Power troubleshooting strategies?

[BigJoeScientist]

Hello, everyone. This is my first post... be gentle

I am trying to repair a Roland PG-200 programmer (a sort of breakout box with switches and rotary controls) that hooks up to a Roland JX-3P synthesizer (keyboard). Here's the rig: http://2.bp.blogspot.com/-5qb9ksPx8io/VPUdoAexD5I/AAAAAAAJYF4/2NxHTKF-vG8/s1600/1.jpg

Here's the PG-200 schematic for reference: **broken link removed**

Here's the situation:
- The PG-200 It was working (powered up, sending data to the JX-3P). I did some repairs to the JX-3P (nothing that should have impacted the PG-200). Then the PG- 200 stopped powering up.
- The PG gets its power (+15v, -15v, +5v) from the keyboard via a 6 pin cable. The cable's continuity is good on all pins. The keyboard power supply seems to be outputting power as specified.
- However, when connected, the PG's +5v rail is only .33v - so the PG isn't powering on (the 5v rail drives the switches/CPU, the 15v have something to do with the rotary encoders, I believe)
- I assume there is a faulty component (I really don't see any bad traces, and I've reheated all the solder points). I have inspected, pulled, tested the majority of caps, diods, transistors, etc. I could continue doing this, but I wonder if there isn't a better way.

My question:
- In situations like this, where a whole rail or board is dead but the power supply seems to be working, what is your approach to trouble shooting? Should I freeze the board and inject some power into the rail and watch for thaws? Should I systematically isolate sections of the rail by cutting traces? Should I continue pulling and testing components, more or less at random?

 

[ronsimpson]

The keyboard power supply seems to be outputting power as specified.

However, when connected, the PG's +5v rail is only .33v - so

I think you are saying;
The power supply is good but there is no power at the next board. (cable seems good)
Start at one end and work your way to the other. Start at 0.33V and work your way to 5V (or reverse) Somewhere you will fine a open wire.

good luck

 

[BigJoeScientist]

Thanks for the quick reply! So, you're suggesting that I check continuity beginning as far away from the +5V input and working towards it? Ok. I neglected to say that the ENTIRE +5V rail is at .33V. With the PG-200 disconnected, the power supply (from the keyboard) to the cable/connector is about 9V. With the PG-200 (the "next board", as you say) connected, that drops to .33V. Seems that the 9V is supposed to be regulated to +5V at the input to the PG-200, with a Zener/transistor/capacitors circuit. I have checked all those out of circuit and they seem to be good.

 

[ronsimpson]

I can not see what you see so this might be hard.
On the connector there probably is 7 volts. (might be 9V)
Q3 should make the 5 volts. There is no 5V.
>Maybe no 7 volts!!!
>Maybe bad Q3!!!
>Maybe bad capacitor C3!!!
>Maybe bad D12.
>Maybe R8 1.5k ohms.
Resistor R8 probable connects to +15V and pulls up on D21.
D21 should keep the voltage at about 5.5V
C3 filters noise out.
If Q3 base is at about 5.5V then its emitter should be at about 5V.

 

[BigJoeScientist]

Ron - I just noticed that I failed to thank you for your help on this. THANKS! I gave up shortly after you replied, but am now back at it. Amazing how time flies.

 

[Ylli]

They also could be a issue with the keyboard such that it can't deliver the current required by the PG200.

 

[BigJoeScientist]

yes, thanks... looking at the ps now.