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Old 20th Aug 2013, 5:33 pm   #1
JoshWard
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Default Roberts R200- Distorted Output

Hi folks,
I have an R200 on my bench at the moment but I am struggling a bit overcoming a problem with distorted output.

I am working from Trader sheet 1602, the set has a serial number above 70,000.

My problem appears to lie within the bias circuitry. With X3 removed or reversed I can easily adjust the bias to the required 4mA and the output seems fine but when in it's correct position (it is actually a PNP transistor wired as a diode, base and collector are the cathode and the emitter is the anode) I cannot get the bias above 3.4mA (that is with the pot maxed out) and the output is distorted.

X3, TR4, TR5 and TR6 have all been replaced with AC128s but to no avail.

I must be missing something obvious here but not sure what it is...!!

Thanks in advance,
Josh.
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Old 20th Aug 2013, 5:53 pm   #2
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Default Re: Roberts R200- Distorted Output

It sounds like you need a bit more volt drop across X3 to get the bias current up a bit. You could try replacing it with a silicon PNP or just a 1N4148 or similar. You will still be able to dc connect the output bases and emitters at the low end of the adjustment range.
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Old 20th Aug 2013, 7:10 pm   #3
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Default Re: Roberts R200- Distorted Output

Has R22, the 5.6 ohm emitter resistor for the output pair gone high? Worth checking.

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Old 20th Aug 2013, 9:06 pm   #4
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Default Re: Roberts R200- Distorted Output

Thanks for the suggestions. I popped a 1N4148 in, adjusted the bias and it seems to be working well

Will check that resistor as well Ron, just in case it avoids future problems!

Josh.
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Old 20th Aug 2013, 9:32 pm   #5
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Default Re: Roberts R200- Distorted Output

Hi Josh

Yes check R22 as Ron suggests, it is very likely to be the problem. Then remove the 1N4148 and replace it with an AC128 with collector and base joined together. X2 provides bias stabilisation with variations in temperature and a silicon diode is not suitable in this set.

You will see (from the Trader sheet) that the early versions of this set did not employ temperature stabilisation of the output bias, so presumably they suffered from thermal run away.

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Old 20th Aug 2013, 10:04 pm   #6
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Default Re: Roberts R200- Distorted Output

Thanks Colin. Just checked R22 and it was indeed high. Replaced it and refitted the AC128- once again I can't get the Q current above 3.2mA or so, there must be something else amiss somewhere.

X2 is the detector in this set, X3 is the bias diode, be careful there are two circuits on the same Trader sheet! In the other circuit (for sets with serial numbers below 70,000) the bias diode is indeed marked as X2. Trader obviously didn't think putting circuits in chronological order was a good idea

Josh.
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Old 21st Aug 2013, 7:10 am   #7
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Default Re: Roberts R200- Distorted Output

Hi Josh

Yes X3, sorry.

With the current set to as high as you can get with R21, measure the voltage across X3 and then measure the voltage between the anode of X3 and the slider of R21. If these voltages are not about the same it could indicate a poorly bias pot R21.

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Old 21st Aug 2013, 8:51 am   #8
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Default Re: Roberts R200- Distorted Output

Hi Colin,
Across X3 I get 179mV and between X3 anode and R21 slider I get 178.5mV so not a lot of difference.

I think I will have to put the silicon diode back in and try gently heating the output stage whilst monitoring the bias current carefully. I never seem to have much luck with bias arrangements, I have also been working on a Ferranti set which uses a thermistor to stabilise the bias, this has failed, a normal resistor works but then you end up with the problem of thermal runaway and of course correct thermistors are no longer available

Josh.
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Old 21st Aug 2013, 11:28 am   #9
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Default Re: Roberts R200- Distorted Output

Is there any realistic thermal coupling between X3 and the output devices? If not, then the bias compensation will only really be for ambient temperature rather than effective runaway control.

So far as silicon vs germanium is concerned, silicon has a higher mV change per degree than germanium (about 2mV vs 1.5mV) so if anything, using a silicon sensor will overcompensate rather than fail to compensate.

If the output stage emitter resistor goes a bit high, the effect will also be to overcompensate since its dV/dI will be higher. At least this is on the "safe" side for the output devices though it will marginally worsen the audio power output available from the radio.
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Old 21st Aug 2013, 12:21 pm   #10
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Default Re: Roberts R200- Distorted Output

Hi Chris,

Perhaps there would not be enough voltage across R21 to use a silicon diode? Using the emitter voltages given in the Trader sheet, Ohms law calculations suggest there is only 5mA flowing in the previous stages which gives only 0.5volts across R21.

The approx 0.18volts Josh is reading across the germanium device seems a fraction low to me.

