Testing 130V voltage regulator

[Reece]

Due to an accident with the high voltage cable, the "zap" took out two components, the HOT and the voltage regulator. I have them removed from the board. The HOT is definitely shorted and will be replaced. I have also gone over most of the caps and resistors etc. in the surrounding "boardscape" and found them OK.

The regulator is an NTE 1777 and checks as below with the DVM set to diode, first with test leads one way / then with leads reversed:

Pins Reading
2-3 open/open
3-4 13/13
2-4 open/open

,,so it's bad. I have three used candidates to replace it that I will call A, B, and C, all are STR 30130. They read like this:

A
2-3 600/open
3-4 open/600
2-4 955/open

B
2-3 590/open
3-4 open/610
2-4 1050/open

C
2-3 590/open
3-4 open/613
2-4 1117/open

The three appear to be good to me but would like others' opinions. Did I test these correctly and do the readings look reasonable? If you had to pick one to use, which one would it be, or does it matter? Thanks!

[radiotvnut]

I don't think it will matter. Just try one and see what happens.

[Zenith26kc20]

Is this a Zenith chassis?

[Reece]

It's a 1996 Magnavox by Philips.

[rca2000]

If...after replacing that 30130 chip, the B+ STILL seems low....you may have blown the resistor in line with the output pin of the reg. It is usuallu something like "4.7 ohm 2 watts" or so--and often opens--and if so--the large series resistor will carry the load but it will NOT be high enough and will vary a LO)T.

[Zenith26kc20]

also check Q403, R432, R435, Q402 and the 100 ohm resistor on the collector of Q403. All of these can fail when the regulator fails. C432 is also known to fail and the high voltage rises and the picture looks scrambled. I change C432 whenever I get one of these Magnavox chassis in the shop.

[Reece]

Thanks, all. I had already changed most of the electrolytic caps around the power supply section but will check again the parts mentioned. I will have to order the HOT as I don't have an NTE 2353 so won't be able to get this going for a while, but that will give me time to "fine-tooth-comb" all the surrounding parts. I will put a light bulb in series with pin 4 of the reg. and use a variac when I finally power up.

[rca2000]

You MUST change that 100MFD cap. It likely CAUSED this problem. I did not realize which chassis this was--till someone mentioned it. IT WILL cause a loss of regulation AND then blow the H-out, along with blowing that 100 ohm resistor someone spoke about and also kill a couple of medium power transistors, Q-402 and Q-403, in the standby supply. I have fixed MANY of these over the decades. Once fixed--they are not too bad of a set. The way this supply works is..in STANDBY..the 30130 is forced to run at only 16 volts or so, by those 2 transistors and then at power on--it goes to full output. DO NOT forget to check and see if there is another TO-220 transistor close to this chip, it is sometimes used as a "sub regulator" and usually dies if the 30130 goes.

[Reece]

I will recheck all those but had just changed many electrolytic caps in this set just prior to the accident. What caused the problem was the HV wire that should have been fastened to the CRT was not secure and slipped out and arced down below shutting the whole thing down.

[Reece]

I've been away from this project for a while. It's a Philips Magnavox 25X601 00AA. In the meantime I checked virtually all the 400 series and 500 series components in the set, dealing with power supply. All electrolytics in those areas were replaced. All diodes and resistors checked OK, as did all transistor devices except for the following: IC410 STR30130 voltage regulator, Q420 NTE399 start up switch, Q501 NTE399 video output, Q502 NTE2353 HOT. All were replaced with the NTE numbers shown. In many cases I had to remove transistors and lift one leg of diodes to test them properly. It's been an instructive exercise in component checking (and going blind!)

I plugged all the cables back in and attached a swarm of DVM's to monitor voltages. With unit plugged in but not turned on:

Test point 10, expect 5V standby, actual 4.95V.
Pin 4 STL30130 output of voltage regulator, expect 16V, actual 15V.

Turned set on with remote control:

Test point 10, drops to 1.46V.
Pin 4 STL30130, expect 130V, actual 130.1.
HOT pin C, expect 127V, actual 129.5.

