Author Topic: TurboExpress Screen is White, No Sound, High Voltage from Q502  (Read 453 times)

sulaco

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TurboExpress Screen is White, No Sound, High Voltage from Q502
« on: February 21, 2022, 10:56:32 AM »
I picked up a TurboExpress that was listed as needing a cap replacement, but when I opened it up I found the caps had already been replaced. The screen had green slightly diagonal lines running across it, which doesn't seem to be a common problem. There was some questionable work done, so I decided to recap it anyway.

After a recap, the TE was in the same condition. It seems like VR501 was the cause of the green lines, but after adjusting I just get a black screen with the backlight on. I don't get any sound from a HuCard, however, I get a perfect picture from the TurboVision (noise from the tuner, and a great composite picture from AV In).

I've measured around the X500 circuit and measured ≈78V across one of the sides of D501, 83V across R504, 85V across C503. This the same on the negative side of CC503. The highest voltage I’m aware of for this board should be around 24V for the LCD. At the power adapter for the screen I only see ≈7V and that seems to drop over time. Q502 is getting warm slowly. I've watched it creep up to 100ºF before I turned it off.

I've pulled off T900, but still saw the high voltage. I found a twitter thread indicating that it should measure ≈110 ohms across pins 7 and 6. Mine measures ≈220 ohms. I've pulled T500 and L900 as well. L900 shows continuity. I'm not sure how to test T500.

It looks like someone already tried to replace Q900 and Q901 and those measure fine. Is there any other obvious culprit? Q502 seemed to measure out of spec. Could that cause the abnormal voltage?

MasterofBiscuits

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Re: TurboExpress Screen is White, No Sound, High Voltage from Q502
« Reply #1 on: February 22, 2022, 06:51:30 AM »
Those voltages are definitely too high. Have you adjusted V500? I believe this one controls the voltage for the backlight circuit.

The absence of sound is a bad sign, but make sure you also plug headphoens in and check that as the headphone jack fails very often, and can stop sound coming from the speakers.

sulaco

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Re: TurboExpress Screen is White, No Sound, High Voltage from Q502
« Reply #2 on: February 22, 2022, 03:09:08 PM »
I've tried VR501, but not VR500 specifically for dropping the voltage. I've pulled L900 and T500 and I'm going to wait for a new set of inductors before testing any more. Hopefully they'll show up later this week.

Keith Courage

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Re: TurboExpress Screen is White, No Sound, High Voltage from Q502
« Reply #3 on: February 25, 2022, 01:18:03 PM »
This might sound dumb but make sure a 47UF cap didn't get installed where a 4.7UF cap is supposed to go. I've seen people do this on accident before. In fact, I think I did once upon a time.

sulaco

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Re: TurboExpress Screen is White, No Sound, High Voltage from Q502
« Reply #4 on: February 25, 2022, 06:50:01 PM »
Fortunately, the only 47uF cap I had was a through-hole for the controller board. I'm still waiting on a replacement L900 before I start poking around again. Do you know what's responsible for stepping up the voltage for the backlight?

Have suitable replacements for T900 or T500 ever been documented? I've read mixed messages of scavenging T900 from a game gear, but haven't been able to find much about T500.

I have a TG-CD ROM that's giving me problems too. The spindle gives a short burst and then stops. The motor spins fine when given 6.7V. That one also seems like a questionable recap possibly breaking something else in the process.

Thanks for popping by Keith. I've followed a lot of your posts up until this point and learned a lot about the TE board along the way. Hopefully I'll be able to get this back up and running in the next few weeks.

MasterofBiscuits

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Re: TurboExpress Screen is White, No Sound, High Voltage from Q502
« Reply #5 on: February 26, 2022, 05:29:17 AM »
Fortunately, the only 47uF cap I had was a through-hole for the controller board. I'm still waiting on a replacement L900 before I start poking around again. Do you know what's responsible for stepping up the voltage for the backlight?

Have suitable replacements for T900 or T500 ever been documented? I've read mixed messages of scavenging T900 from a game gear, but haven't been able to find much about T500.

Back when I had a backlight failure I looked into this and couldn't find any documented replacements for the transformers. It seems most people just do a screen replacement when the backlight fails and that's what I would suggest you do here. I think it will be much easier than trying to diagnose the circuit and find replacement parts, since there is no schematic available. I would order one of these RGB screens https://www.aliexpress.com/item/1005001411506600.html as they are relatively simple to hook up compared with the composite method, and it would allow you to check if you're getting a video signal at all.