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Author Topic: R390A detector distortion  (Read 2654 times)

Posts: 5

« on: September 03, 2002, 10:24:54 PM »

All: I have a Collins / Motorola R390A on the bench.  It seems to work OK, but the audio is distorted.  I isolated the various distortion products to two areas:

a. The first audio tube appears to be a little soft and prone to clip in one direction.

b. The detector doesn't reproduce in a linear fashion.

I figure I can correct the first audio tube, but I have looked at the detector and it appears to be operating as the manufacturer intended.  There is no fixed bias on the detector tube.  As a result detector bias seems to be a function of input signal.  This means that rather than recovering a good sinewave from a "clean" input signal, one peak winds up being pointed rather than rounded.  

The only reason I can come up with for this behavior is a dried out capacitor in the detector circuit, but since it's a 150 pf capacitor, it's probably a mica, and those don't generally dry out.

Any thoughts on all this?  The R-390A is a classy receiver (particularly with my Valiant II) but I'd really like to clean up the audio just a bit.

Dave Maples, WB4FUR

Posts: 3160

« Reply #1 on: September 07, 2002, 08:19:08 AM »

Dave -

Chuck Rippel's Problem Capacitors

Chuck's notes from your problems:
"Failure was an R390A which had low audio output until the limiter control was turned on."

The failure cause was that C-549, a 300V Sprague "Vitamin Q" had turned leaky about 2 Megohms.  This leakage was swinging the bias on the first audio amplifier, V-601A high causing the degreased audio output.  Because it is passing audio frequencies, I replaced the faulty capacitor with a 600V, .01ufd polypropylene media Orangedrop.

C-549 is easy to find, it's located on a small vertical board on an internal IF shielding partition near the the tube socket of V-507.   It has about 80V on one side and might not be a bad candidate to always replace along with C-553.

I have seen many R390A's which, for some reason, have varying degrees of audio output which I had chalked up to aging.  It may be that this cap is a habitual leaker and had biased off the audio amplifier as above in small amounts.

Chuck Rippel

One cause of intermittent snapping and popping heard in the audio can be traced to leaking filter capacitors C603 and C606.  These are multi-section electrolytics of 3 X 30UFD at 300V and 2 X 45UFD at 300V.  Also, the as there are sections of the capacitor dedicated to the audio section, audio quality can also be improved by replacing them.

Remember, due to their using a acidic electrolyte, electrolytic capacitors fail with time rather than use.  They can be reliably rebuilt and repotted using new internal components.

Here are some notes from the IF and Audio module notes of Nolan Lee (this was a "excellent quality" 1967 unit)

.. bottom line .. replace the capacitors & measure the micas & resistors.

IF Module:
Tested the tubes and the 3TF7.
Measured the resistance of the wiring, the transformers, the switch contacts, and the resistors. I replaced more than a half a dozen resistors that were out of spec.
Checked the capacitors and resistors inside the IF transformer cans, that could be tested. Some could not be tested in circuit.
I tested the big above chassis oil filled capacitor for leakage and value.
Tested all of the mica capacitors for leakage and value and then ripped all 18 or so of the axial lead paper capacitors out of the module and tested them just for kicks. EVERY "brown beauty of death" tubular capacitor that was in it leaked like hell and a good percentage had microscopic cracks in the bodies within maybe .020 of the seams and paralleling them. Most of these caps leaked at voltages below 50 volts when tested. Only one of the metal can axial capacitors leaked when tested. I replaced all 18 of the capacitors with Orange Drops. For the .1 and the .033 values I used 400VDC rated ones and for the .01 values, I used 600VDC rated ones. The reason that I didn't use 6600V rated ones thru out was their size. It was a pain in the ass to the fit the ones that I used in there properly. If I'd have used the 600V ones everywhere, I'd have had to move the locations of some of the capacitors and a bunch of them would have had excessively long leads. I didn't think that this was such a hot idea in the IF section and figured that the best placement of the parts was in the original locations. ;-)
Let's see, other than checking all of the screws and nuts, I think that was it for the IF deck other than lightly lubricating the shaft extensions where they passed thru the front of the IF module chassis.
I didn't test the mechanical filters.
I tested the blocking cap before I tossed it and it had tested good even at 100 volts over it's 300V rating

Audio module:
One of the original plug in electrolytic caps showed signs of leaking, tossed it in the trash, installed another one from spares. Reformed both, leakage at 50 volts over the rated voltage was less than 1 ma. per section after reforming. I fused them and ran them for a couple of weeks at full rated voltage on one of the HV supplies, they didn't explode and leakage declined even further. Good enough, bubba! (yeah, I know, Doc, but the power factors were good, I even checked that. :-)
Ripped all of the paper capacitors out of the module, and tossed them in the trash. Installed two new .022 400V orange drops in the location that Chuck likes, and NOS Vitamin Q's in the other locations. I have the orange drops on hand and could have used them thru out but didn't like the way they sit on the circuit board. I did use an Orange Drops to replace the one on the chassis under the circuit board. All of the new caps were tested for leakage at their rated voltage and tested to verify their value before installing.
Checked all of the resistors for value, replaced a couple. Tested the mica cap, no problem there.

Tested all of the tubes, they all passed but tossed the 0A2 and stuffed a new 6626 in it's place. I don't trust used 0A2's, had some weird problems with them.
Tested the relay and measured the resistance of the wiring, the chokes, and the transformers. I left the 800 cps filter alone. Probably not a whole hell of a lot of R390A's out there that still have their original 6AK6's. All of the tubes are original except the rectifiers and the regulator. :-)

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