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Project logs / Real world usage of the Part Ninja (or: Repairing a PM5132)
Today I put the Part Ninja to good use, it came in handy when I was troubleshooting an old Philips PM5132 function generator.
While I was working on a project I wanted to take a break and switched the signal gen off, later that day I came back and the damn thing failed on me. I switched it on and the only thing I got from it were -22 volts at the output. Luckily I had disconnected the circuit under test or it would have gotten fried as well.
Next I opened the device. The negative voltage on the output indicated that either the output amplifier or the positive power supply had failed.
IMAG0226 by kajak_attack, on Flickr
The construction is a marvel of european engineering of the late seventies/early eighties. Everything is made of discrete semiconductors, there are just a few oldskool opamps (LM308, uA741), no integrated function generator rubbish like a XR2206 or the likes :)
IMAG0229 by kajak_attack, on Flickr
First I tested the power supply, and my first guess was right: The positive power rail was at 0 and not 22 volts, as it was supposed to be. They used a uA78GCU1C linear regulator, which I never heard of before. Googling turned up a datasheet, it's basically an adjustable 78xx linear regulator with a fourth pin in a crazy package (like a DIP-8 with four pins forming a cooling tab).
IMAG0223 by kajak_attack, on Flickr
As I didn't have one of these long-time obsolete devices I cut some solder links connecting the power supply to rest of the circuitry and sustituted it with my symmetrical lab supply.
I turned on the power - nothing. Bummer.
Now I knew, I had to dig deeper, because the dying power supply took something else with it. Luckily I found a service manual on the internet.
http://www.ko4bb.com/Manuals/Philips/Ph ... Manual.pdf
With the schematics at hand I tried to locate the fault by dividing the system into its building blocks. I checked the main oscillator, which is the heart of the circuit, and it wasn't running. I followed the old principle of "divide et impera": I tried to find parts of the circuit which are not needed for operating the oscillator but in case of a fault might feed back into it erroneously and stop it from working. The TTL output was pretty closely coupled to the oscillator, one failed transistor could have stopped the thing from working, so I unsoldered a resistor to separate the circuits, but to no avail. So I turned to the input section, and disconnected the wobbling circuitry. Then I checked the function of a controlled current source that is converting the voltage from the frequency dial potmeter to a charging current for the timing capacitors. This is where I found the first discrepancies, the output current was too low and was independent of the frequency setting. So I narrowed the fault down to one opamp and one transistor. I unsoldered them both and put the transistor to the test:
IMAG0218 by kajak_attack, on Flickr
I was too lazy to use the multimeter and took the Part Ninja instead, which revealed that one of the pn junctions in this transistor had gone open-circuit, degrading the transistor to a mere diode . . .
IMAG0217 by kajak_attack, on Flickr
I did not bother to test the opamp, my guess is, that it failed first and put a overvoltage to the base of the transistor.
As I did not have a 2N3964 transistor I substituted a BC560B, which is not a very close fit, but good enough. I wanted to replace the (most probably fried) LM308 with a TL081, but I only had 082s and 084s at hand, so I used a LF411 instead.
BTW, the signal gen is a very intelligent design. The slow opamps operate only at DC or very low frequencies, everything else is done by discrete circuitry. The opamps only fulfill support functions.
I'm happy to tell you that the PM5132 is working fine again. I have not yet replaced the faulty voltage regulator, because I'm going to replace both the positive and negative VREG with LM317 respective LM337 and that will need a separate daughter board due to incompatible pinouts.
It was a nice exercise of analog circuit troubleshooting and it has shown me the value of the Part Ninja. It has helped me saving this nice piece of equipment from ending up in the landfill.
Jan
