February 10, 2008 at 4:50 am #848Gerry O’HaraKeymaster
So youve been told that silver mica capacitors are the most reliable in vintage radios and that it is not even worth checking them? Well, they may indeed be the most reliable, but they are not infallible! Occasionally they suffer from the so-called silver mica ‘disease’. This can cause difficult to trace symptoms such as loud crashing sound, crackling, distortion and weak signals. This effect is caused by the creeping (by electrostatic means) of the silvering from where it was originally applied on the mica dialectric to where it should not be, eg around the edges of the mica (in a discrete silver mica capacitor) or between separate areas of silvering on the same side of a mica sheet, as in the silver mica sheet/pad capacitors ‘integrated’ into the IF can construction in the Zenith illustrated here. Below is a brief email string between myself and Phil, members of the CVRS, concerning this phenomena, which we thought would be of interest to others. I have included some photos to illustrate the points made. For more detail, check out the following website: https://www.ppinyot.com/if_transformers.htm (the first post is at the top of the string).
Another thing we could cover is this business of “silver mica disease”. It affects the tuning caps, I think usually later model AA5’s. I don’t think many folks have even heard of it, even those with a lot of experience under their belt, so might be interesting to some others.
I have first-hand experience of the silver-mica disease in a 1950’s Zenith (Model H723Z) – see attached – I took these photos with the intent of writing an article myself (thinking this was an undiscovered issue!), then I came across the articles on the web… The cracking in the Zenith was amazing before I replaced the mica sheet caps with discrete ones per the photo. The set now works very well. Still may be worth a write-up or at least a tech tip as you say. Maybe a post on the forum?
That is an excellent example of the problem.
There must be a lot of radios with this problem, but to a lesser degree. I was just thinking, I bet you took the can apart because it was bad enough to become problematic enough to make you look for it, but how would we know if it just wasn’t “up to par” ? the radio might still play ok, but with less than great selectivity and maybe some crackling? Thunder storming? Is that about it for the symptom? – and how would we know which can might have that wafer, hate to pull the can apart if it doesn’t need it.
A list of radios that have this style of caps would be handy to have.
I guess it is no worse than the old ones which could have a paper cap in them, they can be bad too I assume.
Anyway thanks, interesting pictures. I think I had that problem with a Crosley D25, dashboard radio, that would be about 52 or so as well. I swapped the IF can from a junker and was happy that it fixed the problem. I don’t remember if I opened the can. Yes we should be talking about this in the forum, we need the posts.
Yes, I had never heard of this problem before. I eventually isolated the noise to the first IF transformer with much checking. As you say, there is a reluctance to remove all the wiring from the transformer connections to remove the can from the chassis in order to gain access. When I opened it up, I wondered where the caps were at first! (as I had never really looked at this vintage of domestic radio before – most of my experiences were with 1930’s domestic radios or communications sets which had all used discrete mica caps). I soon figured it out and it then dawned on me what the problem could be – once I saw the mica plate it was obvious. At first I tried just scraping the fine silver lines away and re-assembled/tested the result, but, although an improvement the fault persisted. I ended up removing the transformer for a second time and fitting a plastic washer in place of the mica plate and two discrete capacitors across the primary and secondary coils. The schematic did not have values marked for these caps, so I measured the coil inductance and calculated what the cap should be to resonate the coil inductance at the IF frequency. I think this came out at 120pF, which seemed about right so I fitted these values, checking the resonant frequency with a GDO (one of the photos shows this) – they tuned up like a charm.
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Download DSC00113 [1024×768].JPG. (Caution: This file may not be virus scanned.)February 10, 2008 at 12:02 pm #879philForum Participant
that is a great , informative post Gerry.
The grid dip meter is also interesting. I have never used one. It picks up on the resonant frequency of a tuned circut right? Do you feed the transvormer via a signal generator, and by varying the frequency a bit , see where the meter peaks? I am guessing, am I close? the frequency is a product of a given inductance and capacitance, so you are testing using the grid dip meter to see if the caps you used are the right size to make it oscillate at 455 Khz right?
PhilFebruary 10, 2008 at 5:26 pm #880Gerry O’HaraKeymaster
Your basically on the right track with the GDO Phil. These relativey simple, versatile and easy to use instruments are popular in amateur radio circles but seem to be relatively unknown in radio repair or restoration work – I am not sure why. GDO’s can be used in several ways (I will place a post later describing their use in more detail). In this example, the GDO is allowed to oscillate in the range of the wanted IF frequency. The meter on the GDO shows the level of this oscillation, the frequency of which can be tuned with the ‘tuning’ knob on the GDO (within the range allowed by the GDO coil selected – they are usually supplied with seveal covering a wide range of frequencies). The GDO oscillator coil is then brought close to the tuned circuit for which the resonant frequency is unknown. On tuning through the resonant frequency (by turning the GDO ‘tuning’ knob), a dip will be observed on the GDO meter at the point of resonance of the tuned circuit under test. To get as accurtate measurment as possible, the degree of coupling between the GDO coil and the tuned circuit coil should be as loose as possible to still give a dip on the meter (keep the two coild close at first and once the dip is found, move them apoart and re-tune the GDO if necessary – this is due to the ‘pulling’ effect of the tuned circuit on the GDO. The GDO is calibrated to read the frequency directly for several of its ranges, but the Millen unit shown here uses calibration curves for the ranges below around 1.6MHz (the calibration curves can be seem on the photo). A more accurate frequency determination can be had by coupling a digital frequency meter (DFM) to the GDO coil afer the resonant frequency has been determined as described above (a few turns of wire connected to a coax cable feeding the DFM), reasing the frequency directly on the DFM. I would note though that the simplicity of the GDO circuitry does not allow really precise determinations to be made, but certainly more than enough for the IF transformer example shown here.
The two photos attached here show a similar application – this time to find the approximate tuning range of a radio frequency (aerial) coil from a communications receiver.
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