Quad ii choke

[ToneArm]

My project at the moment is a Quad ii, it was very very dirty but the bodywork has cleaned up really nicely, so much so it won't need refinishing after all. Quite thrilled how well it looks!

Bad points - the choke. Lots of tar leakage around the choke (and possibly the power transformer -TBC) It has not failed but when I popped the lid off the choke innards had sunk, the yellow tape covering the winding is visible and there is no gap betweein it and the paxolin and terminals. One of the wires was brittle but enough remained I was able to snip back to good and extend it. It is back together with a piece of gasket paper between paxolin and choke, and it works fine...

But.. I know I have done a bit of a bodge, I am unhappy with it. I have never unpotted one of these, and trying to avoid it - is it feasible to melt the pitch in an oven with terminals upward, so the choke just sinks back to where it should be? There will be a void, of course, can the old pitch be topped up? Could I use a modern potting compound over the top to seal it up?

Or should I just stop being a wuss and remove all the old pitch?

Cheers
Glyn

[GrimJosef]

I would advise against doing the job in your oven. The bitumen compound is a mix of hydrocarbons - some much more volatile than others - and the lighter ones will generate a lot of vapour before the heavier ones become fluid. In short, the oven will stink for a long time and bubbles in the bitumen may well cause it to spatter around.

I've done this job a few times and I bought a cheapie 'slow cooker' and wired the heater direct to a Variac for more precise temperature control. I'd recommend lining it with kitchen foil before you start and using a thermocouple thermometer to keep a close eye on the temperature. The lightest compounds will start to liquify around 60C but you really need to get up to 120-130C for the whole mass to be fluid enough to extract the can's contents.

If you go too hot you may risk the can's paint. This is A Bad Thing as getting a colour match to the rest of the amp with new paint can be a challenge. Even at 130C there is a real risk of the paint changing colour, not least (I suspect) because over the years it will have picked up a layer of surface deposits which are more prone to darkening than the paint itself. You might consider cleaning these off before you start, but then you'll have to do the same to the rest if the amp to preserve the colour match. Good luck !

My preferred solvent for removing spattered bitumen is paraffin. I'm told petrol works too but I'm not brave enough to work with it. White spirit comes rather a poor second.

The bitumen used to be available in various (temperature) grades from electrical wholesalers as it was the standard insulator used on underground cable joints. But it's been superseded now and I'd be surprised if you could find any new for sale (then again, eBay's always worth a try). Perhaps roofing bitumen might work if you know a friendly flat-roofer ?

Cheers,

GJ

[ToneArm]

Thanks GJ. I need to think of an alternative to the cooker then...

Does the tar soften enough so the choke starts to sink slightly at lower temp? It doesn't need to fully liquify for my purpose.

Here's a pic of the culprit. The void on the side with the leadout wires is the biggest, about 12mm deep, I guess quite a lot of tar has been lost over time.

[Richardgr]

I have read about some repotting attempts, and they mentioned mixing the bitumen with fine sand to bulk it back up.

Do you know why it got so hot that the bitumen melted? I would have thought that is the choke was fixed in place inside the can, and there was no danger of shorting the contacts to the can or the contacts below, then it ought to be OK to leave it. Have you checked what it says on Keith Snook's website? He describes a couple of repairs there.

[Silicon]

Can anyone confirm if that yellow tape original?

Has that choke been rewound?

[dougietamson]

I recently de-potted a Quad II power transformer.
I used our old basic Asda air fryer at a low temp and its mechanical timer, ramped up the temp bit by bit until it came out.

I used a low cost multimeter with its supplied thermcouple probe to monitor the temp of the metal transformer case.

[GrimJosef]

I think the yellow tape might be original. That said, I have only extracted one choke and that was a long time ago.

The softening of the bitumen is a slow function of temperature. It really isn't a sudden sharp melting. Most of my 'extractions' (perhaps 3 or 4) have been for mains transformers. At 120-130C it is quite easy to drag the transformer out of the bitumen using the turret board, to which it's mechanically fastened. But dragging is still required if you want the job to go quickly. Turned upside down it will just slide out, as Dougie's picture shows, but it might need to be a bit hotter and it will take a little longer.

One thing I did wonder about, but never tried, related to the need to heat the outside of the can rather hot to 'drive' enough heat into the transformer. Perhaps there would be some mileage in electrically heating the transformer, or choke, simply by passing current through the windings ? This could cause everything to heat up rather more uniformly, thereby reducing the heat stress to the paintwork

Cheers,

GJ

[ToneArm]

That's interesting. Dougie, can you remember the measured temperature when the innards started to move.

