System 80 Blues

Anything to do with New Zealand Classic or Vintage Computing not covered in the other forums

System 80 Blues

Postby tezza on Thu Sep 23, 2010 1:51 pm

I'm having a go at restoring one of the "parts" System 80s I got off Kenneth (old_system_80_user) a while back.

It's proving to be a challenge. For the main board, I thought I'd had it sussed when I identified and replaced a RAM IC which took it to it's proper 32k. Despite passing all RAM checks the machine does weird things, especially when a key is pressed ...but not when at the command level in BASIC?? More when programs are running. This includes feezing, returning to READY?, inserting prompt lines were they shouldn't be and (most weirdly) simply returning to the command level (i.e. the ">" prompt in BASIC) (Very alarming when you are in the middle of playing Seadragon).

That's just the board. The internal cassette is also proving difficult. It shows a signal sometimes and at other times, not at all? Occassionaly it will have a signal on load, only to drop to zero level 1/2 through loading? I've replace EVERY capacitor on the analogue board PLUS the quad op-amp and transitor AND resoldered every connection. What else is there!

Oh it doesn't record either.

Power levels going into the cassette deck seem to be normal and are very similar to a good working unit?

Regarding the computer itself, I do wonder if those issues above might have something to do with power? There is no obvious sign of power problems (screen is steady etc.) but there might be something intermittent lurking. I'll do some checks tonight (including a check for AC ripple), and also swap it with a "known good" power unit.

Why am I posting all this? Probably just to get it off my chest but if anyone has any ideas, I'm receptive.
Tez (Terry Stewart) (Administrator)
Collection: http://www.classic-computers.org.nz/col ... /index.htm
Projects and Articles: http://www.classic-computers.org.nz/blog/index.htm
Twitter: @classiccomputNZ | YouTube: Terry Stewart
Trade Me: tezza5
tezza
Site Admin
 
Posts: 2292
Joined: Thu Jan 01, 1970 12:00 pm
Location: Palmerston North, New Zealand

Re: System 80 Blues

Postby pavery on Thu Sep 23, 2010 2:43 pm

Thought things have been unusually quiet from you of late. :shock:

Yep, check out PSU for sure. Tape recorder: Depending on PSU outcome, try it in a different machine. Also with the scope, you could compare with another tape deck to narrow down the suspect area.

Philip
pavery
 
Posts: 9
Joined: Tue Aug 12, 2008 9:04 am
Location: Wanganui

Re: System 80 Blues

Postby tezza on Thu Sep 23, 2010 3:10 pm

pavery wrote:Thought things have been unusually quiet from you of late. :shock:

Yep, check out PSU for sure. Tape recorder: Depending on PSU outcome, try it in a different machine. Also with the scope, you could compare with another tape deck to narrow down the suspect area.


LOL, yea most of Sunday and every night this week working on this demon-ridden machine!
Tez (Terry Stewart) (Administrator)
Collection: http://www.classic-computers.org.nz/col ... /index.htm
Projects and Articles: http://www.classic-computers.org.nz/blog/index.htm
Twitter: @classiccomputNZ | YouTube: Terry Stewart
Trade Me: tezza5
tezza
Site Admin
 
Posts: 2292
Joined: Thu Jan 01, 1970 12:00 pm
Location: Palmerston North, New Zealand

Re: System 80 Blues

Postby tezza on Thu Sep 23, 2010 11:27 pm

Ok, after tonight's "session" I can say it's not power issues...at least as far as the PSU and regulators are concerned anyway. I dragged out my other well-behaved two machines and found the power values on the +5, =5. +12 and +9v cassette line are more or less the same. In fact all values on all machines were within about 0.2 volts of each other.

No AV ripple on the PSU to speak of. The 20mV setting started to show something but I figure that is negligable. One of my good System 80 power suppies showed some AC noise at 40mV , the other 20mV.

This erratic behaviour on the board is indeed erratic. for example tonight it spontaneously went to READY? Pressing NEW LINE did nothing. However if I RESET I got the ">" If I pushed reset a few times I got things like >UL error in 1754 or >sn error in 1756 and the like? Weird! Something is screwy on the bus somewhere. Switch it off for 30 seconds then back on again it comes right..until the next time. If it's one of the logic gates failing occasionally on the data or address bus, it will be a real mission to find. I'm not sure if I want to spend every night of the next month doing this.

The weird thing is that if I run a RAM diagnostic test, it happily checks RAM over and over and tells me RAM is fine.

Tomorrow night I think I'll swap all the socked ICs (Z80, ROM and 2114 video RAM) and see what happens. I doubt if they are the problem though. I can't swap out RAM as they are piggybacked and wired to the board. If that doesn't show anything I might call it a day and consign this unit permanently to the spare parts pile!

I may have found something regarding the cassette issue. More testing needed thought..
Tez (Terry Stewart) (Administrator)
Collection: http://www.classic-computers.org.nz/col ... /index.htm
Projects and Articles: http://www.classic-computers.org.nz/blog/index.htm
Twitter: @classiccomputNZ | YouTube: Terry Stewart
Trade Me: tezza5
tezza
Site Admin
 
Posts: 2292
Joined: Thu Jan 01, 1970 12:00 pm
Location: Palmerston North, New Zealand

Re: System 80 Blues

Postby tezza on Fri Sep 24, 2010 6:51 am

The symptom reminds me of those "spontaneous reboots" I used to read about with the TRS-80 Model 1 expansion units caused by oxidized card edge connectors.

Another thought then. Maybe a dry or dodgy solder joint at the bottom of one of the RAM sockets? The System 80 sockets are not high quality and would have been pulled and poked at during the RAM upgrade plus my RAM repair. Hmm...another hypothesis to check out.
Tez (Terry Stewart) (Administrator)
Collection: http://www.classic-computers.org.nz/col ... /index.htm
Projects and Articles: http://www.classic-computers.org.nz/blog/index.htm
Twitter: @classiccomputNZ | YouTube: Terry Stewart
Trade Me: tezza5
tezza
Site Admin
 
Posts: 2292
Joined: Thu Jan 01, 1970 12:00 pm
Location: Palmerston North, New Zealand

Re: System 80 Blues

Postby tezza on Sat Sep 25, 2010 11:56 am

This repair thread will be continued on the new Repairs and Restoration forum...
Tez (Terry Stewart) (Administrator)
Collection: http://www.classic-computers.org.nz/col ... /index.htm
Projects and Articles: http://www.classic-computers.org.nz/blog/index.htm
Twitter: @classiccomputNZ | YouTube: Terry Stewart
Trade Me: tezza5
tezza
Site Admin
 
Posts: 2292
Joined: Thu Jan 01, 1970 12:00 pm
Location: Palmerston North, New Zealand


Return to General

Who is online

Users browsing this forum: Yahoo [Bot] and 4 guests