[NJARC] Detailed notes on Telefunken Opus 7

[amagoun]

This just in from Kent Ringo, who hopes someone has 0.8 amp fuses.
Please email him directly at karingo at juno.com.

I've moved Rick and the Westinghouse to 11 as requested.
Alex

NOTES ON TELEFUNKEN “OPUS 7” RADIO

The set was brought to me by Tom and “TF” McCallum, several months ago
with the comment that it hasn’t worked for a long time, but that when
last tried, the dial lights illuminated and the set gave forth a little
“scratching” noise.  As I recall, when I connected power to the set, the
tube filaments glowed, although I don’t recall hearing any sound.

I put the set aside and didn’t work on it until late November, 2005. At
that time, I didn’t see any visible glow of the tube filaments, nor hear
any noticeable sound.  The schematic diagram, which was stored inside
the cabinet, shows an 0.8 amp power line fuse, which was open.  Upon
replacing it with a 0.5 amp fuse and turning power on, that fuse blew
immediately, so I assumed that there must at least be a power supply
problem.

Following are notes from recent detailed checks, which have not yet
isolated a readily fixable fault:

Search for a short-circuit on the output of the power supply: I had some
difficulty tracing the wiring but focused first upon what appears to be
a multi-section (black-colored) electrolytic capacitor mounted near the
power transformer. It has four wires: Resistance to ground on each of
these leads measured:
    Red: About 10K ohms
    Yellow: 3 K
   Yellow with black tracer: 3 K (I added the black tracer to avoid
ambiguity between the two yellow wires).
    Green: Zero.  I then cut the green wire and re-measured it.
Resistance to ground is now near infinity
    (after absorbing charging surge from the ohmmeter battery). Green is
probably a common ground lead ground lead for three capacitors within
the same can).
     Conclusion: Probably no short in this multi-section capacitor.

 Next; I noticed a two-section capacitor adjacent to the filter choke.
Both sections of it appear to be OK.

 Problem: I haven’t been able to locate the bridge rectifier of this
power supply, and the wiring does not lend to easy isolation of other
possible short-circuits.

 Back to basics: Check from the input side of the power supply: An
ohmmeter check showed no continuity from the AC power plug through the
fuse! Cleaning the fuse holder with super-fine sandpaper didn’t seem to
help – until I used WD-40 followed by a spray rinse with brake cleaner,
which restored continuity through the fuse holder. Then, further
continuity check indicated an open power switch.  (That switch is all
but inaccessible), so I also sprayed the whole AC power switch assembly
with WD-40 followed by a rinse with brake cleaner. This evidently fixed
the switch.  (It may, like the fuse holder, have become corroded).

 At this point, I installed a 1.6 amp switch (I couldn’t find 0.8 amp
fuses) and applied power. The dial lights lighted and, after perhaps
thirty seconds, I saw and smelled smoke at the power transformer.  By
the time I turned the power off, the transformer core was noticeably
hot.

 At this point, I don’t want to amplify any damage to the transformer. .
I would like to disconnect the bridge rectifier to isolate the load from
the power transformer, but since I haven’t located the bridge rectifier,
I don’t know how to disconnect it.

 The only thing that I have done, since the above, is to power the set
from a 6.3 volt transformer, as a possible way to check the power supply
under a much less stressful environment.  I tried that and determined
that the 6.3 volt source is providing power through the line fuse and
line switch. I had expected to see a much reduced DC voltage on the
output of the filter choke but, in fact, measured no DC voltage at the
filter choke.

 At this point, I’m going to reassemble the set and hope that the people
at the RCA Antique Radio Club can further isolate and possibly fix the
problem.

 K.A.R.  1-26-06
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