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 1931 'potted' power supply
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 Return to top of page · Post #: 1 · Written at 5:05:32 PM on 29 August 2020.
Nhanwell's Gravatar
 Location: Mount Lawley, WA
 Member since 12 September 2017
 Member #: 2167
 Postcount: 45

I recently acquired an Emmco Troubadour 7 console radio.
8 valves - 3 RF stages, Detector, AF & PP OP, rectifier.

From first inspection the only work that has been done on it was a new potentiometer (sensitivity/gain).
The chassis & speaker are in excellent condition. All AF transformers & field coil check for continuity.

The main problem area is the Power Supply which is potted in pitch (?) in a metal container. The rectifier valve sits on the outside.
The PSU supplies 2 x 4v for 3 RF valves and dial lamp, Det & AF valves respectively and 2.5v for Output valves and HT.
The rubber in the mains cable and some of the leads has completely perished.
The field coil & power cables all terminate inside the can. So to replace them and check out the rest of the PSU (transformer, capacitors & possibly chokes), I need to get the assembly out of the can, service it and get it back in the can.

As anyone got any tips on best way to do this. I have drilled out the rivets that hold the top two boards in place and tried a heat gun with the can upside down.

There is a solid base to the can held in by 4 rivets - I was thinking drilling these out and trying again. But will a heat gun suffice or will it need to go in the oven ?

Once its out of the can - whats the best way forward - melt off the pitch to get to the capacitors/connections ?

Then when it comes to "repotting" - do I bother with pitch again, some alternative or or just insulating paper/card around the sides?

All suggestions welcome.

Power supply
Power supply
Power supply


 
 Return to top of page · Post #: 2 · Written at 8:38:15 PM on 29 August 2020.
Brad's avatar
 Administrator
 Location: Greenwich, NSW
 Member since 15 November 2005
 Member #: 1
 Postcount: 6352

Photos uploaded.


‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾
A valve a day keeps the transistor away...

 
 Return to top of page · Post #: 3 · Written at 8:45:54 PM on 29 August 2020.
Nhanwell's Gravatar
 Location: Mount Lawley, WA
 Member since 12 September 2017
 Member #: 2167
 Postcount: 45

A few productive hours - unsoldered the paxolin boards and had a peek at whats below the oil paper.

2/3 of the can is pitch Power transformer & filter choke. Other 1/3 contains 3 large electrolytics & two smaller paper (in cans) capacitors (all original)


 
 Return to top of page · Post #: 4 · Written at 1:30:58 AM on 30 August 2020.
Robbbert's avatar
 Location: Sydney, NSW
 Member since 18 September 2015
 Member #: 1801
 Postcount: 1380

I almost want to jump through the screen and clean that valve... they look so much nicer.

That power supply will be an interesting job, that's for sure. Pity you couldn't give it few taps with a hammer and it falls out of its box.


 
 Return to top of page · Post #: 5 · Written at 7:05:39 AM on 30 August 2020.
Brad's avatar
 Administrator
 Location: Greenwich, NSW
 Member since 15 November 2005
 Member #: 1
 Postcount: 6352

When I did instrument fitting work experience with the gas works at Mortlake one of the regular jobs was to change batteries on some sort of monitoring device that was located all over Sydney's gas network. There were three alkaline D cells inside a metal package, similar to that in the photos but a lot smaller of course. The batteries were embedded in a pitch-like substance for waterproofing/anti-corrosion purposes although by the time I did this, it was probably epoxy. We are talking 1987.

The battery holders were shaped in such a way that whatever we were pouring over the cells, would easily come out after it was time to change the cells, about every three years, as whatever the batteries did wasn't a high-current job. We used to just hold the battery holder like we were giving one of our classmates a Chinese burn on their wrist, with a similar twisting action and the gunk would crack and fall apart. In would go three new cells and a sachet of new 'gear' would be opened and poured over the cells - once we knew they were working, of course.

