I was working on and recapped...this nice little ARVIN 544 a few weeks ago and thought it was done. However yesterday when I turned it on again it was sounding crappy and hard to tune. I knew it wasn't that way when I finished it a few weeks ago. How weird is this I thought!!
The tuning was on a "knife-edge" and sounding like it was coming through wax paper over a comb... but it would slowly get better as it warmed up.
Each time I'd turn it off for a while it was just crappy sounding and weird tuning again when I turned it on a 1/2 hour later.
In fact.. this is the very same radio I had a "sibilant-S" wispy sound distortion ...that I thought i had solved when I found and repaired a loose wispy litz wire connection inside the 2nd IF can... and that really did make a big change last time. However... maybe it was this lousy mica cap underlying everything as the real culprit.

http://www.nostalgiaair.org/PagesByModel/243/M0001243.pdf

I thought maybe a bad tube or degrading tube... but after swapping them with NOS I knew it wasn't that.
It was the weird sharp-edged, nasal-like tuning that really bothered my brain.
So I knew it had to have something to do with the oscillator or something.
After measuring voltages and resistor values... and going over wiring and update notes... I couldn't find any errors.
So the only ting left.... was the mica caps!
Especially the 50pf coupling capacitor from the oscillator coil to the mixer.
Sure enough it was LEAKY... about 1.5 megs at first settling in on 10 megs.
Thought you'd appreciate seeing a picture of the villain after I decapitated it!!
I don't think I'll ever simply trust micas anymore!!!

:I was working on and recapped...this nice little ARVIN 544 a few weeks ago and thought it was done. However yesterday when I turned it on again it was sounding crappy and hard to tune. I knew it wasn't that way when I finished it a few weeks ago. How weird is this I thought!!
:The tuning was on a "knife-edge" and sounding like it was coming through wax paper over a comb... but it would slowly get better as it warmed up.
:Each time I'd turn it off for a while it was just crappy sounding and weird tuning again when I turned it on a 1/2 hour later.
:In fact.. this is the very same radio I had a "sibilant-S" wispy sound distortion ...that I thought i had solved when I found and repaired a loose wispy litz wire connection inside the 2nd IF can... and that really did make a big change last time. However... maybe it was this lousy mica cap underlying everything as the real culprit.
:
:http://www.nostalgiaair.org/PagesByModel/243/M0001243.pdf
:
:I thought maybe a bad tube or degrading tube... but after swapping them with NOS I knew it wasn't that.
:It was the weird sharp-edged, nasal-like tuning that really bothered my brain.
:So I knew it had to have something to do with the oscillator or something.
:After measuring voltages and resistor values... and going over wiring and update notes... I couldn't find any errors.
:So the only ting left.... was the mica caps!
:Especially the 50pf coupling capacitor from the oscillator coil to the mixer.
:Sure enough it was LEAKY... about 1.5 megs at first settling in on 10 megs.
:Thought you'd appreciate seeing a picture of the villain after I decapitated it!!
:I don't think I'll ever simply trust micas anymore!!!
:
:
:
:

Hi Peter, I agree everyone says micas are seldom the problem , but the last two radios I repaired were micas in the front end that have gone bad, now I carry a few assorted sizes just in case...
E.

A few years ago I started replacing the mica's in everything I rebuild, I was just finding too many bad ones.

Dave

Peter,
I've found a mix of good/bad/off value micas in all my rebuilds. So I just measure and replace as required. I've given up trying to determine mica or paper in all those strange marked/unmarked pkgs. Sometimes off-values are just paralled with others to arrive at schematic value. I have a high voltage tester, but accuracy is a factor, so I stick with my B & K for accuracy and rely on set performance for any signs of high voltage degradation.

marv

:I was working on and recapped...this nice little ARVIN 544 a few weeks ago and thought it was done. However yesterday when I turned it on again it was sounding crappy and hard to tune. I knew it wasn't that way when I finished it a few weeks ago. How weird is this I thought!!
:The tuning was on a "knife-edge" and sounding like it was coming through wax paper over a comb... but it would slowly get better as it warmed up.
:Each time I'd turn it off for a while it was just crappy sounding and weird tuning again when I turned it on a 1/2 hour later.
:In fact.. this is the very same radio I had a "sibilant-S" wispy sound distortion ...that I thought i had solved when I found and repaired a loose wispy litz wire connection inside the 2nd IF can... and that really did make a big change last time. However... maybe it was this lousy mica cap underlying everything as the real culprit.
:
:http://www.nostalgiaair.org/PagesByModel/243/M0001243.pdf
:
:I thought maybe a bad tube or degrading tube... but after swapping them with NOS I knew it wasn't that.
:It was the weird sharp-edged, nasal-like tuning that really bothered my brain.
:So I knew it had to have something to do with the oscillator or something.
:After measuring voltages and resistor values... and going over wiring and update notes... I couldn't find any errors.
:So the only ting left.... was the mica caps!
:Especially the 50pf coupling capacitor from the oscillator coil to the mixer.
:Sure enough it was LEAKY... about 1.5 megs at first settling in on 10 megs.
:Thought you'd appreciate seeing a picture of the villain after I decapitated it!!
:I don't think I'll ever simply trust micas anymore!!!
:
:
:
:

Thanks for the radio stories Mr. Pete they are always informative. By the way, on the subject of capacitor leakage, I set up a little test for measuring capacitor leakage. I don't have a leakage tester. This has probably been done before. I put the test capacitor in series with a 1 meg ohm resistor. I put the voltmeter across the resistor. Then I connected a variable power supply negative to the low end of the resistor and the positive to the high end of the capacitor. I tried several old wax capacitors and found substantial leakage. I then tried new metalized film capacitors and found no leakage. My power supply can ramp up to 325 VDC. Me thinks this will give a greater indication of leakage as the voltage can be ramped much higher than what the typical ohmeter can supply. Most likely not a new revelation but mabe it will be helpfull

MRO

Have yet to find a leaky mica in any radio I own, but I'm sure it will happen to me someday. I have found quite a few leaky molded paper caps.

T.

I can't tell you how beautiful it is now just to sit here and listen to how much better this radio sounds now. Really ... it's a real a joy to listen to now!!
... so clean sharp and pure.... it's amazing what signal damage that leaky little booger did to pollute the sound...lol

When I was younger, I almost never replaced capacitors. I didn't understand that they got leaky back then. Some had some slight leakage, but I fugured that the old ones were supposed to be like that. It wasn't until I tried over and over again to figure out why on earth my Zenith 5-G-500 would distort after 15 minutes that I started to understand capacitor leakage. I had some radios that would start crackling, and then I'd turn them on and off a few times, and it would go away. That was just the way it was. I still have a couple that I 'repaired' back then that have that problem. When I get around to re-capping them, it will be so nice to not have random crackling.

T.

Thomas:
hmmm... to a purist like you ...I'm surprised you'd put up with even the slightest of any type of noise or distortion.

I didn't understand what was causing it. Those snaps and crackles you can't duplicate on a multi-meter (unless you use high voltage). I thought that it was a faulty tube or resistor or some bad solder connection that I couldn't find.

As far as being a purist, however, I have over 1000 78 rpm records that I've collected since I was about 16, so I'm pretty used to crackling, though I don't really like it. Sometimes a little is nice. Other times it's annoying.

T.

:Thomas:
:hmmm... to a purist like you ...I'm surprised you'd put up with even the slightest of any type of noise or distortion.

I really meant it more like ... I know how you strive towards perfection

Always.

T.