Crazy HV issue on a Tele-Tone TV149

[Kamakiri]

Almost done with this chassis, which will be for my replica pre-war set from this thread:

http://www.videokarma.org/showthread.php?t=265203

I'll start by saying that I've replaced all of the usual caps, the 6KV deflection caps, and all of the caps underneath the HV section. Replaced the 60 ohm Candohm also, along with most of the resistors in the HV section. A few had drifted north, but not by any shocking amount.

So here's the screwy part.....

Powering it up, at about 75 volts, I get a picture and sound. Huh??

The picture is small, as shown in the pic, and doesn't get any larger with more voltage. Lousy pic, but what picture *is* there is perfect. Sound is perfectly clear, picture locks in great, etc. Width and height are at max. I can center the picture easily as well.

I don't have a HV probe, I just use a neon bulb. According to the brightness of the bulb which I've used for years and am accustomed to, this set is putting out a crazy amount of HV for an electrostatic set. The HV adjust is turned fully clockwise, and turning it counterclockwise only increases the brightness of the picture and shrinks it.

I've checked every resistor I can think of in the circuit. I swapped 12SN7s around just for grins. Here's a link to the schematic....usually I'm pretty good at figuring things out, but this time I'll admit that I don't understand how this circuit works or what the suspect components could be.

http://www.earlytelevision.org/pdf/T...Sams-56-22.pdf

I haven't tested any voltages at this point because I don't want to run it for longer than about half a minute and not at full voltage. I don't want to risk overloading anything.

Ideas as to how to proceed? Thanks!

[Zenith26kc20]

Is B++ normal?

[Kamakiri]

See, that's the part that I don't understand. You've got a B+ and a B- , I get that, but I don't recall having worked on a set with a B++ and to be honest I'm not sure what that is....

[dieseljeep]

Originally Posted by Kamakiri

See, that's the part that I don't understand. You've got a B+ and a B- , I get that, but I don't recall having worked on a set with a B++ and to be honest I'm not sure what that is....

One of the pages in Riders explains how the voltages are read. The set uses a voltage tripler type power supply. The Hallicrafters 7" set uses a similar circuit.
The sweep circuits are the only ones that used the 400 volt source.

[Eric H]

Being an r.f. type HV supply isn't it basically limited by the transformer, or will it just keep increasing as the B+ goes up? I would think if the B+ was really way high the filters would start popping.

Does the HV transformer look original? perhaps someone swapped it with one from a 10" set at some point in the past.

[Kamakiri]

Caps letting loose is one reason why I don't want to power it up even at reduced voltage for any length of time until I can nail down a potential issue. I just can't find anything.

This chassis was a train wreck (dirty, filter cans busted off the mountings), but all the components appear to be original. I get the feeling that this set was sidelined originally by this issue because the HV adjustment was cranked all the way down and the lead to the candohm was dangling as if someone tested it....it tested good, and said "beyond that I don't know what the hell's going on".

(yes, I replaced it anyway )

[Electronic M]

The neon HV test method is going to be inaccurate on an RF HV supply like this, if your use to doing it on Flyback based electromagnetic sweep sets. The neon operates off RF. On a flyback that RF is a low power harmonic, but on a RF HV generator that RF is the fundamental sinusoid....

If you have a resistance based HV probe with a meter (something we all should have IMO) use that to check the HV. Your HV is probably close to the right value and your letting a crude indirect servicing aid confuse you.

The deflection shrinkage issue is probably a low B++ line issue as others have noted.

[Kamakiri]

I do have an HV probe, but it's broken (one drop will do it).

For me, a confirmation of an excess of HV was getting a raster at 75 volts....but hey, I could be wrong. Now that I know what a B++ line is, I'll give that a look.....thanks!

[jr_tech]

My Philco will actually make a fairly decent (but small) picture at 75-80 volts... I think that may be fairly common for an electrostatic set.

Agree with EM you really need to find out where all of the supplies are, and a neon bulb may be quite misleading for HV. You should score a replacment HV probe.

jr

[DavGoodlin]

Start at the power supply - make sure the positive sides of C5 and C6 are grounded. The Sams has an error, and shows negatives to ground.

I have to look at a set like this for a long time to figure out how it works. If you're used to mid 50s and newer tube sets, these PS and sweep circuits look truly insane.

[Kamakiri]

Now that I have a better understanding of what I'm looking at here, first thing I did was check a couple resistors, then started measuring resistances on the tubes. Hm, I got absolutely nothing on pin 3 of the 6X5. I'll power it up later tonight and check voltages on the 6X5 and 25Z6.

[timmy]

Same for the Motorola 7 inch tabletops, they also have B+ and B++ IIRC they are 125 Vdc and 150 Vdc.

[Zenith26kc20]

If I remember, the TS-4 is around 140 volts B+ and 280 B++ (doubler circuit).
I can't see how a AC tripler would skyrocket B++ (if it has). Tripling or the existing circuit would already produce the maximum voltage (400?) volts ((wow!)). With tubes instead of seleniums, there is no problem with efficiency differences from silicon to selenium.

[Kamakiri]

This set's been a lot of fun since the start of this thread. Replaced 4 47K 2 watt resistors that tested about 68K. The width got better, but I lost signal. Screw it, I'll pick up on this thing after ETF