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Old 17th Feb 2014, 1:49 pm   #1
oxford33
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Default Roberts R200 - AF117, BC557 and Noisy Output

Hello everyone,

I recently acquired a broken Roberts R200 which I'm looking to get working. I've found this site extremely helpful so far but am a bit stuck on where to look next. Apologies if this has been covered before.

The set is the third version with the AF117 transistors. Initially it made no noise and drew too much current - swapping out the 100uF decoupling capacitors with new ones brought the current down, and the set now worked (but with a lot of static on top of the output) when tapping the AF117 transistors. I replaced these with some cheap BC557 mostly just to see if they would work, testing the set in between each one to see if there was any difference, and was pleasantly surprised that despite the different biasing of the silicon these seemed to work without any modification. I wasn't expecting this and was otherwise planning on buying AF127, so in case it helps anyone else in future: do try the 6p transistors before you start looking for £5 old ones.

The set now works however there is still a fairly loud constant level of noise/static on top of the output, even when the volume control is turned right down. I assume that this means there is some problem with the output stages, but I'm unsure where to look. As a start I started poking around at the voltages, and saw that the base of TR4 is at around 6V, which is much too high. Any ideas what might be causing this?

Thanks
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Old 17th Feb 2014, 2:43 pm   #2
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Default Re: Roberts R200 - AF117, BC557 and Noisy Output

It's not unknown for PNP germanium small signal transistors to go leaky/noisy so TR4 itself is suspect. You could try another BC557 here, though it would probably need rebiassing- just increase R17 (lower base bias resistor) until the emitter volts is same as quoted in the service sheet (1.2V) at which point the emitter current will be correct.

Worth checking the base volts when the transistor is out and the emitter resistor resistance just to confirm the resistors are about right to start with.
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Old 17th Feb 2014, 9:26 pm   #3
oxford33
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Default Re: Roberts R200 - AF117, BC557 and Noisy Output

Thanks - I'll pop TR4 out and see how it goes.
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Old 19th Feb 2014, 9:01 pm   #4
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Default Re: Roberts R200 - AF117, BC557 and Noisy Output

I've noticed that on eBay, the transistor BC557, has a letter after the final number, such as A or B or C. What does this mean?
Thanks
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Old 19th Feb 2014, 9:45 pm   #5
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Default Re: Roberts R200 - AF117, BC557 and Noisy Output

The letter normally indicates the hfe (gain) group, A being the lowest and C the highest.
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Old 19th Feb 2014, 9:51 pm   #6
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Default Re: Roberts R200 - AF117, BC557 and Noisy Output

A following letter can also indicate a different lead out to facilitate PCB mounting.
John.
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Old 19th Feb 2014, 11:09 pm   #7
oxford33
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Default Re: Roberts R200 - AF117, BC557 and Noisy Output

The BC557's I used were BC557B which were a few pence each from CPC.

For this set the OC81 TR4 did turn out to be the culprit (thanks). I tried putting another BC557 transistor in its place and adjusting the base bias, but eventually gave up and transplanted another OC81 from another (currently dead) R200 as a temporary fix, which worked.

Any ideas why the BC557 wouldn't have worked here? I tried various values for R17 and even when the voltages on TR4 were about right this seemed to make the base current on TR5/6 go way too high (over 100mA) which I couldn't bring down to anything sensible with the pre-set resistor. With a working OC81 installed the voltages/currents are normal so I don't think anything else is faulty. It's been a while since I've done electronics so may have missed something obvious.
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Old 20th Feb 2014, 12:45 am   #8
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Default Re: Roberts R200 - AF117, BC557 and Noisy Output

Ummmm, yes. The bias for the output stage involves the total 0V line current for all the set except the output stage dropping a voltage across X3 diode which is tapped off by R21 preset to bias the output stage.

Consequently, if TR4 emitter current is too high, so will be the output stage bias

The important setting for TR4 is its emitter current (effectively monitored by measuring its emitter voltage relative to battery 0V). Other voltages round TR4 for its base and collector will perforce be different from the table voltages because of the change from Ge to Si. I can see this interaction making output stage quiescent setup somewhat fraught!

