Quote:
Originally Posted by rrrhre2s
was it Zenith or someone else that used 2 neon lamps in the High Voltage Regulator circuit that were marked with black dots for polarity (?) Never really got that a neon had a polarity though.
rrrhre2s
|
Zenith indeed used dual neon lamps in series as part of the HV regulator circuit. They were in series with the varistor off the HV adjustment pot. They did this only in 1972 on four different chassis', the 18CC29, 18CC30, 20CC50, and 25CC50.
BigDave,
The varistor in the regulator is almost without a doubt the cause of your unadjustable, excessive, HV.
Zenith started using varistors in the regulator circuit starting with the "X" series chassis models in 1967. These were the first to use the 6HS5 pulse regulator tube that replaced the old 6BK4 shunt regulator used on previous chassis'. Zenith used the same basic circuit in the '68 and '69 models using the "Y" and "Z" series chassis'. The regulator varistor used on all X, Y, and Z chassis was the same part number, the 63-6485, except for the 16Z8C50 which used the 63-8161.
When the "A" line chassis was introduced in 1970, depending on the specific chassis, Zenith used either the 63-8161 or a 63-8214 varistor, same with most of the "B" line chassis' in '71 as well.
"C" line chassis used the 63-9068
The small 14" portable color sets between '68 and '72 all used the 63-7658.
The 63-6485, 63-8161 and the 63-8214 were all replaced by a common substitute part, the 863-1060. However, as previously mentioned, I have discovered that most all of these Zenith varistors, NOS or original, "spoil" with age. Just yesterday, I received (5) NOS 863-1060's which were newer (made in the mid 80's) and found even these to be faulty with 3 of the 5 being defective. This becomes very misleading when troubleshooting as subbing in these unknowingly "defective" NOS varistors one after another would lead a tech to believe the problem is elsewhere in the circuit!
Fortunately, I have two good working original part number varistors and I have been conducting a series of tests on them (before they go bad!), such as voltage drop across the terminals and current draw throughout the adjustable range when in a HV regulator circuit. With this information, I can compare the operating characteristics to those of modern MOV varistors in which to find an acceptable substitute. I have ordered a few NTE MOV's which fall within the operating characteristics of the regulator circuit and hope to report back with a reliable sub that can once and for all cure late 60's and early 70's Zenith sets of the unadjustable, excessive high voltage symptom.