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Hybrid tellies 16th Jul 2012 9:38 pm

Binatone Mustang
I am looking for a circuit diagram for the Binatone Mustang transistor radio. This dates from 1970. I know it's a typical cheap and nasty piece of Hong Kong tat but I am very keen to get it going properly.
The audio is a bit distorted. I have checked the mid point voltage and idle current which appears to be ok. I have also proved that the speaker is ok.

I have checked this sites service data and done quite a few Google searches.

I was wondering if anyone, who has access to the Newnes Service books or the Trader sheets, could take a look for me? In the Newnes Service books, if it is there, it will be in the 1969-1970 or 1970-1971 editions.

Herald1360 16th Jul 2012 11:37 pm

Re: Binatone Mustang
There's nothing in those year's Newnes books.

What sort of output stage does it have? IC amplifier, complementary (no transformers), totem pole (just a driver transformer) or full transformer type (driver and output transformers)?

The last type in particular needs a matched output pair to sound reasonable.

Can you feed the audio out from the volume control to a separate amp and speaker? Does it sound OK through them?

Hybrid tellies 17th Jul 2012 1:59 am

Re: Binatone Mustang
Thanks Herald1360 for looking through the Newnes books, much appreciated.
I have proved the speaker to be ok. The output stage uses Ge transistors, the outputs are transformer driven with the feed to the speaker via a 100uF cap which I have already changed out. I have been able to monitor the 468KHz IF signal which is nice and clear. I have been thinking that the only way round this might be to build a low powered output stage using an IC. But I am going to try and source a circuit diagram first.

Herald1360 17th Jul 2012 12:20 pm

Re: Binatone Mustang
Sounds like a totem pole, then.

The following Wikipedia comment may be relevant:

"Totem-pole push-pull output stages:

Two matched transistors of the same polarity (or, less often, Vacuum tubes) can be arranged to supply opposite halves of each cycle without the need for an output transformer, although in doing so the driver circuit often is asymmetric and one transistor will be used in a Common-emitter configuration while the other is used as an Emitter follower. This arrangement is less used today than during the 1970s; it can be implemented with few transistors (not so important today) but is relatively difficult to balance and so keep to a low distortion (the highly non-linear TTL circuits such as the 7400 use this arrangement)."

It's a relatively simple circuit to reverse engineer but your distortion may be amenable to just trying different output transistors. Have you any way to check matching?

Depending on the bias arrangements, even silicon devices could be substituted.

Hybrid tellies 19th Jul 2012 9:49 pm

Re: Binatone Mustang
Thanks, I can't check the matching but both output transistors have the same coloured band markings on them. I have noticed that there are only 2 electolytic caps.One is is a 100uf which feeds the loudspeaker the other is a 10uf one near the wavechange switch i.e no sign of any decoupling across the main LT rail. So I am going to try and hang some suitable value capacitor across the supply to see what difference if any this makes.

Hybrid tellies 24th Jul 2012 9:12 pm

Re: Binatone Mustang
Think I am getting closer to the distortion problem. I have already checked the mid point and idle current, which both are ok. They were varing considerably but I traced this to an intermittent open circuit 220 ohme resistor across one of the bias or steering diodes. Just by chance I heard a strange form of interference from another nearby AM radio when this Binatone Mustang was switched on and tuned into a station and you could vary the level of the interference with the volume control. It is like the peaks of the audio from the Binatone was radiating right across the AM bands. As I have no circuit diagram a visual inspection shows no main decoupling cap fitted across the supply line and also no capaciter on the output stage to prevent this form of oscillation. I think the only thing I can do is experiment and try different values of capacitor until I clear or reduce the oscillation and level of distortion.

Hybrid tellies 14th Aug 2012 3:40 pm

Re: Binatone Mustang
Tried adding extra decoupling across the supply rails using various values of capacitance but did not make much difference. So after taking a break from it for a while I came back and did some more cold checks with the meter. I found the detector diode, which was a Ge transistor with a leg chopped off, on measuring it I am seeing 5-7k one way and 1-3 k the other way. Ge diodes and juctions should measure 200-300 ohms forward bias so think this needs further checking. If memory serves me correctly I could fit either an OA47 or OA91 diode here. Would be grateful for any suggestions.

paulsherwin 14th Aug 2012 3:56 pm

Re: Binatone Mustang
As you will be aware you shouldn't be seeing those numbers from a Ge diode so the first thing to do is to try another one. You could use a B-C or B-E junction from a Ge transistor as in the original design if you have something handy - the exact type shouldn't matter much. It would be prudent to test before use of course.

Cheapo radios made at that time often used transistors as diodes so the marketing people could increase the claimed transistor count, which was a selling point. They sometimes used reject or marginal transistors so they are always worth checking.

Hybrid tellies 14th Aug 2012 9:10 pm

Re: Binatone Mustang
Thanks Paul,you are right about the transistor number count. This one is suppose to be an 8 transistor circuit but in fact its only got 6 active transistors. Of the other two one is used as the detector diode and the other two are used as bias diodes in the output stage. It could have been sold as a 9 transistor radio but perhaps they thought that was pushing it a bit far. I am thinking of using a Ge diode such as an OA47 for a replacement.
Will be away camping in South Devon for the next few days, depending on the weather but will have a go when I get back.

Hybrid tellies 9th Feb 2015 11:05 am

Re: Binatone Mustang
Its been a while but thanks to everyone for your replies. I have, at last, managed to get this radio going. Its not brilliant but its as good as any radio of this class is going to be and listening to it takes me back to my very early teenage years.
I shall be doing a full write up for Success stories in the near future and hopefully before I start my new job.:)

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