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Old 16th Nov 2020, 1:26 pm   #1
Uncle Bulgaria
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Default Uher CR 240 adjustments - operating point and hub tension

I thought I'd finish working through the Uher service manual, now things are all basically working on mains power. I'm confused by two elements.

1 - point 2.2 in the manual. At 6.6V, or the edge of the green scale, a tape cannot play all the way through as the machine loses puff before the end of a side. Surely this setting should mean the limit of normal operation?

2 - point 4.3 in the manual. I'm using a cassette I've cut slots out of with some thread tied to a hub, and a Correx 0-100p gauge in a loop. It's hard to get a consistent result, but they're all consistently lower than the manual by 20-30p. Either this is a fault in my measurement method, or the adjustment is wrong. However, with fault 1, I would have thought the tension was too high, causing it to stall at the low voltage.

Any thoughts? The parts of the manual are attached.
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Old 16th Nov 2020, 5:44 pm   #2
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Default Re: Uher CR 240 adjustments - operating point and hub tension

Yes agreed that the 6.6V setting should theoretically equate with limit of normal operation but I do know from my experiences with the 4000 Report Monitors that with lowish batteries where the monitored level drops down to the min battery level that RWD/FFD may struggle at times, that is why I prefer to use power adapter/external PSU rather than batteries.

David
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Old 16th Nov 2020, 6:49 pm   #3
Uncle Bulgaria
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Default Re: Uher CR 240 adjustments - operating point and hub tension

Thanks, David. While I don't intend to regularly use it on batteries, it's pretty fun to carry it around in its case and have an impromptu dance or field recording. A little more convenient than a gramophone! Trying to set it up according to the manual is also an exercise I feel would be helpful for me to do.

I've revised my thoughts - perhaps it's to do with the tape-run Hall effect sensor (manual point 3.3) rather than the tape hubs. With the input at 6.6V (measured at Contact I in the battery compartment) it will play the beginning of a tape, but towards the end when 'play' is engaged it automatically stops after about a second. The trace of MP1 is not rectangular as the manual says it must be. On DC-coupled it's a very rounded waveform as shown in the picture, while on AC-coupled it's even less square and more like a rounded sawtooth.

Pk-pk it is approximately 5V, but no amount of fiddling with the sensor spacing affects the regularity of the pulse shape. Perhaps a component failure? Have you checked yours is a regular square-edged wave as the manual shows?
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Old 16th Nov 2020, 7:26 pm   #4
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Default Re: Uher CR 240 adjustments - operating point and hub tension

Yes maybe from your description there is another reason rather than just struggling on the fast wind due to battery voltage being at low end.

No have not monitored this signal, need to get back to the CR 240 (and the dreaded contact arm) to progress it. I will try and monitor the signal to see what it looks like.

David
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Old 16th Nov 2020, 7:30 pm   #5
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Default Re: Uher CR 240 adjustments - operating point and hub tension

Ah - another titbit: With the tape in a position where pressing 'play' causes it to stop after a second, FF works fine. It's slower than normal, but it gets to the end of the tape and stops as normal. So the problem is only with 'play'.
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Old 16th Nov 2020, 7:34 pm   #6
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Default Re: Uher CR 240 adjustments - operating point and hub tension

Does your 'scope display it's calibration square wave ok using the same probe?

Lawrence.
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Old 16th Nov 2020, 7:41 pm   #7
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Default Re: Uher CR 240 adjustments - operating point and hub tension

Good thinking, Lawrence. Alas I've got the same trace on both my newly re-capped Tektronix 2445 and the Crotech 3131. I've gone through the Tektronix manual's calibration checklist and it checks out as far as the calibration signal goes (same spacing on a variety of time intervals etc.) This is the calibration signal showing that a square wave is possible.
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Old 16th Nov 2020, 7:45 pm   #8
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Default Re: Uher CR 240 adjustments - operating point and hub tension

Quote:
Originally Posted by Uncle Bulgaria View Post
Good thinking, Lawrence. Alas I've got the same trace on both my newly re-capped Tektronix 2445 and the Crotech 3131. I've gone through the Tektronix manual's calibration checklist and it checks out as far as the calibration signal goes (same spacing on a variety of time intervals etc.) This is the calibration signal showing that a square wave is possible.
Ok....carry on....

