. Switch off and increase bulb wattage to let HT rise a little more and hopefully hear some low hum from speaker - or better, music, if you've connected an audio source. DON'T run for any length of time when doing this. Switch off and check for any slight warming of smoothing cans etc. Note HT voltage each time. Once you know it's capable of working you can start replacing components that need to be replaced. Note: running this up with the lamps will help re-form the smoothing electrolytics - no need to replace these unless they prove to be bad.

When you come to replace capacitors, those you list for replacement are correct, but I would also advise replacing the small decoupling electrolytic C36, as this is likely to be unreliable.

Last edited by Techman; 28th Jun 2016 at 1:17 am.
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Old 28th Jun 2016, 7:00 am   #113
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Default Re: Restore a Beau Decca

Hi Gents, yes most BB's were wound that way. As were several other UL amps. The better Williamson amps had at least 14 sections.

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Old 1st Jul 2016, 6:12 pm   #114
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Default Re: Restore a Beau Decca

Thanks Techman- super clear advice once again.

I clipped the mains capacitors and removed the rectifier and then plugged in. All valves show orange glow so the heaters appear to be working, and the 2 PX4 look like they have all element working- see photo.

Next step to put in rectifier and monitor the HT. By the first smoother after the rectifier I think you mean C47 (or C48?). I've shown a photo of this capacitor-I just need to confirm which tab (red or yellow) is C47 before powering up with a lamp limiter for short periods, I think this will be written on the capacitor but I cant see it without unbolting it from its mounting. Will confirm this next.
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Old 5th Jul 2016, 12:21 pm   #115
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Default Re: Restore a Beau Decca

That looks absolutely perfect!

That radiogram obviously hasn't been dragged up a rough concrete garden path.

You'll have the 'audio' nutters banging on your door to get their hands on those valves!

If you haven't already done so by now, you'll have to work out from the circuit diagram where the first smoother is. You can see the rectifier which is a 'direct heated' type. Being directly heated, its filament/heater is the cathode, so you'll need to identify the connection of the filament to transformer MT1 winding L28 where it also connects to L27 with R50 and C49, this is where you need to clip your meter.

The capacitor can in your second picture above may not contain C49, but I note that the end cap of it does look very slightly 'bulged', so it may be that this can will need replacement. However, this slight bulging may have taken place decades ago when the gram was run continuously for many hours at a time, so just keep an eye on any slight warming of the can between your tests or lack of the HT rising properly. Check for any signs of slight warming of the cans of all the electrolytic capacitors, and be aware that under certain fault conditions they can hold a lethal charge, so check they're discharged (measure for any voltage with your test meter) BEFORE touching any connections - even when disconnected from the mains.
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Old 5th Jul 2016, 12:44 pm   #116
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Default Re: Restore a Beau Decca

Just to mention - I may have confused you slightly by saying 'first smoother'. I tend to call them all 'smoothers', so what I probably should have said was the 'reservoir' capacitor, as it's the ones that follow that are strictly the 'smoothers' which you have identified in post #114.

Edit: Just to add - you could probably monitor the HT anywhere along the 'smoothing' line, but I think that monitoring at the first stage 'reservoir' capacitor is the best place to clearly see what's going on.

Second edit: Watch out for any sparking about in that rectifier valve. These old directly heated rectifiers can sometimes be dodgy.

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Old 12th Jul 2016, 12:08 pm   #117
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Default Re: Restore a Beau Decca

Thanks Techman. That all looks good.

I've attached a photo to indicate where I believe I should monitor HT. C49 is the (identical) large capacitor behind the visible one. I checked the indicated point 'A' with a multi-meter and it is connected to pins 2 and 8 (filament) of the rectifier valve V11. So I should connect between chassis and point 'A' to monitor the HT, does that seem right?
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Old 2nd Aug 2016, 8:00 am   #118
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Default Re: Restore a Beau Decca

Finally got round to carrying out the HT test that Techman suggested- 'Switch off, replace rectifier and connect a speaker and power up via a 100 watt lamp in series with mains input, with a DVM set to highest DC voltage range (600 volts?) clipped between first smoother after the rectifier and chassis to monitor the HT. Leave to warm up until you see some HT.'
With the rectifier in place and the voltage monitored at point 'A', after approx 2 secs from switch on, the test lamp begins to brighten and the voltage starts to rise. After about 5 secs, the voltage rises to around 160V and the lamp becomes bright (about half full brightness?). After 5 secs I switched off. I repeated this around 10 times without leaving power on for more than about 5 secs.
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Old 2nd Aug 2016, 11:26 am   #119
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Default Re: Restore a Beau Decca

OSRAM and MARCONI are the same vales just different labels. OSRAM was used for commercial sales, MARCONI for the professional market.

The lamp limiter will glow bright due to the high quiescent current of the amplifier.

If you are planning on using this regularly and want it to be reliable and as good as it can be, I would replace all paper capacitors and replace any resistor that is out of tolerance. You might be able to reform the electrolytics but I would want to check their ESR and leakage current at normal working temperature before using the amp.

On powering up without the lamp limiter, you should monitor the PX4 anode current (by measuring across the cathode resistors) to check they are are not taking excessive current and are reasonably well matched. The valves may need a little time for the gettering to absorb the small amount of air that may have leaked in during storage.

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Old 3rd Feb 2017, 11:46 am   #120
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Default Restore beau decca amplifier

I've now got a bit of free time and would like to complete the restoration of my beau decca amp.

I've been following guidance from Techman- at the end of the previous post he advised:

So to sum up - remove mains filter capacitors, remove rectifier, power up full mains and check heaters. Switch off, replace rectifier and connect a speaker and power up via a 100 watt lamp in series with mains input, with a DVM set to highest DC voltage range (600 volts?) clipped between first smoother after the rectifier and chassis to monitor the HT. Leave to warm up until you see some HT. Switch off and increase bulb wattage to let HT rise a little more and hopefully hear some low hum from speaker - or better, music, if you've connected an audio source. DON'T run for any length of time when doing this. Switch off and check for any slight warming of smoothing cans etc. Note HT voltage each time. Once you know it's capable of working you can start replacing components that need to be replaced. Note: running this up with the lamps will help re-form the smoothing electrolytics - no need to replace these unless they prove to be bad.

When you come to replace capacitors, those you list for replacement are correct, but I would also advise replacing the small decoupling electrolytic C36, as this is likely to be unreliable.'


I've now linked the amp up to a speaker and to my garrard record deck (see photo) and am using a lamp limiter. When I switch on, lamp glows medium to low bright and HT voltage rises to around 175V. After about 20 secs, sound comes from the speaker, but is low volume and accompanied by lots of hum. After about 60 secs HT has risen to 185V and seems steady at this. After 2 mins I switch off. No components appear to be warm. I'd be grateful for any advice as to how long I should safely leave it powered for in order to give faulty components a chance to get warm.
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Last edited by Station X; 3rd Feb 2017 at 11:57 am. Reason: Threads merged.
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