Tech-Master NIB TV Kit #5516

[dieseljeep]

Originally Posted by old_coot88

Compliments once again on your outstanding workmanship sir. But offhand, it seems a minimalist, low-B+ design chassis trying to drive a large CRT is not going to deliver optimum performance, particularly with only 11KV on the anode (if your HV meter is correct).

Try this for a moment.. short out the resistor feeding G2 (screen grid) of the 25CD6 to raise G2 voltage a bit. See if this improves HV, width and focus at all. If it does, it suggests that a higher B+ supply voltage on the H output/damper circuit would improve performance. If it doesn't, then maybe a smaller CRT would work better with this chassis.

All good points!
I would only halve the G2 resistor, another one in parallel.
All the low B+ larger screen sets I worked on, seemed to use a 25DN6 instead of a 25CD6. IIRC, Sylvania designed the tube for that purpose. It looks like the tube in your set is the old style 25CD6G. After the resistor trick, the first thing I would try is a 25DN6.

[old_coot88]

Actually the 25CD6 and 6CD6 are identical except for heater ratings.
http://www.datasheetarchive.com/dl/4...3319916/O/6cd6

[Crist Rigott]

Originally Posted by dieseljeep

All good points!
I would only halve the G2 resistor, another one in parallel.
All the low B+ larger screen sets I worked on, seemed to use a 25DN6 instead of a 25CD6. IIRC, Sylvania designed the tube for that purpose. It looks like the tube in your set is the old style 25CD6G. After the resistor trick, the first thing I would try is a 25DN6.

Thanks guys for the suggestions.

I tried another 470 Ohm resistor paralleled with R63 which is connected to pin 8 of the 25CD6. No joy then I added another resistor in parallel and still no joy.

The schematic does list a 25DN6 in parenthesis next to 25CD6.

[Crist Rigott]

I have the proper HV probe to go with my VTVM on order. It'll be here within the week. then I'll know for sure what the HV really is.

[old_coot88]

If nothing else pans out, I would try this as 'proof of concept' for raising B+ voltage feeding the H.sweep circuit: String a few 9V batteries in series, and put this in series with the +130V going to the plate of the damper tube.

If raising B+ improves HV, focus, and width, then consider building a small dedicated supply to put in place of the batteries.

Just a thought.

[Crist Rigott]

I thought I'd do a voltage check according to the schematic. I found some voltages out of wack!

I started with V15 Vert. Output and went down. Here's what I found. First volt listed is what the schematic says, the second is what was measured.

V15 Vert. Output -
3 Plate 125V - 119V
4 G2 130V - 129V
8 Cathode 17V - 15V

V14 Vert. Osc. & Sync Limiter
5 Plate 130V - 55V

V13 Horiz. Damper
3 Cathode 350V - HIGH VOLTAGE! Blew out my MM which was set at 1000VDC

V12 Horiz. Output
5 G1 -25V - -22V
8 G2 125V - 120V

V11 Horiz. OSC. & Phase Det.
1 G1 -46V - -41V
2 Plate 155v - 142V
4 G2 20 - -9V
6 Cathode -10V - -6V

V15 looks good
V14 Plate voltage is low
V13 Wow, blew out my MM
V12 looks good
V11 Pin 4 G2 voltage is wrong