UK Vintage Radio Repair and Restoration Discussion Forum - View Single Post - Uher CR 240 adjustments

Uncle Bulgaria · 16th Nov 2020, 9:04 pm

Perhaps it's the length of tape - I imagine the magnetic ring is polarised by 180 degrees. If the machine only recognises the tape has reached the end by the pulses stopping, if each pulse is spaced too far apart then it will think the tape has finished. Watching the hub with almost all the tape on it, it doesn't have time for half a rotation before it stops.

Hypothesis: Switch IC801 turns on T710 by having an average voltage high enough from enough pulses per second. T710 has some connection to the hold-in solenoid at the other end of board 700, and if it turns off then the solenoid releases and playback stops.

The pulses at the base of T710 are apparently still 0.635V on both sides of the tape (turning it over so it is playing on to the side with no tape, or the side with most of the tape) according to the 'scope readout, but when the hub is turning slowly (holding 'play' down enables it to keep going) the pulses are very noticeably further apart.

There is 6.4V on pin 931 (IC 801 supply). Could C704 be not holding enough charge between pulses?

16th Nov 2020, 9:04 pm	#9
Uncle Bulgaria Nonode Join Date: May 2016 Location: Cornwall, UK. Posts: 2,339	Re: Uher CR 240 adjustments - operating point and hub tension Perhaps it's the length of tape - I imagine the magnetic ring is polarised by 180 degrees. If the machine only recognises the tape has reached the end by the pulses stopping, if each pulse is spaced too far apart then it will think the tape has finished. Watching the hub with almost all the tape on it, it doesn't have time for half a rotation before it stops. Hypothesis: Switch IC801 turns on T710 by having an average voltage high enough from enough pulses per second. T710 has some connection to the hold-in solenoid at the other end of board 700, and if it turns off then the solenoid releases and playback stops. The pulses at the base of T710 are apparently still 0.635V on both sides of the tape (turning it over so it is playing on to the side with no tape, or the side with most of the tape) according to the 'scope readout, but when the hub is turning slowly (holding 'play' down enables it to keep going) the pulses are very noticeably further apart. There is 6.4V on pin 931 (IC 801 supply). Could C704 be not holding enough charge between pulses?