Model 100 Repair help

[bluejay]

I got a TRS-80 model 100 from a flea market for $50, which the owner claimed that it worked. It came with lots and lots of goodies with it(Tandy Portable Disk Drive, manual, leather case, carrying case, etc) but neither the laptop and the disk drive(probably--got no way to test it, but the belt has disintegrated, so I'm assuming it doesn't work) actually works. Both devices power on when plugged in and the computer beeps when I type on the keys, but the screen is blank. I tried the display knob, hard resetting the computer, and pretty much anything I could do without tools. The manual's troubleshooting guide didn't help much either. I even took it to a retro repair shop, but even they said they couldn't repair it. What might be the problem? The screen?

[+arcadeshopper]

I got a TRS-80 model 100 from a flea market for $50, which the owner claimed that it worked. It came with lots and lots of goodies with it(Tandy Portable Disk Drive, manual, leather case, carrying case, etc) but neither the laptop and the disk drive(probably--got no way to test it, but the belt has disintegrated, so I'm assuming it doesn't work) actually works. Both devices power on when plugged in and the computer beeps when I type on the keys, but the screen is blank. I tried the display knob, hard resetting the computer, and pretty much anything I could do without tools. The manual's troubleshooting guide didn't help much either. I even took it to a retro repair shop, but even they said they couldn't repair it. What might be the problem? The screen?

Maybe bad caps. I have a cap kit if you are soldering inclined. At arcadeshopper.com

Sent from my LM-G820 using Tapatalk

[Turbo-Torch]

Turn it on, type BEEP and hit enter.  If you hear a beep, at least the motherboard is ok.

 

Good chance the brightness control is bad or just needs cleaning with Deoxit.  When the LCD screen fails, you get missing lines...it doesn't fail all at once.  Missing lines are bad zebra strips and you can usually tweak the retaining tabs to add pressure and clear up the problem.  If the zebra strips are bad, you're out of luck as I've not been able to find replacements.

I have 7 Model 100s of which 5 work/look great (all free) but most have needed tweaking of those tabs and cleaning the brightness pots.

Capacitors are another issue and highly recommended.

Lastly is a rechargeable barrel battery that can leak and corrode the board.

 

They are very easy to disassemble and work on, but if you have no electronics background and don't own a basic Philips screwdriver, you're better off selling it and getting a known working one.  Disk drives are rare and it's probably worth more than the $50 you paid, even if it needs a belt.

[bluejay]

On 8/15/2019 at 5:35 PM, Turbo-Torch said:

Turn it on, type BEEP and hit enter.  If you hear a beep, at least the motherboard is ok.

 

Good chance the brightness control is bad or just needs cleaning with Deoxit. 

It does beep, tried some basic programs that involve beeping(10 beep 20 beep 30 beep run). Tried pulling out the optional ram chips, and still no luck. Where are the brightness controls? If it's the little blue thing with the Philips "hole" on top of it, spinning it around both directions didn't work.

[Turbo-Torch]

The rotary dial on the right next to the power switch.  I think it says display under it.

 

I don't know what blue thing you are talking about.  Can you post a picture of it?

[bluejay]

2 hours ago, Turbo-Torch said:

The rotary dial on the right next to the power switch.  I think it says display under it.

 

I don't know what blue thing you are talking about.  Can you post a picture of it?

Oh, I've tried the Disp dial dozens of times. Definately doesn't work. I'll post a pic the next time I take it apart(probably tomorrow):)

[Turbo-Torch]

You didn't say you took it apart.  If you meant the blue pot on the motherboard, that has nothing to do with the display.  I believe it's an adjustment for the internal modem.  Hopefully you got it back to where it was. 

Try contact cleaner on the display pot.  DeoxIT if you can find it is the best.

[bluejay]

Ah, sorry!

Maybe I should plan a trip to a Home Depot soon!

[+arcadeshopper]

We just had this issue in the club 100 list the same symptom and it turned out to be a damaged trace on the motherboard due to a leaky capacitor.  Is there a good connection from T23 to pin 4?

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[bluejay]

I may have discovered the problem. But first, do diodes leak too? Anyways, I've got some major corrosion around a diode that's between two capacitors(can't remember their names) and what I thought it was glue on the wire that goes through the middle of the board turned out to be a massive capacitor leakage. How do I get rid of this?

[Turbo-Torch]

Diodes don't leak.  They also don't typically go bad unless something else down line fails.  Post some pics.

[+chue]

Just taking a guess, but it's likely leakage from the M100's internal battery.  Pics would help.

 

If that's the case, soak the diode in white vinegar and let sit for 5 minutes or until the bubbles stop forming.  Scrub with an old toothbrush and dry with cotton buds (Q tips).  Soak with 90+ percent isopropyl alcohol, and dry with cotton buds.

[bluejay]

I tried crc contact cleaner on every corroded parts including the brightness pot but it still doesn't work. Maybe the screen is totally broken?

[bluejay]

And also, how to you remove large quantities of hardened brown gunk off a 36 year old circuit board?

[Turbo-Torch]

The easy things have been checked/tried...with exception of installing a cap kit as has been suggested.  From this point on you'll need a multi-meter and a strong magnifying glass just to start looking and checking for open traces.

 

And how much "brown gunk" is on this board?  The inside of a 100 is relatively sealed from the outside environment and all of mine have been spotless when I opened them.  

Has yours been in a flooded basement or something?  Once again, post some pictures of what you have.    

[bluejay]

I would love to post some pictures but I recently moved and I still don't have wifi in my house, so it's practically impossivle to post pics. (you know how slow Americans work. It's taking more than a week to supply wifi into my house) And the brown gunk seems to be leaking from the capacitors.

Edit: Nevermind: managed to post 2 out of the many pictures!

Edited August 25, 2019 by bluejay

[Bruce Tomlin]

Disc capacitors don't leak, that looks like crappy glue to tack down the wire.

[Jpenner64]

On 8/20/2019 at 9:23 AM, arcadeshopper said:

We just had this issue in the club 100 list the same symptom and it turned out to be a damaged trace on the motherboard due to a leaky capacitor.  Is there a good connection from T23 to pin 4?

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Also, acknowledging the same specific issue…Many caps replaced on the board due to leakage…C28 leaking impacted/broke the trace from T23 to pin 4 of the LCD interface….From the via in the middle of C53 to Pin 4 trace is good, however as the trace is on the surface here it goes under C28, then back down another via to T23…I added a bodge wire underneath the PCB between via points and presto! Back in business. Your mileage may vary…trace continuity..