VDR - MOV - how to test?

[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.