Fostex B16 repair - schematics?

[joakimkarlthomas]

My Fostex B16 won't play the tape anymore. I found one or maybe two burnt transistors (Q33 and Q32) when doing a quick check. At one of them even 1 or 2 of the solder pads was a bit loose.  I guess this if at least a part of the problem, but I would like to found out more. I can find a service manual and a consumer manual but none of them have a schematic.  Does anyone have a schematic for this machine?

The first transistor I'm looking for was marked B892 and T-4B, the second D1207 and T-4G. They seem hard to find where I live, can I use something else?

 

[analogguru]

Both transistors don't have anything to do with the transport control AND are not parts Fostex originally used.

Q32 and Q33 are use for the (Bias) Master Oscillator.  Q32 is originally a 2SC2655-Y and Q33 is a 2SC1020-Y - both should be easily available (if really broken).

 

[joakimkarlthomas]

Okey! Thanks!
Not sure the transistors are broken, but it looks a bit burnt right under them on the pcb and the solder pad are a bit loose on one of them. But it seems like the pads still make connection.

Anyway, if this is not the problem Im looking for I can leave if for later..

I was going to check the voltage at TP2 och TP3 to check the Tension sensor adjustment. The service manual says it should be 8V, but I have 0V at both TPs..

 

[gyraf]

The service manual says it should be 8V, but I have 0V at both TPs..

Have you checked all supply rails first? That would be the first place to look for a global malfunction, with or without a schematic...

Jakob E.

 

[Mylithra]

My Fostex B16 won't play the tape anymore. I found one or maybe two burnt transistors (Q33 and Q32) when doing a quick check. At one of them even 1 or 2 of the solder pads was a bit loose. 

Are you sure its a burnt part or could it have been a poor repair attempt?
In any case,  as Jakob noted, is the power supply working? Are you getting voltages out of it?

 

[joakimkarlthomas]

Have you checked all supply rails first? That would be the first place to look for a global malfunction, with or without a schematic...

Jakob E.

Thanks. Now Ive checked the rails and it's 7 rails labeled on the psu pcb. 5 of them are close to the labeled voltage, but the 15V shows 1,6V and the 26V rail is instead 48V.

 

[joakimkarlthomas]

The 15v regulator was broken, so the 15v rail was not working. I put a new one in and same thing happened to the new one. There’s about 48v going to this regulator, the new I got has got a tolerence up to 35v. Is the 26v rail supplying this 7815 regulator? This 26v rail that I also measured about 48v before?

I also found that the AC legs of one of the two recifiers is open. Can this be a sign of a broken rectifier?

 

[analogguru]

.....
Is the 26v rail supplying this 7815 regulator? This 26v rail that I also measured about 48v before?

I also found that the AC legs of one of the two recifiers is open. Can this be a sign of a broken rectifier?

It was a common fault that D1, the square rectifier creating the +26V got damaged because the heatsink screw loosened.  Since there was no hole in the pcb the only chance to tighten the screw was to unsolder the whole rectifier including the heatsink.

Later on the heatsink was additionaly glues to the rectifier (KB802).

The latest versions was that the rectifier D1 was changed from the KB802 to a 15D4B41 which was mounted on an external heatsink off the pcb and wired to the pcb.

 

[joakimkarlthomas]

The latest versions was that the rectifier D1 was changed from the KB802 to a 15D4B41 which was mounted on an external heatsink off the pcb and wired to the pcb.

Thanks! D1 is 15D4B41 in my machine. Heatsinked externally. I think it’s broken. Seems hard to find a new one? Any replacement that works?

 

[analogguru]

You can use every squared rectifier block rated with 15A, 25A or 35A and 80V and up, e.g. KBPC2506 (25A/600V).

 

[andow]

I'm also looking for B16 schematics, maybe somebody can help me out....

 

[Bjorn Andersson]

I have posted the B-16 service manual to admin to put up in the technical section
And a very important service bulletin for the B-16

 

[andow]

That would be great! Does the SM include the schematic (the one I have does not...)?