• Please review our updated Terms and Rules here

TRS-80 Model 4P needs "Preheating"

J

Jenz

Experienced Member
Joined
Mar 14, 2019
Messages
98
You probably know this: after digging one of your treasures out and power it on ... nothing ... at least the red LED glows. I removed the cover and looked for some damage on the PSU ... nothing obvious ... after disconnecting the floppy drives and waiting for 15 seconds, it powers up! Floppy drives worked fine then and LS-DOS 6.3.1 and Montezuma CP/M booted, but after powering off and start again: nothing ... so I guess, I have to exchange all capacitors of the ASTEC PSU?

Did not find a serial number on the Model 4P, but this handwritten date inside:
m4pdate.jpg
 
The power distribution connector on the PSU should be re-soldered. Examine it with a magnifying glass and you might see dry solder joins.
 
Thanks Ian! Re-soldered the connector (did not found dry solder joins): sometimes it powers up immediately, but most of the time it may take up to 30 seconds (with disconnected floppy drivers).
 
Meanwhile the PSU is dead ... no red glowing Power LED ...
 
Like Ian stated, look at the Power Supply(s). If they have a row of pins that are 90 Degrees to the
PCB (typical of a TANDY Model Supply) remove it and check the Soldering on each Output Pin.
Look with a magnifying glass and look for small circles in the solder where the solder has separated
from the Pin. Re-flux the Pins and Solder with a good Kester 63/37 Solder. It's likely that problem
or a Cap that has a rounded Top (or Bottom) that needs replaced. Also look for leakage on the PCB
under each Electrolytic Cap. It's easy to spot the leaking or Rounded Top CAPS.

Larry
 
A can of freeze spray might help here. Let your machine “warm up” and when it’s working spray individual components on your power supply. when your machine quits working, or reboots, or like that then you’ve found the component. you can try the connectors, too. if you have a DVM look for voltage differences between working and not.

Doesn’t have to be your supply. Does your machine show anything on the screen prior to warm up? try cooling the FDC and surrounding components.
 
ldkraemer said:
Also look for leakage on the PCB under each Electrolytic Cap. It's easy to spot the leaking or Rounded Top CAPS.
Click to expand...

Thanks Larry, but no leakage (just glue) and no rounded caps ...
 
shank said:
Does your machine show anything on the screen prior to warm up?
Click to expand...

Thanks shank, but not even the fan is spinning on power up. After waiting 30-60 seconds, the fan spins and the systems runs flawlessley ... until you turn it off ...
 
On the 4P the light in the power switch is wired before the mains connection to the PSU, so the upshot of that is the red light on the power switch comes on even with a dead PSU or blown fuse on the PSU, making it confusing. Conversely the red light in the power switch can go out, but power still flows to the PSU (the light is in parallel not series) so the computer would otherwise work fine just with no red power light illuminated.

When I get a 4 or 4P in my workshop which seems slower to get going that it should be I immediately try a known good PSU to make sure that I eliminate problems with the rest of the computer from the slow start. Like the others have said, check your capacitors, especially with an ESR meter if you have one on hand. It sounds to me like the "ramp up" is not working properly.
 
But not even the fan is spinning on power up.
Click to expand...
If that is the case, I'd suggest starting at Section 5.4.3 Theory of Operation in the Model 4P Service
Manual (1984) (Tandy).pdf. Check the 160 VDC being created by BR1 (No Open or Shorted Diodes)
and if Caps C29 are good, I'd move along to the Kick Start Latch Circuit. This gets the Power Supply
Kick Started (Soft Start) until it's running off the Pulse Generator, as described in the Control Section
on p.150.

Suspect's could be R26, C14, or C1. That's where you will likely find the problem, if it's not in the
160 VDC circuit.

Larry
 
TRS-Ian said:
On the 4P the light in the power switch is wired before the mains connection to the PSU
Click to expand...

There was also a problem with the power cord ... after replacing the red light glowed again ...

ldkraemer said:
Suspect's could be R26, C14, or C1. That's where you will likely find the problem, if it's not in the 160 VDC circuit.
Click to expand...

C1 was replaced (I had found one), but to solder on old stuff you need more experience than I have, so the whole power supply will be replaced.

The 4P was switched on for the last time in 2010 to check the german keyboard driver for LS-DOS ... since then it has been properly stored ... seems you should let some things rest better ... ;)
 
After replacing the PSU the 4P booted: fan and floppy drives worked, Montezuma CP/M "beeped", but the CRT display remained dark: no power on the electron gun ...
Two dry solder joins on the PSU (TB2 connector COM/V2) were the reason for this ... now the 4P works again:

m4p.jpg

Thanks to all who've helped!
 
You must log in or register to reply here.
Back
Top