Rca 9-t-246

[yagosaga]

Hello,

today I have fired up the RCA 9-T-246 for which I asked for the schematics. Steve Kissinger sent me a copy, thank you very much.

After cleaning the set, it was full of asbestos, I reformed the electrolytics and checked the chassis and the tubes for faults. Nothing significant was visible.

When I switched it on with a variac, I could hear no wistle from the line output unit. I checked input signal of the line output tube 6BG6 at the junction R183 and C160 with the scope. The sawtooth should be 40 Vpp, but I only get 30 Vpp. At the grid, there should be -110V, the reading was -100V.

With signal and a readjustment of the ion trap, I got the first light on the screen (see photo). Like I measured before, horizontal width is too small. At pin 5 (6SN7-GT) of the line oscillator, there should be 54V, the reading was 38V. All voltages in the oscillator unit seem too low. Does somebody has a suggestion?

- Eckhard

[7"estatdef]

Check your B+ at the 5U4 see if that is low. Perhaps your caps arn't reforming as well as they should. Check the value of the resistors feed the osc stages. Check around the damper for low boost voltage. Try replacing the 5U4 and the damper tube .
Good luck
Terry

[Phil Nelson]

That is very encouraging progress for an unrestored TV of that age.

I have had poor success with reforming old electrolytics. Some will reform, but in my experience, they will either perform badly or fail before long.

I would replace the electrolytics to prevent future damage. If you are concerned about authentic appearance, of course you can restuff the original cans with new capacitors.

Until I have replaced electrolytics and performed voltage checks around the set, I don't try to diagnose more specific problems. There are too many possible causes -- including bad paper capacitors, dirty/corroded controls, and so on.

Just my opinion :-)

Phil Nelson

[yagosaga]

Hi,

the power supply with the 5U4 is working properly. All the B+ voltages are in correct values. Even with 50 Hz, there is only a little hum in it, less than I have expected. I have replaced the booster capacitor, but without success. The difference of the cathode voltage related to the grid voltage of the 6BG6 is only 8v. It should be 20v.

In the attachment you find the schematics with the recent readings and a screenshot. Any suggestions?

- Eckhard

[yagosaga]

Meanwhile I have got full width with two changes:

- added a capacitor 20 nF parallel to C159
- changed R184 to 150 Ohms.

But this is not the final decision. I assume that the 6BG6-G, which I tested well, does not provide enough current. I will try to get a spare tube.

Here is a screenshot. Note that the screen color is blue due to the indoor white balance of the camera.

- Eckhard

[wa2ise]

If you haven't already, I'd recap all the wax capacitors. Do several as a batch (each cap one at a time, remove and immeduately replace so you don't forget where it goes) and turn the set on to confirm it still works right. Looks like all the hard to find parts, lyback, yoke, CRT, and such are good.

[Sandy G]

I'm amazed it made it across the big pond intact...Nice score Eckhard !