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 Leaky Mica in HMV 64-52
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 Return to top of page · Post #: 1 · Written at 11:22:08 AM on 9 September 2021.
BurntOutElectronics's Gravatar
 Location: Alexandra, VIC
 Member since 2 October 2019
 Member #: 2392
 Postcount: 238

I was given a HMV in very dirty shape a while ago, cleaned it up and repaired/aligned it.
Did seem to work alright for about a week before a interesting issue did arise.
I noticed that the radio always seemed a little deaf, however after a week of on and off use the audio would suddenly take a sharp nose dive after warming up. I initially suspected a resistor was going open but this morning I finally found the culprit.
There's a silver mica cap between the second I.F. can and the AGC which was quite leaky and would bias the gain off on the stages.
Before cutting out the cap the AGC sat at 3.5v, which I didn't bat an eye at initially, but after cutting it out it went down to -0.8v and the radio came back.
Putting the multimeter from the mica to ground it was leaking 100v open circuit.
First time I've come across a faulty mica myself although I've always known their tendancy to fail over time.


 
 Return to top of page · Post #: 2 · Written at 12:41:32 PM on 9 September 2021.
GTC's avatar
 GTC
 Location: Sydney, NSW
 Member since 28 January 2011
 Member #: 823
 Postcount: 6268

Yep, and it's the ones hidden inside IF cans that are bewildering to the unfamiliar, especially without a schematic.

The sudden silence or attenuation symptom you mention is an indicator of likely trouble with such micas.

Another quite bizarre symptom in that area that I've come across is the volume suddenly rising when the pot is at its lowest anticlockwise position just as the switch-off position is reached.


 
 Return to top of page · Post #: 3 · Written at 2:10:19 PM on 9 September 2021.
BurntOutElectronics's Gravatar
 Location: Alexandra, VIC
 Member since 2 October 2019
 Member #: 2392
 Postcount: 238

Yes, it sure would be a nightmare with the mica's in the I.F. cans, you'd have to remove them and install variable caps underneath and align just so you can find the original intended value.

I find it fascinating how, with high voltages, the silver can migrate through the mica and cause those pops and crackles.
I was surprised with this cap because it was a mere 10 pf. its all replaced now and put back beside my bed and works a treat.

The radio has plenty of scratches, however, it's a black case and at night when I look at it it's relatively unnoticeable.

I'll say that the problem solving in this is the part I enjoy the most. Listening to it doesn't bring much joy when the only two major stations during the day here are 3AR & 3LO. Wish there was a music station.


 
 Return to top of page · Post #: 4 · Written at 4:39:25 PM on 9 September 2021.
Ian Robertson's Gravatar
 Location: Belrose, NSW
 Member since 31 December 2015
 Member #: 1844
 Postcount: 2054

Generate you own AM station with a modulator connected to a DAB radio.


 
 Return to top of page · Post #: 5 · Written at 6:15:55 PM on 9 September 2021.
Robbbert's avatar
 Location: Hill Top, NSW
 Member since 18 September 2015
 Member #: 1801
 Postcount: 1553

He's a long way out of Melbourne, might not be able to pick up DAB. But he can tune into a music station online, and output the audio from his computer to a low-power AM station in his house.

Looks like a nice country area that long ago had a railway that's been closed.


 
 Return to top of page · Post #: 6 · Written at 8:05:23 PM on 9 September 2021.
BurntOutElectronics's Gravatar
 Location: Alexandra, VIC
 Member since 2 October 2019
 Member #: 2392
 Postcount: 238

No, there certainly isn't any DAB out here!
Although I have got myself one of those "mosquito" 1 Watt transmitters which covers the top of the AM band. just have to whip up an antenna for it yet.
It is a very nice bit of country out here, and I live close by to the timber tramway museum which runs either a diesel or steamer every second Saturday. It runs in a loop around the site and has recently been given the ability to switch direction with a new piece of rail installed. Kids love it and I've captured some wide stereo of the steam engine going past.


