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Old 3rd Feb 2017, 12:42 pm   #121
Edward Huggins
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Default Re: Restore beau decca amplifier

That HT voltage seems low, especially after 2 mins - I would leave on longer and see if it rises further. There could well be a problem in the power supply.
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Old 3rd Feb 2017, 3:05 pm   #122
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Default Re: Restore beau decca amplifier

Quote:
Originally Posted by thejazzageuk View Post
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.
I would keep doing it for a minute or two a number of times with a five minute break in between to see whether the hum level that you're hearing reduces due to the smoothers recovering and whether there's any slight warming of any of these electrolytic capacitors in the smoothing section.

With regards to the hum, you need to determine whether it's HT smoothing related or whether it's being picked up by the wiring that you're 'patching' your audio source (Garrard deck) in to the unit with. So remove all audio input wiring for the first test to see if it removes the hum, if it doesn't, proceed with the intermittent powering up and see if it lessens and let us know the results. If there's still a lot of hum, then one or more of the smoothing electrolytics has lost capacitance and will need replacing. Some would advise replacing them all regardless, but you could tack an appropriately rated separate electrolytic across each one to diagnose the faulty one - if you're not sure about any of this, then ask for advice.

It's normal for the HT to be low with the lamp limiting the input current. Volume may be low due to the lower HT and also that it would probably normally go through another pre-amp stage in the radio tuner head.
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Old 4th Feb 2017, 6:22 pm   #123
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Default Re: Restore a Beau Decca

You need to monitor the voltage across the common cathode resistor. Low HT voltage can mean excessive current.
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Old 4th Feb 2017, 11:48 pm   #124
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Default Re: Restore a Beau Decca

He doesn't need to worry about why the HT is low at this stage as the whole thing is being under run due to the 100 watt bulb (lamp limiter) being in series with the supply, hence the low HT.

Once he's established the cause of the hum, and that nothing else such as the mains transformer has any shorted turns, which looks unlikely at this stage, then he intends to replace various capacitors as we discussed earlier in the thread.

However, I don't know if he's done this check yet (I think I advised it earlier in the thread) and that is to remove all the valves, or at least the HT rectifier, and run the unit on full mains for a while to make sure there's no overheating of the mains transformer. In fact, what I would advise is to leave the 100 watt bulb in circuit at first and see if this now remains quite dim with all the valves removed, then go on to the second part of the test with full mains, but with valves still removed. This will make a definitive test of the mains transformer.

So order of tests:-

1) Lamp limiter in series and no valves in.

2) Lamp limiter out of circuit (full mains) no valves in.

3) Lamp limiter out of circuit (full mains) all valves in EXCEPT the rectifier.

He may already have done this test, but due to the lamp limiter and all valves being in place as he described in his last posting, any possible fault in the mains transformer may not show up until it's too late. I think he'll be alright and the transformer will be fine - I'm just crossing all the 'Ts' and dotting all the 'Is' (or the other way round), if that makes sense?
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Old 5th Feb 2017, 6:08 pm   #125
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Default Re: Restore a Beau Decca

I've suddenly had another thought about something you MUST check!

While you've got it powered up with all the valves in place, check the voltage on the anodes of the two output valves (where R47 & 48 go to the valve base pins) to make sure it's near enough the same on each valve. Also check the voltage on the other end of both these resistors on the output transformer side. This will give us an indication of the continuity of both sides of the output transformer primary, the state of the two resistors and even any possible excess current that needs to be worried about when full power with all valves in place is eventually applied (although capacitor replacement will take care of that before then) - possibly even the state of the valves in comparison with each other.

A faulty mains or output transformer would be pretty much a show stopper at this stage, although I think you'd be very unlucky if this happened to be the case, so unlikely.

A last thought - without checking back so not sure if it does, but if there's any HL41 type valves in the earlier stages, these can sometimes develop a heater / cathode short causing bad, uncontrollable hum. Again, I've had this problem with these valves in the past, but I think you'd be unlucky if this were the case here. NB, a heater / cathode short in these valves can sometimes be cured by removing said valve and whacking it on a hard surface, but please don't try anything like that at this stage before checking input signal wiring and HT smoothing.
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Old 5th Feb 2017, 9:11 pm   #126
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Default Re: Restore a Beau Decca

Thanks for all this advice. I have now carried out the checks advised on Techman's original post. I have done about 20 cycles of 1 min on then wait 5 mins. The sound from the speaker has improved- got a bit louder and clearer and hum slightly reduced. Removing the input wiring reduces most of the hum, but some still remains.
I'm very happy to be advised about how to diagnose which electrolytics need replacing- still quite new to all this. I will now carry out the checks in your last 2 posts and let you know how I get on. I've attached a copy of the circuit diagram which may be useful.
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Old 5th Feb 2017, 9:28 pm   #127
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Default Re: Restore a Beau Decca

Unless the input is connected to a magnetic cartridge or otherwise returned to ground, I would expect hum- there's no internal path to stop the first valve grid from just floating around. Rather odd but presumably no problem in its original setting.
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Old 7th Feb 2017, 8:02 pm   #128
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Default Re: Restore a Beau Decca

Thanks. I've now carried out the next tests suggested by Techman.

check the voltage on the anodes of the two output valves (where R47 & 48 go to the valve base pins) to make sure it's near enough the same on each valve.

