Sharp XL-HP500 Fan Modification? - UK Vintage Radio Repair and Restoration Discussion Forum

Deltatango · 26th Mar 2019, 6:53 pm

In another thread I was seeking help on this unit as it had failed a second time from IC module fault, unable to find any reason for it decided to follow some old advice and fitted a STK402-120 (120 + 120W) for the original STK402-071 (60 +60 W).

This was because it may have been overheating through high use plus I did not want a yo-yo unit with that phrase it's gone again. I was also drawn to the recommended supply voltage range for this breed of modules (30v +/- for its '71 and 39v +/- for the '120) now this unit has a supply rails of 37v so??

The 120 needed heat sink modification as it was a larger body, removed alignment posts re-drill fixing screw holes plus tease out its bent terminals to be longer, pin spacing stayed the same.

Thought when completed and working that it was giving off a very pleasant low heat with no output, so a bit more bias current maybe, that should keep its rails steady!!

Figured that I must offset things a bit so forward biased the fan to run when on not just with high music levels, made a right racket at full power with blade noise and vibration from its little metal mount. After some testing got a decent blow going (and was quiet with low volume) with a "3v" collector voltage so 2v on its motor (4v motor I think).

With this information fitted a 1N4148 as a blocking diode to its normal operation leaving a resistance value measured at 8K for that 3v, so 8K2 in fact used. With that voltage tapped of from collector to base we have nice feedback so it will be a stable bias, plus if more voltage comes from its normal route this will bias it harder without any added loading so speed up more.

P.S. It may have more output power but it was "cheaper" to buy..........

Any thoughts............

Davidt

26th Mar 2019, 6:53 pm	#1
Deltatango Triode Join Date: May 2018 Location: Tonbridge, Kent, UK. Posts: 41	Sharp XL-HP500 Fan Modification? In another thread I was seeking help on this unit as it had failed a second time from IC module fault, unable to find any reason for it decided to follow some old advice and fitted a STK402-120 (120 + 120W) for the original STK402-071 (60 +60 W). This was because it may have been overheating through high use plus I did not want a yo-yo unit with that phrase it's gone again. I was also drawn to the recommended supply voltage range for this breed of modules (30v +/- for its '71 and 39v +/- for the '120) now this unit has a supply rails of 37v so?? The 120 needed heat sink modification as it was a larger body, removed alignment posts re-drill fixing screw holes plus tease out its bent terminals to be longer, pin spacing stayed the same. Thought when completed and working that it was giving off a very pleasant low heat with no output, so a bit more bias current maybe, that should keep its rails steady!! Figured that I must offset things a bit so forward biased the fan to run when on not just with high music levels, made a right racket at full power with blade noise and vibration from its little metal mount. After some testing got a decent blow going (and was quiet with low volume) with a "3v" collector voltage so 2v on its motor (4v motor I think). With this information fitted a 1N4148 as a blocking diode to its normal operation leaving a resistance value measured at 8K for that 3v, so 8K2 in fact used. With that voltage tapped of from collector to base we have nice feedback so it will be a stable bias, plus if more voltage comes from its normal route this will bias it harder without any added loading so speed up more. P.S. It may have more output power but it was "cheaper" to buy.......... Any thoughts............ Davidt Attached Thumbnails