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Post by DHockey on Mar 23, 2021 20:11:48 GMT 12
For the past few months I’ve been working on the restoration of a RCA 141 which is the export version of the RCA 140 / GE K80. Its quite an interesting radio but very difficult to work on. Lots of rubber covered wire to replace along with a coil sub-assembly that needs removing to get access to many components. Probably the worst of the early 30’s RCA “deep chassis” sets. This isn’t to say that I haven’t enjoyed working on it, I like a challenge! The radio came from the estate of Ross Paton in “basket case” condition, partially dismantled but with rewound power transformer, field coil and choke.
Here are some photos: The valve line up is: 58 1st RF amp (used on band 4 only) 58 RF amp 2A7 Mixer/osc 58 IF amp 2B7 Detector and 1st audio 56 Audio driver 53 Class B output 80 Rectifier Schematic: I’ve run into a problem with the broadcast oscillator coil in the radio. When I initially checked out the coils with my DMM everything was OK, continuity and no shorts between windings. However when I was doing an alignment on the set I noticed that there was voltage present on the oscillator section of the tuning gang (about 160V) when the radio was switched to the BC band. This voltage was not present when the radio was on the shortwave bands. A leakage test revealed that there was about 2 Megs of leakage between the BC osc. coil primary and secondary which would drop to about 700K when the coil was warmed with a hair dryer. The radio does work, but I’m concerned about the insulation between the two windings breaking down further and causing more trouble. Also I don’t want high voltages floating around on the top of the chassis. I’m wondering what I can do about this, swapping out the osc. coil would be the answer in most other sets, but this RCA is a bit different. The IF is 445kHz rather than 455kHz and the tuning gang capacity is 335pF (full mesh) which is a bit lower than a lot of later radios so I’m thinking that a coil salvaged from my parts stash may not work well if at all?
Any suggestions would be much appreciated, its taken a lot of work to get the set to this stage and its a bit of a bugger to run into a potential show-stopper when the radio is almost finished!
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Post by dada on Mar 23, 2021 21:58:08 GMT 12
What a pain. So long as the 45pF C27 coupling capacitor is good then electrically there is no major to it working. But if you want to reduce the voltage on the tuning capacitor C40 then a high value RF choke across C40 could bleed this leakage to ground hopefully without affecting the tuning too much. As you can see from the circuit all the tuning coils are floating, so even a small leakage will be evident on C40, however this is not necessarily detrimental to the workings of the oscillator and as you say it still works. Interesting that the leakage increases with heat. One option may be to try doping the coil, if accessible, perhaps with polystyrene glue ie polystyrene dissolved in solvent, but that is a one way street with no simple reversal. Have you tried cleaning the coil with a quick drying solvent to see if the leakage elements can be flushed away? I must say the chassis looks very clean and does not seem to be suffering from any of Ross's fire damage. Good luck! Cheers, DC
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Post by DHockey on Mar 24, 2021 20:00:12 GMT 12
Thanks, yes I did notice that the coils are all floating so the leakage is not really affecting the operation of the radio. I did try putting the coil in the oven at 60 degrees for a few hours hoping that it may dry out any moisture that was in there. However that didn't make any difference, it just increased the leakage until it cooled down again!
I think I may just leave things as is for the moment lest anymore meddling cause a complete failure of the coil. I would still like to find a solution to the issue to "future proof' the radio since I'm not sure how long the coil will last once the radio is back in its cabinet and it starts heating / cooling in use.
The chassis was quite rusty on the top and had been poorly painted at some point when I got the radio. I ended up removing everything from the top of the chassis, removing the rust and repainting it with primer and aluminum coloured paint. No sign of fire or smoke damage so it must have been stored somewhere safe at Ross's place.
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Post by Peter Walsham on Mar 25, 2021 8:48:00 GMT 12
Good morning Daniel
What I do with coils with excessive leakage is to place the coil in a tin of molten wax placed on a hot plate element, and leave it on a very low simmering heat for several hours. I have not had a coil that hasn't successfully responded to this treatment.
Peter
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Post by DHockey on Mar 26, 2021 9:56:20 GMT 12
Thanks Peter, I may give that a try. What sort of wax do you use? Paraffin wax?
