Eliminating AC hum problems? - Videokarma.org TV - Video

jpdylon · #1 07-26-2006, 10:47 AM

I got a Setchell Carlson from Adam not too long ago that has a power supply issue. The B+ is about 38-40 volts lower than expected. It is accompanied by alot of ac hum, both in the picture and in the sound, and also making the picture difficult to vertical sync. I think the hum may be part of the problem.

The power supply secondary for the B voltage passes through two small diodes (no tube LV rect. here) and then goes through a dropping resistor (which reads the correct impedance) before going to the filters. If I bypass the dropping resistor the B+ is close to normal, but the hum gets worse.

The entire set has been re-capped. I've heard that a H-K short in a tube can cause hum in the picture and sound, but I'm thinking not to this extent. I'll be gettting a sams for it soon. Does anybody have any suggestions to eliminate this nasty problem? Different rectifiers?

Adam · #2 07-26-2006, 11:25 AM

I remember trying to fix this for a long time and not getting anywhere

:

some information: the original diodes were bad, I used 1N4007's as replacements. Originally it calls for 140mfd filters, I used 100s. But I tried putting 47mfd caps in parallel with the 100s, and it made no difference, so I went back to just the 100s. According to the schematic the other side of the secondary coil should go to a 200mfd capacitor which connects to ground, I don't remember what I had there. According to the schematic, that large dropping resistor (which if I remember is really 2 in series), should read 300 ohms, my notations say it actually reads 200. There should be one filter on each side of that resistor. After that resistor it should read 275 v, according to my notes it reads 225v. There should also be another branch of the power supply which branches off before that resistor, goes through a 2k resistor instead and then has a 20 mfd filter, according to the schematic this should read 200v after the resistor.

And that's exactly what I kept doing, trying to remove that resistor to get the voltage up, then it lets too much AC in and it upsets the vertical sync. Same with a lower value resistor. I thought about trying a choke coil instead but never got around to it.

Odd facts about the power transformer: according to my notes here the secondary coil should read 5 ohms across, and it actually reads 1.2. And it says the primary should be 1.7 and it is really 1.

And becuase the voltages are low, I couldn't get enough vertical height, so I reduced the resistor comming off the center tap of the vert height control from 1meg to 470k.

I'll scan this on Friday and then you can see it all.

Tony V · #3 07-26-2006, 02:00 PM

This seems to be an odd problem. I'm curious how this will pan out. Definately sounds like it will be a brain rattler but we have all witnessed some doozies on here and they were eventually solved with the help of everybody. Good luck bud!
Tony

jpdylon · #4 07-26-2006, 03:11 PM

Thanks again Adam for the scans. I'll be gone between the 2nd and th 17th to see some friends, but I'lll let you know of my progress when i return. if the transformer is indeed the weak point here, My father knows where I can get a replacement. However, i'm not too sure if I'lll be keeping this long but I at least want to get it right again. The CRT is about wasted and reads less than 5% on my B&K. Turning up the brightness or contrast and everything turns cloudy. Its a CRT issue and not an HV issue.

I've run out of space (again) and I may be selling a few things as well.

peverett · #5 07-26-2006, 10:40 PM

Is this one of those Setchel Carlson portables? I am working on one of these and had the same issue. I tested and changed tubes to no effect. I had also tried new power supply diodes. I had the correct new electrolytics in and still had the hum. All resistors in the power supply circuit were the correct values. What I have finally done is add another 100 ufd filter in the main DC supply circuit. This seems to have fixed the problem, but I am somewhat worried about surge current when the set powers up.

The hum was in the power supply area and displayed very well on an oscilliscope.

Another comment about this set is that the CRT mounting is horrible. Only the ring around the front holds the CRT in, there is no support for the back portion of the CRT.

peverett · #6 07-26-2006, 10:42 PM

One more comment about my Setchel Carlson is that the DC power supply voltages are within spec, with and without the hum.

Chad Hauris · #7 07-27-2006, 06:34 PM

I would not go under capacitance on the caps...first thing I would do is make sure all capacitance values are at least as much as the originals. If there is a resistor that is off value I would just replace it.
Then I would try and see what portion of the set is accounting for the excess current draw: try removing branches off the power supply (remove the Horiz and vertical outputs so they don't get damaged by a lack of oscillator voltage) and see if the voltage returns to normal. IF not then check the no load power supply voltage...if that is still low the problem must be in the power supply and not the set circuitry.

What is the ac voltage at the power transfomer secondary? is it in spec?
Also is this a bridge rectifier or voltage doubler. if bridge you might see if one of the new diodes might be bad.

peverett · #8 07-27-2006, 09:03 PM

On the Setchell Carlson that I am working on, it seemed to be the veritical output. I could watch hum waveforms move past each other when the vertical was in sync. As they would reach the point where they added to each other the vertical sync would become very weak. The horizontal also had a hum bar, but did not loose sync.

Althought I had plenty of vertical height and good linearity, I substuted vertical oscillator and output tubes with no effect. I also verified that all vertical section voltages were within the spec given by the Sams photofact. All vertical section capacitors were replaced and the vertical section resistors checked and replaced as needed. None of this affected the hum.

One note is that a multi-section electrolytic capacitor in the power supply was replaced by individual ones. Has anyone seen this causing issues?

roundscreen · #9 07-27-2006, 09:13 PM

Hi
If your setchel carlson is a late 50s early 60s set look for a cap that is wired in wrong.
I worked on one about 20 years ago and it had the same problem. I found I had the polarty wrong on one of the b+ filter caps. {can} The power supply in the sc was not like the other 50s-60s sets that I worked on at the time and did not even notice that I did it wrong untill I turned it on. I still have the set but I have no Idea were it is or I would take a picture of it.
Ed