Sony PVM-8020 repair

[decoder_82]

Hello !

I am trying to troubleshoot a Sony PVM-8020.

Service Manual can be found here: https://archive.org/details/pvm-8020-sm-sony-en_202104

The monitor has the following symptoms:

- No image
- Audio working fine
- high voltage present (if I increase SCREEN retrace line appear)

With a test signal in the input BNC, I have traced the signal until IC253.
Looks like there is no output on the RGB output (pin 26, 27, 28)

I can trace the input signal to the pin 5. The shape of the signal is OK but voltage too high (3.5v while should be 2.5v).

Here are the measured voltages of on the pin of  IC253 (UPC1365C):

pin 1 11.8v
pin 2 1v
pin 3 8.9v
pin 4 8.9v
pin 5 3.5v (should be 2.5v according to manual)
pin 6 2.9v (should be 1.8v according to manual)
pin 7 6.9v
pin 8 7v
pin 9 11v (should be 9v according to manual)
pin 10 2v
pin 11 1.3v
pin 12 9.3v (should be 7.6v according to manual)
pin 13 6.1v (should be 8v according to manual)
pin 14 GND
pin 15 7.9v
pin 16 4.5v
pin 17 8.2 (should be 9,6v according to manual)
pin 18 8.2 (should be 9,6v according to manual)
pin 19 0.2v
pin 20 9.3v
pin 21 3.3v (oscillogram OK)
pin 22 3.3v (oscillogram OK)
pin 23 -
pin 24 2.8v
pin 25 2.8v
pin 26 0v (should be 2.3v according to manual)
pin 27 0v (should be 2.3v according to manual)
pin 28 0v (should be 2.3v according to manual)

Any idea what I could do next to further troubleshoot ?

Many thanks :-)

[decoder_82]

As a next step, I will check the capacitors near the IC and see if there is anything bad.
I am wondering if the IC itself could be bad. I know replacement can be found on eBay but are relatively expensive. So I would prefer to rule out other faults first.

[Runco990]

The 8020 is notorious for the front pots having the wipers break off inside.  Check those first, as you may just not be getting any signal through.  I have fixed that issue on them 20 years ago already.

[decoder_82]

Thanks I will check. Would this prevent getting a signal from the IC RGB output pins ?

[Runco990]

I am not sure, but I would check the simplest and most obvious thing FIRST.  If the pots are ok, which is rare.... THEN move on to the electronics.  I am just telling you the most common failure of no color, no contrast and no video at all that I have seen in the past.  The electronics in the 8" sony's are pretty reliable.

[decoder_82]

Yes what you suggest makes sense. Thanks for the suggestion.I was not aware this was a weak point on those monitors. I will check this first.

[decoder_82]

So I checked the potentiometers on the front. I tested in circuit and for each when turning it, I could see a change in the measured resistance value. So I guess they are good (at least didn't fail open).
Any other ideas ?
Thanks :-)

[Runco990]

Well, you lucked out on the pots, then.  The wipers are just heat staked on a plastic carrier and often break off. 

Unfortunately, my thought process doesn't work well on forums, I have to have the unit in front of me with a scope.

At this point, can you turn up the contrast and brightness up enough to get a blank raster?

[decoder_82]

Here BRIGHT and PICTURE pots have no effect on the screen.
The HA board (which has the front pots) is connected to BA board. The BA-10 connector is about BRT and CONT from HA board. Strangely turning the pots on the front doesn't change the voltage I get at BA-10. I will dig more to better understand what happens.

[decoder_82]

When I move the sub brightness and sub contrast potentiometers I get a difference in the voltage at connector BA-10 not when turning the Bright and Picture ones. Will fully disassamble and check the pots out of circuit.
 

[decoder_82]

Looks like RV502 is bad when testing it alone.
Should be 20K/1K variable resistor.
Is there a way to fix it ? Looks like it was not made to be open

[decoder_82]

I the end with some contact cleaner I managed to get the pot work again. Thanks for the help.