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Post by Peter Walsham on Mar 27, 2021 9:27:55 GMT 12
Good morning Daniel
I use a product called 'Microwax' (Microcrystalline wax), although I can't see why paraffin or beeswax won't work. I believe that microwax has a higher melting point (of 80 Degrees C) than the others (but don't quote me on that). However, I can say that I had conducted some experiments years ago, where I attached some tails to a small cardboard tube (as a mock coil former), and carried out a leakage test at 1000 Volts. The test showed a relatively low resistance (high leakage, but I don't remember the actual figure). After immersing this in to the wax, and letting it simmer for some time, I repeated the test after a while, and a repeat leakage test showed an infinity high resistance. At the time I had an RF coil that was leaky primary to secondary, and this varied with heat (like your coil does). With nothing to lose, I dipped that coil in the wax and left it until all the bubbling stopped (which I assume was trapped air, along with the moisture). This took quite a long time, but I did it on the lowest setting of the hot plate simmerstat. However, the results were perfect. No leakage whatsoever, and the coil was protected from further moisture by the new coating of wax. I have done this many times since, always with perfect results. I got 500gm blocks of the microwax from a place called 'CCG Industries', which is an art & craft supplier, but other places also stock it as well. It's relatively cheap. Microwax is also acid free, so it's not going to attack the coils.
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Post by dada on Mar 27, 2021 9:48:02 GMT 12
Bees wax has a small issue of shrinkage with age, sometimes cracking the sealed surface.
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Post by DHockey on Mar 30, 2021 8:14:30 GMT 12
Thanks Peter, I've decided I will try your wax treatment on the coil rather than leaving the radio as is. I had the radio running on the bench for a few hours and found that the voltage present on the osc section of the gang climbed over that time and could be measured with a 1000 ohms/volt meter. I have ordered some Microwax, it sounds like the right product for this job. I do have some beeswax, but its a bit dirty / unprocessed so not ideal for coil potting. I've worked out the resonant frequency and inductance of the coil (992Khz, 165uH) so I know what specs I'll need if it all goes horribly wrong. Started work on the cabinet while I wait for the wax to arrive. It was partially stripped when I brought it, there was only lacquer remaining on one side and in a few corners. The rest appeared to have been scraped off. Here is a photo of the cabinet after a wipe down with turps after I completed stripping it.
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Post by Philconut on Mar 31, 2021 21:16:29 GMT 12
An option is to supply the 2A7 triode plate via a 22k resistor, connect the plate circuit to the top of the coils via a 100pf cap & connect the coil bottoms to earth. This removes the HT voltage from the coils completely. However, the wax treatment is a good option. I always re-dip all coils in bee's wax and as Peter has said, wait for the bubbles to stop. This drives out any air and moisture that can, and often do cause problems.
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Post by DHockey on Apr 1, 2021 14:52:18 GMT 12
I did look into doing just that but wasn’t sure how well it would work with a 2A7. I have seen that method in circuits using a 6K8 but couldn’t find an example using a 2A7/6A7/6A8. However I’ll give it a try if the wax treatment doesn’t eliminate the leakage.
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Post by Peter Walsham on Apr 5, 2021 22:00:44 GMT 12
That's great to hear Daniel
I will say that I have been using the microwax 'treatment' on leaky coils for over 30 years and it's worked for me every time. I am totally confident that treating your coil to this treatment will solve the problem. Just remember to have the was simmering at a very low temperature. Giving the coil plenty of 'dipped time' is the best way to go about this, and do not try to hurry the process by turning up the heat.
I will certainly be very interested to read about your outcome, that's for sure!
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Post by DHockey on Jul 8, 2021 10:44:29 GMT 12
I logged into the forum and noticed a message from Peter asking how I got on with wax dipping the coil. So I thought I should update everyone on how I have been getting on. The RCA has taken a back seat for a while due to a finishing off another radio that I had located some parts for and lots of non-radio related stuff going on.
