VDR - MOV - how to test?

[maxhifi]

Read videokarma long enough, and one recurrent theme is the VDR.

My favourite subject, the CTC38 uses about four of them (don't have the manual with me), but the two I am most interested in are in the vertical and horizontal sweep circuits.

I have noticed two things, which I am starting to suspect are related.

1. Vertical size must be turned all the way down to get the correct size, with linearity adjusted properly.

2. Line voltage fluctuations/transients (electric space heater coming on, furnace starting, etc) seem to affect the vertical size.

My Theory: the VDR is supposed to keep the vertical size constant with line voltage fluctuations. Since this isn't happening as well as could be expected, is it also possible that a bad VDR could be causing the vertical size control to be adjusted at the bottom of its range, rather than in the middle?

Now another and related question, is how to test a VDR out of circuit?

Digikey and Mouser have tons of MOVs available, but it will take some doing to find a substitution, because the way they are specified appears different. I think some more homework on my behalf is needed, I am posting my thoughts so far though for a reality check to see if I am staying on the rails or not.

I also ordered two interesting documents which may have some answers for me

1. Field Service Guide RCA Color TV Receivers 1969 1970 CTC-22 Through CTC-40

2. 1969 RCA PRODUCT TECHNICAL MANUAL TV RADIO TELEVISION PHONOGRAPH TAPE ORIGINAL

I am hoping I can get answers from either or both of these, to some of the mysteries of the CTC38 chassis which Sams has not revealed to me.

[Electronic M]

If the vertical sweep gets power from the boost, and the horizontal has the 6BK4 style regulator, then changes in line voltage likely will influence the vertical despite the presence of the VDR.
If you dislike the effects of line voltage fluctuation then I'd recommend you look for a saturable reactor line isolation transformer. These transformers regulate their own output voltage. Zenith used this type of power transformer in their CCII SS sets, and the results were impressive.

[maxhifi]

Hi Tom,

The minor fluctuation in vertical size is entirely tolerable, what I am more interested in is why the vertical size must be turned all the way down to get the right size of picture with decent linearity. If the size could be cranked down, say an additional 1/8 turn, a better compromise could be has between size and linearity. I believe the vertical oscillator and output get their B+ from the power supply and not from boost, but I will have to double check.

-Max

[dtvmcdonald]

What's a VDR? Voltage Dependent Resistor?

[maxhifi]

Yeah, this is what RCA calls them, the schematic symbol is a reaistor in a circle with VDR written next to it. An MOV is a metal oxide varistor, which is kind of a modern analog for the same function, but they are made differently.

[Username1]

Originally Posted by maxhifi

Read videokarma long enough, and one recurrent theme is the VDR.

My favourite subject, the CTC38 uses about four of them (don't have the manual with me), but the two I am most interested in are in the vertical and horizontal sweep circuits.

I have noticed two things, which I am starting to suspect are related.

1. Vertical size must be turned all the way down to get the correct size, with linearity adjusted properly.

2. Line voltage fluctuations/transients (electric space heater coming on, furnace starting, etc) seem to affect the vertical size.

My Theory: the VDR is supposed to keep the vertical size constant with line voltage fluctuations. Since this isn't happening as well as could be expected, is it also possible that a bad VDR could be causing the vertical size control to be adjusted at the bottom of its range, rather than in the middle?

Now another and related question, is how to test a VDR out of circuit?

Digikey and Mouser have tons of MOVs available, but it will take some doing to find a substitution, because the way they are specified appears different. I think some more homework on my behalf is needed, I am posting my thoughts so far though for a reality check to see if I am staying on the rails or not.

I also ordered two interesting documents which may have some answers for me

1. Field Service Guide RCA Color TV Receivers 1969 1970 CTC-22 Through CTC-40

2. 1969 RCA PRODUCT TECHNICAL MANUAL TV RADIO TELEVISION PHONOGRAPH TAPE ORIGINAL

I am hoping I can get answers from either or both of these, to some of the mysteries of the CTC38 chassis which Sams has not revealed to me.

