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8050 Repair - Data Bus 6 Half Amplitude

C

Caterman

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Jun 23, 2015
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Having a problem with my 8050. All 3 lights on. Data can be seen on all data bus lines. But only on 6 the wave amplitude is half all the others
Installed known good 6502 and tested all Ram. Nothing seems to be overheating. Presume some sort of short pulling down D6 everywhere!
New sockets installed. Where should I start?
 
First, the 8050 has TWO 6502s, so which part if the board is giving troubles? The most rigorous thing you can do is to remove all parts connected to the data bus and replace the ROM with an EPROM filled with NOPs. plus a jump at the end to the start again. If the 6502 is alright, the signals on the bus should look OK. Then start replacing the other parts. The moment the amplitude changes, you know you have fond the culprit.

Good luck!
 
Looking at my first encountered issue seems a problem with the UL1 ROM Part 901482-07
I've rigged a chip convertor with a 74LS04 to read data as a 27C64
Read data
Images on the Left are my chip dumps and on the right zimmers images
Hex Edit 07.JPG and compared to Zimmers version massive difference.
Must conclude ROM faulty!!!!

I have done the same with the UH1 Rom Part Number 901482-06
Hex Edit 06.JPG
It is very similar but there are small differences.
Do you think it my setup or both roms faulty or that the Zimmers Rom i am comparing is a slightly different revision?
 
Can you read as a NMOS 2764 instead of a CMOS 27C64 as it will be more compatible with the 2364 ROM. Also double check the wiring in your converter circuit.
 
It was my convert circuit.
In the end the best circuit I followed was from http://mess.redump.net/dumping/2364_mask_roms
IMG_8328_1.jpg
8050 all up and running perfectly. It was just the UL1 ROM 901482-07 in the end.
Replaced with a 27c64 - just need to pretty up


IMG_8330_1.jpg
I can source a PCB with a SMD hex inverter.

Do you know of a source for a PCB that can be used to dump 2364 with an SMD inverter? to replace my lash up.
 
Managed to format 2 floppies and then decided to change both rom sockets. Now I get 4 flash error code. Checked sockets and look ok. Perhaps something else has gone wrong
 

POWER UP DIAGNOSTIC FOR 8050/8250 FLASH CODE​

On power up, the three LED's on the drive should blink. The drive LED's go off and the center Tri-State LED remains green. The sytem has a built in self diagnostic which can be read in the form of a flash code. In the event of a failure, the 3 LED's will blink in a repetitive sequence. The number of blinks between intervals will indicate the possible location of a failure.
​
ERROR INDICATIONS
NUMBER OF FLASHESERROR CAUSECOMPONENTLOCATION
1Zero Page6532UC1,UE1
2ROM $E000-$EFFF2364UL1
3ROM $C000-$DFFF2364UH1
4Unused
5Zero Page6530 6502UK3 UH3
6Common RAM $1000-$13FF2114UC4,UC5
7Common RAM $2000-$23FF2114UD4,UD5
8Common RAM $3000-$33FF2114UE4,UE5
9Common RAM $4000-$43FF2114UF4,UF5
10Controller ROM6530 6502UK3 UH3

​
Ref: http://www.6502.org/users/andre/petindex/drives/index.html#blink
 
Problem is it blinks 4 times.
I have a CBM Midwest Service News Dated March 14 1983

20250205142807159_0001.png
With 4 flashes - I have checked all RAM and confirmed operation of UD3 74LS157
I have treble checked and resoldered all new chip sockets.
Double checked the contents of ROM's UL1 and UH1
swapped out the 6502 and swapped around to confirm all ok
Swapped out the 6522
I have made a 6530 replacement
All no change
Bit stummed at the moment!
 
I've tested all RAM chips and all test OK.
Checked connections of all ram sockets and also all OK
The UH3 CPU has Data on all the Data bus lines but looks exactly the same.
No action on any of the Address lines all held at just under 4v!
 
I've got the following:

2114s that work fine in a 40 column PET but not an 80 column

A 6520 that works fine in the IEEE controlling socket, and nearly fine in the User Port/keyboard socket, but outputs no sound

A 6821 that works fine as a replacement in the User Port/keyboard socket but when installed in the IEEE socket doesn't work reliably with a floppy disk drive.

I guess my point is that in certain situations, all these chips tested fine, but the only way I have found to really test them is to replace them and see if there was a difference.

Colin.
 
I take your point.
I have 4 x 2114 I will track down and I can raid some other CPU's
More info later!
 
All up and running, working drive again.
Tricky to put my finger on exactly the cause.
Using a thermal camera found the following
2 of the ram chips on the right hand bank weren't getting any temperature these were replacements I was trying during the swap out process.
So I swapped back the originals.
One of the 6502 was running hotter than i thought was acceptable, so swapped although it was working.
My final thought is that the desoldered RAM chips that were reinstalled in new sockets have legs that are a touch on the short side.
When testing with a probe probably enough downward force to seat the leg sufficiently, however, they may have sprung up enough not the make propper contact.
I have made doubly sure that all IC's are properly seated.
I will watch out next time for short legs!
Also will only use turned pin sockets in future, that should also help a lot!
Thanks for your input
 
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