Another RCA CT-100 discovery B8001599

[dtvmcdonald]

Mouser part numbers 594-S821K29X7RN63L6R or
75-562R10TST82 They have 19,047 of the top one in stock!

If you want NP0 there is 81-RDE5C3A821J2K1H3B but they are blue.

[Yamamaya42]

Originally Posted by Penthode

Can you show me the link? I looked and the 820pf ceramics bore no resemblance to the original. I have previously used Newark, Digikey and Mouser for years.

The NOS I bought from eBay looks a very close match.

I tend not to trust most capacitors, resistors, found on eBay anymore, and especially transistors
just too many fake parts.

[Penthode]

Originally Posted by Yamamaya42

I tend not to trust most capacitors, resistors, found on eBay anymore, and especially transistors
just too many fake parts.

I agree wrt ebay and transistors. I have been burnt a few times. However if the seller is in the US with what appears to be NOS vintage resistors and ceramic capacitors I have had no problems. When wanting to keep the original appearance I have patently searched for just the right part.

It must be just me. I even baulk at newer metal film resistors which do not match the older carbon composition resistors in older vintage equipment. For 1930's sets, I have a store of older dog-bone style resistors.

I used to restuff paper capacitor but do not bother anymore and certainly the gleam of an epoxy dipped capacitor does not look too out of place. I will use modern film capacitors of a higher breakdown voltage in order to ensure the component is of a physical size roughly equivalent or only slightly smaller than the original.I will always restuff electrolytics though.

Which reminds me, I have a question on capacitors used in 50's set vertical sweep circuits minus the blocking oscillator transformer. I have found that it appears the capacitors are critical in that proper capacitor characteristic choice is essential for maintaining good sweep linearity during warmup over a half hour or so. Has anyone else had this problem?

[Penthode]

Been cleaning the considerable gunge off the chassis this week. Removed the high voltage compartment and the selenium rectifier frame to give it a thorough clean. Started replacing paper capacitors in the HV cage and now that it is clean have given it a thorough inspection.

Whilst making checks around the selenium rectifier, found a fuse blown and a jumper wire which had been placed across it. Not good. Then the resistive ballast if that is what it is called I examined and found part of it open. Closer examination revealed it appears to be simply a voltage dropped with an 800 ohm and a 300 ohm resistances to each drop the 400v DC supply to 285v. 115v across the 300 ohms dissipates 41 watts and 800 ohms 16 watts. The 300 ohm section is the one open.

It looks to me a bit of a bad design wasting 56 watts like this. That is a good chunk of the total 475 watts the setg consumes. I guess power was cheap in those days!

Anyhow think what can I do to salvage this? I found a 50 watt 330 ohm and 25 watt 820 wire wound chassis mount resistors back to back will just fit the 30cm diameter ballast container. But that is a lot of power to dissipate.

Has anyone else dealt with this or have any ideas?

[ARC Tech-109]

Personally I'd rather stay true to the original design despite the 56 watts of heat given the rarety of replacement parts in the chain. An open resistor is far cheaper than a smoked transformer or buckled plate when a redesign goes haywire. Just my two sense

[dtvmcdonald]

In my CT-100 I stared at the low voltage cage for days before deciding what to do.
I love the look of those huge orange selenium rectifiers.

But I also noted that the chassis mount can electrolytics were soldered down.
I decided that unsoldering all of them would be dangerous. Cutting them
all off near the base to restuff would be difficult and risky too.

So I decided to rebuild the whole B+ power supply with modern caps, 5 watt
silicon diodes, several fuses in various places, and large old-style Ohmite series 210
tapped (adjustable) resistors to drop voltages. In my
case the original ballast was OK and was left in series with the new Ohmites.

It put all those on an epoxy circuit board and mounted it in the cage.
I used the usual chassis clips to space the resistors at various distances from the board.

I set the line voltage with a Sola regulator transformer and a Variac ( permanently
mounted ... I inherited about 50 Variacs from a scrapped chem teaching lab)
so the filaments were exactly 6.3 and then adjusted the resistors so that the various
B+ voltages were exactly as Sams specified.

The fuses saved the 15GP22 from its first two arcs, as it got gassier and
gassier. But the third one, while it did blow the fuse, was fatal.

[Tom9589]

Have you ohmed out the vertical convergence transformer for shorts and opens yet? IIRC the original transformers were painted black while the modern reproductions were silver in color. You might have gotten lucky and have a reproduction.

[Electronic M]

I forget what the case was on my CT-100 (I think the previous owner addressed the power supply when they did a janky amateurish recap), but my 21CT55 (CT-100 factory modified to drive a 21AXP22) both ballast sections were open so I bought a couple of chassis mount high power resistors off mouser and found places on the rear upright where the resistor screw holes lined up with the cage screw holes. I gutted the original ballast and it now exists purely as decoration.

I also ended up replacing each selenium with 12 modern silicon diodes in series to emulate the original selenium voltage drop.

[Jeffhs]

Originally Posted by Electronic M

I forget what the case was on my CT-100 (I think the previous owner addressed the power supply when they did a janky amateurish recap), but my 21CT55 (CT-100 factory modified to drive a 21AXP22) both ballast sections were open so I bought a couple of chassis mount high power resistors off mouser and found places on the rear upright where the resistor screw holes lined up with the cage screw holes. I gutted the original ballast and it now exists purely as decoration.

I also ended up replacing each selenium with 12 modern silicon diodes in series to emulate the original selenium voltage drop.

