I'm currently working on an AWA 510M. It's in fantastic condition which is one of the reasons I bought it. Initial testing showed the field coil and OP transformer primary were open circuit. A real bummer because apart from that the speaker looks as good as new. I've pulled the covering off the field coil to investigate a bit further but the break looks like it's very deep and the chances of repair or rewind are extremely slim. I've got a couple of spare 5 inch AWA PM speaker/transformers from junkers so hopefully one is the correct impedance and good to use.

I'm hoping I can just replace the field coil with a 1k 5 watt resistor without any issues as I don't like my chances of finding a 1k choke rated at around 60ma. Jaycar does a 47uH 5A choke (LF-1274) that I might try in series with the 1k resistor. Has anyone tried this before and is it worth the trouble?

I assumed the field coil and OP were taken out by faulty caps in such an old radio. It's had some work done in the past, the electros had been replaced as well as C27 and C30 with mustard caps, although these two were only rated at 400v instead of 600v so I've replaced them. All the other caps were original and I was certain I'd find either C31 or C32 shorted. I replaced all the wax paper caps but was amazed that none of them were leaking when tested on my megger tester. A few resistors were out of spec and were also replaced. The two 100 ohm 1/2 watt resistors on the plate of the 6X5GT look to be severely heat stressed and although within spec were replaced with a couple of new 1/2 watt high voltage resistors. Electros not done yet but will be done as soon as I can get suitable replacements and before powering up.

I was still puzzled about what took out the field coil and OP so started testing the valves. The 6V6GT showed a very faint and intermittent plate short but I'd have expected a catastrophic failure to have taken out both coils. I always though 6V6's were very hardy critters not prone to shorts.It's a made in Australia Radiotron and not branded AWA so is probably a replacement at some time.

The 6SQ7GT showed a definite cathode short which might explain the field coil. Could this have taken it out?

I can't find any other shorts or evidence of a screw driver jockey poking around and the wiring's correct and in very good condition. I'm a bit hesitant to risk a good OP on this if I haven't checked all possibilities yet. Any thoughts?

Cheers,

Warren

Rarely have I seen the 70's 400V caps punch through, I have seen more behave like the old hard cased type that cracked. I would be more concerned if it had a 5Y3: 6X5 will not produce anywhere near that voltage in a surge as it is a heater valve.

The most likely cap to take out the OP transformer is a plate bypass from Plate to ground C31 C32. These were a common fail due to the load on them & the voltage rating should be high. Some of that era had an open type Mica cap. rare fail.

However, the big issue was corrosion & heat. I came to the conclusion after doing a run of them was that about one in eight might have a good transformer. There is also a black pitch covered OP transformer found in some HMV Nippers that particular one is probably overheated by the pitch jacket and they have a severe attrition rate.

I also believe that some of that pitch is corrosive and exacerbates the failure of the coils. It cracking is also not helpful.

The 100 ohm resistors are to take some stress off the 6X5 but make good "fusistors". Never heavy those up as 6X5 has some patterns that are notorious for heater cathode shorting.

The field may be open from corrosion, or it could have been cooked by being left running with a shorted, or leaking filter cap, Albeit a heater cathode short will not help. R16 will often burn with a heater cathode short.

You can use a resistor of equivalent value. Chokes tend to be expensive and you may have to get from Mouser or Antique radio supply US. One of them actually had new bobbins for OP transformers. Might get one salvage. Field is not an RF choke. C33 really should have a decoupler of 0.047mfd to 0.1mfd parallel with it for RF, did not see one at a glance.

There are HRSA members that rewind. If you can get the bobbin out?

6V6 will some times show slight shorting, however, if the plate looses supply the screen becomes the plate & it may fail. Normally R13 & R14 will be out of spec.

6J8 is a valve that loves volts and being SW will die at high frequency if all is not well.

You will get away with 450V types there, cheap ones fail EVATCO is one that will have.

Marc

R13 and R14 were within spec, only a handful of resistors were replaced, the most notable being R12 which had been replaced with a 270k instead of a 200k at some time in the past

I managed to salvage a good speaker/OPT from an AWA 449MA which uses a 6BV7 output valve and is physically a perfect match and fit. The impedance of this combination worked out at 8600 ohms, a little bit high but in the ball park for the 6V6GT. Both radios list the same OPT, an XA2. I also replaced the field coil with a 1k 5 watt resistor.

Testing showed the voltages around the 6V6GT were a little off with 12v across R15, 240v on the plate, 250v on the screen grid, 300v on the cathode of the 6X5 and 2.8v across R16 although the radio sounded fine. I was under the impression that if the screen was at a higher voltage than the plate, the screen would conduct. The AWA voltage chart shows the screen should be 240v and the plate 225v, is this something that’s desirable with a field coil speaker? Should I have increased the size of the 1k replacement for the field coil slightly?

Anyway, I found that Antique Electronic Supply do a perfect OPT replacement. With the speaker from the 449MA the impedance is about 5000 ohms and was only $5 plus shipping. The power rating at 3 watts is a bit low but I figure the radio’s unlikely to be run at full volume so it should cope. It’s actually quite easy to get the old OPT out its cover by leaving it in the oven at 120° for about half an hour. It slides straight out and the new OPT is a perfect fit.

This is where I get confused, the voltages don’t make sense now. R15 now has 13.2v across it which is better, 280v on the plate, 275v on the screen, 294v on the cathode of the 6X5 and 3.2v across R16. These plate and screen voltages are way above the AWA specs of 225v on the plate and 240v on the screen but after checking the 6V6 specs they’re ok although at the upper limit of what a 6V6 can handle. The radio sounds exactly the same now but how can the plate voltage possibly be higher than the screen? I’ve checked and rechecked these voltages multiple times as well as rechecking the impedance of the speaker/OPT combo but it makes no sense to me. Because the original field coil and OPT were open I have no way of checking the impedance of the original components.

Any advice would be greatly appreciated.

Warren

I've gone back to the 8600 ohm speaker/OPT from the 449MA for now as the voltages are much closer to spec while I think about this some more.

I still can't understand how the plate voltage can be higher than the screen voltage with the lower impedance OPT when both voltages come from the same place. I think I'll recheck the wiring again.