With set on: no raster, no sound, no HV. Cannot turn set off with remote or with set button. Following troubleshooting procedure in Sams, I got OK as far as Q462 8V source, which should have pin E at 8.7V, actual 0.06V. It says to check that transistor, and it had already checked good out of circuit. After that I'm kind of lost. Would appreciate any suggestions (besides pitching this thing out! I know it makes no economic sense to keep at it, but I hate filling up landfills, and it has a good strong tube and has always made a good picture. I have several good working sets so don't "need" it. If I can fix it for a spare I'd like to.)

[zeno]

Originally Posted by Reece

I've been away from this project for a while. It's a Philips Magnavox 25X601 00AA. In the meantime I checked virtually all the 400 series and 500 series components in the set, dealing with power supply. All electrolytics in those areas were replaced. All diodes and resistors checked OK, as did all transistor devices except for the following: IC410 STL30130 voltage regulator, Q420 NTE399 start up switch, Q501 NTE399 video output, Q502 NTE2353 HOT. All were replaced with the NTE numbers shown. In many cases I had to remove transistors and lift one leg of diodes to test them properly. It's been an instructive exercise in component checking (and going blind!)

I plugged all the cables back in and attached a swarm of DVM's to monitor voltages. With unit plugged in but not turned on:

Test point 10, expect 5V standby, actual 4.95V.
Pin 4 STL30130 output of voltage regulator, expect 16V, actual 15V.

Turned set on with remote control:

Test point 10, drops to 1.46V.
Pin 4 STL30130, expect 130V, actual 130.1.
HOT pin C, expect 127V, actual 129.5.

With set on: no raster, no sound, no HV. Cannot turn set off with remote or with set button. Following troubleshooting procedure in Sams, I got OK as far as Q462 8V source, which should have pin E at 8.7V, actual 0.06V. It says to check that transistor, and it had already checked good out of circuit. After that I'm kind of lost. Would appreciate any suggestions (besides pitching this thing out! I know it makes no economic sense to keep at it, but I hate filling up landfills, and it has a good strong tube and has always made a good picture. I have several good working sets so don't "need" it. If I can fix it for a spare I'd like to.)

Maybe I missed it but whats the model & chassis ??
This set does have a "different" on/off circuit but I would need a manual.
If the 8V reg has the right C & B voltages there may be a dead short
on the 8V line. You have HOT C voltage but may not have hoz osc/drive.
See what source the hoz IC uses. It may not be turning on. If I got
the book on it I can help further. Been too long to remember everything
abt the set but we did see a lot of em.

73 Zeno

[Reece]

Zeno, model 25X601 chassis 00AA. I will try to check more on your ideas tomorrow.

[Reece]

This afternoon I decided to check voltages as Zeno suggested. Some were there, some weren't around Q462. Jungle chip IC270 showed near 0.6V on pin 36, horizontal output, the way it should. That feeds over to Q501, video output, which feeds the HOT. Q501 is supposed to have 33.4 volts on C and it had nuttin'. It's fed through T504 the horizontal output xfrmr which gets its 129V through Q400 reg and ultimately from pin 4 of the STR30130. I checked continuity from C on the video output to the xfrmr and it showed open. I remembered when soldering in that transistor that the pad and trace were a little flakey. I soldered in a jumper and tried the set; it came to life! No sparks, no smells, no smoke. This is out in the shop with no cable so I hooked up a Zenith digital-to-analog box and rabbit ears and watched a number of channels. All looks good. The heat-sinked devices were just a little warm after running for twenty minutes.

I will cook this for a while and monitor some voltages over the next days when I am in the shop before buttoning it up. Thanks to all who made suggestions in the past.

[zeno]

Nice work !
"dead set" problems can drive you nuts on newer sets. So many
things can cause it & so many ways it was done in later years.
Usually a matter of knowing how to force things on & knowing
the quick test points to narrow it down to a stage. I think its
one of the things I miss most
Now go out & get another set for us all to work on

73 Zeno

[Reece]

I have another set about the same size, a Mitsubishi, but it works OK. It's of a vintage that it might have some bulgy caps, and somehow the on/off button on the front is missing (but I have the factory remote ) so will open it up to dust it off and check it out and see what I can do about the mechanical button problem: hopefully it's down inside somewhere.

I'd better not get any more of these things: I've got no place to put 'em!