GT64 no 5V power

[darkhalf]

Recently picked up a CPC464 and GT64. It came with an extra 5V 2A power supply because the GT64 one is not working

Initial inspection shows the 4A fuse was blown. So closest I have here is 3A or 5A. I've gone with 5A in the interim (but the L78MG regulator only outputs upto 3A)

After replacing the fuse, I'm only getting about 1.8V on the CD501 5V power connector. So I've read through this thread and cleaned up VR501 with contact cleaner but the max range is now about 2V-2.4V
https://www.cpcwiki.eu/forum/hardware-related/faulty-gt64-monitor/msg137438

So something else has been blown in this circuit

So we have two PNP-NPN KTC1015/KTC1815 transistor pairs (these switch upto ~50V) and the second pair goes into the L78MH 5-35V regulator. My measured voltage here is 3.8V (!) 

The reg then goes through a 2SD313 switching transistor and I'm reading 2.7V on the input to the transistor

So potentially it looks like one of the transistors is stuffed. Any ideas where I should head with this one? I'm not sure on current day compatibility with those transistors and would like to fix it. Thanks

[darkhalf]

Sorry put this in the wrong section. Mods please move

[Bryce]

These values on the schematic are the expected voltages. Starting from the rectifier I would confirm that these are correct. I'd also check whether any of the resistors have failed. Something must have been pulling enough current to blow the fuse. You can check the transistors using the diode function of a multimeter.

Bryce.

[darkhalf]

Thanks Bryce I will measure those ones out. 

Btw I had some screen wobble. Reflowed joints and ESR tested all the caps and they were in tolerance. Ended up replacing one tant and 1000uf which were on the edge but all stable after that 

[darkhalf]

Q503 BCE = 2.2, 0, 1.8 (post regulator so doesn't matter right now)
Q504 BCE = 0,18,0 (ok)
Q505 BCE = 18,0,18 (ok)
Q506 BCE = 18,18,~3V

So Q506 is the main one which is out. So I need a replacement for KTC1815

Google shows 2SC1815 or 2SC5328 which runs same current but less voltage (we are dealing with 18V here so doesn't matter)

Now I have to do another digikey order...


Digikey 2SC1815

[Bryce]

Did you measure the voltages in and out of the regulator?

Bryce.

[darkhalf]

I put those in the first post 

~3.8V ( Q506 Emitter)  going in and 2.7V going out

It seems to fluctuate 3-4V on that bad transistor

[Bryce]

So you did. My money would be on a failed 506 too. But I'd recommend confirming the resistor values also before you swap the transistor. If one of those has failed short or dropped in value, the new transistor will immediately fail again.

Bryce.

[darkhalf]

I checked the resistor values and all are in range. The high wattage R505 was a bit tricky since the meter shows 1 ohm on the leads, but 1.2-1.3 ohm on the part so that looks fine

The failed 506 is showing 26 ohms between legs (which R509 1.5K goes between) so that is 100% the faulty part. The end of the 5V plug to the CPC shows a little corrosion and it looks like something was spilt inside the CPC near the power which likely triggered the short on the 5V output

Thankyou for your input on this Bryce

[darkhalf]

Pleased to say that just the transistor and the fuse needed replacing on it

btw I found the screen is a bit wobbly only then the neighbouring CTM640 is powered on (must be magnetic interference)