No sync on RCA KCA118A 17" B&W portable. - Videokarma.org TV - Video

farmradio · #1 09-27-2016, 09:18 AM

Hello folks. Got a 1950's RCA B&W Model 170P034. Using KCS118A chassis. It plays well, except there is NO sync. The voltage on the screen & plate of the 6AW8 (Video output, & sync sep. are off a bit plate is too high, & screen is too low. I have a scope pic. of the 6AW8's video output plate. It does not look good to me. Swapped out the 6CU8 sync amp., & the Vid. out, but no change. The pulses on the plate of the 6AW8 do not look good either. So I'm wondering if those couplates are known in these RCA's like Philco to be a problem. Evey resistor tested is in range. Replaced some caps in the area, again with no change. Don't want to over-troubleshoot, but it's something strange. No sync pulses for both sweep oscillators. Just freewheeling along. Anything come to mind on the KCS118? Thank you.
Cathode reads 4.4V (according to SAMS should be 1.5), 4.6V on the control (SAMS shows .8V) Plate reads 175V, (should be 80). Screen reads 80, (should be 180).

DaveWM · #2 09-27-2016, 10:09 AM

how do the voltages look on the sync sep, triode. Those are critical. If that grid is high at all, the tube will conduct video and not just the peaks (sync) which is the sync.

could also be an AGC issue, try clamping the AGC with a variable DC supply.

DaveWM · #3 09-27-2016, 10:18 AM

looked a little closer did not see an AGC circuit. those copulate almost look like they are some kind of AGC (feeding the output from the video detector back the front end of the IF), and the scope reading from at the grid of the sync sep shows a pretty video free signal already. Maybe some kind of cancelling being done from the cathode of the video out.. I am a little lost, not seen that before.

DaveWM · #4 09-27-2016, 10:22 AM

if you have any positive voltage at the grid of the sync sep, check that cap between test point 15 and 27, I cant read the number but it block the DC part of the video signal.

farmradio · #5 09-27-2016, 12:42 PM

Thank you Dave, believe I did change that cap. between the test points with no change, but will double check. Will report back the sync triode portions grid reading, but I think it was high. Everytime I've hit a wall, (with Philcos anyway), it ends up couplets! Wonder?

DaveWM · #6 09-27-2016, 01:34 PM

if its high, that will be the problem. there is a resistor to ground after that cap, cant make it out, looks like high value, but make sure it has not opened.

farmradio · #7 09-27-2016, 02:52 PM

I'll double check, I know I've checked all resistors in circuit, wonder if a leg needs to be lifted for a more true result. I know a bad one can usually be found in circuit also by the voltage on either side too. Well not this one, because it goes to ground. I don't believe the resistors across the coils can be checked accurately either, they just read low due to the coil. Maybe lift one side or, start jumping some values across them. Got to look at the underside of the PC board too, I suppose. That is in a tough place, but must be done too to get a good look probably. The ETF has a good copy of the SAMS on-line.
http://www.earlytelevision.org/pdf/R...Sams-418-1.pdf

farmradio · #8 09-27-2016, 09:46 PM

The resistor to ground is ok, & resoldered the ground as a safe measure. The sync sep. control grid is -11 varies down to -6. the plate is 73V. pic. of triode control grid.

DaveWM · #9 09-28-2016, 08:23 AM

Try feeding a composite signal into the grid of the video amp.

farmradio · #10 09-28-2016, 09:22 AM

Called Moyer, they stocked a boatload of these back in the day by Sprague, Erie, Centralab, & RCA originals. I'm suspect of the sync one, no so much the AGC one, but they are looking for both & will get back to me in the coming days.

dieseljeep · #11 09-28-2016, 09:30 AM

Quote:

Originally Posted by farmradio

Called Moyer, they stocked a boatload of these back in the day by Sprague, Erie, Centralab, & RCA originals. I'm suspect of the sync one, no so much the AGC one, but they are looking for both & will get back to me in the coming days.

If NLA, you can easily sub them with descrete components, as all the values are listed.

Findm-Keepm · #12 09-28-2016, 09:31 AM

FYI, my August 1974 RCA price guide does not show any of the couplates listed in the Sams.

farmradio · #13 09-28-2016, 06:49 PM

Will try a composite signal.

dieseljeep · #14 09-29-2016, 10:22 AM

Quote:

Originally Posted by Findm-Keepm

FYI, my August 1974 RCA price guide does not show any of the couplates listed in the Sams.

I used to be a hastle looking up part numbers for RCA and GE, as they used drawing numbers on the part. You had to go to the distributor with the model or chassis number and look up the part in the factory manual or even the Sams. Zenith, Motorola and most others used the original part number, so it was a bit easier.
Back in 1974, those parts were probably special order.

farmradio · #15 09-29-2016, 02:07 PM

It also appears that K1, & K3's description is reversed in the SAMS, because the one labeled K3 (sync coupling) has no pF capacitors, & that K1 (agc), with all resistors is not used in the 118A, & verified that by not being able to physically locate it on the board. So this has K3, (sync coupling), near the sync separator, labeled K1 in the SAMS, which in fact is most likely K3 2 pF capacitors, & 3 resistors. Help! I need an RCA original service document on this. This is nuts. After that Sentinel nightmare I just did, I've got discrepancies again in SAMS.