Dove right in! Help with RCA 21-D-8628

Originally Posted by Electronic M

You need AC to test the unknown transformer. The UHF tuner may have a 6V heater winding....You can check by looking at the ID numbers of the tubes in it. On most signal tubes made after 1936 the first digit of the tube ID number is the heater voltage...So a 6AG5 is a 6.3V heater and a 12SN7 is a 12.6V heater.

If you use the UHF tuners transformer to power your mystery transformer pull the tubes out of the tuner and if the tuner has only 1-3 tubes don't run it for more than 30second stints as a source for the mystery transformer. Probably still a good idea to use a 1A fuse in series with the connected winding of the mystery transformer....

The UHF tuner (which I suspect is really what collectors call a UHF converter) probably has a small transformer designed to run the heaters of 1-3 tubes...many 5 tube and higher transformers can take a heater winding short for long enough that you'll notice and switch off, but ones designed for fewer tubes are easier to kill.

The UHF tuner has 4 tubes, three are 6s and one is a rectifier. Would that supply the adequate voltage long enough to test?

I replaced three of the four tubes in the tuner for grins and giggles, I think the old caps are still okayish. Where I live you can't even pickup UHF, and the VHF is weak unless you get the right antenna angle, and that only gets you 3 stations! If you turn the antenna just 10° you lose out on 3 CSI's during primetime. Darn.

As to Kevin, I do not yet have a variac. I'll te-test the transformer leads today and see if I did anything backwards. I could be using the multimeter wrong, though I seem to be getting the hang of it.

Both of the elecgrolytic cans register over half their mfd's (or appear to when the meter auto adjusts) which surprised me.

Something happened to this tv that I still can't explain. What caused the transformer to short out, I suspect as has been pointed out that somewhere between low V rectifier and something caused it. If you left the unit on long enough the tubes began to heat, but no sound or picture. Just lots of smoke.

Also, can I use a homemade short check device? The 250 watt bulb in series with a switch, the transformer, and an outlet? Then measure the outputs?

[Kevin Kuehn]

Originally Posted by Electronic M

It might be a good idea to find a 5 tube AM radio and make that your first project. The average monochrome TV is 5x as complex as an AM radio but uses similar parts so if you make a mistake Tracing it down from the schematic is much less difficult.

Phill's old radios on the internet has a lot of good beginner info and detailed write ups with pictures of TV and radio restorations he has done.

Not my intention to rain on your parade, but I highly suggest you heed the above advice and find yourself a transformer powered tube radio so you can learn the basics of electronics on a much less complicated set. I went back and read your introductory post, and IMO having some experience with antique and vintage telephones, cameras, and a few Edison cylinder phonographs are hardly credentials for the project you've undertaken. I understand the temptation to dive right in, but electronics is truly one of those fields where if you don't learn first, your basics, you'll forever be walking in the dark. Up until early computers, televisions were still considered one of the most complex electronic systems in existence. As far as discrete circuitry goes, they're still very high on that list.

[Electronic M]

The UHF tuner probably has decent heater winding current capacity.

You still get analog UHF out where you are at? Our last ones were gone by 2014.

Connecting the transformer to line with a light bulb in series is probably a better safety measure than a fuse.

The original electrolytic caps are almost certainly what killed your first transformer...the high leakage current overloaded the B+ winding and smoked it happens all the time on sets people try to run on original caps.
The thing about testing capacitors is most testers check capacitance at less than 9V....these caps usually work fine at 9V but in the TV have as much as 450V on them....many of these caps don't develop leakage current unless they have 25-200V on them (depends on the cap and the life it's had)...some line powered capacitor testers (like my Heathkit C3) can test for leakage at as high as 450VDC.

I know first hand that trying to use original electrolytic capacitors in a pre-1965 TV is a fools errand. I have Zenith's first c1961 consumer color TV the 29JC20...my set partially worked when brought up on a variac so I changed the paper caps in it, tested the lytics and only changed the lytic sections that measured leaky or open on my Heathkit...it was great for a few weeks then one of the good lytic sections died, I changed that and in a few weeks another 1-2 died, and a few weeks later when the next one died I finally just changed all the lytics to be done with that nonsense.
Do not power your TV chassis with the new transformer until you have replaced the electrolytic caps.

[old_coot88]

Originally Posted by BandDirector

Top 3:

Tan (right 11)
& Yellow Green (7)
=44.6 ohms

Tan 3
&Yellow Green 7
=26.4 ohms

Tan 3
&Tan 11
=18.4 ohms

Lower:

Tan 2
&Black and Yellow 4|
=.1 ohm

Black 1
& Black and Yellow 4
=.6 ohm

Yellow Green 5
&Yellow 6
=.1 ohm

Black 10
&Yellow Black
=.4 ohms


The numbers reflect a left to right order, they have been paired according to continuity and ohms.

So next step is to apply power to the lowest ohm pairs? Then measure output voltage?

Nooooooooooooooo please !! Don't apply mains power to anything yet.

First off, the three tans (on top) would be the center-tapped hv secondary (one side is higher ohms than the other because there's more wire per turn with increased radius of the outer winding layers). And sorry, my guessitimation of ohmic values was quite a ways off.

The .6 ohm pair is probably the primary. That's where you'd apply power. But for starters, only through a variac or a resistive load like an incandescent light bulb.

[old_coot88]

Lacking a variac or a resistive load, what you could safely do is this - apply mains power to half of the hv secondary. The whole xfmr will then be in step-down mode. The voltages coming out of the other windings will be a given percentage of their normal values, probably around 20 - 30% or so. Allowing for the step-down factor, it'd be easy to identify which winding is which.

