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Vintage Tape (Audio), Cassette, Wire and Magnetic Disc Recorders and Players Open-reel tape recorders, cassette recorders, 8-track players etc.

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Old 19th Jun 2018, 4:32 pm   #1
vampyretim
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Default Miniature Stella reel to reel

Hi,

I don't know if this the correct place for this. Anyway, I am repairing a small Stella reel to reel recorder and it uses 6 D cell 1.5 V batteries. But in order to open the thing up to work on it you can't use the 6 D cells. I don't yet have a bench power supply but do have 2 large, long life 6 volt batteries.
So my plan was to put these 2 batteries in series and make a voltage divider to drop the voltage down to 9 volts.
So I calculated that 330 ohms in series with 1k ohms would do this. However as soon as I put the batteries across the battery terminals of the reel to reel recorder nothing happens the voltage on the batteries drops down to less than a volt and the resistors start getting hot. Obviously a big current is being drawn.

Can anyone help explain what's going on?

Many thanks.
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Old 19th Jun 2018, 4:55 pm   #2
llama
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Default Re: Miniature Stella reel to reel

Your sums are correct insofar as there should be 9 Volts at the junction of your potential divider - off load. When you connect the machine it presents a very low resistance across (effectively replacing) the 1k.

A voltage regulator chip could be used to achieve what you want to do, or if you can calculate the current drawn, a power resistor just in series with the 12 V would do, except the current probably varies too much to rely on that.

Other methods of dropping the 3 volts: maybe a power zener or maybe a string of power rectifiers in series.

In your shoes I'd just use 6 D cells wired externally.
Graham
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Old 19th Jun 2018, 4:58 pm   #3
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Default Re: Miniature Stella reel to reel

How did you calculate resistor values for the voltage divider? The R2R may also be drawing more current than you suspect due to a fault. Might be better to use another power source altogether, PP9s or similar in parallel?

EDIT: just seen grahams response, voltage regulator sounds good too
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Old 19th Jun 2018, 5:07 pm   #4
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Default Re: Miniature Stella reel to reel

Just use 1 6V battery. These use 6 cells and designers normally assume an "end voltage" of 1 > 1.1V per cell, so it should just about work on 6V. The resistor potential divider won't work whatever you do as the circuit design assumes a supply impedance of only a few ohms. The current draw varies widely with motor loading, output power (etc) so it is impossible to calculate a suitable resistor value.

Alternatively, buy one of those holders that takes 6 x "C" cells that go in Roberts radios to replace the PP9 battery and wire that in. Or the proper Philips mains adaptor, this plugs into the 6 pin socket at the side. The correct 'period' one is rare but the later (70s) ones are pretty common.

If a large current is drawn you may have the polarity wrong. Check this, as it is easy to ruin the output transistors like this in this model.
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Old 19th Jun 2018, 5:17 pm   #5
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Default Re: Miniature Stella reel to reel

4 1N400x diodes in series should drop about 3V, but investigate the other issues too.
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Old 19th Jun 2018, 5:21 pm   #6
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Default Re: Miniature Stella reel to reel

If it is anything like the Philips EL3585, the current will be pretty constant in the order of 100mA as the motor current will dwarf the current drawn by the electronics. Does it have a meter that shows the battry voltage on playback? This would indicate of 6V were sufficient. It would probably just run slow if 6V was too low for the speed regulator to work properly. My EL3585 used to run too fast when I fitted a set of the then-new alkaline batteries circa 1964, as the speed regulator evidently relied on the higher internal resistance of Leclanché cells to function properly.
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Old 19th Jun 2018, 6:00 pm   #7
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Default Re: Miniature Stella reel to reel

I remember reading a comment years ago that the EL3585 and EL3586 could be run from a 12V car battery with a 39Ohm 1W resistor in series. Your guess of 100mA seems about right (the resistor would then drop around 4V)

As others have said, for testing I would just put 6 D cells in a suitable holder (or holders) and wire them to the machine.
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Old 19th Jun 2018, 6:25 pm   #8
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Default Re: Miniature Stella reel to reel

