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What do you think is wrong with this Model 4?

A

ardsleytank

Experienced Member
Joined
Nov 15, 2015
Messages
282
Location
Tennessee
Found this on Ebay: http://www.ebay.com/itm/SELLING-AS-...9d53d4e&pid=100040&rk=4&rkt=4&sd=141876806356

I'm wondering what's wrong with it. I've seen a lot of these cases here on the forums, but I'm still not sure what causes them to do that.

Also, it says that it has test lines burnt into the screen, does that make a difference in value?


Reason for asking is that if it's easily fixable, I might think about getting it...

Thanks

Peter
 
Peter,
It's hard to guess, what could be wrong with it. At least it does have video, and appears to be functional with some sort of stable display.
That is a good thing.

The best test is to hold the BREAK KEY depressed and power on the Model 4. Then release the BREAK KEY and depress Enter twice.
If the screen is still in a mess it could be RAM, Video RAM, a Buffer IC or something causing the garbage display. It could be something
like a stuck Address or Data line.

My guess is you will need an O'Scope to do a bit of troubleshooting on the address & data lines, after swapping the RAM & Video RAM.

Larry
 
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Well, I do have an old but working Heathkit O1 Oscilloscope...

I'll contact the seller, and we'll see what happens.

Thanks again.
 
Well, I heard back from the seller. Not good. He did the test, and nothing changed.

I think I'll pass on this one, and wait to see what comes next. ;)

Thanks again for your help.
 
The picture tube is operational but the burnt in screen would be annoying during a future use. Even though it's a normally valuable Model 4D the overall condition is dirty and the storage place wasn't certainly the best for the computer. Many cobwebs and the bottom of the case could have been wet some time ago. The are traces of a dirty or rusty dried liquid on the bottom. The electronics and mechanical parts within could be corroded or at least as dirty as the outside. A new owner can expect an incalculable restauration work. I would buy this computer only for parts as advertised but certainly not for $65 or more.

-Rainer
(Germany)

TRS-80 Model 4D ebay 02.jpg TRS-80 Model 4D ebay 01.jpg
 
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Just sold for $83. If the buyer is lucky and a chip reseat fixes the issue then I think they got a good buy. She'll cleanup pretty good. But, that's just my $0.02. I love the 4D. The top-o-the-line small TRS-80. Wonderful machines.
 
I was going to bid the opening price of $65 but was put off with the $69 shipping charge to WA so gave it a miss.

Turns out I would have been outbid in any case. If anyone here gets it I'd check the 2Mhz Model III mode clock first ;-)

Ian.
 
Hi All,

I stumbled upon this site and this thread by happy accident. I purchased this computer with the sole intent as having it as a donor for my beloved (like new) 4D (complete with Ian's terrific FreHD). 4Ds are becoming increasingly rare. I think $83 is not unreasonable assuming the motherboard and drives are in decent shape. Yes, shipping charges are always high, but such is the cost associated with being in this crazy love affair.

Just for fun, I'll try to see if I can get it working again thanks to the input from those above. I'll let you know what I find.

Cary
 
Avia said:
Hi All,

I stumbled upon this site and this thread by happy accident. I purchased this computer with the sole intent as having it as a donor for my beloved (like new) 4D (complete with Ian's terrific FreHD). 4Ds are becoming increasingly rare. I think $83 is not unreasonable assuming the motherboard and drives are in decent shape. Yes, shipping charges are always high, but such is the cost associated with being in this crazy love affair.

Just for fun, I'll try to see if I can get it working again thanks to the input from those above. I'll let you know what I find.

Cary
Click to expand...

Great. Stick around and let us know how it works out. We'll try to help you get it running right.
 
Thanks Pete! I just finished viewing your TRS-Box video...nothing short of incredible! Truly.

I have already experimented with your TRSWiki product on my Model I and love it. My 10 year old son and I were doing some online research on the M1, and I caught myself smiling more than once as I recalled, some thirty seven years ago, first laying my hands on the M1 when I was just a few years older than him.

A sincere thank you for all of your efforts in keeping history alive.
 
Avia said:
I have already experimented with your TRSWiki product on my Model I and love it. My 10 year old son and I were doing some online research on the M1, and I caught myself smiling more than once as I recalled, some thirty seven years ago, first laying my hands on the M1 when I was just a few years older than him.
Click to expand...

I appreciate the kind words. I too get a kick out of seeing my 11 year old son occasionally typing in simple basic computer programs into my Model I rather than just consuming content on his iPhone. Gives me hope for the next generation!
 
