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Daily changes in cassette deck motor speed

 

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Username: Ccurran

Post Number: 1
Registered: Jan-09
Hello, thanx loads for your site -

Got a cassette deck ( FOUR DIFFERENT decks, to be exact) motor speed problem which has been driving me crazy, trying to get
a solution from people online so far to no avail. Not sure if its due to mains power fluctuation or the regulator circuits in my decks.

The decks are 2 Pioneers, a Yamaha and a Fisher: 3 of these are double decks, all reversible, dating from 1985 to 1997,
having standard motor control circuits and internal power supply voltage regulator circuits. I live in a 14 storey apartment building.
(Also, I have schematics for three of these decks).

During daytime hours the tape speed is normal and satisfying in ALL UNITS. However, after I get home from work ( at about 11:30 PM) and play the decks again (around midnite), there is a very noticeable INCREASE in tape speed (in ALL the decks, as I said) with the corresponding change in sound quality, and NOT very satisfying. This speed change seems to occur AS A DAILY PATTERN, and ONLY AT THIS TIME ( around midnite). When I measure the receptacle voltage at 12 noon, it is about 119.5 vac; at 12 midnite, when this happens, it INCREASES by 1 volt or so. However, the deck motor voltages measure (across each motor, connected in circuit) pretty much the same at BOTH times ( +/- 12.15 vdc). There is no noticeable change in lamp/light levels, or in the operation of any other appliance or unit, around 12 midnite. I cannot believe that ALL the voltage regulator circuits in ALL the decks/motors are defective; if this were so, wouldn't this be happening ALL the time? Shouldn't the deck/motor v.r.'s be
able to handle a 1 volt increase in line voltage ? (When not in use, the decks are off ). They have recently done a lot of electrical work ( replace main breaker box, etc.) in my apartment building, but I have had this problem for at least a year or more here.

If this is a building power problem, is there an inexpensive device I can buy on the market which will stabilize the line voltage ? Or, is it still possible that the decks are defective? HOW can I get to the source of this problem ?

Any info, help or other reliable leads/links you can provide for this problem will be very appreciated. Like to solve this problem for sure.

Thanx loads for your time, patience and help,

Chris

P.S. - I notice from the schematics that the power supply v.r. circuits ( do the v.r. circuits inside the motors, which are Mabuchi motors, also have these?) have ZENER DIODES. Wouldn't these tend to cut off small voltage increases above their upper limit value ?
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