| I purchased this 1947 RCA 721TS from a collector in
Canada. The set was originally bought by a tavern owner who used the
set in his establishment. |
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Several decades of use in a bar had taken
quite a toll on the set, coating it inside and out with residue from
cigar, cigarette and pipe smoke. The first step was to take it apart
and clean it. |
| Inside, it looked much worse than outside.
Electrostatic forces from the high internal voltages grabbed the dust and
formed a thick coating on all exposed surfaces. |
 |
| Under the dust, the chassis is in reasonably good shape for
being 55 years old. Electrolytic capacitors are re-formed. All
except one re-formed successfully with less than
I=Ö(CV)/5
(one-fifth square root of rated capacitance
times rated voltage gives the maximum permissible leakage current for that
value of capacitor in microamperes)
|
Voltage and waveform tests show several problems in the signal path.
I typically replace only parts that test as BAD when the majority of parts
test OK. On this set, almost every paper capacitor checked as BAD, so
I'll replace
all of the paper capacitors, one of the electrolytics and a few
tubes. The picture tube is also weak, so an excellent pre-tested
USED type 10BP4 picture tube (bought on eBay) was installed in this set as
a replacement.
|
|
Below is the pile of old paper capacitors that I replaced.
|
Here's a view of the underside of the
chassis, showing the new Orange Drop capacitors.
|
| RF/IF Alignment is next. Nice response
curve.
|
Here's a close-up of the sweep.
|
| Here's the video signal on the scope. I really
need to buy a scope camera or one of those new digitizing scopes. It
will have to be Tektronix, though.
Tektronix scopes are still made in Oregon even in 2002 when most other
brands are made in Japan, Korea or China. Tektronix is the benchmark by
which all other scopes are judged. My 43-yesr-old Tektronix Type 547
(built in 1959) still beats any
brand-new off-brand scope in performance and reliability.
|
The only problems I've EVER had with any of the Tektronix instruments
I've owned (a Type 531 10-MHz scope, this Type 547 50-MHz scope and my
Type 575 Transistor Curve Tracer) were broken tubes from when people who
helped me move into my new home banged them around. When I perform
my annual check-up and calibration, they seldom need much adjustment to
get back to the manual's specifications (judged by how far I need to turn
the internal calibration controls to get them re-calibrated). That kind of reliability
impresses me.
ONLY IN AMERICA!!!
|
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Assembled, aligned and ready for display. It leaves my
workbench with a picture comparable to when it left the factory 55 years
ago. |
