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

wellswang

Member
Joined
Apr 23, 2022
Messages
12
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.
 

wellswang

Member
Joined
Apr 23, 2022
Messages
12
Location
Suzhou, China
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.
 

daver2

Veteran Member
Joined
Jun 19, 2012
Messages
7,731
Location
UK - Worcester
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
 

wellswang

Member
Joined
Apr 23, 2022
Messages
12
Location
Suzhou, China
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.
 

daver2

Veteran Member
Joined
Jun 19, 2012
Messages
7,731
Location
UK - Worcester
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
 

wellswang

Member
Joined
Apr 23, 2022
Messages
12
Location
Suzhou, China
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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