Portacolor early model repair

[stragulus]

So, I got this what looks like the earliest '66 model Portacolor. Now I'm not familiar at all with repairing anything with vacuum tubes in it, and otherwise am an amateur electronics repair hobbyist.

When I received it, I brushed up on repair faqs, early color tv circuits etc. I took out the horizontal tube and vertical deflection tube and brought the set up on a variac (and isolation transformer, I know *that* much!). I measured B+ and at full 120V AC voltage it went up to 360V. The schematic calls for 270V but I foolishly assumed it is probably due to not being loaded down much.

So, I put the tubes back in and fired up the set. After a little while I heard what sounds like HV arcing slightly. After a few seconds more I heard a few popping sounds and switched the set off. I had taken a video of the set in a darkened room luckily and could see that the popping sounds were from what looks like arcing in the neck (area) of the tube. Can't tell for sure but can't really see what else would arc there. It also came with a 'pinging' noise that very much sounded like what it would sound like if you'd flick the crt with your nails. Anyway, I hope I didn't destroy the set just yet. Nothing else seemed out of the ordinary and it wasn't on long.

So analyzing the issue I now figure that the B+ voltage probably *was* way too high. Looking at the schematics (I have the sam's) the circuit looks like a voltage doubler circuit using 2 diodes and 2 electrolytic capacitors. If one of these diodes were shot, I suppose that could explain your 360V. If that were true, I suppose I should also worry about the caps in the doubler? Although I did take temperature measurements while it was on the variac and the big cap with the multiple terminals remained cool.

Anything else I should think of checking? Going to check on the diodes to see if that theory holds up.

[zeno]

High B+ is probably due to no load. Look to the hoz output & damper.
Other circuits dont draw very much. You can try lightly tapping the
hoz out & damper tubes & see if you get a reaction such as arcing or
the set starts working.

73 Zeno
LFOD !

[init4fun]

Originally Posted by stragulus

I had taken a video of the set in a darkened room luckily and could see that the popping sounds were from what looks like arcing in the neck (area) of the tube. Can't tell for sure but can't really see what else would arc there. It also came with a 'pinging' noise that very much sounded like what it would sound like if you'd flick the crt with your nails.

Anything else I should think of checking? Going to check on the diodes to see if that theory holds up.

If you only measured the B+ with the sweep tubes removed and not with them in place that would easily explain the 100 volts too high reading . Also , many times arcing in the neck is an indicator of a gassy CRT . You may want to determine if the CRT is actually good , or if not that you can source a good replacement , before spending the cash to recap & restore the set ....

[stragulus]

Originally Posted by init4fun

If you only measured the B+ with the sweep tubes removed and not with them in place that would easily explain the 100 volts too high reading . Also , many times arcing in the neck is an indicator of a gassy CRT . You may want to determine if the CRT is actually good , or if not that you can source a good replacement , before spending the cash to recap & restore the set ....

It does explain the high voltage? I thought that was more common with PWM power supplies but that's good to know!

Yeah, I want to get it basically functioning before I consider any recapping. So I measured all the various sources with the sweep tubes pulled again and they check out OK if I supply it with 100 VAC. Nothing runs too hot, so I put the tubes back in and fired it up with 100 VAC. I got a bleak red-tinted raster so basic functionality is there.

Once things started heating up the B+ also dropped and I could increase VAC to 110VAC without any arcing. So perhaps there's still live in it. The set doesn't look high hours to my untrained eye, but I don't have any equipment to measure the tubes or CRT so I'll just try and see if I can get a basic picture out of it.

[damen]

GE sets were notorious for "tin wiskers" in their control pots. I've had the AGC, vertical height and screen controls all sound and look like the 4th of July.

[Electronic M]

Portacolor sets used series string heaters so if you pull any tube (except the HV rect which gets heater supply from the flyback and CRT which IIRC has its own heater transformer) there will be no load on any of the B+ supplies and none of the stages will draw B+.
You pretty much shouldn't pull any tubes from a series string heater set when variacing it, or if you do then at least monitor the highest B+ rail and stop the variac at the rails rated voltage to prevent damage to the lytics

There are 2-4 white ceramic tubular paper caps (IIRC Elmenco brand) that go bad and the lytics. Tin whiskers in the rear pots is definitely a common PITA.

[JohnCT]

Those TVs "regulated" by actually running..

I would remove the 2nd anode lead from the tube, aim it well away from anything, and take a HV reading on it. If you get HV, reconnect it. If the HV drops, disconnect the CRT socket and move it well away, and run the HV meter's probe on the pins of the tube. If it spits HV out of the pins, the tube has gone to air.

John

[stragulus]

Originally Posted by Electronic M

Portacolor sets used series string heaters so if you pull any tube (except the HV rect which gets heater supply from the flyback and CRT which IIRC has its own heater transformer) there will be no load on any of the B+ supplies and none of the stages will draw B+.
You pretty much shouldn't pull any tubes from a series string heater set when variacing it, or if you do then at least monitor the highest B+ rail and stop the variac at the rails rated voltage to prevent damage to the lytics

Good to know. I only very briefly ran it like that and dialed it down to about 100VAC where voltages were more in line with the schematics.

There are 2-4 white ceramic tubular paper caps (IIRC Elmenco brand) that go bad and the lytics. Tin whiskers in the rear pots is definitely a common PITA.

I have indeed found 2 of these. I had already cleaned the pots because the flyback casing looks like it's coated in these little things. The inside is clean though.

So I experimented a little more. The set behaves ok if I leave the variac around 110VAC. I can get it to produce a picture with a VCR connected to the tuner, which is devoid of any color. The primary colors themselves are all present if I play around with the pots. So looks like color demodulation is not working. Also, the PCB seems to need some touching up as tapping it lightly distorts the picture.

Looks like at least the tube is alive, but the set does not seem to like the full line voltage.