RCA 21" triage CTC20 CTC16x CTC12

[DavGoodlin]

Im pretty sure yokes for 15 and 12 are the same. There are identical on the RCA schematics.

If you don't already have it, RCA vol. 1 Color TV service Manual (Tab Books #496)
https://picclick.com/Vintage-Tab-Boo...177180696.html
is a very important text if you want to understand all about these. .

Carl Babcoke never included a foreword like other Authors in this series BUT he must have really been listening in the service seminars RCA must have had. The troubleshooting steps for the ever-common issue of red-plate/No HV so common to RCA New Vista color TVs.

[oldtvman]

After the development of Color television it seems like some of the later models became victims of Corporate greed. I remember several chassis runs that had on going problems. They were good money makers for service, but didn't seem to hold up well to Zenith.

[SueAnders]

It's been fascinating to read the play-by-play on the CTC-20... the lovely Early American 1967 Zenith "Harvard" now alive and sitting in our living room was it's predecessor at Dave's!

[DavGoodlin]

Originally Posted by oldtvman

After the development of Color television it seems like some of the later models became victims of Corporate greed. I remember several chassis runs that had on going problems. They were good money makers for service, but didn't seem to hold up well to Zenith.

I think the late 60s saw Zenith dealers proliferate for that reason, even here just 5 miles from Lancaster's New Holland Avenue CRT and orthicon plant.

[JohnCT]

Originally Posted by DavGoodlin

I think the late 60s saw Zenith dealers proliferate for that reason, even here just 5 miles from Lancaster's New Holland Avenue CRT and orthicon plant.

Zeniths were more reliable, that's a fact. RCAs outperformed Zenith, that's a fact.

My recollection is that in our town, Zenith shops outnumbered RCA shops 8-1. We were only one of three that sold and serviced RCA. I remember the older shops just didn't like printed circuits and far preferred the hand wiring of the Zenith chassis. I suspect that was a big reason why there were more Zenith shops.

John

[nasadowsk]

Why were TVs so hungry for 6GH8s? For that matter, why did every pentode/triode combo out there suck? I can't think of one that didn't have dome stupid issiue or another...

[DavGoodlin]

Originally Posted by nasadowsk

Why were TVs so hungry for 6GH8s? For that matter, why did every pentode/triode combo out there suck? I can't think of one that didn't have dome stupid issiue or another...

I think the manufacturers were trying to reduce tube count and except for GE compactrons, number of different tubes. The CTC16 had only two 6GH8A and it was more for convenience, combining 3.58 mhz osc and reactance, killer and killer and bandpass, a two in one tube.

Then...some wizard figured half a hard-driving 6GU7 could be replaced by half a 6GH8A and that led to higher demands on the pentode section. Maybe the relative success of the 5GH8/6GH8 in smaller screen sets led to mis-application in 25" models.

Then as you say, 6BL8s in Motorola and Philco, 6KT8 in Zenith, 6GH8 in Magnavox, RCA etc. Seemed to fill tube caddys I collected over the years. I often read the 6U8 and 6EA8 were tougher of the triode-pentodes but the substitution is never guaranteed.

[zeno]

Originally Posted by JohnCT

The resistance between the plate cap of the horiz out and the HV rectifier is the first quick test when checking the fly, but if it passes that test, there's still the possibility of a shorted turn. An ohmeter treats coils like resistors and a single shorted turn renders a coil useless and won't be discernible with an ohmeter. The VA48 will ferret out those with shorted turns.

John

Spot on. The only way to be 99% sure the FBT is good or bad is
is to totally remove it & ring it. If it dont resonate its NFG. Shorted
turns were the most common fail on FBT's & yokes.

73 Zeno
LFOD !

[zeno]

Originally Posted by nasadowsk

Why were TVs so hungry for 6GH8s? For that matter, why did every pentode/triode combo out there suck? I can't think of one that didn't have dome stupid issiue or another...

