Found at a reasonable price in Henley-on-Themes last year, It looked in good condition, so I expected the worst electrically and I was right. This unit cost the earth when released in 1981, it was an all singing and dancing modern looking system with 2 horizontal tracking ceramic cartridges fitted one for each side of the vinyl. It was also an early user of micro processors in a hi-fi (HD44750A35) made by Sharp themselves. The nice trick this unit performs with help from computer is to record the whole of an album to a cassette without any intervention from a human. The tape player/recorder is high quality using metal tapes and Dolby noise reduction. With audio outputs from the rear of the unit, this was ready for making CD's that would arrive just a few years later. Although the unit is quite wide, but it is narrow, making it fit on a small table unit as in Press photo from the time.
Sharp did also make a few portable versions, but you would need to be a body builder to be able to carry one. Just rest the album, EP or 45 in the slot of open door, press play and the door closes while you sit down and starts playing either; side 1 or side 2, or both depending on what buttons had been selected. A light inside allows you to find next track with another push button either forward or back.
The repair
First off I tackled the loudspeakers; front clips had broken and cones had parted company with the outer housing. Next I glued the broken pillar clips of fronts and next ordered 2 x rubber outer supports for the speaker cones. Both with postage I found for £5, so quite pleased with that (China). The original fixing were a type of foam that degrades with time.
 | | Before and after gluing |
|
 | | Carefully scrape foam/old glue off paper cone |
|
The speakers come out from behind the front cover easily, 4 screws removed I glued new cone rubbers in place and re-fitted and tested fine. The unit houses a 50 watt amplifier chip split 25x25 for either stereo channel (
STK451, top left below) , so theses speaker handle some volume.
 |
| In side front with turntable unit removed. |
Taking the unit apart is a little tricky, remove all knobs on front open cassette door and release screws inside rear. If the turntable is jammed shut, use a pencil and insert in hole at rear top right to release door catch (don't force it - it will break) 4 fixings and two plugs hold turntable assembly on board. Two large pins through rubber saddles found underneath and two screws either side of the top of the deck. Make sure plastic covers removed from front as well, these give access to many light bulbs that will have gone.
 |
| Rear view, showing fixing screws and bolts for turntable assembly |
Once turntable is removed you can check the 2 main belts; large drive belt and cartridge and door loading belt. Both I had to replace. Remove sticky rubber with turps before fitting new ones, both deck and spindles. I thought this would be the fix all, but sadly I think the main fault goes back before belts went. After fitting belts and cleaning selector switches I found that it would only play a record for about 10-15 seconds, then jam in groove of record on either side of vinyl.
 |
| B=Turntable motor A=loading/cartridge motor C=Main micro HD-44750-A35 |
I found the cartridge motor although worked to find start of record it would not engage any further to play record to the end. I spent many hours checking transistors, diodes and i/c's and eventually decided my fault was the main microprocessor, non could be found on-line anywhere over a 3 month period. I sent messages to the USSR, Italy, you name it, got no reply's. Eventually I found a guy selling parts from a broken down unit and brought his old control panel with micro fitted and that cured my fault. (he had dropped his unit and broke cabinet), he'd also damaged my panel in the corners, but I repaired broken print and it worked again. Apparently the door switches play up, but mine were OK. Remember the loading motor does two jobs, open and close door and shuttle the audio cartridge along it's cable railway. It moves about 0.5mm per second. The record start position is found with light sensors, make sure these are dust free.
My left hand door latch was missing a pin, I expect it was broken when door forced open. I made a pin with a 6ba bolt after removing head, I make a few of these for use in old radio knobs. You need to file a slot at one end for the screwdriver. The circuit manual is a 53 page book, details are to fine to load here, if anybody need PDF copy let me know.
A short video I made before the micro was replaced - so partially repaired here
No comments:
Post a Comment