12.12.2022, 04:12 Uhr
|So I had problem that during power on, two fuses on back of the power supply unit (PSU) of PC 1715 were blown up repeatedly. The culprit was SU179 power transistor (the first one, from the power input side). It was damaged, shorted out. So I replaced him.
Now I can start PC and measure voltages next to lighting red LED diodes on motherboard. Measured values are for 12V and -12V rail: (11.33V, -11.79V) and for 5V and -5V rail: (4.90V, -4.90V) respectively. Computer is not booting form floppies, sometimes floppies just turn on motors and keep spinning. But this problem I would like to solve later hopefully.
Currently I hear fast rattling sound (which I think was still there but I thought that it was coming from computer fan, but I unplugged the fan and sound is still there). This sound comes I think from that already replaced SU179 transistor. I think it is not normal. You can listen to the record of that sound here: http://sndup.net/x4tk (I recorded that with my phone, so sound has a bit unstable volume as I moved a bit my hand left to right like 10 centimerers above PSU, but in reality that rattling sound is of constant unpleasant volume.) I would even say it is comming from back cooling plate of transistor, maybe it is vibrating, but why?
I hear this sound when PC mainboard is connected, but also when I put resistors (3ohm and 33ohm) as fake load in place of of motherboard (on 5V and 12V rails). So I presume problem should be located on PSU board.
The advice I got from Rudiger (Admin) on this forum was:
"Ticking sounds come usually, when device is switching off and restarting cyclical because too high current, possibly because a secondary capacitor or rectifier having a short or crowbar circuit is igniting.
Power off the unit and feed it backward using a laboratory power supply with appropriate +5V or +12V.
If you find golden capacitors from producer "Roe", it will be likely the reason."
I have to say I have not find golden Roe capacitor. From current picture of my PSU here https://imgur.com/a/j40cJo6 (you can even download that picture and see the details when magnified) , you can maybe see little bit suspicious blue Tesla 220uF/25V capacitor second next to the SU179 transistor. I removed it and measured it, it reads 228uF and 0.05 ESR so this sems OK. No other problematic like leaking or burn out cappacitors are detected by my eyes. The termistor (black rectangular plate next to thegiant silver capacitor) coating looks also suspicious, maybe I should measure voltage drop there too.
I did not backfeeded motherboard with laboratory power supply as I do not have one, yet. But I think failure is inside PSU itself as I explained earlier.
My only idea now is to take all capacitors from main PSU board out and measure them one after another, also rectifying diodes (the very first diodes from voltage input side were ok) ... Also I think I will measure stability of output voltages with osciloscope, because I wonder about those possible cyclical restarts which Rudiger mentioned, but I did not noticed them when measuring by voltmeter (and as I wrote here https://www.robotrontechnik.de/html/forum/thwb/showtopic.php?threadid=20150 , I was even able to get picture on my LCD TV, so I do not think there are restarts of the whole device.)
Do you have any other idea how to diagnose it easier?
P.S. I'm not skilfull electrician, just trying my best to get this old PC working again.
Dieser Beitrag wurde am 12.12.2022 um 05:14 Uhr von milan editiert.