I just rebelted two Sanyo M9990s for the first time.Took a stupidly long time and involved removing and refitting the tape heads to get good access. About a year ago I bought a couple of belt kits for the Sanyos from a well-known online place. So far in my other BB repairs I've always got the old belts out, measured and ordered individually, but I thought well, there's a kit out there, so why not? When they arrived I checked the circumferences against the Russell Industries belt specs and they were ok. But when I got the old belts out I noticed that three of them were about half as thick as the new replacements. Hmm.
Anyway, fitted the new belts - they seemed a bit tight, but everything turned ok....or so I thought... until I came to measure the speed and wow&flutter.. On one the speed went from 3002 to 3100Hz and the other from 3000 to 3120Hz . Then the w&f measurements: BB1 went from DIN weighted 0.12 to 0.62 !!! and BB2 from 0.13 to 0.2.(now to complicate things two of the belts were actually clearly the wrong size in the second kit, so the top flywheel belt and the counter belt were bought elsewhere and are slightly different in length and cross-section to the 'official' kit - this may account for the difference in the new figures, but hard to say until I strip them both down again for investigation).
I've learnt then that cross-section really matters. If you go from a 1mm to a 2mm belt thinking it'll be ok, it won't. You have now effectively got two 1mm belts on pulleys doubling the tension at least. Bad news and it gets worse...on the motor to flywheel drive belt, a thicker than spec belt effectively increases the radius of the small motor pulley by a bigger % than the radius of the large flywheel - so the capstan runs faster. I've read that that fact can actually be used to set tape speed in the case of motors with no speed adjustment pot. Interesting!
Luckily I plucked the old belts out from the bin just in time - now I'll go back to measuring and replicating the old stock as close as possible. Every time I try a new belt though I have to remove and replace the heads before testing the result......so it would be great if someone knows the EXACT belts for this model from a supplier. With W&f figures to back them up.
going for a lie down
Anyway, fitted the new belts - they seemed a bit tight, but everything turned ok....or so I thought... until I came to measure the speed and wow&flutter.. On one the speed went from 3002 to 3100Hz and the other from 3000 to 3120Hz . Then the w&f measurements: BB1 went from DIN weighted 0.12 to 0.62 !!! and BB2 from 0.13 to 0.2.(now to complicate things two of the belts were actually clearly the wrong size in the second kit, so the top flywheel belt and the counter belt were bought elsewhere and are slightly different in length and cross-section to the 'official' kit - this may account for the difference in the new figures, but hard to say until I strip them both down again for investigation).
I've learnt then that cross-section really matters. If you go from a 1mm to a 2mm belt thinking it'll be ok, it won't. You have now effectively got two 1mm belts on pulleys doubling the tension at least. Bad news and it gets worse...on the motor to flywheel drive belt, a thicker than spec belt effectively increases the radius of the small motor pulley by a bigger % than the radius of the large flywheel - so the capstan runs faster. I've read that that fact can actually be used to set tape speed in the case of motors with no speed adjustment pot. Interesting!
Luckily I plucked the old belts out from the bin just in time - now I'll go back to measuring and replicating the old stock as close as possible. Every time I try a new belt though I have to remove and replace the heads before testing the result......so it would be great if someone knows the EXACT belts for this model from a supplier. With W&f figures to back them up.
going for a lie down
