old trs80 computer lots of smoke

[shelby5041]

hi
my neighbor gave me an old radio shack trs80 model3 computer...it was stored in his attic for 35yrs...i powered it on and after 5min lots of smoke was coming out of computer...i took it apart and didnt notice any fire evidence so i assume a cap split open...i looked around only saw 1 suspect part...see photo...could this be the smoke?

[petehall347]

yes that is one of the famous RIFA caps .they do that for fun . replace with something suitable . lets hope the rest still works . just plugging things in after such a long time is never a good idea . it should be brought to life slowly whilst monitoring it .

[stj]

might be a good idea to check adrians digital basement on YT, he fixes a lot of that stuff

[shelby5041]

i may be right

i found this info online

Most Common Issue … Did you Model 3/4 Go Poof!

If you turned on your Model III/4 and smoke came out, it is possible your power supply caps blew. This is very common.

[stj]

RIFA filter caps are in most old quality stuff,
they were seen as the best at the time - loads of HP test gear is full of them!

problem is they didnt use a case, they just molded epoxy around them.
as it gets old the epoxy shrinks and cracks - then moisture gets in and booooom.
i have seen a box of them from the 80's , never used.
ALL CRACKED!

[PeteS in CA]

From a 1980ish context and perspective, in the US, Rifa PME271 series were what was available. Wima, Siemens, and other European manufacturers came into the US a few years later. Some folks here regard 40 year-old parts failing dramatically as defective. To me, failing after years of surges and being 40 years old is just a realistic, if bad-smelling and inconvenient, outcome.

To your failed power supply, just replace all Rifa PME271 caps in it (even ones that have not yet failed) with polypropylene or polyester X and Y caps. If it's a switching power supply (original TRS80 power supplies were linear, IIRC), give the output capacitors a good look-over. If you have an oscilloscope, look at the ripple, especially the switch frequency (probably 20-30 KHz) ripple. Those output caps are reeeeeeally old. If any are swollen or the output ripple is much greater than 1% or 2%, the output capacitors probably should be replaced (possibly use Nichicon PM or PS series, as newer types might be too good and cause stability problems).

[redwire]

Rifa PME271 are unfortunately available to this day (they have oddball lead spacing). For OP's blown cap C21 original replacement but I say outright replace them with another brand, they are terrible. Every old one I have seen, the epoxy bulged and cracked even sitting there new in a parts bin.
The other X-cap is C2 0.01uF 250VAC, also a Rifa.
More at this website but they are advocating putting in new Rifas which makes me cringe. I would stuff in anything other make.
The two Y-caps are ceramic disk so shock hazard not a Rifa concern. C1 Y-cap 2,200pF 250VAC, C3 Y-cap 2,200pF 250VAC

TRS-80 Model III Service Manual, Schematics http://cpmarchives.classiccmp.org/tr...hack)(pdf).pdf

[shelby5041]

thanks for all the info

i just want to replace the failed cap or caps with anything suitable..i think it is 0.1uf 275vac 50-60hz ...can someone send me some links so i can see what to buy...digikey preferred....note none of the alum caps are bulged.....for now i just want to remedy the one that smoked out.and others similar to that style...there may be 3or4 total....and thanks for the service manual im sure will come in handy.....hey red can u suggest alternative caps for the 0.1 and 0.01 X caps?

[redwire]

A modern part has a bit wider lead spacing 22.5mm vs 20.3mm but you can bend the leads in.

Original C21 Rifa 0.1uF 275VAC X2 size 24*7.6mm, height 14mm, LS=20.3mm
Consider Kemet R413N310050T1K
0.1uF 300VAC X1 size 26.5*8.5mm, height 17.1mm, LS=22.5mm so a little bigger part but I think can fit, it has long leads.

Original C2 Rifa 0.01uF 275VAC X2 size 18.5*5.2mm, height 10.5mm, LS=15.2mm
Consider Kemet R474I210050A1K
0.01uF 440VAC X1 size 18*5mm, height 11mm, LS=15.0mm, it has long leads but you don't really need that.

[shelby5041]

thank u red

i know this is not a trs80 forum but is anybody here familiar with the old trs80 model3?...when powered on should there be any booting up beeps?..and should anything show up on the screen ?...mine has no beep nothing on screen but the floppy drives do spin.( prior to the cap smoking)

[stj]

you should get something,
i remember using them when they came out!
at the very least you should get a boot message - didnt it have basic in roms?

if we assume the computer is doing stuff,
time to look at the monitor section.
is the neck glowing at the back of the crt?

[shelby5041]

not sure..ill look at it once i get the replacement caps will be few weeks

[shelby5041]

i know the monitor is powering on because when i turn computer off the screen does the line dot thing in the middle

[R_J]

Did you try adjusting the brightness and contrast controls? It might display Cass? on screen

[redwire]

I sure you can find this and other suggestions on the web:

"After powering it on hold down the <BREAK> key and press the orange reset button.
With any luck you'll see the Cass? prompt.
Press <ENTER> twice to accept the default cassette speed and top of memory.
Then you'll be in the BASIC Ready > prompt.
If you get that far then you know most of the machine works and all you need to do is find boot disks.
Drive cleaning and alignment will likely be needed.
If you don't see Cass? then maybe the <BREAK> key is broken.
Not likely you'll fail to see Cass? because of brightness/contrast issues.
But those adjustments are underneath the "rim" just left and forward from the keyboard."
--------------
I've also read that bad RAM is a common problem, if you can't do much in ROM.

[RJARRRPCGP]

yes that is one of the famous RIFA caps .they do that for fun . replace with something suitable . lets hope the rest still works . just plugging things in after such a long time is never a good idea . it should be brought to life slowly whilst monitoring it .

That's like Apple II.

[shelby5041]

thanks red

at the time i didnt know there were bright cont adj...however both pots are siezed up anyways..they dont rotate..will have to use contact cleaner and pliers to try and loosen up.

[redwire]

Uh no no, no need for pliers you are not pulling teeth! That will just break something. Vintage gear is fragile.
What did you want to do - most people want a quick flip and sell to make money as opposed to enthusiasts who want to run the old gear.

[shelby5041]

once i replace caps i want sell it ....even if monitor no work.

[stj]

it will get a lot more $ if it works though.