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Hameg 203-5
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Qa3qa3qa3:
Hi,
I bought a Hameg 203-5 scope, which had flickering traces whenever the controls are manipulated, it looked like it needed some cleaning so i went ahead and disassembled the unit, gave it a good clean and all worked fine.
The issue is, when i was putting it back together, i couldnt remember which of the TO220 packages devices was screwed down to the chassis without insulation, one of them is the TIP47 npn transistor in the regulation circuit for the 170v rail, i ended up guessing it, i screwed the TIP47 down to the chassis with no insulation shorting its collector to GND, when i powered up the unit it only showed three dots in the center of the CRT, i started measuring the PS voltages, the 170 and 140v rails were showing 25v and 23v respectively, i think i have damaged something in the circuit.
I tried troubleshooting the regulator circuit replacing the TL431 and the BF423 with no success. I still didnt replace the TIP47 yet though.
I decided to inject a 170v from an external source and see what effect it has on the unit, when i did the traces showed up and everything was normal but i noticed a resistor (R707 circled in red) was heating up in an adjacent circuit.
My question is if anyone has run into a similar issue how did u manage to fix it.
And basically where should i start.
Thanks
TaylorD93:
Do you have a photo of how you currently have the transistors fitted?
Have you screwed them all down to the metal chassis?
the 170V rail is referenced to the 140V rail, but the TIP47 datasheet says the collector is connected to the "tab" of the TO-220 package, the schematics shows the collector of the TIP47 should be connected to ground/chassis.
All of the output stage transistors, T708-T711 should be isolated from chassis. It may be that one of your transistors is shorting to chassis, and drawing too much current from the 140V rail.
Qa3qa3qa3:
Turned out that the 170v rail is regulated on the negative side and T1001 is the pass transistor that for some reason failed open and the load was being handled by R1005 which was getting hot, replaced T1001 which is a TIP47 and everything is back to normal, thank you so much and yes only T1001 and IC 1001 are tied to the chassis without insulation, other TO220 are insulated.
Thanks a lot
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