They are almost the same 154v and 155v.

Also check the voltage on the other end of both these resistors on the output transformer side.

Just 1v difference (see diagram).

I have already dome some transformer tests earlier in the post. However after measuring the anode voltages, I then removed the rectifier and powered the amp up, still with the lamp limiter in place. The heater elements on the PX4s (and other valves) glow dim-ish orange, and the lamp in the limiter is not noticeably illuminated at all.
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Old 7th Feb 2017, 8:55 pm   #129
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Default Re: Restore a Beau Decca

Great news, that sounds perfect

I'd forgotten that you'd done some (presumably mains) transformer checks earlier in the thread - it's been a long time and I couldn't read through it all again.

I think you could get on with the capacitor replacement as discussed earlier in the thread now.

You could try grounding the audio input (as Chris says about that floating grid) and see if it reduces that hum to nearly zero. However, if it doesn't, you can always investigate the cause at a later date, as it's then likely to be one or more of the smoothing electrolytic capacitors. If you keep giving the unit little 'run-ups' on the lamp limiter they may well improve with time, that's if it is actually them that are causing the hum.
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Old 7th Feb 2017, 9:28 pm   #130
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Default Re: Restore a Beau Decca

That's good news.

Grounding the input does not noticeably reduce the hum, but I can look into that later (the link to the cartridge is long and not shielded)

How do you advise I proceed with replacing the capacitors- do we mean C45 and C46 plus the ones I listed earlier?

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?

Do I check first or just replace- and any particular order? I guess I should do them one by one and do a power up with lamp limiter after each one to check it is still working?
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Old 7th Feb 2017, 10:32 pm   #131
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Default Re: Restore a Beau Decca

It's never a good idea to replace all the capacitors in one go unless you're very confident that the values you're fitting are correct and that you're not going to make any mistakes.

C45 & 46 are the most important capacitors and I would suggest that you could replace both of these at the same time, then do a little power up with the lamp limiter to check things are still operating as before. Nearest modern values and working voltage of at least 400 volts. Then do the rest one at a time.

Mains filter capacitors could probably be left out, but if replaced then they need to be special 'X' type capacitors - don't use ordinary 400 volt ones.

The tone control capacitors are unlikely to make any noticeable difference whether replaced or not, so I wouldn't bother at this stage.

Be very careful you don't bust the glass of those output valves while you're working - and take your time.
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Old 9th Feb 2017, 2:12 pm   #132
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Default Re: Restore a Beau Decca

Thanks Techman. I will go ahead and replace C45 and C46. The are both 0.25MF, the nearest I have is 220nf- will that be OK?
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Old 9th Feb 2017, 2:15 pm   #133
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Default Re: Restore a Beau Decca

Yes, 0.22uf is the nearest preferred / modern value.
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Old 9th Feb 2017, 6:31 pm   #134
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Default Re: Restore a Beau Decca

I've re-capped C45 and C46. Checked functionality after. Very similar to previous with music after about 15 secs accompanied by some hum. HT voltage is 199V which is slightly higher.

Next changed C37, C39 and C35 and tested after each one. Similar -although music volume seems slightly louder, and if you turn the volume up it distorts.

Here is a photo of the AMP after re-capping. I notice that C38 seems to have some leakage around the terminal, and C43 appears to have burst slightly.
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Old 9th Feb 2017, 7:12 pm   #135
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Default Re: Restore a Beau Decca

That looks great.

The HT being now slightly higher indicates just how leaky those first two coupling capacitors were, causing the valves to conduct harder than they should, even with the reduced voltage, so a good job done.

Hunts actually made very good electrolytic smoothing capacitors, but in the case of those you've now pointed out, I would suggest that you replace them before giving the unit full power - you could tack another electrolytic of similar value and voltage across each of them in turn and see if the hum is down to one in particular - if you're not sure about this, then ask.

The distortion is probably down to the reduced voltage via the lamp limiter.
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Old 9th Feb 2017, 7:54 pm   #136
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Default Re: Restore a Beau Decca

Thanks Techman. I would be grateful if you could advise me regarding the electrolytic capacitors. Thanks
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Old 10th Feb 2017, 4:13 pm   #137
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Default Re: Restore a Beau Decca

I have now connected the AMP to the 3 original speakers in the cabinet. The audio is now louder and good quality, however it is accompanied by a hum. The hum volume is about the same level as the music. Even with the input connector removed completely the hum remains at about the same level.
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Old 13th Feb 2017, 12:23 am   #138
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Default Re: Restore a Beau Decca

Hi, sorry for the delay in replying.

