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Tech Chat BBC Tone Generator GE1/3/265

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crustytv
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I bought this ex BBC device getting on for 10-years ago, and it's just languished in the stores. It requires a -ve 24V supply, which I did manage to get, but again that too sat with the generator gathering dust.

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Originally, I planned to use it as a tone source for one of my 405-line feeds. At that time, I had the aurora (which has tone) and another 405 generator that did not.

In the middle of the night whilst asleep, I find my brain besides doing the usual housekeeping reserves an area for thinking about things (I've found I'm not alone in this matter @jayceebee) and/or problem-solving. Anyway, it decided wouldn't it be nice to use that BBC tone generator in the signals rack, and even nicer would be to incorporate the C.O.W. into the unit. There's just enough space to remove the Pi hardware and mount it in the BBC unit.

My subconscious monitoring this idea thought, hmmm like that idea! So this morning into the store to try and locate the GE3, the PSU and see if I can get some life into it.

As mentioned above, the BBC unit appears to need a -24V supply and draws around 65mA. I do have some paperwork and this is a little bit confusing, but I think I've figured some of it out.

BBC 01
BBC 02
BBC 03
BBC 04

I decided to use my bench PSU rather than the dedicated PSU I bought for it, this way I could current limit and easily see what was happening. I set the PSU for 24V with OVP on and current limit to 70mA. The positive of the supply to what I believe to be ground, being PL14 and the negative to PL4. I checked the voltages on the emitters of TR1, 2, 3, 4 & 9, and they pretty much tally.

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This should mean I have tone out on PL7 & PL8. I was not sure what to expect or quite how and where I should connect the scope, I connected the GND lead of the scope to pin8 and the probe to pin 9 7. The resultant wave form, not what I expected for a tone. I placed the counter near and there does appear to be 10Hz, which cannot be correct.

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As I say, I'm a little confused as to how or where I scope for the tones, so that's as far as I've got. There should be two tones, A & B at pins 7 & 8, so I'm sure I'm not scoping correctly. Also, assuming the tones are now present and correct on pins 7 & 8 how would I get that to be audible? Any ideas or rocks thrown to express anger at my stupidity, are most welcome. 🤡 

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Posted : 28/08/2023 11:01 am
Lloyd
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Just a thought, does anything change if you connect anything to the input terminals 11 and 12? I’m wondering if it’s sitting quietly waiting for an input! I was looking at the bit that says test procedure, where there’s a table of what it should do depending on what’s connected to 11 and 12. As for hearing what’s going on, do you have an audio signal tracer? Or maybe some old amplified PC speakers connected to the output might produce some noise!

Regards,

 Lloyd 

 
Posted : 28/08/2023 12:15 pm
Cathovisor
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I don't think this is going to be of much use to you, but as it has a balanced output you would connect your scope across pins 7 and 8.

I'll have a greater study of the circuit later but I doubt you'll get 1kHz tone from it.

 
Posted : 28/08/2023 12:20 pm
crustytv
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Posted by: @lloyd

do you have an audio signal tracer?

I do my old Taylor, If I can find it 👍 

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Posted : 28/08/2023 12:23 pm
Lloyd reacted
crustytv
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Posted by: @cathovisor

I don't think this is going to be of much use to you

Oh, that's a shame

Posted by: @cathovisor

as it has a balanced output you would connect your scope across pins 7 and 8.

Good, that's where I had connected it, my typo above.

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Posted : 28/08/2023 12:25 pm
Cathovisor
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@crustytv best thing you can do is follow the test procedure to see if it's actually doing anything first.

The tone outputs seem designed to be around 7.5 to 15kHz.

 
Posted : 28/08/2023 12:26 pm
crustytv
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Step one I have already completed +ve of supply (24V) to PL14 and -ve of supply to pin 4.

Step 2. Ah-Ha, I don't have ANY of the frequency forming capacitors C4, C6, C10, C12 or C15 fitted.

Looking at the print side, I can see C4 & C6 have at some point have been fitted but now removed, but the rest (C10, C12 & C15) have never been populated as the print is still virgin. This might explain the lack of tone. The Taylor sniffs nowt!

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Posted : 28/08/2023 12:56 pm
Cathovisor
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Here's a thought: try the suggested values for C4 and C6 and see if one oscillator bursts into life. Even fitting just C4 should produce something.

I have a feeling someone in the past has tried to use this for a frequency other than intended - like 1kHz...

I think our old friend 1 over 2Pi(root)LC may come in useful here.

 
Posted : 28/08/2023 1:09 pm
crustytv
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C4 (.0068uF) & C6 (.0056uF) fitted, looking for near 8kHz still nothing on 7/8.

Can you confirm the power is correctly applied, I have the PSU set to 24V. The +ve lead going to pin PL14 and the -ve lead to PL4. That give -24 and it's drawing 71mA

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Posted : 28/08/2023 1:47 pm
Cathovisor
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@crustytv Power's correct.

Firstly, what's on the emitter of TR1?

 
Posted : 28/08/2023 1:55 pm
crustytv
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-3.76V

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Posted : 28/08/2023 2:02 pm
Cathovisor
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@crustytv And is there any sign of oscillation with a scope?

 
Posted : 28/08/2023 2:04 pm
crustytv
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Scope across 7 & 8, scope gnd on 8.

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Posted : 28/08/2023 2:08 pm
Cathovisor
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@crustytv Sorry Chris, I wasn't clear - is there any sign of oscillation on the emitter of TR1?

After you've checked this, connect pin 12 of the edge connector to the -24V supply as per section 2 of the adjustment instruction.

 
Posted : 28/08/2023 2:14 pm
crustytv
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Sorry, yes, 7.26kHz which is near as damn it what the two caps C4 &C6 should have set it to.

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Posted : 28/08/2023 2:18 pm
Cathovisor
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@crustytv Excellent - now it seems that if you apply -24V to pin 12 of the edge connector, it should appear on pins 7 and 8 on the edge connector. But before you do that... check what's on pin 15 on the PCB first.

 
Posted : 28/08/2023 2:23 pm
crustytv
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Voltage or waveform?

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Posted : 28/08/2023 2:27 pm
Cathovisor
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@crustytv Waveform.

 
Posted : 28/08/2023 2:28 pm
crustytv
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Posted : 28/08/2023 2:29 pm
Cathovisor
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@crustytv Okay - now connect pin 12 of the edge connector as described to the -24V supply and see what you get.

 
Posted : 28/08/2023 2:32 pm
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