dkedrowitsch
Experienced Member
So I recently purchased a pretty clean PC 6300 and matching CRT 313/H monochrome monitor. The motherboard had some slight battery damage but I've got quite a bit of experience dealing with that thanks to a number of Amiga repairs I've done. I mitigated the leak with some mild acid (Tarn-X is my preference) then replaced the charge current limiting resistor with a diode so I could safely install a coin cell holder. Besides that, the system pretty much looks and works like new--with a very quiet and healthy ST-225 being the cherry on top.
The monitor however has been a different story. At first, besides a somewhat soft focus it worked OK. But after a few minutes it developed an annoying horizontal scan jitter. Here's a short video of the issue:
https://youtu.be/JkAwy-igt6E
I went ahead and replaced every electrolytic with new 105c rated parts from Nichcon and Panasonic which restored the focus, but the horizontal jitters persist. They temporarily cleared up after the monitor was on for a few hours, but returned to stay after the next power/thermal cycle.
The cause seems pretty obvious, with my scope triggering on H-sync the 12v power source to a TDA2593 horizontal drive IC looks like an EKG readout. There's a sharp 200mv dip that coincidences with the leading edge of the drive pulse to the HOT which is then followed by a 200mv bounce as the dip recovers. These ugly transients must be the cause.
At first I assumed there might still be some decoupling issues, but then I noticed the 15 volt supply from the PC 6300 video board is actually 13.4-volts under load. 13.4 volts seems hardly enough for a 12-volt linear power supply to keep out of drop-out under heavy current spikes such as when driving a CRT yoke and flyback. And this particular CRT 313/H is a 12-volt chassis with a discrete linear power supply (zener reference, adjustment pot, bipolar output transistor, etc) that's powered by the computer PCU's 15-volt "CRT" output.
So I my questions are: Is it normal for the 15-volt "CRT" supply of an Olivetti M24/AT&T 6300 to sag down into the mid/low 13-volt range under load? If not, I may need to dig into the power supply.
Would anyone happen to have a service manual for the CRT 313/H? It's a Hantarex monitor--a sticker on the HOT heatsink says so...but unfortunately no model number can be found anywhere. It's a very different beast from the 1231 chassis most M24 CRTs seem to be based on.
Thanks!
Dieter
The monitor however has been a different story. At first, besides a somewhat soft focus it worked OK. But after a few minutes it developed an annoying horizontal scan jitter. Here's a short video of the issue:
https://youtu.be/JkAwy-igt6E
I went ahead and replaced every electrolytic with new 105c rated parts from Nichcon and Panasonic which restored the focus, but the horizontal jitters persist. They temporarily cleared up after the monitor was on for a few hours, but returned to stay after the next power/thermal cycle.
The cause seems pretty obvious, with my scope triggering on H-sync the 12v power source to a TDA2593 horizontal drive IC looks like an EKG readout. There's a sharp 200mv dip that coincidences with the leading edge of the drive pulse to the HOT which is then followed by a 200mv bounce as the dip recovers. These ugly transients must be the cause.
At first I assumed there might still be some decoupling issues, but then I noticed the 15 volt supply from the PC 6300 video board is actually 13.4-volts under load. 13.4 volts seems hardly enough for a 12-volt linear power supply to keep out of drop-out under heavy current spikes such as when driving a CRT yoke and flyback. And this particular CRT 313/H is a 12-volt chassis with a discrete linear power supply (zener reference, adjustment pot, bipolar output transistor, etc) that's powered by the computer PCU's 15-volt "CRT" output.
So I my questions are: Is it normal for the 15-volt "CRT" supply of an Olivetti M24/AT&T 6300 to sag down into the mid/low 13-volt range under load? If not, I may need to dig into the power supply.
Would anyone happen to have a service manual for the CRT 313/H? It's a Hantarex monitor--a sticker on the HOT heatsink says so...but unfortunately no model number can be found anywhere. It's a very different beast from the 1231 chassis most M24 CRTs seem to be based on.
Thanks!
Dieter
