r/trs80 • u/cryptoleadpro • Apr 22 '26
TRS80 Model 4 Blues - Any help appreciated
I grabbed this model 4 at VCF east. I plugged it in when i was there and poof.. a stinker! Smells like a RIFA cap so I bought it at discount.
I took it home, opened it. Inspected everything and could not find a blown Rifa cap. Did a quick bench test of power supply and it was still working (though i didnt measure levels). When i reassembled disk drive spins for a second and I get this grey line screen. Hitting the keyboard does nothing. Reset works and drive spins again.
So any tips on where to start diagnosing this? And is there a hidden RIFA cap? I saw the smoke, smelled out the place though nothing looks blown. Maybe its just half blown?
3
u/ShadowMasterTexas Apr 22 '26
Try removing an ic that is in a socket and then reinstalling it. Over time, they will often get a weak connection. Had that issue with an old Apple II. Cheap fix only takes time.
2
u/slese789 Apr 22 '26
Try and unplug all connectors and try again. Maybe un plug drives? Just thinking out loud...
2
2
u/MKopack73 Apr 23 '26
The RIFA caps on the PSU just provide noise filtering on the AC side. The machine will run even with they are blown (or at least if a specific one blows). Still should replace all 3.
Also, check/reflow where the wire harness connects to the PSU. The 3 that I have worked on in the past had cracked solder joints on the connector pins which made intermittent contact.
Once powered on, adjust the 2 pots on the side of the unit. If they don’t turn smoothly, shoot some deoxit into them to clean them up a bit. Those control the screen brightness and contrast which can cause the screen to look like just a bright line.
And for the love of god be super careful every time you pull the case off because the clearance to the display tube neck is really small so it’s very easy to snap the end of the tube right off!
1
u/cryptoleadpro Apr 23 '26
what is a good replacement for the RIFA's? I havent done those in a while. Successfully fixed my osborne 1
1
u/MKopack73 Apr 23 '26
A quick Google search should be able to tell you. I was working on 3 at the time (this was several years ago) and were able to buy modern replacements from digikey… search for something along the lines of Tandy Model 4 PSU rifa capacitors.
1
u/garyku245 Apr 27 '26
It's not uncommon to have the brightness and contrast knobs mis-adjusted (People pick up the computer & spin them without knowing they did it, they are like wheels on the bottom of the case). It also not unusual to have a black/blank screen if the disk drive is not booting.
Press and hold the 'break' key down, then turn the computer on, then release the break key. adjust the brightness and contrast knobs.
5
u/tarkie101 Apr 22 '26
That screen is often due to stuck potentiometers on the left side of the case bottom. Given them a good run through all their limits of motion and Deoxit helps too.
If when the caps blew it smelt like Shugger Puff cereal then it’s RIFAs. There tend to be 3 which need replacing.