Bush TR130 distortion and overloading.

[davidgem1406]

A friend sent me a Bush TR130 to look at for him with the comment that the AGC was not working and strong signals were causing distortion.

There are 2 versions of this radio early 1966 to 1967 and late 1967 to 1968.
Differences in circuit a minimal, I have only noted 1 so far and additional capacitor from the VC wiper to ground.
The PC board is another matter the layouts differ considerably. I have the later version.

Well there certainly is considerable distortion at high volume that seems to be regardless of station or noise.
And as the output of this set is just 1 Watt the output power is far in excess of that, it is way to loud to listen to in sheer volume alone.

As a routine I have changed all the electrolytic's. As there is one (C19) in the AGC I thought maybe it could be this, but no.

I have checked all voltages for the output and RF stages and they are within what one would reasonable expect them to be. They are quoted on the schematic.

The other problem I have noticed is that the volume control does not suppress the audio when turned to fully down.
I found that there was a 10R resistance between the wiper and ground so removed it, stripped it, cleaned it, checked it. The 10R resistance is present between the wiper and either end of the track. Nothing can be done about that then. Back it went.

However even with it sitting at 10R above ground when turned down that aught to be sufficient to kill the audio completely.

I checked the AGC to see what was happening there.
First at the top of the VC and the result was -4.7V with a tuned signal and -0.5V at no signal.
Next I checked the other side of R11 (18K) and found corresponding voltages of +0.4 and +0.6, a change of 0.2V at the base of VT2.
The 18K resistor has been replaced.

It seems that there is far to much audio available to drive the output stage.

Am I missing something here?

Help and comment would be much appreciated.

Thanks
Dave.

[Nuvistor]

I cannot remember the TR130 volume control but if it is one of the moulded track type and a carbon wiper they never reached 0 ohms consistently and as a volume control in whatever set I saw them they always allowed a little audio through at minimum.

Have you a scope, you can easily tell if the audio from the detector is distorted?

The transistor is an AF117 in my circuit, we know what those can be like but usually stop the set working.

If it is distorted at high volumes whatever the signal strength high or low I would be checking audio first.

Frank

[davidgem1406]

Hello Frank,

Thanks for the reply.
The audio is fine up to maybe half volume and then starts to distort. By the time it reaches near full volume it is way over the top of a mere 1 Watt and very heavily distorted. I cant imagine that it should produce that sort of volume. It is way to much for me to listen to.

It seems to be that there is just to much audio signal available that just drives the output stage well above what it is capable of.

Dave.

[Nuvistor]

One watt can be loud in an efficient speaker but I take your point.

You need to confirm output from the IF stage is clean/distorted, from there you can decide if you do have an AGC fault or audio. Without that information it is guess work.

I have just looked at the manual I have and it does look like a Morganite ( I think that was the make) moulded control, Bush used them in most of their sets. Very reliable and rarely went noisy in their service life but suffered from not turning the audio completely off.

[Herald1360]

If the audio is fine at any level, it's unlikely the RF/IF/detector is faulty. The VC is after that. If there's too much audio gain, I'd be looking more for an AC feedback problem, though do check the output stage dc voltages make sense.

[paulsherwin]

I'm not sure about the TR130, but many manufacturers designed their radios to produce some sound with the volume at minimum. This was to prevent the radio being left on inadvertently and draining the battery.

It's normal for any radio (valve or transistor) to be overdriven with the volume turned up all the way when tuned to a strong station. Presumably the behaviour you're describing is worse than this.

[davidgem1406]

Hello Herald 1360,
I have checked the output stage voltages and they are just about what the service info states. The biggest variation being about 0.05V on the output pair collectors.
I have also checked and reset the O/P stage bias.

I don't see any AC feedback path in the output circuit. All I see is R31 and C34 in series connected from VT5 collector to VT6 collector. That looks like some compensation to me.


Hello Paul,
Yes it is worse than just at full volume. It starts at maybe half volume and then gets worse at higher volume until it becomes absolute distortion.


This is what I have just done to try to prove either the RF stages or the Audio stages to be at fault.

I used another radio, namely a Roberts R700 as the test equipment.
However I do not know what the output power of the R700 might be, I can not find it given anywhere.

