View Full Version : Magnavox 25P601


colectorad
04-21-2016, 06:20 AM
The right edge of the screen is twisting:

http://s31.postimg.org/r8loac9yv/Picture_Bending.jpg (http://postimg.org/image/r8loac9yv/)

Some repair notes: C432 has been replaced with a 150uF 200V capacitor. Q502 has been replaced with a NTE2640.

zeno
04-21-2016, 07:18 AM
IIRC the P6 had an odd ball pass regulator. Both a transistor
& an STR. A small cap would open & a resistor would burn.
I dont have a manual & my memory is not there.
So attack the regulator & look for the resistor. IIRC it was
near a heat sink.
BTW careful not to run it too long or the HOT may short again.

73 Zeno:smoke:

rca2000
04-21-2016, 04:41 PM
You are correct. The P6 chassis is an updater of the S1 chassis and the 25" version does use a pass regulator transistor AFTER the STR reg. It usually is on the sink next to it--and will be a TO-220 style transistor. THINK it is a darlington. You should see a few volts of drop through that trans..if NOT it is either shorted...or the STR is bad. ..

zeno
04-21-2016, 07:11 PM
You are correct. The P6 chassis is an updater of the S1 chassis and the 25" version does use a pass regulator transistor AFTER the STR reg. It usually is on the sink next to it--and will be a TO-220 style transistor. THINK it is a darlington. You should see a few volts of drop through that trans..if NOT it is either shorted...or the STR is bad. ..

More IIRC's ! The STR gave the reference voltage to the passes
base. the on/off for the set was by turning on/off the STR. Over
kill if you ask me but NAP did some really crazy PS's over the years !
He should thank god its not a C-3 !!!! I hated those but it turned out
most had a bad FBT.

73 Zeno

radiotvnut
04-21-2016, 08:20 PM
Usually, what happened in those is the 100uf, 200V cap behind the flyback would fail, causing a picture that sometimes looked like it was out of horizontal sync. If the owner took it in for service as soon as it started acting up, a new capacitor would usually get the set back in business. However, many owners kept watching them and the bad cap would eventually cause the HOT to short. When this happens, it often blows the STR, the TIP47 transistor on the heatsink, and a resistor (1Kohm, IIRC; but, it's been a long time), and the solder-in B+ fuse (a tiny picofuse). In other cases, a few more parts in the power supply will get fried.

colectorad
04-26-2016, 09:03 AM
IIRC the P6 had an odd ball pass regulator. Both a transistor
& an STR. A small cap would open & a resistor would burn.
I dont have a manual & my memory is not there.
So attack the regulator & look for the resistor. IIRC it was
near a heat sink.
BTW careful not to run it too long or the HOT may short again.

73 Zeno:smoke:

Use this X6 service manual (http://www.videokarma.org/showpost.php?p=3056238&postcount=4), it's close enough.

R432 is fine. Q400 isn't shorted. Measuring between IC410 pins 2 (BASE) and 4 (OUT) shows 2.75K ohms in-circuit. Is this normal?

colectorad
05-17-2016, 03:03 AM
None of the voltages I've checked seem quite right. Here are my readings with the schematic values in parenthesis:

IC410 [STR30130] pin 2: 133.3V (131V)
pin 3: 160V (157V)
pin 4: 132.3V (130V)

Q400 [TIP47] base: 132.1V (130V)
collector: 136V (130V)
emitter: 131.7V (129V)

--

I swapped out the TIP47. Same results as above.

colectorad
05-31-2016, 03:52 AM
The collector of the horizontal driver transistor (Q501) is reading 60V. The schematic shows 33.4V as normal. R524+R504 read 1.7K (good) and R503 reads 5K (also good). What else could cause the voltage to be near double?

zeno
05-31-2016, 11:43 AM
be shure you have the right ground. The CRT dag is NOT grounded
on most newer NAP's . Best to use the tuner or IF sheild
as ground. If there is more than one ground such as hot & chassis
the manual will show different symbles.

73 Zeno:smoke:

colectorad
06-02-2016, 03:55 AM
^I haven't used the CRT dag, it's not grounded on this chassis. I'm using the heat sink, it's properly grounded.

colectorad
06-05-2016, 02:00 AM
1.) I've checked C502, C503, & C504 out of circuit and each tested good.

2.) The voltage drop across R503+C503 is only 3 or 4 volts. Is this normal?

3.) I increased the resistance of R504+R524 from 3.6K+3.6K (1.7K) to 4.7K+4.7K (2.3K) and the voltage dropped from 58V to 48V. What are the downsides to increasing the resistance?

4.) As the set runs, the voltage at the collector of Q501 slowly decreases, but doesn't reach the specified voltage. What's lowering the voltage?

colectorad
06-15-2016, 03:34 AM
It would appear that the SAMs has some errors and omissions. The Philips 7553 service manual (http://download.qrz.ru/pub/hamradio/schemes/tv/philips/PHILIPS-20X600/20lj26-20x600-service-manual-7553.pdf.pdf) should be used as the primary reference instead, with the SAMs as a back-up.

According to the 7553, the voltage at pin 1 of the horizontal output transformer should be 59V and 56V at pin 2, which is more or less what I'm getting. So, the h-driver circuit seems to be okay.

The beam limiter voltages are off:
Q661 emitter: 2.6V (0V)
collector: 3.4V (2.6V)
base: 0V (0V)

C661 is good.

The raster only straitens when I turn the G2 control all the way down.

Some other problems: C406 was bad, It's 10V-rated in an 8V circuit, so a 16V-rated replacement is preferred. C419 was bad, It's 16V-rated in a 14.5V circuit, so a 25V-rated replacement is preferred.