sick Emerson 600

[dtvmcdonald]

My Emerson 600 (7jp1 electrostatic) lost horizontal sync suddenly.

It appears very hummy (i.e. 60Hz FM on the oscillator). Shorting the
control voltage to ground doe not kill the hum. Tubes in that circuit are good, the all voltages look OK.

If I disable the upper (on the schematic) heater chain by removing v19 and v7, the hum goes away. There is no bleeder resistor on B+

Also, there is -6 volts DC on the chassis, versus the ground bus. The resistance of this is 100K or so by paralleling a resistor to halve the voltage. This voltage in quite hummy.

Disabling all heaters makes the voltage on the chassis be B+.

So a question: the schematic shows no DC connection between the chassis and ground. Disconnecting wires to the tuner and HV cage show no shorts in those circuits. Does anyone know if this is in fact normal?

Could a heater to something in a tube cause this? I will test all the tubes.
The ones I have spares for seem OK.

Barring good suggestions it looks like the nightmare task of disconnecting
and checking every 1500pF ceramic bypass to chassis cap.

Suggestion?

[jr_tech]

Heater to cathode short in one of the tubes in the upper string?

jr

[dtvmcdonald]

Nope, not a tube or capacitor!

I decided to test every tube for shorts, so I removed all of them.

I think: "hmmm ... I did not test EVERY tube pin resistance to circuit ground" because of inaccessibility.
Now all were accessible, so I tested every pin to circuit ground. Except the heater chain, which was broken, all
measure WELL within specs, with the controls set as given in the Sam's.

Then ... hmmmm again ... I decide to test all tube pins to chassis. Chassis to
circuit B+ is 110 k when cold, chassis to circuit ground is higher so I test to B+.
Most are 110k to 135K or almost open. But one spot in the actual horizontal sync circuit is 103k.
So I check each and every spot in that circuit. One spot was 30k. It was tied to the hold pot.
The circuit in my TV is not exactly as the schematic, the wiper is not connected to one end of the pot.
As I turned the pot, at one spot the wiper to chassis (not circuit ground) resistance dropped
to 110 ohms! The problem was found.

Then I notice that the front of the pot is glued down with Quik-Stik glue.
After getting that off and the control out, yes, its a 110 Ohms short.
So I take the pot apart. There was no obvious reason for that short.
No tin whiskers, no loose junk. But it did look like maybe some track
material had migrated to the area of the short on the insulator ... the
clearance was much too close, by design. So I carefully cleaned the insulator.
In addition, the case was only attached to the mounting threads by one
of the clamps, so I removed that, leaving the case floating. Both of the dual pots are near
ground, so this is harmless (one could of course ground the case to circuit ground.)

The problem is fixed. Sorry to bother you folks! But it is a good learning experience.