Help with Power supply to Florescent display circuit fault

[Jimkarl]

Have a Tascam Portstudio 244 (MKII) 4 track cassette recorder. Initially, it had no power at all, tape transport mechanism not moving, no display,etc. Dead. I checked the power supply board and found several leaky/bad caps and replaced. Was enough to get the tape mech working but still couldn't resolve the display. Couldn't find freebie schematics anywhere (MKII is a unique beast from the other MKi/MKiii models with the power supply being internal) so broke down and bought the service manual. Unfortunately, it hasn't help me resolve so I'm hoping someone here with more experience than I (that's everyone probably) can see what might the cause.

So this is the bottom portion of the power supply board that feeds the florescent display board. The 3 lines at the bottom right go off to the display (F1, -30V, F2). I have replace the 3 caps around the Q6 circuit (though they tested good). I had killed Q6 on a previous repair attempt so that has been replaced as well and tests fine. I checked the diodes .678/0 for two of them and .522/0 for the D12. All resistors measured good in spec (R14/R15 are 4.7 ohms, reading 5.1) I am getting slightly lower than -30V at the Emitter of Q6 - I'm seeing -25.9v there, with -26.6v on the base and -36.8v on the collector. For F1/F2 - I'm seeing -23v



This the FL board (display) below. The service manual doesn't tell me what F1 & F2 need to be to turn this display on. In previous repair attempts, I had accidentally slipped my probe between the -30V line and F1 and/or F2 (different times) and the display actually lit when that happened. That was before some component replacement - the display no longer lights when I do that now. I'm not sure if the voltage differential isn't enough now/too much, my -25.9v is lower than it used to be, or if I possibly did some permanent damage to the florescent circuit.



So I guess my multiple questions include:
1) Any idea why I'm only seeing -25.9v instead of -30v and is that relevant?
2) How am I getting -23v on F1/F2? Is D13 (RD5.1ESAB2) dropping the voltage correctly?
3) Probably most important to understand - how does this florescent work - What should be the expected voltages on F1 and F2?

 

[73's de Edd]

Sir Jimkar . . . . . . .

Go back to the power transformer and power up and measure for the AC voltage across terminals 12 to 14 and then move over past R14 and R15 series resistors to see what your AC voltage is there.
Pass us the AC voltage measured in both places . . . . . . . expecting it possibly around 3-4 VAC.
I can't quite make out your central zener diodes markup . . .but go to DC mode, power up the unit and probe across it and see if it isn't either 5 or 6 VDC .
(My mistake . . .just found that the image would mag up and that then reveals it being a Schottky . . .but what is its DC voltage being read across it ? )



73's de Edd

 

[Jimkarl]

You are "Da Man"!! I had no ac voltage on the transformer side of those resistors (but had 5+ VAC at the transformer) Bad traces. Created to jumper wires and display it lit up lie a candle now!! And Zener showing 5+ VDC across it now. Thank you very much!
Can you explain briefly how this display & circuit works? I do want to understand the "how" of it all.

 

[73's de Edd]

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Sir Jimkarl . . . . . . .


but . . . but . . .BUT . . .BUT . . .BUT! . . . . . . . .You be "Da Man" . . . . . since YOU fixed it !

Even with my 100 wpm keyboard poking . . . . I am pressed for time at the present, in needing to analyze my "numbah one " son . . . . .Karl's . . . . Carrier Infinity . . . control board electronics . . . to get his AC working again.

In the interim, see if this info below doesn't answer all for you.

Basics . . . .The filaments need heating and a switched electrode NEGATIVE supply voltage presence.


http://www.explainthatstuff.com/how-vacuum-fluorescent-displays-work.html



BTW . . .if your service data is being in electronic format . . . might I get a copy of it . . .to add to my 75 yr accumulated Million Dollar Archives.

73's de Edd


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[Jimkarl]

No - definitely you pointing me to the 5VAC. Interesting read on the florescent. I guess my question was more basic. How do you know to expect the 5VAC reading from the transformer? I get the voltage drop across the resistors but it never occurred to me that I was looking for AC voltage, given the -30V dc reading I was chasing.. In fact, that's the reason I ended up buying the service manual - I had no way to tell what the expected readings where on the transformer to begin with and turn the the schematic didn't say anyway, so that was disappointing. Had the schematic stated the output , I would have certainly checked it.

Unfortunately this was a paper copy - and an unwieldy one at that the schematic are HUGE pullouts and I barely got the portion I needed to for this post on my little printer scanner (after several tries). I have others that are electronic that you are welcome to though! Most from Hi Fi engine though so readily accessible.

 

[73's de Edd]

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Sir Jimkarl . . . . . . .

Mark up as mission accomplished.
I didn't get involved but is the now dormant Kenwood TK-40Q1 still on your to do list ?

73's de Edd


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[Jimkarl]

LOL - Please don't remind me. That thing has been sitting (covered) on my "workbench" (Crappy plywood pool table) for what seems like a year now. I just never seem to find the time, or the will, to get back to it. Way too many unfinished repair projects that are higher priority. I don't even remeber what the problem was it's been so long. The truth is, I've learned a ton from people on the forums since that one and I may actually be able to solve it now LOL, but as I recall the board tracing was a lake of solder nightmare and was darn near impossible to find the right component pins in the sea of solder. That's another reason why I'm avoiding it. One day I will get back it though and surely be posting on that thread again...

 

[73's de Edd]

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Sir Jimkarl . . . . . . .

You had broken a lead totally off from a regulator xstr.
And that was probably because that odd transistor is like a TO-92 but has a 3/8 width of 1/8th of an inch thick . . . . . . . . .repeat . . . . . 1/8th of an inch thick aluminum conforming heat sink formed around that transistor case and then sticks out as a tab about 1/2 of an inch.

I can see you grabbing that tab and cranking it to reposition the transistor, and easily levering off a lead !

I'll tell you a much better . . .AND AVAILABLE . . .unit to replace it, when you get . . . A ROUND .

73's de Edd