Heathkit SB-610 Problem SOLVED...

[Eddy Swynar]

Seasons Greetings Fellow Heathniks...

Last night I ran into a situation with my beloved SB-610 that appeared quite
vexing initially,but which ultimately was solved. I'm sharing it with you to
file away mentally for possible future consideration "...just in case".

Delete this message if you're already a 'scope pro! Here goes:

PROBLEM---

The trace on my SB-610 suddenly appeared as a diffused fuzzy ball (about the
diameter of lead pencil) in the trapezoid position. Likewise, the normally
sharp line with the scope in the envelope pattern position was dim, wide,
and fuzzy. When the sweep speed was cranked low,  instead of a normally
semi-rippling line, I could only see 4 or 5 1/4" tall vertical fuzzy lines
evenly spaced across the screen.

I could also just detect a trace of an acrid odour descending the stairs
(the rig was warming up downstairs), as if something had "smoked".

TROUBLE-SHOOTING---

Read the manual BEFORE you do anything! The exact first step in the
troubleshooting guide was the key: in my instance, the resistance was only
several thousands of ohms, a far cry from 5 megohms!

I de-solded the transformer's blue & yellow wires from their respective
moorings on the tie strips, & measured the a.c. voltage from them to confirm
that the "iron" hadn't expired...WHEW! It was well in excess of 600 vac.
What a relief THAT was!

REPAIRING THE THING---

The output voltage at the juncture of R605 & C606 was less than 300 volts...

Time to replace the rectifiers, I thought. I've learned the hard way that it
pays to check the continuity of solid state rectifiers both ways BEFORE
soldering them into position: I do this with the tone continuity position of
my Radio shack FETVOM. Anyway, diodes D5 & D6 are SILENIUM units, & I
automatically replaced them with back-to-back pairs of modern
run-of-the-mill I kv/1 ampere silicon units.

The fuzzy pattern remained.

I next noted that while resistor R605 was SUPPOSED to be a bright banded
orange-orange-yellow affair, it appeared almost all black/brown. EUREKA! l
removed the offensive part, & noted that it was, indeed, semi-charred, &
displayed a resistance of several megohms. A new replacement was soldered in
its place, & the power activated...

After about 30 seconds, smoke poured off the body of the poor thing, & R605
self-destructed yet again. What next?

I replaced R605 a second time, AND snipped off the lead from C606 "just in
case"---I fired-up the scope, & a beautifully clear pattern returned to the
screen! It was pin-point sharp in the trapezoid pattern, too. And best of
all, the resistor survived a 30 minute survival test unscathed.

FINAL SOLUTION---

My C606 displayed a RESISTANCE of several thousand ohms. I replaced it with
a junk box 0.1 ufd. 1600 vdc capacitor on-hand, & all is well again...in
fact, the trace is brighter & sharper than it's ever been!

Should anything similar crop up in the future, my suspicions will fall upon
that bank of similar black capacitors alongside it in the SB-610. Perhaps
just as in receivers, capacitors in scopes should be replaced every twenty
years or so, too! Hi Hi.

Anyway, I'm glad it's back on line & working fine. The experience also
taught me that it's time to replace all of the tubes in my '610---with one
exception, they all tested in the "red" zone of my Stark tube tester! It's
amazing the scope works as well as it does, testament to the longevity of
"no frills" engineering, I guess!

~73~ Eddy VE3CUI - VE3XZ

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