Replace or not to replace, that is the question.

[oldticktock]

Hi,

Was just pondering the above.

Started on another Dac90a and after cleaning up the chassis from a thick layer of dust it looks in remarkable condition even the wiring is good. HT smoothers are fine, besides changing 'That Cap",RF By-pass and substition of the Tone cap for brighter sound, if all the others are ok and the radio functions, should I leave the other waxies alone? it seems a shame to remove if they work.

What do you all do? is it a replace on sight for all regardless or a replace what needs to be replaced. I guess it probably comes down to preference and where you are coming from a "Repair" or "Full Restoration".

OTT

[kalee20]

Personally, I replace all the wire-ended capacitors. The HT can electrolytic I'd leave.

When I've finished a job and screwed the back on, I always like to think it'll stay put for another 20 years!

[HamishBoxer]

Me too,i would not trust those wax caps long term.They have had a long life now.

David

[KeithsTV]

Likewise I always replace all the waxies usually before switching on. Many sets are a pain to dismantle to access the caps so the fewer times it is done the better.

I also try to reform the HT electrolytics only replacing them if they fail to reform.

Keith

[oldticktock]

I too have been replacing on mass, except if the HT electros reform. It just started me thinking if this was the right approach and so far it would seem so.

I'm sure I read a thread on here where this was debated and there was some oppostion to this approach but I cannot find it again.

[brianc]

I too replace all waxies on site. I maust add though, that your DAC90A looks so original and clean that it would be worth rebuilding the waxies with the bright yellow replacements hidden inside the old case. I wish the modern caps weren't such a garish colour. On the pre-war TV I'm working on at the moment, I have replaced the non-original caps with polyprops disguised in black Hellerman sleeves, as it reduces the visual impact somewhat. The originals have been rebuilt.
Cheers
Brian

[PJL]

We have covered this many times before but...

Many on the forum have tested wax caps for leakage and almost without exception they have unacceptably high leakage currents at maximum working voltage from 10's of uA to almost a dead short.

The impact of this will vary dependent on the circuit position. 10uA leakage at 100V in an audio coupling capacitor with 1M grid resistor results in a 10V increase in grid volts and say 70mA increase in anode current. On the other hand a decoupling capacitor on a heater rail is only going to impact performance after it explodes and this might take 1A leakage at 6.3V.

[Boom]

Up to date I have been a wholesale 'change the waxxies on sight' man. The DAC10 that I am currently working on is totally original however internally.

I spent so much time on the cabinet that I didn't think the set was working so was going to perform the same operation on it. This was the first DAC10 that I have ever played with so I pushed a couple of buttons (the labels were missing) , heard a vague hiss with no stations when the tuning knob was turned and assumed that the set was in need of waxxie changing.

It was only when I sat down with the manual that I realised the push buttons were three presets and two manual LW/MW! Pushing the MW button the set came on loud and clear. An easy mistake if you are not familiar with the set and the labels are missing. (Well that's what I've told myself anyway.)

I've decided not to replace the waxxies and to keep the chassis as it is to preserve originality. I'll check the grid of the UL46 to see what sort of + volts are on it later but it sounds so clear and sensitive even after 2 hours that I am not convinced the set is in any danger by using it.

Dave

[GMB]

I have always looked at the circuit and imagined how bad it would be to have a few hundred K resistor in parallel with each capacitor. If it wouldn't make much difference then I leave it by default, except if across the mains.

Having said that I don't ever regard vintage equipment as totally safe so I wouldn't leave anything running where I wasn't there to keep an eye on it, and I generally don't run things for extended periods.

But now that I have the option of restoring the wax capacitors to near original condition I now do that rather than replace them - but there is the extra difficulty of removing them from the set and then getting them back in again. Often hard to do without extending the leads.

[Leon Crampin]

As has been suggested previously, I consider the effects of leakage and the consequences of outright failure before replacing original capacitors.

Mains bypass and audio coupling capacitors must be changed. Wax capacitors in AGC circuits must also be changed - their leakage is of the same order as the circuit impedance. Cathode decoupling capacitors can be left; after all they are shunted by a few hundred ohms anyway.

Decoupling capacitors fall in the middle. There's only one in a DAC90A and the source voltage and circuit impedance are such that no damage will be done even if it fails dead short. I generally leave it.

There is an obvious proviso that any capacitor you leave in there must have about the right value of capacitance - with or without leakage.

Leon.

[Boom]

Originally Posted by Leon Crampin

Mains bypass and audio coupling capacitors must be changed.

Why mains? The worst that can happen is that they go bang.

Dave

[Leon Crampin]

And cause damage to a newly restored chassis.