The feed to the audio stage was disconnected and wired to the Bush on the detector output at the junction of C26 and R11, whilst disconnecting from the volume control.
The chassis were connected together. Each ran from its own supply.
This resulted in the Roberts output being distorted when the volume was turned up and producing far more volume than when using its own RF stages.

I then reversed the procedure and drove the Bush output from the Roberts RF stages. At full volume the Bush produced about the same output as the Roberts when it was driven by its own RF stages. There was no distortion.

It appears to me that what is being produced by the Bush RF stages is well more than there should be.
Having said that the voltages in the RF stages are near as can be that as is stated in the service data. The AGC also seems to be working.

I also changed R7 (120K) as this was out of spec. That resulted in a change in the available AGC voltage increasing the amount of change between no signal to full signal by about 0.2V.

I don't have much of an idea as to what to look at or do as the next stage.

Thanks
Dave.

[G6Tanuki]

Is the AGC actually working properly?

If it isn't [remember these radios use the tin-whisker-prone AF11x transistors in the IF-strip, and also invariably--leaky Hunts brown decouplers] it's possible that a less-than-efficient AGC is not properly controlling the gain of the IF-strip and so is delivering sufficient excess audio to the volume-control that at high settings the audio-stages *will* overload.

In its era the TR130 was known for its sensitivity - various variants having 'bandspread' or '208' settings to help you pull in stations-other-than-the-BBC, where every last bit of gain helped. if the AGC is not able to throttle-back the sensitivity when receiving a local station then the audio-stages will be hammered.

[Sidenote: the TR130 was styled by Tom Karen of Ogle, who also did the design-outlines for my 'fun' car]

[julie_m]

Remember this is a transistor radio .....

In a valve radio, AGC action is pretty much implicit, as long as the IF amplifier is a variable-μ valve.

In a transistor radio, the AGC voltage turns on a transistor which is the bottom of a potential divider in the signal path; just because you're getting a DC voltage on the AGC line, doesn't mean that that transistor is actually doing anything. If this transistor is not conducting, you will have no AGC action.

[Herald1360]

Doesn't basic tranny agc simply use the fact that transistor gain varies markedly with collector current over part of the range of useable current? If so, and the standing bias for the IF/RF devices is wrong, the agc won't work as intended.

Use of a transistor as a variable resistor in a pot circuit sounds rather sophisticated for a '60s domestic set.

Quoted max output for the 707 is 1.5W.

A reasonable estimate for unknown sets with transformerless outputs is to take the rail volts less a couple of volts for circuit drops as the max p-p output voltage applied to the loudspeaker.

So 12V supply set with 8ohm speaker will deliver about ((12-2)^2)/(4*8) or about 3W max. Probably a bit less in reality.

[davidgem1406]

I have now done some more check but they were not very conclusive.

Yes it uses the AF117 transistors all of them suffered from intermittent whisker problem when knocked but I have disconnected the earth to the cans.

Maybe the one doing the AGC has other problems as well even though the voltages are correct.

Herald1360 has I think given me the clue that the AGC is not as it should be even though it would appear to be working.
The Roberts R700 has an output of 1 Watt as calculated by the formula given by Herald1360. 12V supply and a 25R speaker calculated to 1 Watt. That is the same as the Bush.

That says to me that the results I obtained in my previous posting show that the AGC of the Bush is not working as it overloaded the Roberts output stage equally the same as the Bush.
The RF stage of the Roberts did not cause that problem for either set.

I think my next thing is to change the AF11x transistors and see the result of that.

Thank all,
Dave.

Ill be back ...

[Herald1360]

I got a bit muddled as to which set was which (hence my blither about a 707).

Anyway, about a watt sounds right for the Bush (if only as the FSD of the wattmeter suggested); OC81s won't do any more. For output guesses, all bets are off with a transformer stage unless the ratios are known- it's possible to get high powers from low rail volts with the right transistors and transformer ratios. (I built a 6V accumulator powered noise box once that got about 10 watts from a pair of 2N174s(!) to be able to hear a tranny at work over the racket of a canning line as a holiday job student).