Richard, I don't know why the bitumen melted. It is not an uncommon thing it seems, and that choke is bordered on two sides by GZ32 and KT66 so its a toasty place to be!

It is possible it's been rewound although I don't think it has. I bought the amp as a project, it had broken off valves in the octal sockets and the 9-pins were so full of dirt you couldn't see the pins.

Can anyone think of a reason not to seal it off with a layer of modern polyurethane potting compound over the top of the bitumen, assuming I can get the contents to sink to the bottom of the can?

Cheers
Glyn

[dougietamson]

Glyn, as GJ confirmed the bitumen was soft enough at 120-130C, it wasn't runny so little change of getting it in places you don't want. I used some aluminium trays to catch it just in case.

The old air fryer was also handy for softening the wax to get the laminations apart. I made sure to pre-heat before placing the PT inside to avoid scorching, they ramp up with a very hot blast.

Do you have the Quad pre-amp and radio tuners or plan to use them?

These add a bit more load onto the power supply.

[GrimJosef]

Originally Posted by ToneArm

... Can anyone think of a reason not to seal it off with a layer of modern polyurethane potting compound over the top of the bitumen, assuming I can get the contents to sink to the bottom of the can?

If anything should go wrong at a later date you won't be able to work on it again, at least not without risking the can.

Also I suspect the bitumen might expand when it gets hot, which it still will. If you seal it you may find the resulting pressure drives it out through the can seams, which weren't always completely fluid-tight.

Whether or not those things matter is your call of course.

Cheers,

GJ

[ToneArm]

Hi folks.

Dougie, it will be used with a modern preamp. I do have a 22 but despite my best efforts I can't get it reliable. The push buttons are tempramental and often lose one channel five minutes of bashing on and off with all the switches and it'll be fine again for a bit. One day I'll dismantle it again, maybe try an ultrasonic clean or something. But it is not an easy thing to work on, and also quite noisy compared to the competition.

GJ, thats a good point, hadn't thought of that. My worry was the heat of the potting compound curing might make the bitumen melt and mix with unexpected consequences! I'll keep pondering!

Cheers
Glyn

[Richardgr]

If it is encased in bitumen, and not resting with a possible metal-to-metal contact, so sitting on a good insulator like vinyl floor covering, then I can't see what harm could come of it. You would have dealt with the failure scenario, and it is dampened by the potting compound.

[Silicon]

I was pondering on this issue and I thought of three reasons for potting compound.

It holds the choke in place and dampens any acoustic 'buzz'.
It helps to keep out moisture.
It helps to conduct heat away from the windings to the metal case.

Electrical contractors can purchase a silicone gel to fill the void in externally mounted weatherproof boxes. It seems relatively expensive.

Could silicone gel find any use for potting chokes and transformers?

[kalee20]

I've used silicone resin for potting transformers.

Big advantages are that it will cope with a wide temperature range. It doesn't go hard at low temperatures, so stresses on the internal component are not an issue.

Against that, it does expand rather, on heating - you need space for it to expand to. I have had soldered-together metal cases burst at the seams at high temperatures (150°C and upwards).

Another downside with silicones is that adhesion is not very good (nothing like epoxy, for instance). So moisture can creep along leadouts. It just slows, without eliminating, moisture problems - you still have to seal the case.

There are thermally conductive grades available, generally by adding suitable fillers. Electrolube or Dow Chemicals are the people to look at.

[Ed_Dinning]

You could try the effect of warming the case, when inverted with a large weight on the core for a number of hours (even pass current through the windings to assist heating.

This may pust it back sufficiently that it takes a further 30 years to decend again

Ed

[ToneArm]

I placed the choke in a plastic bag to keep it dry in a can of boiling water from the kettle. Replaced the water every 10-15 mins to keep it hot, and pressing firmly on the core I felt it move after about an hour. Another 15 mins or so and I had pressed it in enough to give about 10mm clearance.

Pleased with the result. The paint was not damaged at all (although i decided to give it a bit of a polish with Brasso wadding which lightened the colour a bit.) renewed the leads with silicone covered wire and re-installed.

I haven't added any new potting, new or old type, the void is still a void. I think Ed is right and it will probably take decades to sink again. Or I may go back and fiddle with it again one day, knowing me..

Thanks to all for advice.

Cheers
Glyn