Rca ctc-5

[old_coot88]

Originally Posted by DaveWM

the ring is still there, slight but still there (its a ring not a ghost as you can see multiple diminishing rings. Its tunable so I am pretty sure its in the IF.

For the heck of it, try subbing the 3rd IF again (i know you subbed it before and the main culprit was the 1st IF). I've had a number of cases of ringing being caused by the 3rd IF tube.

[DaveWM]

I will try a few more OC and report back later.

[DaveWM]

did not get to more tube swapping but did notice some odd voltage on the video amp (6aw8). the screen was way high, like 210 vs 180 and the plate was low 110 vs 150. I checked the voltage divider on that tube and found the 15k was 11k and the 56k to ground was 65k. Not a lot but in those direction it would add up to too much screen voltage. I went ahead and replace those two ( had some old school 2watt carbon comps on hand) and check it again, much better screen at 178 and plate at 138. Seemed to help the video out, brighter and the contrast control is not as sensitive anymore, but still has a good range.

this all stemmed from what I thought was a bit dim of a pic, I started with checking out the video out, then moved to the video amp. Interesting effect of the change was before the fix I thought I had a slight tracking issue, as increased the contrast, the green highlights seemed to worsen (was checking by watching a B&W program. I just assumed it was a CRT issue. After correcting the video amp tube (and the contrast seems to work "better") I don't notice the green highlights happening. I spent a LOT of time trying to setup the the CRT drive pots to get rid of problem before. Perhaps it just not as much drive at the CRT cathodes, but it def is better.



Its fun to check things out, make a few changes check again, to slowly see improvements. I think it may add to overall trouble shooting, by being able to see direct cause effect. Not perfect, and certainly no substitute for good trouble shooting technique, but fun non the less.

[DaveWM]

more intresting tidbits, looking over the schematic, I can see the brightness control gets it neg voltage for low brightness from the horz out grid (filtered thru a .22 cap and 470j resistor. I am pretty sure I increased this neg voltage from the -40 to something less (more neg) when I was changing the plate load resistor in the horz osc in a attempt to increase drive to the horz out. Later CTC get this neg voltage from the grid of the blanker tube. I think I will check the later CTC-7 and see if it makes any changes for the great neg voltage at the horz out tube shown on that schematic (and if there is any changes in the circuit that taps this to supply the brightness control with its neg voltage).

[DaveWM]

tried a few more 3rd if tubes, found one that got rid of the rings. I think any more improvement will have to come from a full alignment.

[DaveWM]

while checking on brightness level I checked the cathode and grid voltages at the CRT. I don't have my VTVM handy, but the green grid was maxed out at 240v while the schematic has it a 255, the cathode voltages were much closer.
I don't know if the load of the DMM is more than a VTVM, but I may just check that 1 meg resistor that feeds the B+ to bias the grid thru the background pots. I would think that I should be able to hit the 255 at something less than max setting, as opposed to the max of 240v. the other drive controls seem to have no problem getting the the correct bias setting. While I am able to get good gray scale, its at a reduced brightness. Later I plan to install my CRT test socket so I can see where the G2 voltages are (they are not easy to get at like the cathode and G1). I am pretty sure I should get a brighter pic, the CRT test fine, and the HV is solid at 22.5 kv, I know the early CRTs were not as bright as later but I would at least like to see how it looks when the voltages are closer to what is in the schematic.

[DaveWM]

chasing low brightness down the CRT bias voltages turned up some interesting notes. The blue background when maxed out would only bring up the G1 to 240v the schematic calls for 255v and it makes a big difference here. So I started by checking the resistance of the 1 meg resistors that couple the pot adjusted boost voltage to the G1s. I checked the blue G1 1 meg out the circuit and it tested at 6 megs. Replaced that one. The other two G1s had no problem getting two the specified voltage so I will leave them be for now (can't test in circuit and I don't want to replace something that is working, even if it has drifted as the pots can make up the difference).

While I was at it I check the 10k resistor that provides the boost to the pots, it was 16k. I have noticed that when it comes to voltage dividers you need to be pretty close to the correct values to have the correct ratios to divide the voltage by. Since I was low on G1 voltage and a drifted high resistor would make that worse I went ahead and replaced it as well.