Help needed diagnosing and repairing a PC engine LT, currently looking at oscillator

[soop]

Hello all. I'm currently trying to repair a damaged PC engine LT (a portable games console circa 1991)I got a few years ago. The thing had 10+ lifted pads and three lifted traces from corrosion from bad caps when I got it, and the first step was to clean it and repair the traces and pads as best I could using some copper and a high temperature two part epoxy, then replace the capacitors. Of course it would be amazing if that had fixed the console, but things are never so simple. I've double checked the capacitors, and I'm (90%) confident that they have connectivity where they need to have connectivity, but we still have no video out and no sound. I've checked the voltage and ground to the CPU and graphics chips, and both are correct. I've also checked the connectivity from the chip to the external bus, which, while not linked to the screen, is nearby and traverses the motherboard from the chip. There's also a fairly good 1-1 correspondence from the GPU pins to the pinout of the bus.

The next thing I attempted to measure was the oscillator, and here, I'm not seeing what I'm expecting. I see a wave, though it's not perfectly square (straight up or down, then a slope immediately back to 0v, flat line, repeat), but the issue is the frequency is not what I was expecting. I'd expect to see the pattern at 50ns, but instead I'm seeing it at 5ms. The oscillator should provide a 3.579545 MHz clock signal (NTSC video).

I'm a complete newbie having never used an oscilloscope before, so it may be that I'm doing something wrong, but the signal is well defined at that range, so it looks like there's an issue. How would I go about diagnosing the issue from here?

 

[soop]

This link may help you. http://archaicpixels.com/HuC6280

thankyou
I've already found the pinouts of the main ICs though and checked the continuity to everything I can think of that's relevant (evidently I might well have missed something though)

 

[rjenkinsgb]

Schematic:

https://gamesx.com/wiki/lib/exe/fetch.php?media=schematics:turbografx-16_unit_service_manual_-_smtg16.pdf

The CPU clock is the 21.477MHz one.

 

[soop]

so I checked through a load of different components; the CPU, the voltage regulator, the CPU clock, the DC input pins. Everything, if it displays a wave, displays that same square wave. I soldered a short lead to a ground point so that I could clip the ground connection in and free up a hand to measure the pins on the CPU more easily. I was hoping to see something different on the audio outputs.

could I be looking at a ground loop? all the voltage measurements I took look to be good, all across the board, it's just this weird frequency propagating through the board somehow.

Oh, and just to make sure it wasn't the oscilloscope, I did a quick check on my bench PSU and got a different signal

 

[Nigel Goodwin]

so I checked through a load of different components; the CPU, the voltage regulator, the CPU clock, the DC input pins. Everything, if it displays a wave, displays that same square wave. I soldered a short lead to a ground point so that I could clip the ground connection in and free up a hand to measure the pins on the CPU more easily. I was hoping to see something different on the audio outputs.

could I be looking at a ground loop? all the voltage measurements I took look to be good, all across the board, it's just this weird frequency propagating through the board somehow.

Oh, and just to make sure it wasn't the oscilloscope, I did a quick check on my bench PSU and got a different signal

5mS is mains hum - so you probably haven't got a ground connection between scope and unit - if you put your finger on the end of the probe you will get a similar effect.

Solder a wire to ground on the PCB somewhere, and connect the scope probe ground wire to that.

 

[soop]

5mS is mains hum - so you probably haven't got a ground connection between scope and unit - if you put your finger on the end of the probe you will get a similar effect.

Solder a wire to ground on the PCB somewhere, and connect the scope probe ground wire to that.

ok, this is good to know. I did actually solder a lead to ground but... perhaps it's an issue with the probe after all. It would explain a lot

*edit* and there we have it. A short continuity test reveals the ground lead to the probe is broken. Well that explains a great deal, I'll have to order a replacement and retest everything

 

[Nigel Goodwin]

ok, this is good to know. I did actually solder a lead to ground but... perhaps it's an issue with the probe after all. It would explain a lot

*edit* and there we have it. A short continuity test reveals the ground lead to the probe is broken. Well that explains a great deal, I'll have to order a replacement and retest everything

Just use a croc clip lead from the body of the scope (most scopes have a ground connection, or socket, on the front for that very purpose) and use that for your ground connection. For most purposes the ground doesn't need to be from the end of the probe.