Yep, I will take a break again and just work on putting the old stuff back on the chassis as a practice step.

And review how to use a multimeter again so I can test the transformer.

I do have an old tube radio floating around somewhere that my grandfather had from the airforce. It was a set that opened like a little medical bag, halfway up from the front. Neat looking thing I need to track down, it's here somewhere.

[Kevin Kuehn]

Originally Posted by old_coot88

First off, the three tans (on top) would be the center-tapped hv secondary (one side is higher ohms than the other because there's more wire per turn with increased radius of the outer winding layers).

Normally I would expect more like 2-3 ohms increase for radius on the center taped HV windings. 8-9 ohms difference would indicate a lot more turns.

[old_coot88]

Originally Posted by Kevin Kuehn

Normally I would expect more like 2-3 ohms increase for radius on the center taped HV windings. 8-9 ohms difference would indicate a lot more turns.

True that. But I still think it's worth putting mains voltage on half of the hv winding, record the other windings' readings, then put mains voltage on the other half, and see if the readings are the same. Also see if voltage out of the unconnected half remains the same both ways.

Real hillbilly troubleshooting

See pictures. I used a RCA Senior VoltOhmyst

Perhaps seeing readings on a VTVM will make the readings easier.

I am about to tackle the RCA UHF tuner and see how to ground up see how something works. I still haven't found a radio to work on. My grandmother has a tube radio intercomm so I may be pulling that out of the wall. Yep. Pulling it out.

[old_coot88]

That's a timeless and venerable piece of gear. Are you zeroing the meter for the ohms measurements?

Mimi's Rangeaire Intercom AM/FM radio

No wires or components have been removed. This unit works well, except that all intercoms in the bedrooms and door have bwen removed. The master bedroom com no longer works, the door com speaker may still be behind the grill outside, however it has not worked in my memory. This will not be missed, and when restored, will be saved as my own intercom system for later in my own home!.

Originally Posted by old_coot88

That's a timeless and venerable piece of gear. Are you zeroing the meter for the ohms measurements?

I did before I ran the series of tests. I downloaded the manual/schematic Wednesday when it arrived. I followed the process to 'tune' it and I have only two days experience with it. The ac/dc probe came today, from a B/K unit, just a random eBay find. I have just begun this whole process in a random way. I don't really now what tools to assemble, so it is just a scavenger hunt as I make small steps. I don't even know what to call the probe's connection to the chassis, a screw type sort of coax?

I love the whole nature of this ordeal and I am hell-bent. Just need to stay calm.

Tools I have:

CRT tester
RCA Senior vtvm VoltOhmyst
B/K solid state 2-channel oscilliscope
B/K Dynascan 1070 (does not have test pattern)
RCA Bias Supply
HP VHF signal generator

Projects:

1957 RCA KCS116C 21"
Rangeaire AM/FM tube radio from 1964 - learning project radio

I have been told to put the tv chassis components back together, and to find a compatible transformer. A smaller project was advised to first tackle how to restore a tube electronic, hence the Rangeaire.

Problems:
I do not know how to measure resistance in ohms, therefore it will be difficult identifying the resistors I removed from the tv chassis.
I know how to callibrate the vtvm via the RCA manual.
I know how to setup the oscilliscope via the B/K manual.
I do not know how to measure a capacitor using my DMM, even assuming the cap is dead, I cannot pinpoint whether my reading is accurate. Therefore I cannot accurately place the old tv caps unless I can trust what is written on each one. They are clearly marked with volts and +- % for identification.
The tv crt is rated good.
The Rangeaire radio does work, a little low on volume but it does work. I can study and test the caps and resistors, tubes, and voltages to learn AC/DC measurements.

Now what?

[JohnCT]

Originally Posted by BandDirector

I do not know how to measure resistance in ohms, therefore it will be difficult identifying the resistors I removed from the tv chassis.

Order a fistful of brand new 1% film resistors ranging from less than an ohm to over 1 meg. With known good resistors of exact values, you can practice with your meter until it's second nature. Order resistors with the value printed numerically on them so you don't get confused with color codes. Known value resistors can also tell you if your meter is working properly..

In addition, you have to become familiar with ranges. For instance, a 680 ohm resistor is also known as a .68K resistor. A 820K resistor is a .82Meg, etc. Using known good resistors takes the guessing out.

I do not know how to measure a capacitor using my DMM, even assuming the cap is dead, I cannot pinpoint whether my reading is accurate.

If you're talking about a typical DMM that measures AC/DC voltages, resistance, and current, then you can't use such a meter. Some people use a resistance "kick" to determine a cap is good or not, but that's a waste of time. However, many newer DMMs though have a rudimentary capacitor function that checks for value.
If this is what you're referring to, then order a bunch of capacitors and learn the differences between pf, uf, and nf. NF was not used in the old days, but is more European than U.S.

There are on-line conversion calculators so if your meter says a capacitor is reading 68nf, you can plug that into a converter and find it's also known as a .068uf.

Most of these meters don't test for leakage as a good leakage tester must put the same voltage across the cap as it's rated for. For instance, a typical 0.05 400V cap may read .05 on a capacitor DMM tester, but become electrically leaky at 100V or more, well beyond the voltage a typical capacitor checker will test for.


John

[old_coot88]

Regarding the VoltOhmyst, you might have to re-zero it a few times until it warms up good.