Josh

What is the distortion like? Low Q-current causes crossover distortion which sound worse at low volume and difficult to hear at high volumes. If itís not crossover distortion one possibility is that you have low current in the driver stage due to a bias fault or a faulty transistor and that is this that is causing the distortion. The low Q-current in the output stage being a red herring. If this is the case you will, of course have to set up the output stage bias (quiescent current) when you have fixed the driver fault.



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Old 21st Aug 2013, 12:56 pm   #11
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Default Re: Roberts R200- Distorted Output

The distortion is present regardless of volume. I keep thinking it is a transistor fault but I have replaced the driver and output transistors with no change. It must be a bias fault but I can't seem to find it.

It works fine with the diode/transistor removed or with a silicon diode fitted, but as soon as I fit anything germanium I just can't get enough bias current.

Josh.
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Old 21st Aug 2013, 3:56 pm   #12
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Default Re: Roberts R200- Distorted Output

Could be running on only one output transistor for some reason. Will you post the voltages you are getting on TR5 and TR6? You may as well do the same for TR4 while you're about it.

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Old 21st Aug 2013, 7:21 pm   #13
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Default Re: Roberts R200- Distorted Output

OK Colin, I am getting:
TR4
Emitter: 1.25v
Base: 1.3v
Collector: 7.5v

TR5
Collector: 7.8v

TR6
Collector: 7.8V

TR5 and TR6 bases and emitters have nothing to speak of on them.

The only obvious thing I can see is that the collectors are low.
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Old 21st Aug 2013, 8:14 pm   #14
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Default Re: Roberts R200- Distorted Output

Those voltages look normal and healthy Josh and the fact that you get no distortion at 4mA suggests there is nothing drastically wrong with the output stage. It may be that this circuit is a bit marginal in its design and tolerances have pushed it over the edge.

You could leave a silicon diode in and set it up for 4mA and then monitor it for a good while, but I suspect the silicon diode will not be conducting.

Another approach may be to add an 8.2k resistor between the battery negative rail and the cathode of the germanium diode; this would give an extra 1mA thought the R21/X3 combination which may be enough to get the output stage current into range.

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Old 21st Aug 2013, 9:01 pm   #15
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Default Re: Roberts R200- Distorted Output

Hi Josh,

what were those voltages measured against- Batt+ or signal ground?

I would expect to see about 23mV across the output emitter resistor for a 4mA standing current.

The bias problems with replacement devices could just be down to the new transistors being "better" ie less leaky than the original types, thus needing some actual negative bias between emitter and base to turn them on. It wasn't unusual for pnp germanium output transistors to need positive bias to actually turn them off.
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Old 21st Aug 2013, 9:26 pm   #16
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Default Re: Roberts R200- Distorted Output

My Trader sheets do not go that high.

TR4 has a low emitter/base voltage. I would expect about 0.2V not 0.05V.
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Old 21st Aug 2013, 10:36 pm   #17
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Default Re: Roberts R200- Distorted Output

I tried an 8.2k resistor in there and I can now get the bias up to 4mA OK with the AC128 in there. I think I will try it with a silicon diode and do some tests though just in case it does work, silicon diodes are a lot cheaper than AC128s so it would be good if it does work.

The voltages were measured with positive probe to chassis. I had the same problems with the original transistors which is why I tried replacing them, with no luck it would seem!

Josh.
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Old 21st Aug 2013, 11:00 pm   #18
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Default Re: Roberts R200- Distorted Output

Quote:
Originally Posted by TrevorG3VLF View Post
TR4 has a low emitter/base voltage. I would expect about 0.2V not 0.05V.
If it's a bit leaky, 50mV may be fine. It's a classic pot on the base and emitter resistor circuit, so the current will just rise a bit to balance emitter volts with base volts minus whatever bias is needed (strictly speaking plus here I guess since the devices are pnp).

Voltages- I guess chassis is the heavy line at the bottom of the circuit, so you would expect the bases and emitters of the output pair to be pretty close to that, since it is itself sitting at the max possible bias voltage for the pair.
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Old 22nd Aug 2013, 11:02 am   #19
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Default Re: Roberts R200- Distorted Output

There is plenty of emitter voltage on TR4 so it could be replaced by a silicon transistor. If this cures the problem, then a germanium transistor could be sought.

A scope is the easy way to go, it would be good to find if the output stage is running one-legged.
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Old 22nd Aug 2013, 3:44 pm   #20
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Default Re: Roberts R200- Distorted Output

Hi Josh, have a complete set of correct Mullard driver and output pair here f.o.c. that I can pop in the post for you. Always seem to have more trouble with AC128s than OC81s.

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