We used to buy 6GH8's by the gross every few months. The tubes just
seemed to suck BUT a lot of the problems were tube sockets.
Sets with 5GH8's were the 14" & 19" plastic cabinet sets. Same problem
with those BUT in the late chassii they got much better. Those sets ran
real hot & cooked up all the low level tubes. Add to that the low focus
CRT's & you have a real POS TV in a few years. That being said a good
one could throw a fantastic pix if you damn near retubed it gave
the CRT a clean & balance.
To add to the fustration if you "swap" GH8's the symptoms often changed.
You may start with no color, test the tubes then put them back
& get different symptoms. This drove the home tube tester types to drink.

enuf fer now
73 Zeno
LFOD !

[dieseljeep]

Originally Posted by zeno

We used to buy 6GH8's by the gross every few months. The tubes just
seemed to suck BUT a lot of the problems were tube sockets.
Sets with 5GH8's were the 14" & 19" plastic cabinet sets. Same problem
with those BUT in the late chassii they got much better. Those sets ran
real hot & cooked up all the low level tubes. Add to that the low focus
CRT's & you have a real POS TV in a few years. That being said a good
one could throw a fantastic pix if you damn near retubed it gave
the CRT a clean & balance.
To add to the fustration if you "swap" GH8's the symptoms often changed.
You may start with no color, test the tubes then put them back
& get different symptoms. This drove the home tube tester types to drink.

enuf fer now
73 Zeno
LFOD !

The RCA distributor sold a box of 100 Raytheon Japanese tubes for $98.00.
In the later model sets, the color osc tube was made by a different supplier. The only one, the other ones were RCA made.
I have an RCA CTC51K that has OEM 6GH8's that are Japanese. The horiz output as well.

[old_tv_nut]

Originally Posted by zeno

...
To add to the fustration if you "swap" GH8's the symptoms often changed.
You may start with no color, test the tubes then put them back
& get different symptoms. This drove the home tube tester types to drink.
...
73 Zeno
LFOD !

I know it's mean to say it, but this made me laugh.

[DavGoodlin]

I remember an acquaintance got his 15" RCA (CTC22) serviced and gave me 5 (?) 5GH8s in the boxes new RCAs came in. I thought it was odd that 5 of a same type were in one set! But at 10 yrs old, he figured the set was still worth it.

[Electronic M]

Originally Posted by old_tv_nut

I know it's mean to say it, but this made me laugh.

I found that hilarious myself.
I don't think it's mean...if they were not smart enough to realize that a tube is bad by substituting it with another of the same type in a different circuit and watching symptoms switch circuits then that's on them.
I literally used that to verify an intermittent that had fought me for years. My hybrid Zenith had a bad red video output transistor that was intermittent and REFUSED to malfunction while metered. Switching the identical red and blue channel transistors and watching the intermittent move to the other color was how I finally found it.

[DavGoodlin]

The 6GH8A madness continues...while sweep problem is all but gone, but what remains appears to be weak sync AND video, likely AGC.

So far it's crummy-looking video but chroma-locked color bars are good to see, the AGC control has no effect using the Sencore VA-48 at both antenna & tuner link going to first IF amp.

Unlike the 4-board color chassis like CTC-21/25/28..., the 2-pcb CTC19/20/24/31 video is taken off before a 6GH8 pentode section amplifies chroma/sync and AGC.

This is the last place to look before I move on to the CTC16X. I'm getting bored for now and need a change, that's why I line up triage of at least two similar sets.

[DavGoodlin]

Time for a change. The CTC20 began exhibiting a vertical issue along with its funky shaky video with color. Voltages on the 6GH8s are close enough that I suspect the video detector is possibly bad. This could really lead down along road to get this set right before daring to assemble it.

CTC20 jig.jpg

I unplugged everything and put the CTC16X chassis up, plugging in all the tubes and powering up from 70-100 volts over an hour.

The difference in CTC16 and 20 chassis is remarkable.