Zenith made that funky transformer related voltage regulator as a way to
control supply voltages in the tv as line voltages fluctuated. I believe RCA
decided they did not need to put that much money into a power supply
immune to voltage changes, so they decided to use the VDR's to control
on screen artifacts that would cause the customer to complain - if they did,
when the picture would jump around when the frig turned on or off.....
After all, it was only the picture that was effected, not sound.... So
why fix the entire power supply..... Just stop the picture from changing
size...

I think they needed something they could use as a non linear device that kinda
tweeked the gain of those amps and react rather quickly - thus do it in a
gain circuit.... Their solution was for sure cheaper than Zenith's....

If you don't find new ones with the exact specs, I bet you could parallel them
with a small resistor to make them more stable.... Kinda divert some
of the current..... Down side lessen the effect it had on the circuit...

I think it would be neat if you could put manual items in those circuits and
test each to see what they do to the picture visually, and document the
effects..... And are they in fact linear, or non linear to provide a quicker
response..... Sony's picture use to jump around with extra bounce when
the voltage levels were disturbed..... and they were not damped in their
effects....

Or the dam things are an RCA designers fudge device.....


.

[Jon1967us]

What's a good replacement for the one VDR (adjacent to the thermistor) in a CTC16 power supply?

[zeno]

They used to come in a kit. IIRC the number was Workman FRT2. Feel free to
correct me. You got both but you could get them separate also.
Anyhows the thermistor would fall apart & all the TV current would flow
through the VDR & smoke it up.

On MOV they are metal oxide varistors. There purpose is to die to save the TV.
They are put in the AC line just AFTER the AC fuse & as close physicaly
& shortest leads possible. If a surge hits it will DEAD SHORT & blow the
AC fuse to protect Your set will not have one unless someone modded it.

73 Zeno
LFOD !

Originally Posted by Jon1967us

What's a good replacement for the one VDR (adjacent to the thermistor) in a CTC16 power supply?

[Jon1967us]

Found a FR066 Workman "Globar". I don't see any specs on it but it lists RCA as a "replacement for" hopefully it works.

[JohnCT]

Originally Posted by Jon1967us

Found a FR066 Workman "Globar". I don't see any specs on it but it lists RCA as a "replacement for" hopefully it works.

The globars were a term we used to describe the thermisters used in the degaussing circuit. I don't recall anyone using that term to describe any other type of thermister or VDR or MOV.

EDIT: oops, I see this discussion switched from sweep to power supplies.

John

[bhegges]

To pick this thread back up I am looking at early color Zenith sets and they also use VDRs and thermistors. The degaussing circuit uses a pair and some sets have a VDR in the yoke.

To start somewhere I am interested to understand the the function of the VDR in the AGC Keying circuit and how to test.

• 29JC20 chassis AGC Keying control grid voltage used a 3meg fringe lock pot feeding from the low voltage source
• 26KC20 through 25MC36 chassis used VDR #63-4906 in place of a pot
• 24MC32 and several later chassis used VDR #63-5494, while the part number changed the circuit looks the same

Looking at the snippet from the 25MC30 chassis the 105v source should be stable making it easy to measure the voltage drop and current through the VDR (assuming I have a good VDR). This info could then be used to set up a test circuit for trying substitutions. I will assume with voltage applied it comes in somewhere around the same value as the middle of the pot used in the earlier circuit. How much can this low voltage supply vary, enough to warrant using a VDR I guess.

[zeno]

From Zenith 1978 parts catalogue

63- 4906 Unlisted ( often available til stock is gone) , no sub
63-5494 Listed, 41 cents, no subs.

I would put a pot there. IIRC fringe lock ( also on Motos ) have
to do with the RF AGC but its been a LONG time ! Adjust per Sams.
BTW some NOS VDR's are going bad in the pkg

73 Zeno
LFOD !

[Notimetolooz]

The VDR may not be there for supply voltage regulation, it may be there to compensate for other component variations.