Anyway, I did try the wax treatment I melted the wax in a double boiler and put the coil in for about 3 hours. I removed the coil, left it to cool and checked the leakage between the windings again with my meter. Unfortunately, there was no change. So I then tried putting the wax straight in a pot and simmering it on a low heat for a few hours, still no change in leakage. I get the impression the coil is very tightly wound and possibly lacquered rather than waxed and the molten wax just can’t get in between the windings. The next thing I tried was warming the coil with a heat gun and squeezing the windings while monitoring the leakage reading with my meter. Doing this I was able to get rid of the leakage, however once the coil cooled down again the resistance between the windings returned.
I then decided that any further messing with the coil would likely result in damage, so it was returned to the radio and I tried John’s suggestion of removing the HT from the primaries of the oscillator coils with a 100pF cap, grounding the coil bottoms and feeding the 2A7 with a 22k resistor. This resulted in the radio not working well at all on any band, just static and the occasional garbled sounding station.
The radio is now back in its original configuration. It does work, but there is close to full HT present on the oscillator portion on the tuning gang. I think my only options to solve the issue are a rewind of the original coil or installing one of those slug-tuned “universal” oscillator coils.
A belated thanks to everyone for their suggestions, I have enjoyed trying them out and I have learned a lot about oscillator stages working on this fault!
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Post by Peter Walsham on Jul 8, 2021 11:13:47 GMT 12
That is a real bummer! Darn - I had full expectations of a 100% success on that.
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Post by DHockey on Sept 13, 2022 20:37:01 GMT 12
Time for an update on the RCA 141, the original oscillator coil failed completely not long after I last posted about the radio, at that point I put the radio back together and put it up on the shelf for another day and did some other projects. However, last week I was parting out a butchered Collier and Beale chassis when I noticed it had a replacement oscillator coil,a “universal” type with the adjustable slug. I decided to investigate if I could use this in the RCA.
I built a version using a 6SN7 rather than FET’s and powered it from my high-voltage bench supply. It’s a very useful device, enabling you to work out the resonant frequency of a coil and the inductance.
I connected the salvaged universal coil to the tester and found it resonated at around 1200 KHz, I then adjusted the slug in the coil until my frequency counter read 992 KHz, the same frequency as the original RCA coil. Now that I knew the coil had enough adjustment to match the inductance of the original, I decided to work out if I could fit it into the RCA chassis. Mounting in the original position would not be an option, the original coil and trimmer forming a sort of integrated unit, the-trimmer screw being accessed by inserting a screwdriver down the centre of the coil. The new coil with its adjustable slug in the centre would not work here and I wanted to keep the original trimmer, so the coil would have to go somewhere else.
The 141 was available in two versions, a 5 band and a 4 band. Mine is the 4 band and there is vacant space in the coil unit next to the BC band coils, I made up a small aluminum plate and mounted the new BC oscillator coil where the oscillator coil for the 5th band would have been and ran short connecting wires across to the original BC oscillator trimmer.
I powered up the radio and was greeted with…silence. The oscillator was not running, so I powered off and reversed the primary wires to the coil. This resolved the issue and now when I switched on I had stations present across the dial pretty close to where they should be! I then performed an alignment which sorted the dial tracking. Checking the oscillator plates of the tuning cap revealed there was no longer voltage present, the original fault. I can’t detect any change to the performance of the set on the BC band, it is still very sensitive, picking up stations with no aerial connection at all.
Really happy to have this radio working again, albeit slightly modified in an easily reversible way, should I ever get around to attempting to rewind the original coil.
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Post by Philconut on Sept 13, 2022 21:51:35 GMT 12
Great news Daniel. I am working on a Philips 594 which was a complete mess. I have replaced all the components, including the ones hidden inside the 2nd IF can. I had to rewind the oscillator coil completely which took a while but it ended up going really nicely. Tonight, I had it going while I did another job & it stopped abruptly. Cause, the oscillator had stopped. Soooooo after spending most of the day & evening on it, I turned it off & shut the door albeit with a few bad words! But that's not my reason for writing. I have been investigating the specifying of driver transformers and while I have collected quite a bit of info, I don't seem to be any closer to any answers. I notice that your 141 has a 56 driving the class B push-pull 53. Would you please measure the voltage ratio between the primary and HALF the secondary. I do it be putting 50 volts across the primary & measuring the secondary volts. It will be interesting to see what the ratio is & if it sheds any light on my questions. Cheers John
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