I dare say these days they use an SLA battery and a solar panel to do this job now.


‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾
A valve a day keeps the transistor away...

 
 Return to top of page · Post #: 6 · Written at 10:48:30 AM on 31 August 2020.
Nhanwell's Gravatar
 Location: Mount Lawley, WA
 Member since 12 September 2017
 Member #: 2167
 Postcount: 45

Partial success so far - still more to do.

Unsoldered & removed paxolin tag strips and the waxed paper below.
1/3 of can was unpotted capacitors which came out of the can easily - then the transformer & choke could be removed.

But the choke & transformer were totally covered in pitch. A heat gun is OK in small areas but not suitable for larger areas.
Pitch melts at >> 400F (200C) so placing in an oven for an hour or so is not a sound option - how would it effect the enamel on the wire windings?

So .. I tried an alternative strategy .. placed the transformer (wrapped in plastic) in the freezer overnight.
This makes the pitch brittle and it fractures when tapped.

It may need a couple of cycles through the freezer but this seems the easiest way to get the pitch and expose the lead connections.
Once the majority of the pitch is off, maybe use the heat gun in small areas.

Just hope the Transformer is OK. The filter choke was O/C but no surprise given the state of the downstream capacitor.
Fingers & soldering irons crossed.


 
 Return to top of page · Post #: 7 · Written at 12:57:01 PM on 31 August 2020.
Trobbins's avatar
 Location: Melbourne, VIC
 Member since 11 July 2012
 Member #: 1179
 Postcount: 42

Do you really need to extract or 'un-pitch' the transformer/choke? If lead in wire has deteriorated, then maybe that can still be managed without de-pitching.

Certainly the caps need to be either disconnected (and alternatives shoe-horned in otherwise), or removed and replaced.


 
 Return to top of page · Post #: 8 · Written at 1:42:14 PM on 31 August 2020.
Nhanwell's Gravatar
 Location: Mount Lawley, WA
 Member since 12 September 2017
 Member #: 2167
 Postcount: 45

The LT leads are all fine.

Unfortunately the Mains power cable has totally perished. Its crispy & crumbles on touch. This connects to the transformer inside the pitch (there was even a knot in the cable in the pitch).

All primary fly leads (neutral, 240, 220 & 200) & HT secondary leads are multi-strand wire and the insulation is perished and the wires badly corroded to such an extent that they break if touched. I did look at trying to make the connections outside the pitch but given the state of the corrosion/perished insulation it was not possible.


 
 Return to top of page · Post #: 9 · Written at 5:08:46 PM on 31 August 2020.
Trobbins's avatar
 Location: Melbourne, VIC
 Member since 11 July 2012
 Member #: 1179
 Postcount: 42

I did a 'short cut' with one transformer, and just carefully soldered new cable on to where the pitch was mechanically holding the lead out wire fairly well in position, and then taped the new wire down so it wouldn't move and did a megger test. But also done the full pitch crack and melt - a very ugly path to take, and the only redeeming aspect of that recovery was that the lead in wires terminated on tabs on the outer winding layer (rather than being buried in to the winding layers).


 
 Return to top of page · Post #: 10 · Written at 10:23:31 PM on 31 August 2020.
Marcc's avatar
 Location: Wangaratta, VIC
 Member since 21 February 2009
 Member #: 438
 Postcount: 4346

My experience with pitch is not with a cabinet that big but needless to say, cunning & perseverance will win. A seriously hot knife can work wonders. To free it from its shell its the wrong way up. I would suggest, without looking at the top that the pitch is not all that thick.

If the box can be suspended the right way up for a few mm with suitable protection that will cushion a small impact, soak up the pitch and not ignite. You may only have to melt what is on the side and one end for the inside to slide out under its own weight.

Most of the thermal energy from a heat gun will go into the pitch to melt it, if you are careful.

