TRS-80 Model 3 - mice damage, CPU and ROMs removed

[gerha7]

Hello. Newbie to the forum and to the TRS-80 computer line. Although, I have been using the trs80gp emulator (for BASIC, exploring LDOS, playing Robot Attack, etc.). I couldn't resist anymore and I finally bought a TRS-80 Model III, 48K (model# 26-1066) that includes two internal disk drives. Given the power cord had been partially chewed trough by mice, I didn't plug it in or turn it on. Upon opening the case, I see several cables (power and data/ribbon) chewed on/through. I also noticed that the CPU (Z80), the three ROMs, and the character/font ROM are gone. Thankfully all the RAM chips are still there.
What I'd like to know is where I can get replacements for (enumerated for reference):
(1) CPU
(2) 3 ROM chips
(3) character/font ROM chip
(4) keyboard ribbon cable
(5) drive belt for floppy drive
(6) disk drive ribbon cable
(7) RIFA capacitors
(8) black and white wires (power) to and from On/Off switch
I'd love to give this battleship-grey beast a new life. Any guidance would be much appreciated.
Thanks.
-Neil

 

[ldkraemer]

Z80 -- https://www.jameco.com/z/Z80B-CPU-ZILOG-CORPORATION-IC-Z80B-NMOS-CPU-6MHz-DIP-40-pin_35705.html

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https://www.mouser.com/

3 ROM OEM parts likely hard to locate - With Adapter Board you can create EPROMS of 8K, 4K & 2K from ROM's File.
Character ROM - Jay
Cables - Keyboard & Floppy -- Jay
Jay -- may have some ROMS

Home | newSOFT

jaynewirth.wixsite.com

Rifa's -- Info & Parts

Mike's TRS-80 Pages

If it's a Tandon Drive there may be Belts available.

Larry

 

[ldkraemer]

I do have the Character ROM File and it can be burned into an EPROM
- Mitsubishi M5L2732K 24 Pin. Should be Plug and play for the Character
Generator. (Pin 21 - A11 is tied to +5VDC which selects the Top 2K of the
4K EPROM for Plug-N-Play)


Larry

 

[gerha7]

Thanks Larry for all of the great info. Looking at the video/CRT side of things, I noticed what looks like oil lightly coating on and around the red rubber cap (anode cap?) and on part of the CRT. I also noticed some chewing off the flyback transformer...I assume that would have to be replaced. I did some searching for a replacement video board but can't really find anything. So if anyone knows of where to get one, would be great. Thanks.
-Neil

 

[VERAULT]

Thanks Larry for all of the great info. Looking at the video/CRT side of things, I noticed what looks like oil lightly coating on and around the red rubber cap (anode cap?) and on part of the CRT. I also noticed some chewing off the flyback transformer...I assume that would have to be replaced. I did some searching for a replacement video board but can't really find anything. So if anyone knows of where to get one, would be great. Thanks.
-Neil

You dont replace a flyback unless its not functioning. The chewing portion can be easily remedied by painting on a little liquid electrical tape: Link here

Amazon.com

 

[grumpyx]

I should have most any part you need for a Model 3. It's probable that the board had other issues and was just used for parts. I should have a complete board if you want to go that route. Jay may have some parts as well https://jaynewirth.wixsite.com/newsoft may not be listed but he has other stuff.

 

[gerha7]

Hi grumpyx, can you DM me on what you would charge for the Model 3 motherboard with the A, B, and C ROMs and the character ROM? Also, what you would charge for just the 4 ROMs? I'm still a new forum user so I don't have access to initiate DM's.
Thanks.

 

[gerha7]

Hello. Looking at the video board for my Model 3, I see there is a loose wire. It seems it should be attached to pin E302 (see picture). The wire leads to an in-line resistor then goes to the deflection yoke assembly. Just wanted to confirm it should be soldered to E302. Also, there is a transformer (T502) that looks like it's been chewed on. Does it look like an issue where it needs to be replaced?

Also, while I'm working on other issues on the video board, is there a way to connect video from the motherboard to an external monitor? I've seen on Adrian's Digital Basement (8/13/2022) where he connected the M3 motherboard to a VGA cable, then to a RGB-to-HDMI converter. I don't need to convert to HDMI, just to RGB/VGA or composite since it would be temporary. Thanks.

 

[Trash-Eighty]

Attached are 3 photos from my Model III. Hope this helps.


The transformer is ok if the core underneath the green label is still ok. It seems to me that there is only cosmetic damage on the T502.

A smart project for connecting an external monitor is the composite video board from https://blue-print.be/

 

[gerha7]

Hello all. Here are better pictures of the 'green' transformer (T502) and the high voltage wire coming off the flyback transformer.
The insulation around the green transformer is chewed a bit on one side (see pic), I assume I can apply some liquid tape.
The wire coming off the flyback transformer has insulation chewed down to the bare wire, but I don't see any frayed/broken wire. I assume I can apply high voltage putty, like GC Electronics 10-8880 Hi-Voltage Putty, Clear Isolating Compound.
Let me know if the liquid tape or putty aren't suitable repairs.