I also expect a set I've restored to work pretty much as originally specified, for a reasonable length of time. Of course, with old equipment there will always be odd and sometimes unexpected events but given the time it takes to restore a set properly, changing this capacitor is near the top of my list.

Leon.

[Radio_Dave]

Originally Posted by David Tilley

Why mains? The worst that can happen is that they go bang.

Along time ago, before I knew what I was doing, I had one explode in my face. A dollop of wax and wire brushed pass my temple and stuck itself to the wall, about 10 feet away . It sounds funny now, but it really could have taken my eye out!

David

[Boom]

Originally Posted by Leon Crampin

And cause damage to a newly restored chassis.

I can't see that a wax mains filter going bang is going to do any damage other than possibly spread a small amount of wax about.

I just checked the .01 in the DAC10 and the grid had 1.5V on it so very reluctantly and as I want to use the set I have stuffed the cap'. The rest of the waxxies (including the mains filter) can stay as they are until they stop the set working or its performance drops off. It certainly looks far better with its waxxies than with out of place bright yellow modern caps'.

There is another side to leaving it alone as much as possible. Strangely an unrestored set usually commands far more than a restored one when sold.

Dave

[GMB]

There is a significant fire risk with a wax capacitor exposed to powerful AC.

This is a consequence of the internal heating that occurs due to the AC current flowing through it, which is likely to be much higher than the initial leakage current. But the leakage current goes up exponentially with temperature and as it overtakes the current due to the reactance the whole thing can go into thermal runaway.

[ppppenguin]

Nobody would argue (I hope) that "that" cap should be retained for originality's sake. I don't see that there's any debate to be had about the mains filter cap either. It's a safety critical part and the old waxies aren't up to the job.

[kalee20]

I've had a couple of across-the-mains waxies fail, both dripped molten wax out and then shot their ends out, rendering them open-circuit. No damage was done.

However, the possibility for disaster is there - what if the shot-out end, with its lead attached, contacted something? With full mains voltage, there's the potential for a big spark to ignite the hot wax.

The chance of this happening may be small, and it's not happened yet. I just don't want there to be a 'first time'.

[Sideband]

Originally Posted by David Tilley

I can't see that a wax mains filter going bang is going to do any damage other than possibly spread a small amount of wax about.

....then you have not had the experience of a mains switch being completely destroyed by a faulty mains cap! I've had 3 DAC90A's where the switch contacts were blown apart beyond repair due to the owners switching on after 30 odd years of non-use and then wondering why it went bang. OK if the pots are readily available...not so nowadays. I would far rather have a good reliable set that I don't have to pull to bits every few months.

Replacing all the waxies will also help prolong the life of valves. Also especially with the DAC90A you need to make sure that the current through the output transformer is minimal otherwise that is another component that will fail.

At the end of the day, it's your set and there is no right or wrong but I hate taking sets to bits after restoring them. I believe they should be as reliable as possible and many of my sets are working 20 years later with no problems. It's no guarantee that other faults won't develop but they are more likely going to be due to valve holders, switches, drivecords and odd resistors going high, none of which is likely to cause major problems for other components.


Rich.

[Boom]

Originally Posted by Richard_Newman

I've had 3 DAC90A's where the switch contacts were blown apart beyond repair due to the owners switching on after 30 odd years of non-use and then wondering why it went bang.

Surely a correct 1A fuse in the mains plug would blow before the switch blew apart?

Now that the .01uF has been stuffed I am probably reluctantly going to stuff the others but I do feel that history is being destroyed. Maybe two sets is the answer? One restored and the other not.

Dave

[oldticktock]

See I also find the stuffin procedure hard to reconcile too. Whilst I fully appreciate some may wish to, I think why hide just to preserve the view.

If your going to replace then make a good job that the original engineers would 'doff their hats' to and show it, adds to the history of the sets lifecycle. Sometimes can we not take it "preservation" too far, I expect i'm going to be flamed now, but I do appreciate each to thier own.

This restoration https://www.vintage-radio.net/forum/...3&d=1230506945 to my mind is as good as it gets, some may not go for the circuit modifications but the quality of the restore is equally pleasing (in my mind) to the eye as the originals left in place.

On some buildings restorers take the stance that the restoration should be visible the old and the new. I'm not sure but on St Pauls did they not make it quite evident where new stone had been added. They were quite capable of making it invisible but the approach was taken to let the building speak about its history, perhaps sets should do the same.

I suppose it all boils down to ones own preference and perhaps what the set is your working on and how it should be preserved. I guess with a round echo you would want to maintain authenticity to the letter but maybe on lesser sets its free hand, who knows it's obviously a can of worms which the lid should have been left firmly on

OTT