I've finally looked at the TR130 circuit (manufacturer's)- it looks like agc is only applied to VT2 via R11/C19 and bias resistor R7. Increasingly positive dc output from the detector will bias back VT2 and reduce its gain. Wrong polarity detector diode (unlikely?) or problems with bias/agc resistors/capacitors could all affect things, as could a dodgy VT2.

HTH

[davidgem1406]

I have now replaced all the AF117's but no change in the situation, but they would need doing anyway.
Also changed all components in the AGC circuit R7, R11, C19. C19 I had already replaced much earlier but I changed it again just in case but the problem remained.

I than started to look at other more obscure reasons.
Checking the resistance of IFT2 the secondary between pins 5 and 6 is O/C
I checked this against IFT1 as they are identical to be sure that what I expected was correct, it was.

Well thats now a rather bigger problem I think!!!

Dave

[Herald1360]

Sounds like a S/H IFT would help.

Try this bodge and see if it does anything useful....

Disconnect VT2 collector and link it to IFT2-5 via 3K3 resistor. Link VT2-C to IFT2-2 via 100pF capacitor. Mark IFT2 cores and have a twiddle. This won't do a lot for selectivity, but there's several other IF tuned circuits left. If it doesn't help, nothing lost and a proper job is still on.

[davidgem1406]

Hi Chris,
Sorry but I have given a bum steer in my previous post about the IFT.
I was looking at the circuit diagram for 66 to 67 version as it is much clearer than the 67 to 68 version.
The set is the 67 to 68 version but the circuit only has some small mods to the earlier version. But IFT2 turns out to be one of them. (IFT2 is the same as for the TR132 in schematic anyway)

IFT2 only has one winding for the secondary not a tapped winding as in the earlier version.
However the resistance of that winding is showing at 0.5R, does that sound rather low?
The primary winding is tapped 6.2R + 2R = 8.2R, thats the same as the earlier version.
IFT1 is 8.2 for primary and secondary, thats good for both versions.

I wonder why IFT" was modified in that way??

Dave.

[Herald1360]

OK, forget the bodgery.

Many trannies of the period had single tuned IFTs with just a coupling winding for the following stage- probably because cheaper and also with two IFs, three tuned circuits were good enough. Domestic valve sets with one IF had four but not the five of the early TR130.

Running out of ideas..... have you changed the capacitor from the VC wiper? If that's leaky it would upset AGC, especially as the volume was turned up and it would be "closer" to the AGC feeder R11. Also the extra capacitor from VC wiper to ground could have a similar effect.

[Thyristor]

The TR130 is a very loud radio, for what it is (small radio, large speaker), so some distortion at full volume is likely.
I've got one, that I repaired many years ago; just tried it, sound distorts at max, and volume doesn't fully mute at minimum.

I'd definately check the audio (sine wave) with a signal generator and a scope, into a dummy load, that will be a quick litmus test.

Another thought, your speaker may be u/s?

[Boater Sam]

Thyristor is probably right. The speaker is very small and shallow on the cone for any output approaching 1 watt. The case is pretty well sealed sonically, all the sound has to come from the front of the cone. Is it different with the case opened and closed?
I have restored several TR130s and they will all over drive the speaker even if it is in top condition.
Because the output is so high, if the radio has been used a lot at high volume it is very possible that the speaker inner suspension is getting shot.
Try hooking it up to a large speaker, it could be impressive.

[davidgem1406]

Hello Chris,
I assume the capacitor from the VC wiper you are meaning is the electrolytic C30, yes it has been changed.
The cap from VC wiper to ground (C15) I have not changed, I will disconnect it and see what happens.


Hello Thyristor,
The distortion comes in before reaching maximum.
I will try an audio stage check using a sine wave as you suggest.
The speaker has been substituted for one the same size as the original, the result was the same.


Hello Boater Sam,
I will try and change the speaker for a larger one, dependent on what I have available, but the impedance could preclude that possibility as it is 15R.

Thanks all.
Dave

[Herald1360]

You could use a four ohm speaker and ten ohm resistor. You're listening for distortion, not worried about loudness. Or any other suitable combo- two eight ohms in series....... Rs or LSs. At least one LS though