This RCA is driving me nuts! - Videokarma.org TV - Video

Big Dave · #1 05-05-2005, 06:33 PM

I recapped my 8" RCA portable (8PT7010 series). I have major problems in the vertical section. With the 6CM7 in circuit, I get the following:

Pin 1 110 volts where the print calls for 440

Pin 6 Nothing, where the print calls for 68

Pin 7 About -1 volt where the print calls for -29.5

Pin 8 0, where the print calls for -19 volts.

I have rechecked whe recap and found nothing miswired as of this writing. When I enable the high voltage, R 165 (3900) gets hot. This does not happen when the HV is disabled. When I remove the 6CM7, pin 1 shows 440, but as soon as the tube warms up, it drops to about 110. The part numbers are based on the RCA manual. Any help is appreciated. This is driving me nuts.

Chad Hauris · #2 05-05-2005, 07:49 PM

I am looking at the sams photofact for that model...it is based on a slightly different model (8-PT-7030) but that is where the sams index directed me. The vertical output stage gets its power from boost voltage...when you say disable the hv do you mean remove the damper or HO tube...if so then the vertical stage is running on B+ alone as boost is not being developed.

Sounds like the stage is not oscillating and is drawing too much current and dragging down the supply voltage as no bias is being developed...you might try feeding 60 hz ac filament voltage into the grids of the tube through a .047 600 volt capacitor and see if you get any vertical deflection or negative bias voltage on the stages...may need to check the feedback loop between the discharge and output sides of the oscillator.

Big Dave · #3 05-05-2005, 10:04 PM

Can you upload the SAMS? i have the RCA schematic and it's very hard to read. I am convinced I made a mistake when recapping, since I had a picture on it before. I'm starting to think I miswired something when I rebuilt the power supply. I did find a few errors after recapping and corrected them. I pulled the top cap off the HO so I would not get blasted. The HO and damper are in place. I've walked away from it for now. Any help is greatly appreciated.

Chad Hauris · #4 05-06-2005, 06:39 AM

The sams does not use a 6CM7 but calls for a 6CG7 for vert. osc/output..It may be a little different but I can probably scan it tomorrow.

Also...if the HO tube plate is not hooked up you won't get enough boost voltage to really do the job at the vert stage. You will get b+ flowing through the damper tube and the flyback but if the horizontal output is not working you won't have the AC oscillation in the flyback that gets rectified by the damper and added to the B+ to develop the boost.

***Eric H*** · #5 05-06-2005, 10:49 AM

Phils Old Radios has an article on this set. He had a similar problem with the vertical and it had something to do with the 6CM7/6CG7 swap

http://antiqueradio.org/rca16.htm

Big Dave · #6 05-06-2005, 06:32 PM

I remember Phil's article. The SAMS is wrong. A 6CG7 will not work. I left the top cap off the HO so i wouldn't get blasted. I can yank the 1V2 (ding, ding, ding). I am now convined I miswired something else, like the power supply. Since i have a side job this weekend, I'm giving it a rest for a few days. A new perspective will help. I will not let this RCA beat me.

Big Dave · #7 05-07-2005, 07:52 PM

I've decided to take a break from this one. I will eventually get it going. My thanks to Eric for uploading the print. Just so everyone knows, it wasn't the quality of the print or the scan that made it hard to read, it was my printer. Next set, My RCA 21 incher with a 21AP4.