Philips-Magnavox chassis 25X601 00 AA

[Reece]

I keep learning as I go on solid state stuff. This set just quit dead one day, no smoke, no clicks. Fuse OK. HOT was shorted across all legs. Crossed to NTE equivalents and replaced HOT, chopper transistor, bunch of electrolytics in the HV "cage," HOT snubber cap, big 680mfd 'lytic. All fusible resistors are OK. Still nada. I have no 5V standby voltage. I have no schematic so going blind chasing traces. What produces the standby voltage on this chassis? Appreciate any suggestions and a schematic etc.

[Dangler]

Try checking C432 (100 mfd @200V), R432 (1K 1/2W)

I have the Sams 3724 on CD that I can email to you if you want.

[Reece]

That cap is one of the ones I had replaced, and the resistor I had already checked, reads 990 ohms. Thanks, sending you a PM.

[Dangler]

I sent it to your other email address, but just to be sure, here's the Sams.

[Reece]

Thank you! I printed it out and am saving it on my laptop for easier viewing while working. Already I found F401 open, a little wired-in fuse that looks like a diode hiding among some resistors. Would never have found that without the schematic. Of course, now I have to find out why it opened and go carefully.

[Dangler]

Sounds like the regulator is shorted (IC410) (STR30130). The fuse is probably a pico fuse, 2.5 amp. Shouldn't be too hard to find if you don't have one on hand. I think I still have a good supply from my repair days.

Sometimes when the HOT shorts out, it will take the regulator with it. Another thing to check is the horizontal driver transformer (T504) for bad solder joints. That could be why the HOT shorted.

If you need a regulator, let me know, I have a few NOS ones for free.

[Reece]

I am suspecting the regulator: it's supposed to read like a diode between legs 3+4; that is, reads a value one way and not the other, but it shows open both ways. I had already checked those transformer terminals and they were well soldered, as were all the ones on the FBT. The different FBT windings ohm out OK per the schematic. Will send you a PM.

[Reece]

In checking over the devices suggested in the Sams power supply troubleshooting section, I find one transistor "iffy." It's Q462, identified as "8V source," and marked 554-3 and marked under that the Motorola batwing symbol and 617. I can't find it or a cross for it anywhere. Anyone know of one?

[Zenith26kc20]

I HOPE I found the correct notes...
C432 opens, killing Q402 and R432. It can also kill R435 which will give a small picture after the repairs are thought to be done. Q403 also can fail and I think it caused the set not to turn off. The regulator does not usually fail but Q400 will fail if high voltage rises too high from C432 opening. This sets regulator IC is actually a driver for the true regulator (Q400). Pin 2 of the regulator is pulled down to 17 volts when the set is off and rises to 133 when on. Kind of a twisted power supply but it works!
A shorted HOT will kill Q400.
Be careful with vertical electrolytics also, they will open and kill the vertical out IC.
I'm pretty sure these are the correct notes on this set. It applies to many of the Magnavox chassis out there. The first sign of failure on this sets power supply is a scrambled picture when C432 starts to open. Keep playing and R432 will start to overheat. Keep going and the fun begins!
Never had any flyback issues on this one.
Otherwise very reliable sets.

[Reece]

Thanks, the set just quit without any "drama." C432 wasn't open or shorted or bulged but I replaced it anyway along with all the other 'lytics in the horizontal section. These that you mentioned were OK: R432, Q402, Q403, set doesn't have an R435. HOT was shorted & replaced along with Q400 & Q501. I checked C550 and it's OK but C556 & C558 are iffy, so will replace all three as long as I'm in there since they're so cheap. Also will replace IC410, Q404,Q464,Z462. Still looking for an = to Q462 per my last post.

You mention reliability: yes it has been, and it's good to see that they used all Nichicon original electrolytics. I usually order them or Panasonic.

EDIT: thinking back several years ago this set did have a scrambled picture pulling every which way and changing C432 fixed it. However now I realize that while that was going on some of these other components got stressed but not yet burned up, and now some of them finally failed.

[Dangler]

Originally Posted by Reece

In checking over the devices suggested in the Sams power supply troubleshooting section, I find one transistor "iffy." It's Q462, identified as "8V source," and marked 554-3 and marked under that the Motorola batwing symbol and 617. I can't find it or a cross for it anywhere. Anyone know of one?

According to the NTE web site, it crosses to an NTE 399. I really don't know if that is correct or not. It lists two 554-3's. The first is an RCA part and the second is a NAP (North American Philips). You might check it out and see for yourself.

I got the regulators and fuse out in the mail yesterday, so you should have them by Thursday.

[Reece]

Thanks, Dave. Yeah, I saw the NTE cross, but don't think they are right. They don't jibe with the functions. The RCA is a big IC, and the other sounds like it ought to be right, I'm guessing NAP is North American Philips, but it doesn't ring true per the description. And can't find the Motorola reference anywhere. This is just a little three-legged xsistor.