You don’t say whether you’ve still got the limiter in line now you’ve got it connected up to the original gram speakers in the cabinet. If you’ve still got the lamp in line, then I wouldn’t connect full mains just yet. It may well be ok with full mains and I have to say that if it were me I would have done by now, but I would certainly be feeling all those large electrolytic capacitor cans for any signs of warming.

So, assuming that you’ve still got the lamp in line, the next step is to see which of the large electrolytic capacitors in the power supply smoothing circuit has failed either o/c or just low capacitance. The ones that show signs of distress may not actually be the ones causing the hum and I’ve certainly run some of my old gear with capacitors in that state with no problems. It could be that the set was left switched on for several days sometime in its life and the capacitors just vented a little bit due to heat, but it could also be that they’re just not in a very good state and really need replacing.

The electrolytic smoothing capacitors all seem to be 8uf with a 16uf reservoir. You could buy a nearest value 10uf and either solder it or attach it with crock clip leads across each of the ‘can’ capacitors in turn to see which one significantly reduces the hum. If you bought a couple, you could connect them in parallel to make 20uf and connect them across the 16uf can to test that. It may, however, be easier to buy all the ones you need in one go and replace most/all of them – they’re not that expensive. You need the ‘axial’ type with the wires coming out of each end for the 8uf type. They’ll need to be no less than 450 volt working and should be 105 c temperature rated for best reliability. For permanently replacing the 16uf reservoir, you’ll ideally need the best fit type you can get, paying particular attention to the ‘ripple current’ rating and this one doesn’t look like it wants to be an axial type. This is all, of course, if you decide to replace them all, it’s up to you. I myself, have various ‘test’ capacitors to hand for doing this type of testing so I don’t need to buy anything in until I actually know what I need. Some would advise you to just replace all of them regardless, the choice is yours, if you’ve got the time, then why not do the tests to see which one/s are faulty – you’ll learn a lot more doing it that way. A little word of warning – always make sure you connect electrolytic capacitors the correct way round (the same way as the originals) as they go off with an awful bang if you don’t and can be quite dangerous. Also, always make sure they’re fully discharged before touching the connections, as a shock will, at the very least, make you jump!

You’ll have to decide how you’re going to replace any faulty electrolytic capacitors. The replacements will be a lot smaller than the originals. You could fit them inside the hollowed out cases of the originals (re-stuffing). I’m a bit ‘iffy’ about re-stuffing. I never ‘re-stuff’ capacitors, although I wouldn’t say that I ‘never’ would. Replacement capacitors are designed to be in the open for cooling and not sealed inside an airless extra container. I suspect that many of the sets that have re-stuffed electrolytic capacitors don’t get continuous all day long use, or the slight heating of a capacitor is insignificant enough for it to make little difference. It is also likely to cause confusion for the future – we’ve already encountered cases of newcomers starting to restore a vintage radio where the old wax capacitors they’ve removed have turned out to have already been re-stuffed with new components and this situation is only going to get worse. I like to see a good honest repair, which is what you’ve done so far, as all the capacitors you’ve replaced have actually needed replacing. At least one of the electrolytics shown in the picture is fitted under a clamp. What I tend to do in this situation is disconnect it electrically from the circuit and just leave it there and fit the replacement along side it in the circuit. Depending on how the others are fitted, you could perhaps do the same with them if you’re going to replace them. All original smoothing electrolytics that are being replaced should be electrically disconnected from the circuit if they’re going to be left in place for historical reference. This means that the positive connection should be disconnected and isolated, the chassis side connection can be left in place if desired. So the choice is yours, test and replace as necessary, replace all, re-stuff, leave originals in place but disconnected and replace along side. Remember, nearest preferred value, working voltage as high or higher than the original (check that 16uf capacitor can in case it’s a 500 volt rating) if you need to replace it, and check the replacement for temperature and ripple rating. Be mindful of polarity as they’ll explode if you get it wrong and be careful of them holding their charge and giving you a shock, especially the new replacements.

Last edited by Techman; 13th Feb 2017 at 12:40 am.
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Old 13th Feb 2017, 1:59 pm   #139
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Default Re: Restore a Beau Decca

Just as an additional note - I may have been referring to the wrong capacitor as the reservoir and it's actually one of the 8uf ones, so check carefully.
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Old 13th Feb 2017, 3:12 pm   #140
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Default Re: Restore a Beau Decca

The reservoir is two 8u 450v in series with 100k across each. Max surge voltage could be 600v or so so modern 10u 400v types would be OK for this location. The rest of the filtering uses 450V rated caps.

Don't forget that the midpoint needs to be insulated from chassis if using metal can types!

Hmm, ripple current may be a problem with small modern aluminium caps. You might be better to use polypropylene film types.
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