"Damian Menscher" bravely wrote to "All" (22 Dec 04 05:15:13)
--- on the heady topic of "Tek 647 Scope CRT Circuit"

DM From: Damian Menscher

DM Long story, but I'm getting desperate for advice....

DM About 10 years ago, I picked up a Tektronix 647 scope for $100 from
DM someone at a computer show. I thought it would be a way to learn
DM about electronics. It appears to be a tube/solid state hybrid. (It
DM has 6 tubes in it -- 5 to generate the HV for the 6th, the CRT.) So
DM it takes a few minutes to warm up. Then it works wonderfully for
DM about 1/2 hour. Then it goes into this strange mode where the screen
DM blanks, flashes 5 times at four-second intervals, then it comes back
DM to normal for a minute or two before repeating the blanking/flashing.

DM Looking in the manual, I discovered this has a "protection circuit"
DM that turns off the oscillator if the -2.2kV supply is overloaded.
DM This is accomplished using a "multivibrator" -- a matched pair of
DM transistors, one if which is normally off, the other normally on.
DM The behavior of blanking the screen and flashing after four seconds
DM is consistent with proper operation of this protection circuit being
DM activated. The problem is, I don't know what's activating it.

DM I replaced one of the tubes (5642) which was not lighting with the
DM others, since that seemed an obvious problem. When that didn't help,
DM I contacted a former Tek employee, who sold me a replacement for the
DM transformer in the HV circuit. (He said sometimes those fail and
DM cause this problem.) Next, I tried swapping the two transistors in
DM the protection circuit, thinking that maybe the "matched" set wasn't
DM so perfectly matched. Putting them in each others' sockets didn't
DM help, though.

DM The only remaining idea is to replace the rectifier tubes in the
DM voltage tripler circuit with solid-state diodes, in hopes that the
DM improved efficiency will somehow reduce the load on the circuit. But
DM I'd really rather not do that, since I somewhat like the idea of
DM "restoring" this, not just "fixing" it.

DM Another desperate attempt might be to simply disable the protection
DM circuit. It's entirely possible that there's no problem with the
DM scope, but only with the protection circuit. If something blows,
DM then I'll at least have something to fix. If nothing blows, then I
DM can refocus my effort on studying the protection circuit.

DM If anyone has any advice on what to try, it would be greatly
DM appreciated. I've had a lot of fun with this thing, but failing to
DM repair it for 10 years is starting to become a bit frustrating. I
DM can scan the schematics of the CRT circuit if it would help (assuming
DM 1963 copyright date + 28 years has passed without renewal).


Damian,

Increasing the efficiency of the rectifiers is apt to make the problem
worse. You really must trouble-shoot the input to the protection
circuit. Your multivibrator configuration is perhaps setup as a
Schmitt and is sensitive to a voltage trigger level. If every resistor
and capacitor checks okay in there (especially the sensing input)
then and only then should you look elsewhere.

Something is getting hot and causing the problem, as you stated this
happens after about 1/2 hour. May I suggest investing in a bottle of
freeze spray and patiently freezing a small area of circuit at a time?
Maybe it's a "resistor going open" from the normal circuit heating? If
you can't find anything the last resort will be to replace *every*
resistor in the protection circuit. It's stupid but sometimes a
shotgun approach saves your remaining grey hairs.

A*s*i*m*o*v

.... Dunno if we'll get that past the CSA und UL 'owever.