1981 Toshiba T250 Monochrome CRT board issues

[daverand]

I was bringing up my old T250... power supply came up fine (no RIFA caps), 8" drives came up fine (well, I did have to disassemble the spindle bearing and grease it), motherboard works... but the monitor failed after a few minutes.

I pulled all the semiconductors and tested them - they seem fine. The caps test ok (but I will replace them anyway). Found two tantalum caps that didn't short (!). One of the two fusible resistors on the CRT driver does appear... suspicious. It obviously got hot, and the value is unreadable. The other one is 0.5 ohm.

Does anyone have a lead on a service manual for a Toshiba monitor? The flyback used is a DH-1183 (T-WGS14) and the board is labelled "PB-811B". Suggestions appreciated.

I am creating a schematic if this helps anyone else.

 

[daverand]

The CRT itself seems to be a Toshiba E2710B31-3MHT, if that helps... anything similar will allow me to do a better guess of the fusible resistor...

 

[Chuck(G)]

First things first--does the heater in the CRT light up?

 

[daverand]

Yes. Heater is fine. Since the flyback is not being powered (due to the suspect fusible resistor, or the horizontal oscillator not running), no higher voltages present. Nothing getting very hot (any more).

 

[daverand]

I did up a quick-and-dirty schematic... a couple of uncertain values. I'll refine this over the next couple of days, and check everything.

 

[daverand]

Huh. After finding nothing in particular, I shotgunned all of the electrolytic caps. To no one's surprise, that fixed nothing. Tracing through the circuit, I found good vertical deflection on the yoke, but no horizontal (and, no high voltage). Good HSYNC from the computer's board, clean drive into Q104, good signal into T101. Solid drive into the HOT (Q105) and... nothing on the collector. Just +12.

Pulled Q105 - tests fine.

Sigh. I've ordered some.

Here's the updated schematics, a bit more readable.

 

[gertk]

It seems like a contradiction if the base drive of Q105 is ok and there is no collector current (note: through T101 the base is mostly current driven since 'high voltage' transistors of that era have very low hfe or gain values).
If the collector is at 'solid +12V' it could mean:

a) the transistor is not doing anything
b) the LOT is dead short but then Q105 should get quite hot..

 

[daverand]

Q105 does not get hot, at all. It just looks like it is not turning on. What's worse is that it tests fine - hfe of 31 according to my cheap tester, which seems to be in line with the datasheet (10 to 80). "at solid +12" means I scoped the base and collector of Q105, and while there is a pleasant square wave at 15khz on the base, the collector is at +12v with no ripple at all.

Possibly T101 has a shorted winding, and it is not delivering the full current to turn on Q105? I'll know more when the replacement transistors arrive.

Again, it did work when first switched on, but the CRT was somewhat dim, and I did not get a chance to tweak it before it failed. R115, which I thought was a fusible resistor, got very hot at some point in its life, and there are no longer any markings on it. 20 ohms was its measured value (out of circuit). I will also measure the voltage across this when I replace the transistor, to see how much current is going into that primary on T101.

If you have similar schematics using the 2SC2335, I would sure like to look at them...

 

[daverand]

Well, after replacing the HOT, the power supply cratered. And, after replacing the power supply with a modern equivalent, I now have my original T250 back running, booted from a 40 year old disk!

 

[gertk]

Good work!