Lawrence.
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Old 16th Nov 2020, 8:04 pm   #9
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Default 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?
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Old 16th Nov 2020, 9:04 pm   #10
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Default Re: Uher CR 240 adjustments - operating point and hub tension

Quote:
Originally Posted by Uncle Bulgaria View Post
Ah - another titbit: With the tape in a position where pressing 'play' causes it to stop after a second, FF works fine. It's slower than normal, but it gets to the end of the tape and stops as normal. So the problem is only with 'play'.
From your previous Post I incorrectly thought it was a fast wind issue, although you had clearly stated it was a Play issue.
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Old 16th Nov 2020, 11:41 pm   #11
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Default Re: Uher CR 240 adjustments - operating point and hub tension

It led me to check fast winding, David, and thus the pulse-speed - a serendipitous mis-read! I've got further interesting information: holding a 2.2F electrolytic in parallel with the existing 2.2F capacitor C704 means the tape doesn't stop. It does so immediately on removal of the paralleled capacitor.

The capacitor in circuit is one that I think I must have replaced already when trying to sort the solenoid - it's marked as a tantalum on the diagram but is a small aluminium electrolytic. I replaced it with a tantalum to check, but the behaviour was the same - working with the capacitor in parallel, stopping with it removed. All these capacitors are tested as in tolerance, by the way.

I've now put a 4.7F tantalum in, and it plays to the end of the tape!

Was my hypothesis likely to be correct? It's just a coupling capacitor so there won't be any downside to changing its value, will there? As there isn't a square wave on the 'scope where there should be, is there likely to be a fault with the Hall-effect switch, or the magnetic ring? I don't want to disguise an underlying fault...
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Old 17th Nov 2020, 12:06 pm   #12
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Default Re: Uher CR 240 adjustments - operating point and hub tension

You have done well to get it working. I do not think there will be a downside as long as it works OK in Play and stops when it gets to end of tape. Difficult to say if masking an underlying fault.

Found my CR 240 (buried under loads of various R2R bits) and was going to have a look at the MP1 waveform but it will not power up, just found a wire broken off the back of the battery box connector, so need to work out where it connects to.

May be quite a while before I can check MP1 as going out soon.

David
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Old 17th Nov 2020, 12:25 pm   #13
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Default Re: Uher CR 240 adjustments - operating point and hub tension

Doubling the value of C704 is probably just masking the cause of the symptom, the solenoid isn't holding because something's causing the flip flop to be re-set.

I haven't got a schematic for the CR240 AV only one for the CR240 so just a basic explanation of how it works for the CR240 if it's any help:

When the function control is moved to Start, the Start contacts make, that pulls the collector voltage of T707 down, that sets flip flop (collector voltage of T706 goes high) that in turn enables the high current pull in winding of the solenoid to be briefly energized due to the charging of C701 and the resultant turning on of T705 and T704....At the same time the lower current holding winding of the solenoid is also energized as a result of T703 being turned on by the same setting action of the flip flop, the Start contacts can now be allowed to break because the flip flop has been set, that condition will continue unless the flip flop is re-set.

Ignoring the flip flop re-setting due to the external Pause function or the FF and Rew functions, the flip flop is re-set by turning off T707, this can be done by the Stop or Pause function or by the sensor circuit when the tape stops at the end, either of those functions removes the bias from T707 and thus re-sets the flip flop, this is done by grounding T707's bias supply via the respective function switch contacts or in the case of the sensor circuit, by T708 being turned on.

With the tape moving the pulses from the sensor charge up C703 via T710, the voltage developed across C703 is enough to keep T709 turned on so long as the pulses keep coming (with sufficient amplitude) T709 being turned on means that T708 will remain turned off thus the bias supply for T707 is maintained and the flip flop remains set, when the pulses stop or are not of a sufficient magnitude the voltage across C703 will fall below the turn on threshold for T709 thus T709 will be turned off and T708 will be turned on thus removing the bias supply to T707 which causes the flip flop to re-set ......so far as I can make out.

Lawrence.
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Old 17th Nov 2020, 1:21 pm   #14
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Default Re: Uher CR 240 adjustments - operating point and hub tension

Thanks Lawrence! I'll work through that once I've done today's work. It's just a CR 240, not an AV so your diagram will be correct. SDS sent me one for the right serial number so it included the Hall-effect switch rather than the photo-diode assembly on the PDF I found online.

