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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 16th Jun 2012, 1:10 pm   #1
sp10mk11
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Default Automatic Level Control By-pass

Hello

The cassette deck section in my Sharp music centre has ALC for setting the record levels.
I would like (if possible) to by pass this and fit a record level pot.

I have tried jumping the circuit from R849/850 (signal in) to R851/852 with a 10K pot but without much success

Can anybody here have a look at the schemetic and possibly advise me
Thanks
Gary
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Old 16th Jun 2012, 7:00 pm   #2
ricard
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Default Re: Automatic Level Control By-pass

At least when I viewed it the resolution of the schematics is too low to really see the component designations. Also, a more complete schematics might help too, although it does focus on the amps and alc circuits.
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Old 16th Jun 2012, 8:24 pm   #3
sp10mk11
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Default Re: Automatic Level Control By-pass

Hello i have uploaded a larger image hopefully this will help
Thanks
Gary
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Old 16th Jun 2012, 9:42 pm   #4
AndiiT
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Default Re: Automatic Level Control By-pass

Hi,
I'm sure that there will be others in the forum who can advise you better than I can but you need to look at two things here, one is how to disable the Automatic level control circuit and the other is where to insert the level control potentiometers (you will require two of these, one for each channel)

It may be possible to trace the record/play switching to see if some of the contacts actually bypass the playback volume control and re-wire these to allow the control to operate as a level control in record mode.

Regards

Andrew
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Old 16th Jun 2012, 10:35 pm   #5
cmjones01
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Default Re: Automatic Level Control By-pass

OK, I'm game for an attempt at this...

The automatic record level control seems to be done by D803/D804 rectifying the amplified audio signal. This results in a voltage across C803 which starts to turn on Q813 as the audio signal gets larger. This in turn, via the balance adjustment VR803, turns on Q811 and Q812 which each attenuate the left and right channels by shunting R847 and R848.

To disable the automatic level control, I'd just remove Q811 and Q812. This will leave you with maximum record level all the time.

To add a manual level control, you could do various things. As AndiiT says, it may be possible to trace the record/play switching and modify it so that record level control is done nicely with the volume control. I don't think there's enough of the diagram to do that though, and it would be quite a complex job.

You could put a stereo pot wired as a pair of variable resistors across R847 and R848 (wire it to the collector and emitter holes where you've taken Q811 and Q812 out). Try 22k as a starting point for a value, but anything between 4.7k and 100k would probably work to some extent. The action of the control would probably be a bit crude, though, and hard to control smoothly.

A 'nicer' way of doing it would be to disconnect C821 from R825 (left channel) and C822 from R826 (right channel) and introduce a pot. Bottom end of the pot to ground, top end to R825 or R826, wiper to C821 or C822. That should give you a reasonably good record level control.

Others may have better ideas...

Chris
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Old 17th Jun 2012, 2:58 pm   #6
sp10mk11
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Default Re: Automatic Level Control By-pass

To Andrew and Chris

Thankyou so much for clearing my foggy head about this project.
Last night in bed i was thinking about how the signal flows through ALC circuit and i had gotten it round the wrong way, this was confirmed by Chris explaining how the ALC worked, once i got this into my head it all made sense

I followed your advise chris as regards fitting dual can pot between the input to the record amp and the output form the 2nd stage amplifier, and with a bit of fiddling with various value resistors across the track on the pot (1k across a 10K track) I now have a smooth level control which when at maximum so to speak is just begining to overload the tape. what i have been doing to check the record levels is to play back the recordings made on the sharp on my reference cassette deck (nakamichi BX300) and they are peaking around +3db so i might do a bit more fine tuning to reduce the pots track resistance so maximum setting is spot on record level wise.

I will then mount the record level pot somewhere suitable for it and find a suitable knob to put on it.

All this might seem silly when it is all for a non-dolby very basic music centre cassette deck, but i have been improving this system the last month (better turntable and cartridge) and re-capping the audio circuits.

Whilst i like top end hi-fi i always like the underdog so to speak and like to see what i can get out them
So again thankyou so much for helping me out on this one
Gary
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Old 17th Jun 2012, 10:58 pm   #7
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Default Re: Automatic Level Control By-pass

I'm glad to be of help, and I'm glad the advice worked!

Chris
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