Hi All,

As promised, here's a quick update. I received the computer yesterday and got a chance to begin troubleshooting. Despite serving as a compelling lesson on the dangers of second hand smoke, the machine cleaned up very well both inside and out. In fact, extremely well. No corrosion whatsoever, and the motherboard looks like new. In fact, the original warranty sticker was still intact when I received it. So far, so good!

As you know from the original poster's comments, the screen was filled with random characters. While drive 0 powered up on boot, there was no attempt by the machine to read from the diskette. I tried a direct boot to cassette - no luck. Just random characters and a spinning drive. Oh well, at least there were signs of life.

Following the advice here, I reseated all the socketed chips. No luck. Next, I replaced all 64K of RAM with known good RAM. No luck.

Next, I decided to go right to the heart of the machine and replace the Z80 chip. SUCCESS! The familiar Diskette? prompt appeared and up came LSDOS!20160121_071949.jpg20160121_071839.jpg

While I originally had in mind to keep this computer for parts, I am thinking I may be able to make it fully functional again in the meantime. A couple of quick questions:

1/ The screen image will occasionally jump, momentarily expanding and contracting in size by about +/- 1/2 inch. Most of the time the image is stable. Power supply seems good, so I was wondering if anyone had any thoughts.

2/ I have to use one of the disk drives from this machine as a replacement for my primary 4D, so I am looking at options for disk drives. I have found the TEC FB-503 drives are very hard to come by, and this was the primary reason for me acquiring this "parts" computer. I have a brand new (IBM) Tandon TM-100-2A disk drive (tests good), but I cannot get the 4D to recognize it (drive powers up, spins but no head activity). Is this drive suitable for the 4D? (I have tried it with the resister pack installed and removed but no luck). If not this drive, can anyone suggest a suitable DS/DD drive?

Thanks for everyone's help!

Cary
 
My opinion on why the Tandon won't work is you may need a fully pinned cable. Not the Tandy with some of the pins pulled out of the cable to handle drive select. Can use a PC cable just need to undo the IBM "twist" and make the A: drive cable straight through. Also be advised that the Tandon may be set to DS1 by way of a zero ohm resistor may have to de-solder it and move to DS0 if you want it to be drive :0. Just my thoughts.

Oh congrats on getting a display on that other system.

St. Augustine? Hey just up the road a bit. I am near DeLand.
 
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Thanks for the quick reply Frank. Yup, looks as though we are almost neighbors!

Regarding the cable, I inspected the stock 4D cable and it appears to have a full set of teeth. I tested with another cable as you suggested with the same results. What's odd is that the Tandon drive works perfectly fine as drive 1, but not as drive 0. So it does seem to be set as Drive 1. Hmmmm...

I am attaching a pic of the drive boards. I have removed the (yellow) resistor pack but still no luck. Not sure what other drive designations settings there are?

20160121_085834.jpg20160121_085852.jpg

Thanks again!
 
Cary,
For the Model 4, if I remember correctly the Internal drives use no Terminator Resistor on their drives, but I could be
remembering wrong. They may just use a Terminator on the last drive on the cable........I forget.

For the Drive Select you need to locate the "1E SHUNT" which is somewhat close to the Terminator, but closer to the edge
of the circuit board. It is shown in this photo jumped from 3 to 14 (for DS1) and will need to be changed to 4 to 13 (for DS0).
A trick I use is to buy a 8 position DIP switch and just select the proper switch(es) for Drive select.

I've attached the schematic of 1E Shunt also, so you have a clue as to the signals. I'd think that one change in the shunt
would fix your DS0 problem.

Glad you got the Model 4 going with a new Z80.

Larry

100-2-1E_Jumpers.jpg100-2-DS0.jpg
 
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Congratulations! :D

Glad to see that the Model 4D is in better condition than I thought.
I have attached a page from the Tandon manual which also shows the location of the 1E shunt and the drive selection.

Edit:
I just saw that there is a discrepancy from Larry's schematic. In my attachment it's the position 2 to 15 for DS0.
Cary, have a try which position is the right one.
 

Attachments

  • Tandon TM-100-2.jpg
    Tandon TM-100-2.jpg
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Thank you to both Larry and Rainer for your guidance with the disk drive issue. It did indeed require position 2-15 to be jumped for D0. Also, in case anyone references this thread in the future, I did NOT have the terminator pack installed. I am still not clear on the role of the terminator with regard to the M4, but again....IT WORKS!

Thanks for the follow up guys.
 
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