Hi Folks;
I have been given a Hallicrafters S36A, which has been attacked by the magic soldering gun.
The issue I have is this: All of the 8500 mmfd mica capacitors (20 each) have been replaced by orange drops. I know that I can replace some of the micas with orange drops, but I do not have the knowledge to determine which can be orange drops and which have to be micas or ceramics. I think all of the caps in the RF and the oscillator need to be either mica or ceramic Is there anyone out there that can guide me through this or do I have a parts radio?
I looked at the price of mica caps in the .01 uf at 600v and almost had a heart attack. They were not cheap.
Thanks

I can't imagine why Halco used 8500 mmf mica.

http://www.nostalgiaair.org/PagesByMode ... 008860.pdf

In this receiver, those capacitors wouldn't be used in tuned RF tank circuits, so mica was not really needed. I would guess you can use film caps for all the 8500 mmf (pF) units.

Mica in that rating would be large, expensive, and hard-to-find today.

Maybe someone else has another insight?

Rich

PS: Does the radio work correctly with the changed capacitors?

That model was cutting edge technology when it was designed and built. The use of mica caps for high frequency coupling and bypass was necessary because a paper cap has rather high inductive reactance at the frequencies involved here. The last thing you would want to do is put in an inductor where you need a cap. I suspect that today's modern poly caps would be fine for non-resonant circuits. I agree with Rich that 8500 pf would probably not be used in a resonant circuit (like oscillator, RF and IF tuning) in this radio. I'd say leave the orange drops in if they are of recent manufacture.

Dave

Rich
The radio works great on FM, but is deaf on AM. I can force a AM signal through the radio using my signal generator. The signal generator output is on the one volt range at about .45volts. Not much sensitivity for AM. The IF aligns right up at 5.25 MHZ. with a low input through the converter tube grid.
Sorry about forum placement.

Id use only disc ceramic for bypassing as no film cap will be very effective and OD's are the worst of the lot at RF frequenies. When that radio was built there werent any discs and the paper caps certainly were useless at the frequencies involved. Halli likely used the largest compact mica available at the time altho I do have a couple of .01's in the later dipped package.

Id start with .0047/.005 discs in RF stages and go to another in parallel if neeeded. For the IF's the .01 to .02 is fine.

Pay close attention to resistor values also.

Considering its age and technology a rebuilt version does very well. I was also given a S-36A.

Carl

I would listen to Carl because he has had much more experience with receiver designs than I have.

Dave

I agree. Ceramic disks would be the way to go for that value of mica cap.

8500 pf equals .0085 uf.

-Steve

Thanks for the replies and help.
I will get a list of the orange drops in the RF and order some ceramics and replace the orange drops. The resistors have been checked and any out of tolerance has been replaced. Thanks to the forums I have gleaned a whole bunch of tips pertaining to these old radios. I may be back for more help if the radio is being difficult to get going right.

Common values then were not the common values now. You will not find 8500pF caps. You will find 8200, but for bypass, there's a wide latitude, and .01 ceramic disks, which are cheap and plentiful, will do fine. You might want to avoid the cheapest Z5U/Y5V types and their ilk if they will see a significant fraction of rated voltage, because their capacitance will decrease. If you can find X7R's for a good price, use them instead; they are much more stable.

If you want to save yourself a bit of work, leave alone the OD's in IF and AF, and just rework the RF. (That's assuming a 455 IF; I don't know the S-36A.)

Dave Wise

S-36 IF frequency is 5.25 MHz.

--Chuck

For a basic screen or cathode bypass any .01 1000V disc will be fine; I dont believe they make them in a lower voltage anymore except for SS levels.

Carl

Yes, at 1000V rated, the applied voltage is a small fraction, and there won't be enough loss to cause trouble.

Maybe this effect isn't so severe at lower ratings.
It's possible that I have a skewed viewpoint. Modern 3kV Z5U's (Murata, CeraMite, TDK) have only 35% of their rated capacitance at 2kV. I discovered this while recapping a Tektronix 535 high-voltage supply. When Tek themselves went to ceramic in the 535A, they picked and chose carefully, or maybe the caps were better back then - one old Erie Y5V I have doesn't drop at all.

Dave Wise

How do you measure that?

Ive used 500V discs for decades and bought over stocks or auction lots of 1000 at a time for use in up to 300V circuits and nary a problem.

Considering its the ractance at the frequencies of interest that is important a bypass has an extremely wide range of tolerance while still doing an acceptable job.
An example is at 5.25mc the XC of the original 8500pf mica is 3.6 Ohms, a .01 is 3.0, a .02 is 1.5, and a .05 is .6. Any is acceptable and for manufacturing the only issue is cost.

At 140mc a .005 is .227 Ohms and a .01 is .114. At 27mc a .005 is is 1.2 and the .01 is 1.2 Ohms.
Its important at those frequencied to have minimum lead lengths and .5" or less is a good target.
Either of the above is fine on paper but Id prefer the .005 since lead lengths and self resonant frequencies come into play.

Self resonance is the reason paper and film caps are poor bypasses at VHF and even high HF since above that frequency they act less and less as a capacitor as the frequency increases. This is why the HRO-60 built with Mylar caps, which are the same as polyester OD's, is much less sensitive on 10M than the later ones using disc ceramics. Ive not had the pleasure of evaluating older production HRO-60's using paper caps since by the time I saw them in 1963 they were already massive leakers. Its the only receiver I know of that had such a long production life, 1952-68, that went thru all the phases and combinations of caps.

Carl

I have a General Radio Type 1680-A Automatic Capacitance Bridge. (Any automatic C meter will work, if both terminals have a DC return.) It has a Track Continuous mode which gives a constantly-updated reading. Across its Unknown terminals I string a series pair of the suspects, and connect a 100Meg high-voltage resistor from their junction to the -1950V test point in my Tek 453 scope. I note the reading, turn the scope on, and watch the digits fly. If you give me your email address, I'll send you a video clip tonight. I want to upload a set of them to YouTube, but I haven't had time to edit them yet.

Dave Wise

I tested some 1kV .01's last night. They're old, so like the Tek HV caps, new ones might be inferior.
Most of them lost only 10-20% going from 0V to 500V. A Sprague (circle-2) .01 P-tolerance Z5U lost 40%, but since it started out high, it ended up at 75% of rating.

The loss is a gradual process; it takes several minutes to peter out.

FWIW,
Dave Wise