That diode X3 helps to stabilize the output stage quiescent current since its voltage drop will fall as temperature rises. It's an unusual way to do it, though.
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Old 20th Feb 2014, 10:58 am   #9
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Default Re: Roberts R200 - AF117, BC557 and Noisy Output

Thanks for the detailed explanation - I hadn't realised how critical TR4 current was for the rest of the biasing. I'll give this another go when I get the chance and see if I can make it work, but the easy answer is certainly to stick to germanium for the audio stages.
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Old 20th Feb 2014, 4:52 pm   #10
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Default Re: Roberts R200 - AF117, BC557 and Noisy Output

Quote:
Originally Posted by Herald1360 View Post
Ummmm, yes. The bias for the output stage involves the total 0V line current for all the set except the output stage dropping a voltage across X3 diode which is tapped off by R21 preset to bias the output stage

Consequently, if TR4 emitter current is too high, so will be the output stage bias
Herald1360:

Are we talking about the same radio?
The schematics is different from what you are talking about!
The bias of TR4 does not influence the bias of the final stage, only of TR3, and there is no X3 diode neither R21.

oxford33:

From the voltages and resistors, the emitter current of TR4 is 1.92mA. If TR4 is changed for a silicon transistor, and one would want to keep the same emitter current, it would take R11 of 8.95K (neglecting the base current, and using the voltages from the service sheet).
So one could leave R11 the same, the emitter current would be smaller only by a little bit. A silicon transistor also would have a higher beta than Ge transistor, that would also help.
As Herald1360 mentioned, measure the emitter voltage of TR4, and keep it the same.

Peter

Last edited by Station X; 20th Feb 2014 at 9:38 pm. Reason: Forum rule compliance.
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Old 20th Feb 2014, 5:58 pm   #11
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Default Re: Roberts R200 - AF117, BC557 and Noisy Output

I'm not sure now......

I was referring in my most recent posts to the version described in Trader sheet 1602 as the third and final version of the set, which has AF11X devices and has the biassing arrangement described which also has no interaction with TR3, unlike the earliest model .

There's also an intermediate model which has biassing like the third but still has the OC RF devices. Here the bias diode is X2 and preset R20.

All very confusing, not least because for my original suggestions about going silicon for the OC81, I too was referring to the first version- I didn't twig about the later variants until I picked up on the AF11X references.

There's also possibilities for yet more confusion if someone has the manufacturer's or other data from the Trader sheet because there's no guarantee that component references will be consistent
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Old 20th Feb 2014, 8:37 pm   #12
oxford33
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Default Re: Roberts R200 - AF117, BC557 and Noisy Output

The set I was looking at was the third version (confusingly this is listed second in the service sheet) so the component names Herald1360 used made sense.

So! Embarrassing story time. I tried this again and figured out why the current was quite so crazy last time: I'd naively assumed the insulation colours on the old transistor legs was consistent with TR1-3: turns out it wasn't (at least in my set), so I'd installed it the wrong way round. Oh dear.

The good news is that the right way round a BC557 does seem to work in place of TR4, even without changing the biasing (but I suppose this should be changed to do things properly...).

Thanks for the help
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Old 20th Feb 2014, 9:53 pm   #13
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Default Re: Roberts R200 - AF117, BC557 and Noisy Output

I would certainly expect it to work reasonably well in this sort of transformer coupled circuit, and was about to suggest double checking the wiring.

We all make mistakes like this.
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Old 20th Feb 2014, 11:30 pm   #14
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Default Re: Roberts R200 - AF117, BC557 and Noisy Output

Quote:
Originally Posted by Herald1360 View Post
I was referring in my most recent posts to the version described in Trader sheet 1602 as the third and final version of the set, which has AF11X devices and has the biassing arrangement described which also has no interaction with TR3, unlike the earliest model
The trader service sheet number I looked at is 1449, that explains the differences!
I did not realize that the radio had different versions.

Peter
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Old 21st Feb 2014, 1:09 am   #15
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Default Re: Roberts R200 - AF117, BC557 and Noisy Output

On the AF117 equipped R200 there is also a 4k7 resistor R25 that draws 2mA from the battery (approximately equal to the emitter current of Tr4). This increases the current through the bias diode X3 and therefore stabilises its forward voltage, particularly when strong signals reduce the IF stage current. It was 3k9 on the middle version so even more current was 'wasted'.

Ron
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