I can make a better scan of the diagram when I get a moment later today and post a link, if that's helpful.

I was up most of the night trying to track down the bizarre fault that has now reared its head - thinking all was well I tried recording some test tones. The fluttering warble is extreme on a tape I record, and on tapes I've previously recorded on this very machine that a couple of months ago were impressing me with their high quality. However, when playing a commercial tape extended notes are not at all warbly. I'm going to do a recording on the only other cassette deck in the house and play it back on the Uher to see if that narrows it down.

I've checked the motor electronics at MP2 which are of an even amplitude and frequency with only a slight wavering up and down as if carried on a lower frequency sine wave. The tape path has been cleaned. The only change since picking it up again after a few months' hiatus has been changing that capacitor...
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Old 17th Nov 2020, 7:50 pm   #15
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Default Re: Uher CR 240 adjustments - operating point and hub tension

Great circuit analysis from Lawrence.

After fixing my wiring had a quick look at MP1, looks very similar to yours, so unless we both have a similar issue then yours is probably OK, I will capture some scope examples later. Have not yet done it at the 6.6V setting.
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Old 17th Nov 2020, 11:34 pm   #16
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Default Re: Uher CR 240 adjustments - operating point and hub tension

Set up my CR 240 6.6V setting for operating voltage to the left edge of the green field on the left VU meter, it was way off around -10dB, had to adjust the R 1001 trimmer fully clockwise to just about get it to reach the left edge.

Seriously struggled to capture even a half reasonable scope picture. On Play AC coupled, signal very spikey like differentiated pulses. On DC coupled sort of a square wave with quite rounded leading edge. On FFD and RWD the signal was somewhat better.

No improvement to signal with increased operating voltage.

Did not attempt any hall effect sensor/magnet adjustment.

With numerous attempts at different sections of the tape, Play never stopped until it reached the end.
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Old 18th Nov 2020, 10:21 am   #17
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Default Re: Uher CR 240 adjustments - operating point and hub tension

Quote:
Originally Posted by Uncle Bulgaria View Post

2 - point 4.3 in the manual. I'm using a cassette I've cut slots out of with some thread tied to a hub, and a Correx 0-100p gauge in a loop. It's hard to get a consistent result, but they're all consistently lower than the manual by 20-30p. Either this is a fault in my measurement method, or the adjustment is wrong. However, with fault 1, I would have thought the tension was too high, causing it to stall at the low voltage.
I have never done cassette measurements but find reel to reel tension/torque measurements difficult/subjective/hit and miss.
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Old 19th Nov 2020, 2:52 am   #18
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Default Re: Uher CR 240 adjustments - operating point and hub tension

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Originally Posted by DMcMahon View Post
Set up my CR 240 6.6V setting for operating voltage to the left edge of the green field on the left VU meter, it was way off around -10dB, had to adjust the R 1001 trimmer fully clockwise to just about get it to reach the left edge.

Seriously struggled to capture even a half reasonable scope picture. On Play AC coupled, signal very spikey like differentiated pulses. On DC coupled sort of a square wave with quite rounded leading edge. On FFD and RWD the signal was somewhat better.

No improvement to signal with increased operating voltage.

Did not attempt any hall effect sensor/magnet adjustment.

With numerous attempts at different sections of the tape, Play never stopped until it reached the end.
I've not had a chance to go over it further yet. Did you make sure the 6.6V was set at point I in the battery compartment, not 6.6V at the Auto/Radio input? It seems odd the variable resistor has to be hard over, as mine has a lot of leeway.

It's heartening that the trace looks so similar to mine - why then do they make such a point of it being rectangular? Neither of ours are close!
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Old 19th Nov 2020, 9:20 am   #19
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Default Re: Uher CR 240 adjustments - operating point and hub tension

Yes measured at point I, it does seem unusual, sometime I will see what the voltage is arriving at the meter board.

I guess just the idealised representation of a square/rectangular wave. Maybe this is normal/typical for this type of analogue hall effect electronic switch output.

David
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Old 19th Nov 2020, 9:21 am   #20
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Default Re: Uher CR 240 adjustments - operating point and hub tension

If not already mentioned, is the tape speed within spec? If the EOT Sensor senses take up spool rotation then with more tape on it, the spool will slow down. If (capstan) tape speed is slow it may be enough to trip the EOT circuit towards the end of tape.
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