It was a very good thing you replaced the selenium rectifiers with modern silicons, as seleniums can short and give off a very unpleasant-smelling gas when they fail.

I did not realize, however, that you needed to replace the selenium rectifiers with 12 silicon diodes, for a total of 24 silicons. I am thinking your TV used two selenium rectifiers, as a lot of 1950s TVs did; my folks' second TV, a Crosley Super V from about 1955, had two seleniums in the LV power supply. (Their first television, a 1954 RCA Victor 21" console, had a 5U4 LV rectifier, IIRC.) These selenium rectifiers, IIRC, were still good when we finally put the Crosley Super V in the basement in 1965 (where it, and the cabinet of the 1954 RCA, remained until 1972; long story and OT), replacing it with a Sears Silvertone all-channel 17-inch b&w portable shortly thereafter (at my mother's insistence, as she wanted an all-channel TV by this time; the reasons for that are OT for this thread).

Now that your TV has silicon LV rectifier diodes to replace those seleniums, you won't have any more trouble with the rectifiers for a long time unless they short again, giving you one less thing to be concerned about. However, this shouldn't be an issue; if the rectifier diodes short again, the TV's line fuse(s) or circuit breaker, of course, will open immediately, so you will know beyond the shadow of even the most unreasonable doubt that there is something wrong in the power supply.

[nasadowsk]

I never understood why selenium rectifiers caught on - they seemed to be a headache, even then. Was the filiment requirements for two 5U4s that much?

[init4fun]

Originally Posted by nasadowsk

I never understood why selenium rectifiers caught on - they seemed to be a headache, even then. Was the filiment requirements for two 5U4s that much?

With the 5U4 filaments at 3 amps per tube, I'll bet saving 6 amps with the selenium rectifiers made for an easier life for the power transformer

[Penthode]

Originally Posted by dtvmcdonald

In my CT-100 I stared at the low voltage cage for days before deciding what to do.
I love the look of those huge orange selenium rectifiers.

But I also noted that the chassis mount can electrolytics were soldered down.
I decided that unsoldering all of them would be dangerous. Cutting them
all off near the base to restuff would be difficult and risky too.

So I decided to rebuild the whole B+ power supply with modern caps, 5 watt
silicon diodes, several fuses in various places, and large old-style Ohmite series 210
tapped (adjustable) resistors to drop voltages. In my
case the original ballast was OK and was left in series with the new Ohmites.

It put all those on an epoxy circuit board and mounted it in the cage.
I used the usual chassis clips to space the resistors at various distances from the board.

I set the line voltage with a Sola regulator transformer and a Variac ( permanently
mounted ... I inherited about 50 Variacs from a scrapped chem teaching lab)
so the filaments were exactly 6.3 and then adjusted the resistors so that the various
B+ voltages were exactly as Sams specified.

The fuses saved the 15GP22 from its first two arcs, as it got gassier and
gassier. But the third one, while it did blow the fuse, was fatal.

I have stared at this set's power supply and haven't decided anything yet. I would like to keep it original especially if the selenium rectifier are good. I have generally had no problem with the seleniums unless there was a B plus short. But the fuses are there for protection.

I was thinking of fitting a 50W wirewound resistor in the ballast tube. But 41watts? I wonder if the resistor would survive? Maybe remove the seleniums and use the space to mount some old style porcelain 100w Ohmites?

[Penthode]

Originally Posted by Tom9589

Have you ohmed out the vertical convergence transformer for shorts and opens yet? IIRC the original transformers were painted black while the modern reproductions were silver in color. You might have gotten lucky and have a reproduction.

Yes that was one of the first thing I did. I lifted the secondary and did not see the VTVM needle quiver which suggests the leakage resistance is greater than 1000 Mohms.

[Penthode]

Only received the rivets for the tube socket remounting yesterday. The rivets are 3/32" x 1/4" and the 1/4" length was needed for the phenolic wafer socket + tube shield bracket + chassis thickness. Had to drill each rivet core by an additional 1/32" and reduce the diameter of rivet head from 0.200" to 0.150 to match the original over heads.

Supported the front chassis section with books and a punch was clamped in a vice below the chassis for the head of the rivet to rest. Then used the hammer and rivet tool on the topside to spread the rivets. Two sharp taps and a couple of finishing tops to ensure the tube socket is firmly secured.

The tube sockets are back in place and along with the IF and Audio Ratio Detector transformers. Currently replacing the potentiometers and soon will commence rewiring.

So much cleaning to get rid of the mess left by the previous tenants. Looking much cleaner now. And in case you forgot how bad it actually was, go back to page two of this thread to have a look!

[Penthode]

This week's update: I have completed the reinstallation of the video and sound IF and audio amplifier wiring to the previously mouse encrusted region. I am awaiting a couple of capacitors and that's it. I shall reattach the front section of the chassis next week and reintegrate the two. I reckon it has taken about 60 solid hours of effort to get this far.

All of the resistors in the mouse infested area are new as are the capacitors. I reused the pots after dismantling and cleaning. All of the resistors are NOS old style brown body resistors.

The high voltage box which was removed has been cleaned and all capacitors (apart from the doorknobs) have been replaced.

Have been meticulously following the dozens of photographs taken from different angles as well as refering to the schematic to confirm connections. I took copious notes when dismantling but have hardly needed to refer to them.

Still have to finish replacing the paper dielectric capacitors on the front section and a large bunch on the main chassis.