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Old 22nd Aug 2013, 7:16 pm   #21
JoshWard
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Default Re: Roberts R200- Distorted Output

Thanks for the offer Rob, it is much appreciated. I am not sure if I will need them though as after fitting the resistor that Colin suggested in post 14 I haven't noticed any distortion.

I have just monitored the bias for fifteen minutes or so, it went down from 4.03mA to 3.92mA. When I touched the output transistors the bias climbed again.

Whereabouts would I need to stick the scope and what would I need to look for to test that both output transistors are working? I'm still not convinced it is a transistor fault, after all it was the same with the original transistors.

I can't get the damned thing to play ball with my MP3 player either, even with a 4k7 in each channel with another 4k7 going into the volume pot there is distortion, if I add any more attenuation I won't be able to hear anything

Josh.
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Old 22nd Aug 2013, 10:15 pm   #22
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Default Re: Roberts R200- Distorted Output

With a scope, you are better off with a sine wave oscillator, there are many simple circuits, phase shift, Wien bridge or tuned circuit.

Even without a sine wave you may be able to see distortion. Start at the volume control and work through to the output. The collectors are the places to look. You may be able to see clipping or crossover distortion. Clipping is seen at high volume, crossover distortion at low volume.
Without the circuit it is not possible to be more specific.
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Old 23rd Aug 2013, 12:02 am   #23
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Default Re: Roberts R200- Distorted Output

I believe snippets are OK to post-

If not this'll disappear soon enough.....
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Old 23rd Aug 2013, 12:35 am   #24
G8KBG Tony
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Default Re: Roberts R200- Distorted Output

Josh, have you checked R25? It may have gone high or o/c.

Tony
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Old 23rd Aug 2013, 10:49 am   #25
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Default Re: Roberts R200- Distorted Output

Thanks for the snippet.
You can check the standing current in each of the output transistors by measuring the voltage drop across the output transformer.
I do not think that the standing current of the output pair is very critical for distortion.

With a scope and a sine wave input, the voltage across R22 emitter resistance will show balance.

There is a feedback resistor R24 47R going into the nether regions so the distortion should be very low. Disconnecting this from the speaker and connecting to earth will give the plain amplifier distortion.
There will be an earlier transistor TR3 ? which will affect the distortion.

I do not see what R25 4k7 does other than waste power.
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Old 23rd Aug 2013, 12:16 pm   #26
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Default Re: Roberts R200- Distorted Output

TR3 is second IF, audio is as shown except for base bias pot for TR4.

R25 may be there so there's enough voltage drop for bias, but it does seem to be a strange way of doing it.
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Old 23rd Aug 2013, 3:52 pm   #27
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Default Re: Roberts R200- Distorted Output

Quote:
Originally Posted by G8KBG Tony View Post
Josh, have you checked R25? It may have gone high or o/c.

Tony
Well spotted Tony. R25 is electrically in the same place as the 8.2 k that I suggest Josh added in post 14. My mod would add about 1mA to the bias circuit. R25 (which I hadn't noticed) should be providing about 2mA and yes, if it is o/c it could well be the problem.

Let us hope R25 is indeed high or o/c and then the added 8.2k can then be removed.

Colin M
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Old 23rd Aug 2013, 6:50 pm   #28
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Default Re: Roberts R200- Distorted Output

I have already checked R25 and it is fine (not what you were wanting to hear )
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Old 23rd Aug 2013, 9:55 pm   #29
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Default Re: Roberts R200- Distorted Output

No.
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Old 23rd Aug 2013, 10:30 pm   #30
JoshWard
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Default Re: Roberts R200- Distorted Output

Your 8k2 resistor trick works well though, so I have put the set back together. The bias is at 4mA, there is no noticeable distortion (will leave on test for a few days and see if it comes back), and the Ge diode protection passed the hairdryer test (gently heated the circuit whilst monitoring the Q current).

Thanks for all your help everybody
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