1986 Fisher television
 

hi all, i finally found a Fisher PC-420 television i've been hunting for and it has some issues as expected.

the tv has what looks like barrel distortion the corners are pulled in on all 4 sides. also has some lines at the top of the screen that look like vertical comb over.

the vertical/PCC control board is separate from the mainboard. it has two electrolytic capacitors and a film capacitor. all were good but i replaced anyways. it uses a C2168 and A958 transistor which all check out. it appears the transistors are getting ~27 volts and the board itself is getting 89-90Volts from the mainboard.

the PCC transistor is okay as well.

i do not have a service manual for this TV and cannot find one. i'm not an expert in CRT's but i have some electronic knowledge.

anything else i should look for?

thanks

Is the yoke firmly seated forward against the funnel of the crt? If it has slipped back barrel distortion and corner shadow can occur. :scratch2:

jr

https://www.stereomanuals.com/man/re...ls_p-88_xx.htm
They list it. I know nothing about them.

If this is the chassis I am thinking of it has a SMPS with 2 ceramic modules.
They went DISCO in the 90's. There are 2 'lytics in the HV in series. One
is the HV filter the other the 200V filter for RGB outputs. They went &
are near the FBT. Try them first.
The supply should put out a solid & clean ? 130 ? VDC. There is
probably a marked TP for it. BTW If the modules go usually the set goes dead.

73 Zeno:smoke:
LFOD !

thanks for the replies. i did find that website with service manuals but they are long gone.

i've attached some pictures. the vertical board has a test plug that's plugged in not sure how that works.

the tv picture looks great outside of the distortion.

i'll see if i can check some voltages.

the yoke is up against the tube.

Chassis # A7R-42000

There are two test point posts near the flyback. i grounded my meter probe and i'm reading 85 volts dc on both pins.

looks like your b+ is low check filter cap in ps. i would try that first.

The TP for the main B+ is the cathode of a diode, off the SMPS transformer.
( rear edge of HV PCB ). There may be other diodes off it to run lower
level voltages. The main B+ can be 120 - 140 VDC depending on the set.
Also on the right edge of the HV PCB there is a big black cap. I think its
one of the trouble makers. BUT its been a LOT of years ! The
top vert problem may be another 'lytic IN the vert out. Find the main
problem first. There is probable a beefy diode in the PIN thats in the
hoz out area that shorts on many brands. Lift one end to test.

Zeno

thanks Zeno, i checked all the diodes and the remaining capacitors i didn't change and all check out okay. i've checked all the resistors.

kind of at a loss now. the Vertical/pcc board had two caps i replaced them with no change.

looks like i'm getting 158v dc past the transformer.

i replaced all the electrolytic capacitors with no change. still have 4 lines sagging down from the top. it appears the h width adjustment has to be maxed out to even fill the screen. also have a pair of very small lines rolling up the screen. thought for sure it would be a capacitor.

+1 for suspect PSU.

For indirectly checking the PSU, measure the video amp supply. Is derivated from flyback and usually have 175 to 200V. Suspect if less than 175V. All other flyback derived supplies will be low also then.

Is very uncommon but I've seen sometimes the H tune pulse capacitor (the ones with 1600V ratings in parallel with HO transistor) changing value to less, due to defect (near self destrucion), making image to shrink, but normally this makes all flyback derived supplies to rise (including the video output supply), and affects only the H size. Not seems to be the case, but anyway, if you have a capacimeter...

I found the service manual!!

anyhow it is low B+ 85volts manual says 110-112v. looks like the SMPS is regulating that way. looking through the schematics now.

that's good turn it on and get a hair dryer and heat up around those tree caps in power supply and watch the screen and see if the screen gets bigger and that's a bad cap in there. see it 100's of time. heat up caps in the horizontal caps too. of you do that sometimes you can find the culprit easier.

here's the schematic of the power supply. i'm not getting the 111v specified. i checked the circuit to the best of my knowledge and cannot find anything wrong. does anyone see something i should check?

i see you should gut it and put a fish tank in it. we have telling you where to look, but you don't listen. sorry, but were not there, so you have to try to find it. nobody can help without these measurements it hard to point you in the right direction. look its has a bad cap has either power supply or after to horizontal. to get true b+ measure the collector of h-output. i know everybody say's its a cap, but caps gave trouble when they new.