Quote:
Originally Posted by vampyretim View Post
I don't know if this the correct place for this. Anyway, I am repairing a small Stella reel to reel recorder and it uses 6 D cell 1.5 V batteries. But in order to open the thing up to work on it you can't use the 6 D cells. I don't yet have a bench power supply but do have 2 large, long life 6 volt batteries.
So my plan was to put these 2 batteries in series and make a voltage divider to drop the voltage down to 9 volts.
So I calculated that 330 ohms in series with 1k ohms would do this. However as soon as I put the batteries across the battery terminals of the reel to reel recorder nothing happens the voltage on the batteries drops down to less than a volt and the resistors start getting hot. Obviously a big current is being drawn.
Ohms law should tell you that 330 ohms is a no no.

Lawrence.
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Old 19th Jun 2018, 9:09 pm   #9
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Default Re: Miniature Stella reel to reel

Thanks for all the replies. I really just wanted to understand what I was doing wrong. And I can now see that in the voltage divider you're putting the load across the 1k resistor and the voltage divider is no longer 330 ohms in series with 1k as the load is in parallel with the 1k thus lowering the total resistance.

If ohms law would have told me that 330 ohms was a no no what then was the correct way of going about it?

The way I did it was by dividing 12V by 1330 ohms and timesing that by each individual resistor. I was obviously in error because that is the open circuit voltage.

Thanks again.
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Old 20th Jun 2018, 10:36 am   #10
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Default Re: Miniature Stella reel to reel

Quote:
Originally Posted by vampyretim View Post
If ohms law would have told me that 330 ohms was a no no what then was the correct way of going about it?
Ohms law.... V/I....Voltage drop/load current, the problem is defining the load current, but say quiescent is 100mA then a series dropper would be 30 ohms if the tape recorder was operating at normal quiescent, for any other current using the same resistor value the voltage drop will change, that change can be reduced by using a potential divider but the current flowing through it would have to be significantly higher (10x for instance) than the load current to get a reasonable voltage stability, that would also be a no no for your two 6volt batteries unless you're quick.

In the absence of a proper power supply or bit's to make a regulator my solution would be 6 D cells, a plastic tube, two nails, a spring and a couple of fly leads.

Lawrence.
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Old 22nd Jun 2018, 5:37 am   #11
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Default Re: Miniature Stella reel to reel

I have 3 of these machines (EL3585) and I made one working unit from the 3.

One of them had a s/c motor and caused a high current drain.

Here's the working one I put on YouTube. The long rectangular square section tube was actually supplied with the PSU and not something I knocked up LOL.

https://youtu.be/y_abOCQT17c
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Old 22nd Jun 2018, 11:26 am   #12
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Default Re: Miniature Stella reel to reel

My EL 3585 used to get a lot of use and needed a new motor every 3 or 4 years @ around £5 each in the 1960's. The phosphor bronze brushes used to wear down and carve grooves in the commutator. The rubber tyres of the feed and take up spool hubs eventually wore down so that they no longer engaged enough to be driven reliably. At this point I heard a friend's EL 3302 and bought one myself rather than repairing the EL 3585, which I still have. I had previously fitted a socket to turn off the motor to provide remote control operation and another wired across the record/Playback head to allow it to be used as an amplifier for my transistor radio.

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Old 22nd Jun 2018, 12:37 pm   #13
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Default Re: Miniature Stella reel to reel

I repaired one of these around 1970 for a school friend. From memory there was a problem with the centrifugal contacts on the motor. There was an article in PW about fitting a transistor to switch the motor current and using the contacts to switch the base current to the transistor which I fitted and brought it back to life.

Keith
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Old 22nd Jun 2018, 2:14 pm   #14
TonyDuell
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Default Re: Miniature Stella reel to reel

From the circuit diagram given in an earlier post, it seems this machine is closely related to the EL3586. That one has a second DIN socket (5 pin 240 degree) for a PSU or remote control switch (like on the EL3300, etc).

I have a pair of EL3586's, I must get round to working on them...
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Old 23rd Jun 2018, 8:57 pm   #15
jamesperrett
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Default Re: Miniature Stella reel to reel

I've got one of those Philips portables which has had the main drive belt replaced by a length of elastic. One day I'll get around to fixing it.
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