High voltages on Admiral 19A11. - Videokarma.org TV - Video

bigaudioal · #1 02-18-2014, 09:49 PM

Good evening gents,

I have been working on my Admiral 19A11 for some time now. This set has been completely recapped and all resistors replaced. Horizontal sweep transformer and filter choke have also been replaced. The set has been working fairly well, except for a vertical linearity issue (picture squashed at the bottom), which it has always had. I began to test voltages to try and track down this vertical linearity issue. However, I immediately noticed that all voltages were high across the entire set. Even at the 5Y3 tube (L.V, rect.). Pins two and eight should be 240VDC and are running around 266VDC. Pins four, and six should be 240VAC and are running around 292VAC. Any ideas where my issue may lie? My untrained, novice opinion is the power transformer. Nothing in the circuits really before the 5Y3 tube other then the power transformer. The transformer stays cool to the touch for about 40 minutes, then starts to warm up. Never gets too hot to touch, but does get fairly warm.

Any advice would be greatly appreciated. Thanks.

bandersen · #2 02-18-2014, 10:36 PM

What's the AC line voltage on the set ? They typical spec out the voltages assuming it's 117, but it's often more like 125 these days. That will increase all the voltages in the set.

bigaudioal · #3 02-18-2014, 11:24 PM

SAMS lists it at 117. The ACV at my plug is reading 115.5 right now.

kramden66 · #4 02-19-2014, 12:21 AM

I always seem to get higher voltages in sets and the ac voltage is 113 to 115 .

your lin issue could be the 6kv caps , what did you put there ? sometimes changing the value a little corrects the lin issue

mike

bigaudioal · #5 02-19-2014, 06:48 AM

In photo from left to right the SAMS called for .005, .001, .001, .005, and .005 (all microfarads @ 6KV).

Replacements values are .0047, .001, .001, .0047, and .0047.

Thanks!

Very interesting regarding voltages when AC is 113 to 115. My house typically runs around 115.

Zenith26kc20 · #6 02-19-2014, 08:15 AM

Looks like ceramic discs for 6KV caps. They can be non linear. That may be your problem. I did that years ago on my Motorola TS-4 and it worked. One finally failled and the newer tubular 6KV caps perform much better.

bigaudioal · #7 02-19-2014, 08:55 AM

Ah! Had no idea about the ceramic discs having possible issues. I will try replacing with tubular ones. Any ones specifically that I should be looking for? Brand? Assuming Mouser carries them.

Thanks again everyone. Guess this is how new guys learn, huh?

bigaudioal · #8 02-19-2014, 10:00 AM

I just did a search on Mouser for 6kV caps and all that came up was ceramic disc type. Any suggestions for where to get the tubular kV caps from?

Thanks.

bandersen · #9 02-19-2014, 11:55 AM

Ceramic caps have a peculiar property that causes the capacitance to vary as the voltage on them increases. So when you try to pass a sawtooth waveform through one, it gets "bent" causing the non-linearity.

DavGoodlin · #10 02-19-2014, 12:01 PM

Thanks Bob, I was wondering why a capacitor that measured correct on my checker would not work well in a signal-coupling or wave-shaping circuit.

Someone else on this forum has suggested that ceramics were good for bypass applications and not much else.

bandersen · #11 02-19-2014, 12:15 PM

Here's a more in depth discussion on this topic. http://www.videokarma.org/showthread.php?t=260749

When I popped open my National TV-7, I was surprised to see original ceramic doorknob caps for the horizontal. So it seems that ceramics work fine in the horizontal, but not the vertical.

Using a much larger ceramic cap for the vertical should reduce or eliminate the non linearity.

bigaudioal · #12 02-19-2014, 01:57 PM

Bob,

Thanks SO much for the explanation. Very interesting and noted. Cannot wait to swap out and see the result. I ordered the caps through Allied.

Thanks again everyone. Will post update in a few days with result.

N8NM · #13 02-19-2014, 02:20 PM

Re: the higher voltages: I've found that it's not unusual for the measured voltages to be a bit higher than those in the Sams (or Rider's) manuals and don't sweat it as long as everything works OK, current draw is reasonable and the transformer stays cool.

bigaudioal · #14 02-19-2014, 03:03 PM

After about 40 minutes, my power transformer does heat up. It does not get too hot to touch. It heats up so that if I put my entire hand on it, it is nice and toasty. I can keep my hand on it indefinitely without burning myself. Once it gets to this temp, it seems to stabilize there.

Phil Nelson · #15 02-19-2014, 04:00 PM

Not sure about this set in particular, but a hot tube next to a transformer can also warm it somewhat.

Phil Nelson