First there are 2 grounds. One is the primary ( hot ) of the SMPS & the
other the chassis ground. gotta use the right one or voltages
will be wrong.
If the 2 lines ARE from the set its 120 cycle hum & most likely a cap
on the primary side.

Zeno

thanks Zeno

i got rid of those two hum lines. the service manual tells me exactly which two test points to check. which i get ~88 volts manual says 111v.

i tried the heat gun trick near the octocoupler and the b+ goes down as well as the voltage to the octocoupler. manual says 40v in but i'm getting 32v.

i wonder if it's one of those error modules which i cannot seem to find.

your smps is running its has a oscillator like a horizontal sectioned the problem is after bow hot air on the capacitors. just fallow along the traces on the board and check the voltages.

I dont know your level so dont take this wrong !

Primary side ( hot) ground for your meter is the negative end of C011. Use it
for measurements on the RIGHT side of the SMPS transformer.

The chassis ( cold) ground is the tuner & IF shields, CRT dag etc.
The left side of transformer windings are the only cold grounds.

The isolator is to feed back the cold B+ to the hot side for a reference voltage
to control the PS output.

In the olden days we saw quite a few of these. If the transistor was OK we
would change the 2 modules & isolator. This fixed almost every one.

If you have a variac you can try turning it up & down a bit & see what happens.

Enuf fer now
Zeno:smoke:
LFOD !

thanks, i had another isolator i swapped out no change.

i don't know a lot about televisions but know my way around electronic components.

i disconnected the main resistor to pin one on the flyback and the b+ went to 107 which is still less than the manual states.

do you have any clue what those modules are called? i did a search and cannot find any info.

thanks.

have a great weekend!

Some testing to try...
1 - measure the photocoupler diode voltage (pin 1 to pin 2, IC001). If is less than 1V, the primary switching is soft, no longer providing sufficient power, and secondary comparator A003 no catch it and not makes the diode to conduct. Suspects: the A002 module or capacitors near it, including the C011 and all small ones (since some couple pulses and high ESR on then upsets the regulator).
2 - if measures 1V or a little higher in the photocoupler (indicating conduction of internal diode), the A001 module was in error in it's internal reference. For testing this, a dangerous proposal: sub this module for a series string of 110V zeners (or a 110V zener) only for testing, removing the A001 module (and changes the photocoupler connections for proper conducting, I can draw it if you wants to test it), to check if restores near 110V regulation. Is not recommended to run continously the zener string due to temperature voltage drift.

i'm pretty sure the A003 module is not working correctly. i'm trying to source the part but having no luck.

optocoupler pin 1/pin 2 measures 0.963volts

If is impossible to locate the A003, a replacement to consider will be something analogous to my explanation, but using low drift devices: 3x series 33V tuner voltage regulator (like TAA550 or equivalent), plus 2x 5V6 zener, all in series with the optocoupler anode, and optocoupler cathode wired to ground. This will results in a sufficient low drift substitute.
I made some voltage regulator based on these 33V tuner devices with very good results. One just for a TV, with failed regulator, for saving it from dumpster.

thanks Alex, it's not looking good on finding a replacement. although i can put things together. i'm not a design from the ground up circuit inventor.

it seems over in india they have these power modules that will solve what i'm trying to do. none in the usa

i took out the IC. it has a resistor, zener diode, and an npn transistor on it. are the flat black blocks just trace connectors? the schematic shows a wiggle line.

see pic.

Just to update.

with the help of Alex i determined it is the error detect module. i substituted some zener diodes and resistor and the picture is perfect at 111 volts on the B+

Excellent! One more TV working! :banana:

For completness, below I include the drawing of the low temp drift option I mentioned; needs to use it ONLY if common zener string changes +B voltage too much when chassis is hot.

Ops... a error: where reads "TAA320" is TAA550 or equivalent (33V tuner reg. IC).