Replacing "micamold" caps in oscillator section - Page 2 - UK Vintage Radio Repair and Restoration Discussion Forum

Silicon · 15th May 2023, 3:10 pm

Will you crack open the low value Micamold capacitors to see if they are paper?

Alternatively you could test them for leakage at 250V DC.

Alan Bain · 25th May 2023, 8:56 pm

Well I put them on a capacitance bridge (low voltage AC at 1kc/s) set to read ideal capacitance with parallel resistance

520pF read 535.9pF || 120M
1500pF read 1571pF || 10M
2700pF read 2943pF || 8M
2700pF read 2700pF || 12M
3000pF read 2990pF || 7M
3000pF read 3033pF || 84M
3900pF read 4117pF || 1.5M

The capacitance values are remarkably accurate, but the 3900pF is starting to look a bit suspect but the 520pF may well be mica.

Probably worth checking leakage at HT, clearly need a better bench PSU, but somewhere I do have an HT supply and yes I'm tempted to open the 3900pF one to see what I find.

A randomly chosen one of the "special" decoupling caps we all know is quite different
4700pF read 19nF || 20K (umm we know all about these....)

FrankB · 26th May 2023, 8:20 am

The Micamold capacitors are all to be suspect. I had a nightmare situation with them.
I was restoring a Magnavox B&W TV set and had H osc problems,
The shop had a box of NOS Micamolds and I tested them with an LC53 Sencore unit, a military capacitor tester, an eye type tester and a digital tester. They all tested perfect. I replaced the ones in the circuit with them and it still wouldn't work. Replacing them with new epoxy coated silver micas solved the problem. The other tech had the same situation with an RCA TV tuner, Caps tested perfect, but the Micamolds were punk.
IIRC, the Micamolds were all paper mica.
Since then, if a set has Micamolds in it they are replaced.

Alan Bain · 1st Jun 2023, 10:21 am

Well with those micamolds replaced quite a lot of the AR88 seems to work. There is good output from oscillator on all bands. The RF gain pot was an intermittent open circuit - with a temporary replacement the RF amp stages work with gain control as does the BFO (although there are a lot of harmonics in the output).

The RF gain is only a divider so I can abandon the unfindable 66K value and adjust the resistor to chassis, but it seems that 1/4 metal shafted pots with long shafts are becoming an endangered species...

AF gain seemed similarly mangled (and why do all the 2M-log pots in my box have integrated on-off switches...?).

The IF amp has an issue and I see the xtal phasing cap is pretty much at max capacity which looks wrong. I'm assuming I tune to the xtal peak rather than setting my synthesiser to 455.000000kHz?

This is all putting off the hardest problem of what do I do about the AF output transformer until last. I am tempted just now to introduce a cathode resistor to the output stage and use a radiogram transformer (I don't really need 600 ohm line out).

However, notwithstanding all this, it is nice to see what was once a rusty wreck looking and starting to function like a radio again.

15th May 2023, 3:10 pm	#21
Silicon Nonode Join Date: Jun 2007 Location: Coulsdon, London, UK. Posts: 2,163	Re: Replacing "micamold" caps in oscillator section Will you crack open the low value Micamold capacitors to see if they are paper? Alternatively you could test them for leakage at 250V DC.

25th May 2023, 8:56 pm	#22
Alan Bain Pentode Join Date: Jan 2012 Location: Heswall, Merseyside, UK Posts: 175	Re: Replacing "micamold" caps in oscillator section Well I put them on a capacitance bridge (low voltage AC at 1kc/s) set to read ideal capacitance with parallel resistance 520pF read 535.9pF \|\| 120M 1500pF read 1571pF \|\| 10M 2700pF read 2943pF \|\| 8M 2700pF read 2700pF \|\| 12M 3000pF read 2990pF \|\| 7M 3000pF read 3033pF \|\| 84M 3900pF read 4117pF \|\| 1.5M The capacitance values are remarkably accurate, but the 3900pF is starting to look a bit suspect but the 520pF may well be mica. Probably worth checking leakage at HT, clearly need a better bench PSU, but somewhere I do have an HT supply and yes I'm tempted to open the 3900pF one to see what I find. A randomly chosen one of the "special" decoupling caps we all know is quite different 4700pF read 19nF \|\| 20K (umm we know all about these....)

26th May 2023, 8:20 am	#23
FrankB Heptode Join Date: Oct 2005 Location: Olympia, Washington, USA. Posts: 664	Re: Replacing "micamold" caps in oscillator section The Micamold capacitors are all to be suspect. I had a nightmare situation with them. I was restoring a Magnavox B&W TV set and had H osc problems, The shop had a box of NOS Micamolds and I tested them with an LC53 Sencore unit, a military capacitor tester, an eye type tester and a digital tester. They all tested perfect. I replaced the ones in the circuit with them and it still wouldn't work. Replacing them with new epoxy coated silver micas solved the problem. The other tech had the same situation with an RCA TV tuner, Caps tested perfect, but the Micamolds were punk. IIRC, the Micamolds were all paper mica. Since then, if a set has Micamolds in it they are replaced.

1st Jun 2023, 10:21 am	#24
Alan Bain Pentode Join Date: Jan 2012 Location: Heswall, Merseyside, UK Posts: 175	Re: Replacing "micamold" caps in oscillator section Well with those micamolds replaced quite a lot of the AR88 seems to work. There is good output from oscillator on all bands. The RF gain pot was an intermittent open circuit - with a temporary replacement the RF amp stages work with gain control as does the BFO (although there are a lot of harmonics in the output). The RF gain is only a divider so I can abandon the unfindable 66K value and adjust the resistor to chassis, but it seems that 1/4 metal shafted pots with long shafts are becoming an endangered species... AF gain seemed similarly mangled (and why do all the 2M-log pots in my box have integrated on-off switches...?). The IF amp has an issue and I see the xtal phasing cap is pretty much at max capacity which looks wrong. I'm assuming I tune to the xtal peak rather than setting my synthesiser to 455.000000kHz? This is all putting off the hardest problem of what do I do about the AF output transformer until last. I am tempted just now to introduce a cathode resistor to the output stage and use a radiogram transformer (I don't really need 600 ohm line out). However, notwithstanding all this, it is nice to see what was once a rusty wreck looking and starting to function like a radio again.