Meck XB-702 restoration

[Electronic M]

It's possible the issue is a H-K short or bad H-K hum coupling in sweep or sync tube(s)...If a cathode voltage is hum modulated it would be harder to change it with B+ filtering than B+ hum.

[Kevin Kuehn]

Originally Posted by Electronic M

It's possible the issue is a H-K short or bad H-K hum coupling in sweep or sync tube(s)...If a cathode voltage is hum modulated it would be harder to change it with B+ filtering than B+ hum.

Yet another thought are the caps bypassing the filaments to chassis ground. If any are shorted or leaky they could place AC on chassis ground which could in turn back feed into other circuits through their associated bypass caps. In other words the supposedly floating chassis could be at some AC modulating potential.

[bandersen]

Thanks for the suggestions.

I've checked and rechecked the tubes for leakage. I could try swapping them all out.
This is a hot chassis set, not floating. There are a few ceramic caps along the filament string to the chassis but I'd expect they'd pop if they were leaky or shorted.

You can get the Sams for free here: https://www.earlytelevision.org/pdf/...sams_61-16.pdf

[Kevin Kuehn]

The thing is that the picture is solid with the vertical sync referenced to the AC mains as with the BK analyst, so it could be inherent to the design of the set, and likely more of a problem with half wave voltage rectification. Possibly solid state diodes make in more so because they have such a low impedance.

[Edit] I just looked at what I assume is the correct schematic in Riders 3-2. The power supply is not what I was expecting.

[bandersen]

A selenium, 6X5 and 25Z6 - what's surprising about that

[bandersen]

Here it is in all its glory.
My favorite part is pin 4 of the 25Z6. It goes to a couple filter caps that have their negative going to the hot side of the AC line.
Then the hot (negative in this case) goes along with that rectified AC to power the self-contained HV generator.

[Yamamaya42]

This is the one you said has the "weird power supply" ?:P

[dtvmcdonald]

Power supplies are not normally designed to intentionally explode caps
as this one appear to be.

[Kevin Kuehn]

It's a different configuration voltage doubler than usual but I think the caps are correct polarity for that portion. What bothers me is the 12vpp ripple that you measured on the B++. More ordinary would be .5-1vpp? It looks to me that B++ only supplies the tuner, horizontal, and vertical oscillators, so it's surprising there's that much ripple.. something does not add up. I'd be tempted to break the line were B++ exits the cathode of the 6X5 and insert a current meter.

[jr_tech]

C5 and C6 appear to be backwards

jr

[Kevin Kuehn]

Originally Posted by jr_tech

C5 and C6 appear to be backwards

jr

Ah. I was looking at Riders, which has those correct.

At any rate I think Bob found the smoking gun in an unsuspected area.

[Yamamaya42]

Originally Posted by Kevin Kuehn

Ah. I was looking at Riders, which has those correct.

At any rate I think Bob found the smoking gun in an unsuspected area.

always where you least suspect

[bandersen]

Originally Posted by Kevin Kuehn

Ah. I was looking at Riders, which has those correct.

At any rate I think Bob found the smoking gun in an unsuspected area.

Yeah, this felt like an HK short issue all along. The one tube I never thought to test was the CRT

[Yamamaya42]

Originally Posted by bandersen

Yeah, this felt like an HK short issue all along. The one tube I never thought to test was the CRT

[url=https://flic.kr/p/2n3GguK]

OUCH!

finding another 7JP4 will be impossible.

risk trying to burn it clean or, use a separate transformer for it?

[Electronic M]

Originally Posted by Yamamaya42

OUCH!

finding another 7JP4 will be impossible.

risk trying to burn it clean or, use a separate transformer for it?

Bob's in Chicago within driving distance of me in Milwaukee...And I do have a spare good 7JP4...(heck I have the three amigos of 40s CRTs 7JP4, 10BP4, and 12LP4 sitting in my spare pile.)