Mitsubishi HS-423UR SVHS VCR heat-sensitve problem with video...
 

I have a 1987 Mitsubishi HS-423UR SVHS VCR. It currently has a problem with video. First part it works and the video is clear, then the video slowly deteriotes after 30 minutes, and then the video is dark, distorted, and not clear. What is wrong with this unit and can this one be repaired? Is the problem coming from the video board?

I think the problem location will depend on what is all affected. Does the deteriorating video happen only to tapes being played? Or, is video also bad from TV channels tuned on the VCR's tuner and watched "live"? Did you try composite video output from the VCR and also try its Channel 3 output into a TV antenna input, and if so, were they both affected the same way?

Tuner works fine with clear picture. I tried both using RF output and composite video output, and it still shows distorted video if using the unit longer.

Too high of tape tension will cause the heads to clog and create those symptoms, same thing with worn heads-tape path,I do beleave the head cleaning roller engages during eject and the unit will prob work for a few more mins then the same thing will happen,all too common,prob needs a complete overhaul

Wow, so far I have technicians that appear to know nothing about VCRs. Tape tension has nothing got to do with heat-related problem in the video board. Chassis was already restored.

How long have you been repairing vcr's?

Wiseguy has a valid point. I have seen tape tension problems show up as the tape plays. Sometimes from worn clogged heads, and sometimes the problem is related to not being able to keep the proper tape tension as the supply reel empties.
So far you only know the problem is not in the tuner, and that it is time related. It could be heat related as you said, if that is the case some careful use of freeze spray, or a heated soldering iron should help locate the problem. Note I said careful use, you would not want to get it on the tape path, or heads.
The next step is to drag out the service manual, you do have a service manual don't you, and scope the video from the heads to the output to see what is going on.
Most of us here would have to actually see this and do some basic troubleshooting before we could offer any more than general suggestions. How you handle those suggestion is up to you.
Good luck, I hope you find the problem since those were not bad vcr's quality wise.

Bill R