Mitsubishi HS-B41 VCR switches off after one second

[Quantumplate]

Hello out there - my first ever posting so y'know....
Digging into the power supply board on this 31 year old unit that my dad bought back in the day. Replaced the caps (that didn't solve the issue) and have now tested all the components and the voltages on the two transistors shown in the pic. Everything's ok apart from the collector of Q901, which is showing 49.2v ! (btw emitter is also 49.2v as the mains here runs at 247.5v, base v was 48.5)
I removed it from the board and it tested fine...of course. Confusing for my beginner's brain    Anyway I stuck a new one in...and same rogue voltage on the collector. Can anyone enlighten me as to what's going on?

[shakalnokturn]

I expect the +31.5V & -30V supplies are for the VFD?
The stated +31.5V should be the typical working voltage, if unloaded because display is not lit it it would make sense this voltage is too high.
Do you have about 6V AC on transformer's bottom winding? (Filament)

Can you better describe the sequence when plugged in?
Display, sounds, drive movements...

[fzabkar]

As I see it, Q901 is an 8mA current source. The voltage across R903 is 0.6V, so I = 0.6 / 82 = 8mA.

[Quantumplate]

Thanks so much for your help - motivated me to get it on the bench once more.
you're probably right about these being the VFD voltages. This manual schematics are way outside my comfort zone in terms of complexity and run over lots of separate pull-out pages! Hard to digest.
The symptoms are: I switch it on at the front (or push a tape in to activate the power) and within a second the red on light goes out again. Seems the microprocessor is not happy about some condition. The display briefly comes on during this period. If I push a tape in it will suck it in and lower it, but won't unload the tape as it's turned itself off by then.  I've taken apart and cleaned the mode switch and replaced all the caps on the main power board. There is a sub power board with an electrolytic on it I've yet to get to.
The transformer's bottom winding is giving me 4.9v ac rms, so about right - 4.5 in the manual (you can't see the other half of the schematic for the power boards, I can post if you want?)

[shakalnokturn]

It's a long time since I worked on VCR repairs and even back then I don't remember going through many Mitsubishi's.

A mechanical fault (jammed or out of sync) could cause the CPU to detect a fault condition or cause power to drop.
It can be worth checking drive belts and maneuvering the transport by hand to make sure nothing jams.

How many of the power rails are actually switched? I'd expect a permanent 5V and maybe a switched 5V and 12V, if switching is done by transistors check for weak solders there.

Edit:
If there's a clock backup battery it could also be an idea to check that isn't dead, low backup battery voltages can cause odd symptoms sometimes.

[Quantumplate]

It's the first time I've ever worled on a vcr apart from a belt change. Been a reliable machine for 31 years.
I think all the mechanics are ok, at least nothing obvious, and it was quite happily running until very recently. Thinking back I do remember it doing this power off thing a few times in the past before righting itself if I just kept turning it on.
Hmm not sure how to answer your question re switching. A bit beyond my abilities. I've been warned by a vcr engineer that this problem could be a lot of work to track down. He doesn't have time to help me any more sadly.
Btw I meant to ask what you meant by 'filament' above?

I'll try and measure that collector voltage tomorrow and see if it dips when I switch on. Your original thesis did cross my mind. Thanks

[timenutgoblin]

Everything's ok apart from the collector of Q901, which is showing 49.2v ! (btw emitter is also 49.2v as the mains here runs at 247.5v, base v was 48.5)
I removed it from the board and it tested fine...of course. Confusing for my beginner's brain    Anyway I stuck a new one in...and same rogue voltage on the collector. Can anyone enlighten me as to what's going on?

What voltage are you measuring on the anode of D915? If Q901 is a current source or current-limited source then there should be some resistance between the collector terminal of Q901 and ground. If you're measuring 49.2V at collector and emitter of Q901 then maybe you have an open circuit between collector terminal and ground. Maybe a circuit is switching on, but then goes open circuit.

As noted on the schematic, 31.5V on the collector at 8mA is equivalent to 3K9 of resistance. That being said, there isn't necessarily a 3K9 resistor in the circuit.

[fzabkar]

Check the sensors:

http://repairfaq.org/sam/vcrfaq.htm

[shakalnokturn]

....not sure how to answer your question re switching. A bit beyond my abilities.
.....
Btw I meant to ask what you meant by 'filament' above?

I haven't found the full schematic online.
If you have it, what happens to the 3 unregulated outputs from the power supply? I'd expect some regulators are used before supplying 12V and 5V IC's.
There will also be some power switching for standby mode done by transistors or by dedicated voltage regulators.

The "filament" relates to the VFD: They are basically a triode with a phosphor to get some colourful glow on the anode, there are several parallel filaments (heated) that also serve as the cathode from which the electrons are emitted.
Between filament and anode are multiple fine meshes, one for each segment of display, they're used as on/off control grids by the display IC.

[Quantumplate]

Thanks. Atached is the other half of the power schematic. Hopefully this will answer your questions as I can't say anything with confidence myself. But as the 31.5v gets cut down to 5.1 as an input to the opamp in the IC with a 15K resistor, then I make the current just over a milliamp...is that right?

Only connector PD is attached when I'm doing the voltage tests as the other connector leads are so short.

[Quantumplate]

Update - after a 'fun' evening juggling with too short connecting leads, I freed the sub power board with the large heat sink attached. The diode and capacitor on it checked out ok. Then i was able to reconnect the the main power board and carry out measurements on it.
So I don't know if this will turn out to be an intermittent fault ( for which i apologise!), but when i switched the ON button, this time it stayed on. I quickly measured the Q901 collector V and it was 32.0v as it should be. In the excitement I forgot to turn OFF and measure the collector to see if it rose to 49.5v. Told you I was a beginner..

So I'm nun the wiser. I blew some dust off the sub board and rehoused a spider. Maybe just a good bit of dismantling and reconnecting has done something.  Anyway we'll see in the morning, not dancing on the ceiling yet.

Thankyou to everyone who's chipped in. 

[Quantumplate]

For anyone searching for the answer to this issue on these machines, the HS-B__ range, the problem WAS the 33uF 16v little cap. Although it tested good it wasn't.