 
 Return to top of page · Post #: 7 · Written at 8:50:07 PM on 9 September 2021.
Marcc's avatar
 Location: Wangaratta, VIC
 Member since 21 February 2009
 Member #: 438
 Postcount: 4602

64-52 series was seriously popular & I have one worker and a few chasses & bits. I must have overhauled over a dozen. The most common nasty in those apart from wire & caps and a batch of bad resistors that crept into several makes in the late 50's, was the bean counters.

Early models had two parallel 22K resistors on the screens up front & somehow that became 10K . This seems to be 1Watt ensuring that current squared by resistance, saw their demise. Never, had much problem with Mica in them but like all; If free one end I will put 500V on them to ensure they are good. Then a dob of paint to show I tested it.

Standard antenna length is normally 25ft. Workshop main is around 37m carried on three poles. All insulation hardware & line taps are Electric fence. Wire is 2.5mm "Tye Easy" fencing wire. Constant tensioning is by dead weight & pulley. That works quite well.

There is a guy in that area I know, who is a Ham radio operator: Worth knowing.

DAB has serious issues around here.

Marc


 
 Return to top of page · Post #: 8 · Written at 9:49:52 PM on 9 September 2021.
BurntOutElectronics's Gravatar
 Location: Alexandra, VIC
 Member since 2 October 2019
 Member #: 2392
 Postcount: 238

Yeah I've noticed that they are quite common.
It's the second I've fixed, first one was for my dad as it was the same cream model my great grandfather used on the daily when my dad was young.
Not a bad performer, but could do with a bit more sensitivity out here.
I don't have a long wire for old radios unfortunately.
I believe that it's generally only micas with high voltage across them which break down. So I normally only check the ones with DC across them. I did make a HV cap checker but it needs a service.
Both I've seen have had those parallel 22k resistors you spoke of.
Although the other one had a large resistor in a wax cap enclosure. Took me a minute to work out what they did.


 
 Return to top of page · Post #: 9 · Written at 12:12:28 AM on 10 September 2021.
Marcc's avatar
 Location: Wangaratta, VIC
 Member since 21 February 2009
 Member #: 438
 Postcount: 4602

The 1938 Valve and circuit tester uses 225V for leakage testing, in conjunction with a neon (pass fail). I normally use an Insulation Tester and test any Mica out of circuit. Any NP cap that leaks (99.99% of wax paper & Oil filled) is a dud.

Unless it has a "loop stick" (ferrite) antenna you are not going get much. Electric fence insulators are often cheaper than those Radio ones and the Ceramic ones are good for better than 10kV: Quality items.


 
 Return to top of page · Post #: 10 · Written at 8:28:36 AM on 10 September 2021.
BurntOutElectronics's Gravatar
 Location: Alexandra, VIC
 Member since 2 October 2019
 Member #: 2392
 Postcount: 238

Luckily for me it does have a ferrite bar antenna.
I actually do have a insulation tester myself so I'll have to try that out.
I do intend to make a long wire eventually but it's not something on the table at the moment.

Lance


 
 Return to top of page · Post #: 11 · Written at 10:45:30 AM on 11 September 2021.
Marcc's avatar
 Location: Wangaratta, VIC
 Member since 21 February 2009
 Member #: 438
 Postcount: 4602

It is a well established fact that if the set has been sitting for more than a couple of years,& I have a couple plus, electrolytic's that have been in stock a while, I always hook them to the reformer part of the bench PSU before using them. They loose form with time, when not used. It is never uncommon for them to present as a short.

Around 50 years of experience says powering a set, of unknown history, "to see if it goes", is a recipe for disaster. Equally fault finding where there are Waxed paper and some oil filled caps still in it; Can also lead to a lot of wasted time and a set that will never run properly irrespective of its "B" Voltage.

As before you can make a "go, no go" NP cap tester using a Neon & appropriate resistor. even using a Neon fitted screwdriver /probe.

If you have valve radio coupling caps under 200Meg is a dud. If its a decoupler on a screen 50Meg and here if it leaks which 99.99% of papers & oil filled will: It goes in the bin. Test is at rated voltage, B voltage, or quoted test voltage. Mica I usually test at 500V. Wax paper rarely as the probability of getting one that does not leak above 1Meg says time is better spent replacing it.

Bad caps can kill valves & seriously impinge on performance.


 
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