So ive been trying to diagnose my Fostex,
It started working sluggish and then started blowing fuses, then I got some magic smoke after opening it up. I started to tinker and had some problems with U4 always seemed to by smoking.

I decided to forgo everything and buy a new board with all new components, its a euro board( I live in America) but it seems completely the same besides a different transformer and fuses.

I replaced the board, and still magic smoke(which I believe is from U4 again) and fuses blown. Interestingly one of the fuses still tests for continuity but the other one doesn't. The only thing I can think is that the transformer is bad.

Anyone think otherwise? and also can't seem to get the transformer out of this shell too

 

The schematic is not shown fully.
What part of the schematic is the new board?
Since that is not clear, I would suggest tracing the output of U4 and where it goes to. Check those components.
Transformers of such types rarely fail, unless the ferrite core has been broken by mishandling.

 

Here's the full schematic, View attachment Fostex-ModelR8-tape-sm_Page_58.jpg

ah interesting, im kind of at a loss then. I noticed one of the capstan motor wires was off, not sure if this could cause fuses being blown, and the problems described

What part of the schematic is the new board?

What do you mean by this?

 

As stated in a previous reply, I would look downstream from U4 to see what is wrong. I am guessing that U4 is a voltage regulator that is struggling to maintain a voltage level and burning up. You likely have a short circuit downstream from U4. This will take some investigation, but try removing U4 and measuring from the pin 3 trace to ground. I suspect that you will see low/no resistance.

Dan

 

D

As stated in a previous reply, I would look downstream from U4 to see what is wrong. I am guessing that U4 is a voltage regulator that is struggling to maintain a voltage level and burning up. You likely have a short circuit downstream from U4. This will take some investigation, but try removing U4 and measuring from the pin 3 trace to ground. I suspect that you will see low/no resistance.

Dan

Do you think a bad motor could cause this?

 

If the motor is on that rail, yes it is possible. What connects to U4?

 

it looks like the preamp section u4 goes to a molex that supplies 15volts to it

 

Im looking at it now, if the connection to u3 to the Motor was bad would it send the voltage down to u4 causing it to overload?

 

I am assuming here that both U3 and U4 are voltage regulators. Each have common grounds and input voltages from what I can see here. If only U4 is getting damaged, look for problems downstream from there. As I initially suggested, check for shorted components (Q8,Q9 for example). Also, what model of Fostex so we can get the entire schematic. Only seeing parts is not helpful.
What is the part number forU4?

 

Here's the full schematic, View attachment 327028

ah interesting, im kind of at a loss then. I noticed one of the capstan motor wires was off, not sure if this could cause fuses being blown, and the problems described

What do you mean by this?

I replaced the board, and still magic smoke(which I believe is from U4 again) and fuses blown.

What board? Which part of the schematic does it replace?

 

Since both U3 and U4 are 15 V regulators, I still say that the problem is likely not on this board but whatever is connected to pin 6 of J2 which is somewhere else. Disconnect this pin and I suspect that the regulator won't fail. Check for a direct connection to ground at that point.

 

I Also, what model of Fostex so we can get the entire schematic. Only seeing parts is not helpful.

Its a Fostex R8 here's the full schematic https://medias.audiofanzine.com/files/fostex-r8-service-manual-482511.pdf
all the electronics start on page 36

What is the part number forU4?

U4 is an L7815

What board? Which part of the schematic does it replace?

I replaced the whole System Control pcb with a new old stock board complete with components. So it shouldn't have any faults as it was pulled from a working unit.

Since both U3 and U4 are 15 V regulators, I still say that the problem is likely not on this board but whatever is connected to pin 6 of J2 which is somewhere else. Disconnect this pin and I suspect that the regulator won't fail. Check for a direct connection to ground at that point.

Ok thanks! I will try this

 

Its a Fostex R8 here's the full schematic https://medias.audiofanzine.com/files/fostex-r8-service-manual-482511.pdf
all the electronics start on page 36



U4 is an L7815



I replaced the whole System Control pcb with a new old stock board complete with components. So it shouldn't have any faults as it was pulled from a working unit.



Ok thanks! I will try this

U4 drives the bias oscillator, but it also leaves the board (via W3) and goes somewhere else not shown on the schematic. Disconnect the plug for W3 and see if that removes the short. Also check the input voltage for U4 - the supply is wired somewhat unconventionally, and a fault somewhere else could cause the input voltage to be over 35V which is the maximum allowable for a 7815 regulator (it would normally be no more than 25V or so).

 

So while reassembling the machine today I found this, it seems maybe u3 might be the actual problem as this happened on my old board as well. I'm just trying to trace why this would happen. Seems like it supplies 15 volts to the take up reel motor? if this motor were to fail could this cause an overload effectively destroying the board like this?

 

You have the schematic and the (badly damaged) PCB so you're the only one who can fault-find. Note that the damaged ground track also appears to go to the chassis from C3 (via a link), and it's possible that there's a supply shorted to the chassis somewhere. A motor failure is possible (although unlikely IMO), but it's also easy to disconnect it and test it separately with a bench power supply.