ProbCaps

Problem R390A Components

IF Deck

I am often asked if there is a "Laundry List" of problem capacitors that should be replaced in the R390A. Until recently, the answer has been limited to C-553, the capacitor which blocks B+ from the mechanical filters plus "any others which test defective." There are now several which I pre-emptively replace because they consistantly turn up as defective across the many receiver restorations I have performed. While most are in the IF deck, the RF deck and Audio Deck also have a few.

To the right, you see 3 capacitors which have proven to be a problem and were replaced with Orangedrop types. C-531, C-547 and C-549. These have all been the cause of audio and Limiter function problems.

C-553 is the plate blocking capacitor for V-501. It is rated .01ufd at only 300VDC. Failure of C-553 will cause catastrophic failure of the 4 Collins mechanical filters as B+ is applied to them then shorts to ground until the filter burns open internally. I routinely replace this capacitor with a .01ufd, 600V Orangedrop type. Notice how the new orange colored capacitor is dressed into the corner of the filter compartment in the bottom of the IF deck. Because of the voltages involved, the leads are also covered with teflon "spaghetti." The plate choke, L-505 can bee seen directly to the left of the "new" C-553.

Audio/Power Supply Deck

A common failure that is often overlooked involves the 2 multi-section, electrolytic capacitors mounted on the AF deck. No matter what the age
or condition of the receiver involved, these capacitors are probably electrically leaking causing undue stress on the power supply and, as they also serve as cathode bypasses in the audio section, some measure of audio distortion. In the worst cases, as in the picture at the right, acid actually seperates from the from the electrolyte and begins leaking out the bottom of the capacitor.

While electrolyte leakage in itself is not indicative of a filter capacitor failure, it does indicate the capacitor is at the end of its service life and must be must be changed or rebuilt. Click here for information about having the electrolytic filter capacitors rebuilt.

Another high-failure capacitor is C-609, also located on AF chassis about midway on the main (and only) circuit board. C-609 is an 8ufd electrolytic and serves as the cathode bypass capacitor for V-601A. Again, the acid leaches from the electrolyte and will destroy the capacitor sometimes actually only the 2 soldered in leads.

I replace this component with a 8ufd 35V available from various commercial parts houses. However, a 10ufd, 35V replacement is available at your local Radio Shack.

Note in the picture that I have placed a "+" on the end of the board. Electrolytic caps are usually polarized and you should make sure to orient the replacement capacitor with the positive end as shown.

RF Deck

There are several concerns in the RF deck. By far, the most common, re-occuring one involves the receiver not working well or perhaps not at all on the lower bands. Checking the output of the first crystal oscillator circuit of V-207 at J-221 reveals a peak to peak output of perhaps a few hundred milivolts instead of the 3+ volts. Tuning the transformer, T-207 will not correct the condition.

This is caused by C-327, a 100pf mica capacitor failing. It is across the primary of T-207 and is accessed from the underside of the RF deck as below. It should be replaced by a 100pf, 1KV ceramic disk capacitor and the circuit re-aligned.

Page prepared by R. Charles Rippel

E-mail to: Charles Rippel
URL: http://www.R390A.com
Revision: 03 March 18, 2001
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