SOLVED - Turbo Express / PC-Engine GT missing 24 V, Q500 heats up

Xenogears · 04/29/2018, 10:11 PM

Hello Everybody,

I have a Problem beyond my knowledge. I´ve got a PC-Engine GT, fully recapped. When turned on, Q500 starts heating up slowly but really hot. Also the Coil L500 gets warm... I already replaced Q500, no change. I replaced Q900 and Q901 - no change.

What I found so far:

D501 / TP 500 should read about 30 Volt. It does not. Starts with about 20 Volt and slowly drops down to 11 V. The same time it takes, Q500 to get warm.

TP 501 should read -24V. I do not get this voltage anywhere. Does anyone know the beginning of that Voltage ?

Backlight turns on with the dark, slightly green apperance instead of a bright white light. Any clue would be glue to my shattered-to-pieces-brain. The Unit is just a few seconds away from a meeting with my Power-Bender 2K. :-)

ClodBusted · 04/30/2018, 02:51 AM

Sounds like there's a short somewhere. This would explain the voltage drop at high currents (excessive heat).

I had a similar issue with my Turbo Duo, please see this thread for more information:
https://www.pcengine-fx.com/forums/index.php?topic=17134.msg512999#msg512999

Xenogears · 04/30/2018, 03:16 PM

Quote from: guestSounds like there's a short somewhere. This would explain the voltage drop at high currents (excessive heat). I had a similar issue with my Turbo Duo, please see this thread for more information: index.php?topic=17134.msg512999#msg512999

I measured all Day without even an Idea where to start. I swapped parts into a working one - these worked fine. Coils and and nearly every Testpoint has the same Value as the working one. All okay exept:

approx. 30 V / -28 V / -18V are missing...maybe I could use the GT to light some Cigars with the Hot-Transistor... just turn on, use the Heating time to cut the Cigars ends...and GT is ready to fire...great Idea. I should immediately start smoking...

ClodBusted · 04/30/2018, 04:18 PM

Have another look at your GT and see if there are bare leads or component legs touching metal shields (if there are any), look for accidental shorts due to solder blobs shorting severeal conacts, etc. It might not be obvious in the first place, but you'll get behind it if you're persistant enough.

Xenogears · 05/01/2018, 05:45 AM

Quote from: guestHave another look at your GT and see if there are bare leads or component legs touching metal shields (if there are any), look for accidental shorts due to solder blobs shorting several contacts, etc. It might not be obvious in the first place, but you'll get behind it if you're persistent enough.

Okay----a short. If I measure the negative Pad of CC501 to GND I get 1,5 MOhm Resistance.
Compared to my working one - there is infinitive Resistance / no connectivity. Indicates a short. The Board is completely cleaned, no conductive Cap-Leak on it....cleaned beneath Board and Coils.

soop · 05/01/2018, 05:59 AM

Do you think it could be a blown resistor?

Xenogears · 05/01/2018, 06:34 AM

I measured em all... fine. Values checked - Fine. I will check again but if there is no other clue, I will rip off all the Parts of the PCB and just put in an LED Backlight. Thanks for your replies so far, I still need to understand, how 30 /-28 /18 V are created. Is it just the Transistors /Parts in the 5XX-Circuit ?

soop · 05/01/2018, 06:43 AM

That's beyond me, I'm afraid. Maybe one of the experts can help

Xenogears · 05/02/2018, 05:24 AM

So I supported myself with a Big Bar of Chocolade and found the solution. 2 Issues:

The first one was a broken Trace between R500 / R501 they are connected like this:

[R501]---[R500]

The "---" marks the connection that was broken

So the Heatup of Q500 was fixed. -24V still missing. And as long as they fail you get no +30V.

The second issue was also a broken Trace:

[R502]--- -(CC503)+

Thats it. Now I am that fed up with fixing on the fly, I am encouraged to build a Circuit Schematic.