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Hazeltine 1500

Gary C

Gary C

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Joined
May 26, 2018
Messages
2,297
Location
Lancashire, UK
After failing with my PS/1 monitor (for now)

I am looking at my broken Hazeltine terminal. The video side isn't running at all. First thing I note is that I dont have the NDOT & DOT clocks but I do have the VID clock which suggests something around U99 & U105 in the service manual drawing.

Not yet had chance to poke around more, but I note that on the drawing there are several 'CLAMP' connections that don't seem to terminate on any drawing.

And in writing this, I suddenly see what the circuit is actually doing. There are three XOR gates connected to test points that are pulled high, ground the test point and it pulls the XOR gate output down which inhibits the relevant part of the circuit.
 
Ouch.

While diagnosing, the transformer started to sound like one of our 11kV to 3.3kV transformers and the magic smoke appeared. Fortunately the failed rectifier left a small tell tail to ID itself. Fitted was WL02 which is simply replaced with a W02 rectifier from RS (no idea where the L went !)

So back up and the 8080 processor is running nicely with lots of activity on its bus and sync pin but the video clock is still absent or more accurately the video clock is there at 16mhz but isn't being transformed into the DOT clocks.

Just changed the 74S161 counter and yay ! dot clocks are back.

Just need to reconnect the monitor to see if there is any image.
 
With the new counter installed, all the video clocks are now restored and the monitor has leapt into some sort of life, but its displaying a screen of rapidly flickering what look like characters.

A step forward at least.

While the processor is running, its noticeable that CTRL-G doesn't sound the bell, and selecting offline doesn't put the online LED out so it appears all is not right on the digital side.

Scoping about, bit 1 of the data bus is stuck at 0. On the CPU side of the bus controller, its active but on the other side, nothing.

So a couple of RAM chips, a couple of ROM's, the 8228 bus controller, a few logic chips, keyboard controller could be the culprits which will be a pain to desolder.

Not sure what else I can do though.
 
The drawing interestingly shows that a 74LS125 sits on bit 1 and testing, it should be in tri-state. Taking it out of service, the machine now sometimes gives a beep and I can see some brief activity on bit 0 when reset is pushed.

The chip tests bad out of circuit so that's next up for replacement but I'm not sure that will solve the issues either.
 
So the 74LS125 swapped with a working unit and its doing a little more but basically the databus on the 8228 side looks really odd. bit 1 has a voltage of about 2v so something is pulling it down.

I think it might be worth writing a program to do a memory test and disable the video but need to change the jumper settings for a 2716.
 
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