Tandy TRS-80 Model 4

[mnbvcxz]

I cannot give you any advice on troubleshooting, hopefully someone more knowledgeable will help you.

 

[Se87]

I cannot give you any advice on troubleshooting, hopefully someone more knowledgeable will help you.

FOUND IT! It was this paper looking square thing, looks as if it was a capacitor. Photos:

 

[Druid6900]

Thank you, I'd bet money on the power supply smoking. Would it be cheaper to replace a cap or just replace the whole thing? I heard you could replace it with a normal power supply out of a modern desktop machine.

There is a place called Hosfelt Electronics in the US that sells the original Astec Model 4 65w power supply (it has a metal case, but, you just take that off and throw it away) for about 5 bucks. They probably still have them.

It would be a lot less hassle than repairing the old one and it would still be original.

Just take a picture of the old one, before disconnecting it, so you know where all the plugs go.

 

[Chuck(G)]

FOUND IT! It was this paper looking square thing, looks as if it was a capacitor. Photos

I'm assuming you're pointing to the thing with the "0.1 úF" on the top.

That's the infamous made-in-Sweden Rifa paper capacitors. Your system will probably run without it, but you need it to keep RF from the power supply off the power line. You can replace it with any like-valued polyester/ceramic/polypropylene non-polarized capacitor.

This is a very common failure on these old PSUs.

That round electrolytic capacitor in the corner doesn't look healthy either. You should probably replace it.

 

[TRS-Ian]

Just to give you a bit of a heads up, thats not a Model 4 Astec 65W power supply, thats a Model III Astec 35W power supply (as fitted to most Model IIIs and cassette based M4s). The power connections are different. M3 supply has 3 connectors with 4 prongs each, whereas the M4 supply has one long connector with 12 prongs and one missing.

On the plus side the M3 Astec 35W supplies are nearly indistructable so just replace the 3 Rifa caps with modern MKT caps and they power supply with filter like it was supposed to.

I'm betting your computer started life out as a cassette based M4, and its cat. no. is 26-1067. To put a M4 Astec 65W supply into your system will require a wiring loom from said disk based M4 computer.

Hope this helps,

Ian.

 

[Se87]

Just to give you a bit of a heads up, thats not a Model 4 Astec 65W power supply, thats a Model III Astec 35W power supply (as fitted to most Model IIIs and cassette based M4s). The power connections are different. M3 supply has 3 connectors with 4 prongs each, whereas the M4 supply has one long connector with 12 prongs and one missing.

On the plus side the M3 Astec 35W supplies are nearly indistructable so just replace the 3 Rifa caps with modern MKT caps and they power supply with filter like it was supposed to.

I'm betting your computer started life out as a cassette based M4, and its cat. no. is 26-1067. To put a M4 Astec 65W supply into your system will require a wiring loom from said disk based M4 computer.

Hope this helps,

Ian.

Yes. The gentleman who owned this machine took very good care of it and had it matenienced in 1988, for disk drive failure I believe. He may have had various other machines, I wouldn't be suprised if he is someone very important on this very forum. I also got 5 boxes stuffed with 5.25 disks, most in original box never used.

 

[Druid6900]

Actually, when I was working for Radio Shack, we would regularly retro-fit Model III and 4 computers having one or two 35w power supplies with one 65w power supply in cases of adding drives or damaged power supply after Aztec discontinued that particular model of PSU.

The 12 pin strip still handle the drive power plugs and the main board and auxiliary board plugs were still on the PSU logic board.

Different or additional cables weren't needed and everything would reach the power supply if it was mounted on the inner shield.