Not a job to do in a confined space, nor where pitch accidentally igniting cannot be dealt with, or cause the ignition of any material close to it.


 
 Return to top of page · Post #: 11 · Written at 12:34:18 PM on 1 September 2020.
Ian Robertson's Gravatar
 Location: Belrose, NSW
 Member since 31 December 2015
 Member #: 1844
 Postcount: 1910

I have used hot melt glue for potting things like this with great success. Once assembled and the sticks inserted, warm it up in an oven.


 
 Return to top of page · Post #: 12 · Written at 1:20:31 PM on 1 September 2020.
Marcc's avatar
 Location: Wangaratta, VIC
 Member since 21 February 2009
 Member #: 438
 Postcount: 4346

Even that there is no one to object to some of the stuff that goes into the kitchen oven here, there would be some trepidation in respect of any parts sealed with wax. At this point I have no indication that the works & cover have been separated?


 
 Return to top of page · Post #: 13 · Written at 2:50:39 PM on 1 September 2020.
Nhanwell's Gravatar
 Location: Mount Lawley, WA
 Member since 12 September 2017
 Member #: 2167
 Postcount: 45

The choke & Transformer were totally encapsulated in pitch.

I did get them out of the can after sliding out the capacitors.
The choke & transformer were separated by a metal sheet so easy to part.

I placed the transformer in the freezer over night and then tapped gently with handle of screwdriver and the pitch splinters/shatters and fell away.

The LT leads seen in the photograph are all good - just need to slide on a few new sleeves.

The problem areas are the leads for the HT to the rectifier & the mains. The rubber insulation had totally perished and where exposed the stranded wires (not copper) were badly corroded & fragile.

The next job is attaching new fly leads to the Primary & HT windings and continuity testing.

The choke measured as OC (on fly leads) - just need to expose connections to choke and confirm its not one of the leads.
But given the down stream capacitor looks distressed and there were signs of leaking pitch from can in vicinity of choke ... I think its highly likely to be burnt out.

I'll post a few photos and circuit diagram I have traced out.

There is a curiosity - 0.1μF capacitor between one of the 80 anodes and earth. Not seen this before or in any contemporary circuits.
But the capacitor (dated 08/1930) is original and the wiring too.


 
 Return to top of page · Post #: 14 · Written at 9:49:24 PM on 1 September 2020.
Marcc's avatar
 Location: Wangaratta, VIC
 Member since 21 February 2009
 Member #: 438
 Postcount: 4346

Some of that pitch, I am sure, has a high sulphur content and is quite destructive; Not only from the sulphur, but when it cracks. In cases of fragile transformer wire, I will often place a tag strip/s and wire to it first, with slack wire, this to remove any chance of "pulling" on those fragile wires.

Joints of dissimilar metal (anywhere) should be coated. E.g. Circuit board lacquer, Enamel paint (thick) or Mar. Do not use silicone on wires, it can be conductive, some is corrosive.

Marc


 
 Return to top of page · Post #: 15 · Written at 10:59:07 AM on 2 September 2020.
Nhanwell's Gravatar
 Location: Mount Lawley, WA
 Member since 12 September 2017
 Member #: 2167
 Postcount: 45

Thanks for the tips Marc.

I think you are right about sulphur or some other nasty in the pitch which corrodes the wire. The worst corrosion is where the wires emerge from the pitch.

The tag strip idea is a very good one too - I'll use that! I wasn't game to solder new fly leads directly due to the transformer due to the risk of fatigue/snapping and I had been pondering how to attach and secure a lead. Glad that I didn't rush in.

I'll strap some paxolin board to the top & base of the transformer & attach the tag strips to those Which will keep them in place until I put some of the pitch back. The laminations of the transformer are not bolted together and there is no transformer mounting bracket as such. So some pitch will probably be required when (or if - assuming the transformer is functional) goes back in the can.

Thanks again for the tips.

Power supply
Power supply
Power supply
Power supply


 
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