Wednesday, 11 January 2023

Hitachi P25B Portable Black & White TV fix

I fixed a B&W TV over my Christmas break. Its a 12" Hitachi from the late 70's, small little transistor set.

The TV Powered on with Power LED lit, but didn't do much else. Nothing on screen and no audio, after a fair bit of poking around I got to the bottom of the issue.

The main fault was an open Circuit resistor which stopped current flow through the Base of the Horizontal Output transistor which in turn drives the LOPT windings to create the HV.

The resistor had previously been bodged with someone adding another resistor in series with the original. Once swapped we have HV and analog fuzz on the tube. I tuned the TV to my Sig gen, and then find there's no audio, after prodding around I cannot see anything wrong in the circuit. I connected a set of Headphones to the jack on the side and sure enough I got audio through, looking at the schematic, the jack was switching the TV speaker in and out of circuit based on the connection, everything belled out ok, which only left the speaker, it turned out to be open circuit, I luckily had a very similar size speaker, swapped it over and voila, we have a fully working TV with a lovely bright tube.

Thursday, 8 October 2020

Atari Space Riders Pinball (1978) - Part 5

So the game was up and running and looking great, a bunch of my arcade buddies were coming over the next day so I thought it was best to change the replay settings as they were currently set to win extra credits at certain score points; there was no point in having that as an incentive considering we weren't dropping coins to play games, I though I'd change it to add-a-ball at certain scores, I flicked the dip switches on the CPU board and thought nothing of it. Roll on the next day, I turn the game on when my mates arrive and during the second or third game the game froze, all the play field lamps were on with nothing else going on, no coin input or anything worked. I re-powered the game but still it was in this frozen state. Hum ho, no more pinball games for my mates, at least one of them had got a game in! I was miffed at what had happened but that's life with 40 year old machine.

A few days later I wanted to find out what had gone on it was only then i realised that last thing I did was change the dip switch settings. I moved them back to how it originally was and boom, the game started working again, ok that was an easy fix but I wanted to have the game set differently so needed to find out what was wrong.

I played games for a few hours now it was 'working' again but did notice the game rules weren't working correctly and Tilt would randomly be triggered ending a ball for no reason.

Atari arcade games usually have good tech manuals which give descriptions of the different areas of the game circuits, the Pinball ops manual is no different, because it seemed related to the dip switch settings, I read up on the switch reading circuit. All the switches are connected to one side of multiple 74145 IC's, From memory (its been a while since I fixed the problem!) the switch states constantly being read and their values stored in memory. Long story cut short, I had some failing 145's, Initially I removed and tested the 145's that were connected to the dip switches, sure enough they tested as fault in my IC tester. because these had failed I decided to remove the remaining one Six in total I think and test those, a few more came back with errors so I put sockets in and fitted replacement IC's. To date I've not had issues with switches being falsely triggered or the game locking up.

Fast forward most of 2020 and the game has been regularly played, recently the display for credit and ball number are intermittently working, something new to look at when I have the space to remove the glass from the game.

Boring post with no photos, I'll make sure I pout a few in the next post.


Tuesday, 29 September 2020

Sunday, 27 September 2020

Tuesday, 10 December 2019

Atari Space Riders Pinball (1978) - Part 4

So with the electronics and mechanical parts all seeming to be working OK, it was time to give the play field and all the parts a good clean. Time to strip it down and get rid of all that grime.

The worst part of the play field was by both pop bumpers.

So this is where all the play field art was hiding!

 Putting the kitchen sink to good use!

It was a great day to dry all the parts outside

I ordered a selection of cleaning products and some new lamps, balls and rubbers from Pinball Heaven. After giving everything a good clean I used some Novus on the plastics and Milwax on the playfield. The metalwork had a quick polish too. At this point I started reassembling it all using loads of photos I'd taken fir reference.

At this point I was all set to put the legs back on, move the game in to place and invite some mates round for a game or two, what could go wrong? Being an Atari pin, my work was not yet complete...