Pentranic ch-88 red on full red bias not working

[Retroman839]

So trying to get this chassi working

Pentranic ch-288,

All seems ok except red is full on and can not be turned down
The bias pot works for green and blue but not for red..

When I first turn the monitor on I get white screen then after a couple of seconds
The red kicks in on full power…

The potentiometer for red bias does not do a thing..
So am I right in thinking a resistor in the circuit or the pot?

I have been studying the schematic
Here is the circuit for red

Can anyone give me a pointer to which component part maybe Likley culprit?
I’m hoping something on the neck board
Pot, vr, transistor, resistor?

Any help massively appreciated

Retroman8392022-05-25 22:59:02

 

[big10p]

Leaking transistor Q711, maybe? Swap it for the one in the blue or green circuit to see.big10p2022-05-25 23:37:15

 

[gunblade]

one day they will find you fried in the corner of your workshop

is that the original tube/chassis pairing?

 

[Retroman839]

I take precaution.

Well grounded.
And I have a good discharge for removing anode, and discharging H V.

No this is not the original
But chassi is very close

Tube is

Mo sells
CH2948A These where paired in wangan midnight driving game
And I’m trying a ch 288

I had It wired up to jamma loom today with Pandora’s box 4s 15khz
And I had a a good stable picture..
Good focus…
But color bias was off. The Red was just full throttle no way to dial it down, the pot on kneck did nothing.
Retroman8392022-05-26 00:54:46

 

[Retroman839]

Leaking transistor Q711, maybe? Swap it for the one in the blue or green circuit to see.

Cheers I will give it a try..
I will just get a new transistor and try it..

I actually have a NOS of this chassi
So I could swap kneck cards over

I only want to use 15khz on this tube,
I think the wangan midnights where 31khz that used the ch 29 48a digital chassis

I was thinking 15khz only on the ch 288 and I might be ok

Retroman8392022-05-26 07:33:05

 

[gunblade]

surprised yoke is a match

i think the issue will be the crt pinout, show me a photo of the neck card solder side

 

[Retroman839]

Just going to work shop
Looking for cold resistors /transistors
On the red circuit also

This next pic is the nos chassi

Tried soaking in as much as poss
And just can’t get it any better

Chassis must just be out of range ?Retroman8392022-05-26 10:45:12

 

[Fantazia2]

Is it a known good tube?

Could it possibly be the red gun has a shorted internally?

 

[Retroman839]

Hey man
One happy camper here

I’m going to pursue the repair on the other neck..
So the large colour resisters on the known bad neck card where hot..
But the one for the red was mega hot and it’s a different
Colour banded resister?
Retroman8392022-05-26 12:14:17

 

[thegreathopper]

Excellent, Well done

thegreathopper2022-05-26 12:17:40

 

[Retroman839]

Chuffed to bits…
Was all worth while in the end!

Just tweaking and she’s looking sweet!!

Retroman8392022-05-26 12:49:46

 

[gunblade]

does look good, geometry is good and therefore yoke must be a good match, colours and contrast good so heater current must match

 

[Retroman839]

Yeh I think I found the fault also on the bad neck pcb

Large resistor for the red colour is a different value( has different colour bands )

And is burning hot.
Now I see the Charing on the legs

So I’m going to change the transistor & this resistor to match
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Retroman8392022-05-27 07:07:43

 

[gunblade]

yes that sounds the right course of action to me

 

[Retroman839]

Good to know I’m going in the right direction

Finally found a schematic I can read!

Thanks Gunblade

Retroman8392022-05-27 15:18:14