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Please Help: TRS-80 Color Computer 2 can not boot up

wellswang

Member
Joined
Apr 23, 2022
Messages
29
Location
Suzhou, China
Hi,

I have a TRS-80 CC2 which model no is 26-3127. I checked the manual and found that it may simlar to 26-3027.
Please check the mainboard:
mainboard.jpg

When I power on, the display screen twisted at first:
DSC_5112.MP4_20220604_230857.429.jpg

I found +5v of all chips is below 4v, about 3.5v. The AC input is not AC15v, it measured only AC10v.
So i disconnected AC transformer and soldered a +5v DC input on C5 positive.
The display screen not twisted again, but it shows some vertical bars and question marks.
screen.png

I thought RAM chips may corrupt, so I desoldered all RAM chips and replaced with new 4164s.
But problem still exists. All 8 RAM chips passed tested on another system.

Then I replaced following chips:

MC6809: tested on another CC2, and it's OK. replaced with another MC6809, problem still exists
74LS783: tested on another CC2, it's ok. replaced with 74LS785, problem still exists
74LS138: replaced with another 74LS138, problem still exists
74LS244: replaced with another 74LS244, problem still exists
74LS02: replaced with another 74LS02, problem still exists
MC6847: tested on another CC2, it's ok. replaced with another MC6847, problem still exists
MC6822: tested on another CC2, it's ok. replaced with another MC6822, problem still exists
MC6821: replaced with MOS6520, problem still exists
NE555: replaced with new NE555, problem still exists

I replaced almost all chips but problem still exists.
I also tried to use 27C64 to replace the ROM chip and built an DIP28-24 adaptor. It still failed.
28-24 adaptor.jpg

28-24 adaptor connection (red pin number were connected each other):
28-24 ROM adaptor.png

Please help me to find out the problem. Thanks!

BR,
Wells.
 
Finally, I fixed it.
The root cause is AC transformer, I replaced a new AC15V( 7.5v 0v 7.5v output) transformer. It works fine.
I don't know why the original AC transformer is AC 10V.
Maybe I can't use DC 5v input directly to drive the board -_-b.

Thanks all you guys.

微信图片_20220625225107.jpg
BR,
Wells.
 
Sorry, I didn't see your original post.

If you apply +5V across C5, the problem is that you still have an unpowered power supply upstream of C5 that could be affecting the +5 V supply.

Did you actually measure the +5 V voltage after connecting it across C5 or not?

I suspect it could have still been low, or had some strange voltage purtobations on the DC rail.

Dave
 
Hi Daver2,

Yes, I measured it. it still a bit lower than 5v, about 4.6v, I think maybe it dosen't matter. -_-b
I'm WRONG!
I ignored it at first, but I could not find any other problem, so at last I bought a new transformer and replaced it. Problem solved ...

Thanks!

BR,
Wells.
 
If you look at the datasheets for TTL devices they are generally specified for 5V +/- 5% = 4.75V to 5.25V. Hence the 4.6V that you measured is outside of tolerance.

Even at 4.75V I would be suspect myself - because you are still "on the edge" and any noise could affect the operation.

As you can see, ignoring it means that you needlessly replaced a 'ton' of chips I am afraid...

Dave
 
Yes! I just want to save time for buying a new AC transformer and ignore the problem, but it cost me more time to replace and test it.
Learned a lot on this case! :)
There is a saying in Chinese, "欲速则不达", haste makes waste.

Thanks!
 
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