Pentranic CH-888 19" CGA

[Liam]

Does anyone know anything about this chassis? It's labelled 21020888-01 CH-888 Rev: 0, and the part numbering is different from the schematics online for CH-288.

HOT is referred to as Q304.

The flyback is CH--9046 and is referred to as T302.

Chassis is ticking, and flyback seems to be unavailable, making the whole thing appear obsolete.

 

[gunblade]

if its a ch-888 then the HOT is Q302 2SC5250, Have you checked it for a short?

 

[Liam]

Thanks. In this monitor it's a BU2507DX. I took it out, and it appears to be short between base and emitter.

 

[gunblade]

well thats odd as the 2sc5250 does not have a damper diode whereas the bu2507dx does

 

[gunblade]

i have attached the schematic, hopefully this is your chassis

 

[Liam]

Great, the part numbers match, except HOT is Q304 here, and Q302 in the schematic. Maybe that's what the A in CH888A is for. Mine is just CH888.

 

[gunblade]

do all the other components match?
did the bu2507dx look like its the original transistor or do you think someone has replaced it at some point?

 

[Liam]

From what I can see, all other components match, and the component numbers on the board match, except for this one. The solder looked original.

 

[gunblade]

ok i just find it odd that a taiwan built chassis would use a phillips transistor which you only really find on european chassis plus the schematic does not show a damper diode built into the HOT

 

[Liam]

I think I'll change it for 2SC5250. If the HOT is the only change they made when revising the chassis, it might be alright. It's also cheaper and easier to find.

 

[Liam]

New 2SC5250 did the trick, which I didn't expect, as it tested the same as the old HOT. Still some way to go, though. All colours are present, sync controls work, but graphics are saturated, no degauss, apparently no contrast control, and any attempt to change picture height or vertical size results in momentary vertical collapse.

 

[gunblade]

use gain pots in place of contrast but you may need to change the value of the heater resistor - sort degauss out first
does the ptc read ok, you can connect a main lamp to the degauss header to prove, there is an option for a switch as well - can't remember if this is the chassis with degauss switch on remote board
replace vertical size pot

 

[Liam]

No degauss switch on this one, but I have some of those remote boards with the switch in my spares, for some reason. I'll try those things. This will need a new tube if it works properly. Burn is so bad I can read everything from the poker game. I have a couple of Daewoo and Bush 20" TVs from the early 2000s that might suit.

 

[gunblade]

i can't remember if that chassis used a particular remote, i am pretty sure it did use the one with a degauss momentary switch, if you have the wrong remote that would explain the vertical collapse issue - i can't find any photos of this chassis with a remote board attached and i have only had to repair a couple so can't remember. In any case the connectors would not fit if it was the wrong remote

 

[Liam]

I'm pretty sure the remote board is the right one. Degauss is working. It just wasn't connected properly. The vertical size adjustment is the only one that causes fluctuation in height and momentary collapse, but the height is adjusting as it should, and settles down when the knob stops turning, but sometimes changes in height require a change to the v-hold because the picture starts to roll.

The sub-brightness control on the neck card is working, but neither the brightness nor contrast controls on the remote board do anything. It's like they're not connected, but the leads connect to the board as they should, so I'm not sure what's wrong. Contrast is really off, like it's turned up to the last. It's like the picture is over-exposed.

Those two controls go through a separate loom from the remote board, while all the rest go through a larger loom. The V-Lin control also goes through the smaller loom with brightness and contrast, and that appears to work, although the adjustment is very subtle, so it's not the loom itself or its connectors, as far as I can tell.

 

[Georgian2]

I'm pretty sure the remote board is the right one. Degauss is working. It just wasn't connected properly. The vertical size adjustment is the only one that causes fluctuation in height and momentary collapse, but the height is adjusting as it should, and settles down when the knob stops turning, but sometimes changes in height require a change to the v-hold because the picture starts to roll.

The sub-brightness control on the neck card is working, but neither the brightness nor contrast controls on the remote board do anything. It's like they're not connected, but the leads connect to the board as they should, so I'm not sure what's wrong. Contrast is really off, like it's turned up to the last. It's like the picture is over-exposed.

Those two controls go through a separate loom from the remote board, while all the rest go through a larger loom. The V-Lin control also goes through the smaller loom with brightness and contrast, and that appears to work, although the adjustment is very subtle, so it's not the loom itself or its connectors, as far as I can tell.

To me, it looks like the V-size potentiometer is bad. It could also be bad solder joints between the remote pcb and main pcb. So when you touch it, you move that board and cause bad contact. Otherwise, does the vertical collapse always happen when the potentiometer is in the same position?

 

[Liam]

I'm not sure. I didn't get to work on it today. I'll take out the pots and check them first, then trace back through the circuit.