philips: Philips PM5134 with failing 5V supply -- dropped to 1,3V

After some time of not being used, my PM5134 after power up is briefly showing an indication, but it quickly dies. The +12V en -12V are present and the fan is running. Only the 5V supply has dropped down to barely 1,3V.

How can I start troubleshooting this ?

Does the PM5134 have some particular weak spots that could pull the 5V power supply down ?

Many thanks for any hint or suggestion.

Paul

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There is no schematic so it's hard to say. But equipment of these years often used fixed voltage regulators for the 5V rail - like the "7805" or similar. These are getting hot even in normal condidtion. After some years they tend to become faulty or getting 'noisy' (wild oscillations) - I had several faults with these IC's over the years. Also there are often defective tantalum capacitators in devices of this age - they tend to become a short circuit or even explode (which is the easier one because you can tell it by eye).
I hope that helps a little ...

Regards, D. Consales

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Meanwhile I removed the UA7805 regulator from the UNIT 1 (which is the bottom- and main board).

This regulator, when tested with a 20 ohm resistor, turned out to be all right still.

So I connected a 5V external power supply to the small elco at point "K" and ground. This way I could find the overcurrent consumer was in section E on the UNIT 2 board. This section is the display driving part involving ICs #406, 407,408, 424, 420 and 423.

I replaced the tantalum capacitor near 405  as one of the first, by an elco. I was hoping this capacitor would have been the reason for the short circuit, but it was not.

I don't think any of the transistors 302, 303, 304 or 305 could cause a power drain on its own.

On the other hand, the generator seems to work also with the display controlling circuit disconnected, so I might just leave it like that.

One thing I noticed though is that the 5V current consumption could be different when switching frequencies. I don't know if this is normal. It looks like the normal 5V consumption is around 140mA.

The 5V power consumption could swing suddenly to 300mA when the frequency range know was actioned. That is: sometimes after normally switching over to another range. When switching back to the range where it was first only consuming 140mA, the 300mA consumption was still drawn, until touching the knob. Moving the knob slightly without clicking however to the next range, also made the current consumption move up and down.

I don't know if this is related to the original problem however...

I will upload a series of pictures of the generator opened, with the 5V regulator removed however. Need to size the pictures down still.

 

Attachments:

• PM5134_1 (199 KB)
• PM5134_2 (198 KB)
• PM5134_3 (187 KB)
• PM5134_4 (194 KB)
• PM5134_5 (190 KB)
• PM5134_6 (138 KB)

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I'm a little confused now. Why point "K" ? In my schematic the fixed regulator is connected to points "C" and "F" on U1 and then to points "C", "D" and "E" an U2/U3.

Your description on the strange "jumps" of the current leads me to wild oszillations again. I had a very similar behavior at a Sommerkamp FRG7700. Maybe try simply to replace the ua7805 ?

D. Consales

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Many thanks for your anwser Daniel.

The solder bridge points you listed are all correct, but I just hooked up my external current limited power supply to point K, which is just "upstream", directly at the capacitor behind the 7805.

The current jumps are occurring with this external power supply still in place. I have to say though that the current jumps made me think also of spurious oscillations...

I think I will now unsolder some of the previously mentioned ICs, and test them out separately on a breadboard.

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