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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 6th Dec 2019, 2:37 pm   #1
samricemonkey
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Question Sharp GF-575 inputs

Hello,

I'm new to this forum and am looking for some advice on a Sharp GF-575 I recently bought. I had to replace one of the speaker drivers but thats ok. Otherwise, it works pretty well apart from a problem with the microphones. External inputs and internal mics. The problem is that the signal from the microphones is barely audible, even with the microphone and main volumes all up to the max the mic signal is barely audible over the noise. I want to use external mic inputs. The problem is not with the microphone itself; I tested that in another device. If I shout or tap the microphone there is a poor and low-level signal. So its not a connection problem. So my uneducated guess is that the problem is with the mic preamp. But i'm not sure how to test or fix this. I don't have a good enough understanding of circuits and components so don't really know what to do next. I had a look at the service manual and tried to figure out what component might need to be replaced. I thought it might be a transistor C2603F (part Q401, 402) but I don't really know. Then I had a quick go at trying to find the part in the actual machine but couldn't see it without a serious dismantle and didn't want to go any further without confidence in my diagnosis. So, I'm looking for advice what to do next. I expect this is repairable and I'm willing to have a go if its not too complicated or expensive. I should mention one other issue is that the line-in function doesn't seem to work either. There is a switch to select line-in or phono input level. The input woks if phono is selected but not if line-in is selected. So its not a connection problem. Perhaps the switch? but the phono level seems relatively low compared to the internal sources so possibly something to do with an amp component. I wonder if this is related in some way to the problem with the mic signal? Oh, and it also seems that the sound quality is different from each tape deck.

I'd be interested if anyone here can advise how I should go about fixing the mic inputs and possibly also the line-in. Cheers,

Sam

Last edited by samricemonkey; 6th Dec 2019 at 2:53 pm.
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Old 6th Dec 2019, 10:51 pm   #2
Welsh Anorak
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Default Re: Sharp GF-575 inputs

Hello and welsome to the forum.
Two questions. One - what are the level meters doing? That will help locate the appreximate area of the problem. Secondly is it on both channels? If so it must be a component common to left and right, which narrows things down still further.
Silly quaestion, but is the head really clean?
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Old 7th Dec 2019, 1:16 pm   #3
samricemonkey
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Default Re: Sharp GF-575 inputs

Hi Welsh Anorak,

Thanks for the reply. So, to answer your questions I carried out further investigation using headphones because the replacement speaker is still on it's way in the post actually.
Firstly, the level meter works fine.

I haven't managed to find the user manual for this so I'm not sure if its working as it should but here's what I've found out:

The external microphones only work when in tape mode and when recording. This, I'm sure is normal. It turns out that the microphones signal comes through really clearly when the dubbing selector switch is set to the "off" position. The mic recording level slider works well too as do the other sliders. But if the dubbing switch is set to "Normal" or "High Speed" then the signal from the microphones is really terrible, as I described before. I'm not sure if this is normal or not. And if dubbing is off then you can't record from tape deck A on to tape B because no signal comes through from tape A when recording. So the dubbing selector switch basically switches between the mics and tape deck A as recording sources and means that the mic signal can't be mixed with the deck A signal. This is annoying because I wanted to do mic-mix ping-pong overdubbing. I'm pretty sure that it should have originally been able to do this.

As for phono and line-in external inputs - the input works when the switch is set to phono but not line-in. The tape will record from phono mode and when recording the recording level slider controls the level of the phono signal. There is no signal from the external mic in phono/line-in mode.

To answer your question about the channels: With headphones the signals from all the sources come through well in both L and R channels. The microphones L+R signals also come through clearly in each channel .
There is a bit of a difference in the EQ/tone between deck A and B. Deck A has less treble and sounds muffled. The heads in both decks look OK but I haven't cleaned them and probably should.

So, I guess my question now has become should the external mic signal be able to mix with the deck A signal in dubbing mode? If so, how do I fix it so that this works?

Thanks again for replying. Much appreciated!

Sam
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Old 8th Dec 2019, 11:34 am   #4
samricemonkey
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Smile Re: Sharp GF-575 inputs

More detail about the meter: It shows the same behaviour as the sound that comes through the speakers and headphones. So in tape record mode when dubbing mode is off it shows a good response to the microphones and the recording level slider but with dubbing mode on it responds very poorly to the microphone input and will only show a low response to a loud sound source.

Hope this helps.

Thanks again.
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Old 11th Dec 2019, 1:59 pm   #5
Welsh Anorak
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Default Re: Sharp GF-575 inputs

Two things to do - both easy! Firstly clean the heads several times to make sure they are really clean. Might not help with the fault, but will give you much better playback. Secondly clean the mic selector, dubbing and record/play switches using proper switch cleaner - not WD40! These often give rise to odd symptoms.
Then report back and hopefully we'll be getting somewhere.
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Old 12th Dec 2019, 12:47 am   #6
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Default Re: Sharp GF-575 inputs

"So the dubbing selector switch basically switches between the mics and tape deck A as recording sources and means that the mic signal can't be mixed with the deck A signal"

- It seems to me that this unit is acting exactly as it should. Dubbing is just that - tape to tape copying. Few people would want the background noise picked up by a mic mixed in with their copy of Spandau Ballet's greatest hits! Hence the use of a selector switch. In dubbing off /tape mode, the mic is used (note, not amplified, internal mics at any rate were usually piped directly into the recording circuit).

In many units I have seen the 'mix mic' feature is to allow the use of an external mic, fed directly to the amp and bypassing the recording circuit, so you can use the machine as a small PA system.

For proper mixing of signal sources you will need something a bit more high end than this . A hi fi cassette deck with separate mic and line controls such as some of the Pioneer units, or a Tascam or Fostex 'portastudio' will give you the versatility you need.

It is a pity you don't have the user manual, I am sure this would explain quite a lot.
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Last edited by ben; 12th Dec 2019 at 1:15 am.
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Old 6th Jan 2020, 10:38 am   #7
samricemonkey
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Default Re: Sharp GF-575 inputs

Thanks for the advice. I haven't cleaned it yet but will do. The behavior of the switches is very consistent so I'm inclined to think it's the way it's designed. But I don't quite understand the need for a dubbing selector switch because on many cassette recorders you can just press record to start recording by pressing the record button without a separate mode-selector. Anyway, I'm willing to accept this is the way it's supposed to work but I'll post back if I find out anything else after cleaning.

Cheers,

Sam
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