LED drive transistors in a JMP1 preamp

[PRNDL]

I'm troubleshooting a Marshall JMP1 preamp and have narrowed it down.

The LED's only turn on (light) once in a while when the power is turned on. The entire display is out, although it will work if I touch the 5 volt rail with an oscilloscope probe (the 5 volts looks good and steady).

Refering to the schematic:
The emitters of TR13-20 (MPSA13's) go from CN3 (page 5) to DR1-6 on CN1 (page 6).

Using the diode scale, I measured 1.3 BC, .7 BE and 1.3 CE with similar readings for a few others (transistors were not removed for test).

MPSA13NPN Darlington Transistor This device is designed for applications requiring extremely high Current gain at collector Currents to 1.0A.

I'm wondering if the transistors are all bad in the same way, or if there is something else going on, like a bad regulator (7805).

Any advice would be greatly appreciated!!

Schematics can be found with these links
http://www.drtube.com/marshall.htm (3/4 down the page)
http://www.drtube.com/schematics/marshall/jmp1-61-04.pdf

 

[Curtis]

I'd be looking more closely at that 5V regulator. If it only works when it's touched by your probe or intermittantly on power-up, it certainly sounds suspect. Maybe it's sensitive to vibration or temperature? Check for shorts (eg regulator tab to other circuits or chassis, foreign objects underneath the circuit board), or dry/fractured solder joints around the regulator. Try wiggling it with your fingers and see if it turns the LED's on.

 

[PRNDL]

Thanks Curtis.

I was suspicious when all of them would go out.

I re-flowed many of the solder joints and made certain the board lay flat (the last time it bowed up slightly). It still has the issue, but it turns on with two tries.

There's something wrong with the unit that's somewhat complex. Many of the patches have zero volume settings, which are easy to change, but I suspect the ROM chip may be defective.

I've been able to get it to where it works and sounds great, but it may need someone with specific expertise to take it from here.

 

[PRNDL]

I took a closer look and the 12 volt supply has a significant amount of ripple, although the 5V output from the 7805's is steady.

I'm currently trying to find suitable replacement caps (2200uF/25V and 10uF/350V radial cap cans).

I found them on Mouser ... it's all in how you search.