Regarding the external monitor, I looked at the blue-print site but they're sold out of the TRS-80 composite video board. I know I could DIY but that's more cost and time for something that's temporary, and seeing the video stated in my previous post, seemed simpler/cheaper.

 

[gerha7]

What is the best way to clean dirty circuit boards? My M3's main board isn't too bad, dusty/dirty of course, but the circuit boards that sit atop the disk drives are especially dirty, and what I assume is mouse urine. I've searched/seen videos rinse boards off under tap water, or mild soap and water and a toothbrush, or just spray with 99% IPA and not wiping/brushing and just let it dry. I've cleaned my video board with cotton Q-tips dipped in 99% IPA. But I hope there's an easier/quicker way to clean the main board, and the disk boards.

Always striving to be MacGyver, but usually ending up being MacGruber.

 

[VERAULT]

The dishwasher.

 

[Robbbert]

Mice are well known for defecating throughout whatever equipment they have invaded. You'll know because of the smell that hangs around for years afterward. Parts that have been pissed on will go rusty or otherwise fall apart over time.

All I can say is good luck with your repairs - I think you'll need it.

 

[gerha7]

I'm looking for the (40 pin) chips in my M3's floppy disk controller board (U7 - WD1793) and RS-232C board (U2 - TR1602B, UART), as both of them were removed (of course). The only place I was able to find the chips was on eBay. Regarding the WD1793, on eBay it's listed as FD1793...I assume they are similar? Just wondering if anyone else purchased these chips from other sources.

 

[Robbbert]

FD1793 and WD1793 should be the same.

The TR1602 is supposed to be pin-compatible with HD6402, TR1865, AY-3-1015, TMS6011, S1883, 2536, MM5303, COM2502, COM2017, MHB1012 and many more, but you should check the datasheets first.

 

[gerha7]

Making slow progress on my M3. I get the "Cass?" prompt but none of the keys are working, except the Reset key seems to work fine. I know some keys can fail but all of them? Anyway, I tested the continuity/resistance on the adjacent (closest) posts of the '1' key, and I get no continuity/resistance reading. I repeated the test on several other keys including the Enter key and still no reading...it's like I'm not even presssing the key. So I de-soldered and removed the Enter key on the numeric keypad--I didn't find any structural issues. I assume the carbon on the rubber contact has degraded?

What should I use to re-carbon the rubber contacts, and is there an allowable range (Ohms) it should test at? I don't want to go through desoldering, cleaning, re-carboning, soldering then find out it still doesn't work. It's an ALPS keyboard, I2KEOIOC. Is it OK to clean the metal contacts in the plastic casing with 99% IPA...or spray with DeOxIT...or is there something else I should use?

Also, how do you get the rubber contact back in the plastic casing? Of course, it's designed to be slightly bigger than the casing, which makes it maddening to get it back in properly.

Thanks!

 

[ldkraemer]

If you unsolder the ALPS Keyswitch and remove each one, you can use a very thin knife to
spread the two opposing end tabs and disassemble the Keyswitch. Then you can remove the
rubber Dome and clean it along with the metal contacts. But, you need to get the keyboard
functional first. Port 0x3800 is the starting address of the Keyboard ROW & COLUMN
matrix.

Larry

 

[gerha7]

I removed/desoldered the Enter key on the keypad. I've cleaned the metal contacts in the keyswitch case. Not sure what the best way is to clean the carbon contact though. I used a little 99% IPA, and I also dragged the contact for an inch or two along a plain sheet of printer/copy paper to lightly sand it. I tested again, and now I get around 1100 to 1600 Ohms. I assume that's too high? I've seen a video & comments where below 1000 Ohms is OK, but under 100 is ideal.

What product did you use to re-carbon pads on the rubber domes? Amazon has products like KeyPad Fix and CaiKot 44. I just want to get a good product that lasts and is durable on flexible surfaces.

 

[Robbbert]

It's unlikely that all the keys have failed. It might be an idea to make sure that the key matrix is being scanned before pulling keys apart.

For example touch a wire across the enter key's solder pads and see if there's a response to the Cass? prompt.

 

[gerha7]

Yeah, I also thought it's odd that *all* keys (except the Reset button) fail, but I haven't been particularly lucky on this machine. I tried what you said by 'shorting' the solder pads for the Enter key. It works! I am able to get past the Cass? and Memory Size? prompts to the Radio Shack Model III Basic (c) '80 Tandy READY message. I shorted a few other keys and they display a character just fine. I even shorted the Shift and '/' keys simultaneously and it displayed the '?' character. I guess this rules out any defect outside of the keyboard, therefore the defect is most likely the key switches themselves, namely the carbon pads I would assume.