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Old 17th Jan 2022, 4:30 pm   #1
Paul_A_R
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Default Dansette Chorister Repair advice

I picked up a non working Dansette Chorister.

Sure enough it was completely dead. Switch cleaner restored activity but only static.

I pulled the only visible electrolytic on the RF/IF side of the radio. It was not too bad though I replaced it anyway. Most of the working parts are in a modular can LP1154 is the code. I assume this is a Mullard but can not find any data on it. I have found some data sheets for these modules but they start at LP1155.

I removed the module and opened it, unfortunately voiding my warranty in the process. It contains 3 A127 transistors which read OK on a multimeter.

I replaced the only electrolytic and tested a couple of the resistors and 'tropical fish' capacitors which tested OK so I put them back.

With the module replaced I can get one MW station though weakly and with a tendency to float in and out of tune.

The next step would be to try an alignment. The plastic trimmers look like they have been played with. The three closer to the amplifier side of the board look serviceable but the one closest to the tuning mechanism is a mess.

Is it likely that I will be able to complete an alignment with the mangled plastic trimmer or is it more likely that this damage has also been reflected in the mechanism?

There are also a couple of other adjustable components on and by the wave change assembly which I am not sure what they are so I have not tried to adjust them. Might these provide any assistance and if so are there instructions on setting them.

Paul
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Old 17th Jan 2022, 5:33 pm   #2
Herald1360
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Default Re: Dansette Chorister Repair advice

At least they're AF127s not AF117s! The RTVS service info is little more than a circuit, just showing the IF as a module, and helpfully states "No IF alignment is required" though it does offer basic RF alignment instructions.


Try tweaking the IFs for 470 kHz with whatever you have that can turn the cores, then tweak the RF. You've nothing to lose now that warranty's been blown!
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Old 17th Jan 2022, 5:43 pm   #3
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Default Re: Dansette Chorister Repair advice

Have you tried gently tweaking the damaged trimmer? I wonder if it is OK and has been mangled by heavy handed adjustment with an incorrect screwdriver? If it is stuck then that is a different matter.

Certainly from looking on the surface combined with your description, it looks like alignment is going to be required.
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Old 17th Jan 2022, 6:44 pm   #4
Paul_A_R
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Default Re: Dansette Chorister Repair advice

Thanks. I will try some tweaking. I have turned the damaged trimmer while tuned to my one station and it did change the reception so I will get the signal generator out. I am also looking at the Dansette Companion service sheet. It again uses a Mullard module but is a bit more detailed in its description. Hopefully between the two sheets I will make some progress.

Paul
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Old 17th Jan 2022, 9:36 pm   #5
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Default Re: Dansette Chorister Repair advice

I will need to defer realignment for a while.

Even slight movement was causing the radio to cut out. If have reflowed some of the soldering and that has improved things but the on/off switch is still troublesome after several goes with Servisol. I may need to dismantle it to clean it. I will also have to look more closely at the connections around the wavechange switch.

I have also noticed that the crackle and my single station are right at one end of the scale.

After this there is no sound at all for the majority of the the travel of the tuning dial until right at the other end there is a short period of crackle but no further stations.

Any suggestions why most of the dial should be silent? I have visually inspected the vanes and can not see anything that would short them.

Paul
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Old 18th Jan 2022, 1:21 pm   #6
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Default Re: Dansette Chorister Repair advice

Try hanging a multimeter on the ohm range between the tuning capacitor and ground. Then slowly turn it from one end of travel to the other. There should be no reading at all; any reading means the cap is either shorting out (bent vane) or partially shorting which could be dirt.
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Old 18th Jan 2022, 8:12 pm   #7
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Default Re: Dansette Chorister Repair advice

Set the wavebands switch to MW2 before checking the tuning cap for shorts- otherwise you'll just get dc short via the tuning coils!
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Old 18th Jan 2022, 10:49 pm   #8
Paul_A_R
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Default Re: Dansette Chorister Repair advice

Thanks to both of you.

I rechecked the vanes as there is a crack in the case and the radio has probably, therefore, been dropped.

They look OK with a bright light behind them. The light did reveal some muck on the vanes which I did not see previously.

I removed the assembly and cleaned it in IPA. After drying I replaced the assembly and got no resistance reading when tuned to MW2.

The radio now plays static across all three bands rather than just the ends of the MW band.

The trimmers in the Mullard module do affect the level of the static but no signals. They do not appear to have been far out, at least as far as volume of static goes.

The radio also only picks up signal generated tones as static. The level of the static peaks at 465Khz for instance but the is no tone and a scope across the speaker shows only static and no sine wave. I did think that perhaps there was a tone underneath all the noise but I also thought I heard a harmonica playing so I am not hanging my hat on that one.

I will have another go at cleaning the wave change switch and then pull the Mullard Module again.

Paul
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Old 19th Jan 2022, 1:19 pm   #9
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Default Re: Dansette Chorister Repair advice

Possible shorted AM detector diode?
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Old 19th Jan 2022, 3:06 pm   #10
Paul_A_R
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Default Re: Dansette Chorister Repair advice

Thanks. I will check the diode when I pull the Mullard module. I do not think I have any so the radio will slide down the list a bit while I source one.

If the diode has shorted is this an old age thing or might some other fault have caused it.

Paul
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Old 19th Jan 2022, 3:23 pm   #11
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Default Re: Dansette Chorister Repair advice

If the diode has failed then probably just age. I have repaired many a transistor set where there was nothing but crackles and static, and the only issue was a failed detector.
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Old 19th Jan 2022, 3:36 pm   #12
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Default Re: Dansette Chorister Repair advice

I've also known old diodes to break in two - with just a hairline crack across the body - pretty well invisible. A gentle prod will reveal such a break.

Mike
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Old 19th Jan 2022, 5:05 pm   #13
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Default Re: Dansette Chorister Repair advice

Is the diode as shown in the pic attached?


PM if you want a gash Ge diode to try.
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Old 19th Jan 2022, 8:12 pm   #14
Paul_A_R
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Default Re: Dansette Chorister Repair advice

Yes the diode was in circuit.

I have removed it for testing and it does not test short. It is marked OA90. It reads 0.32v on the multimeter diode test which may be a bit high but would that be enough to cause the symptoms?

I have some 1N5819 would these work?

My thinking was if the Schottky should work and produced any functionality in the radio it would confirm a fault with the diode and I would pick up some 0A90 from somewhere like Bowood.

Paul
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Old 19th Jan 2022, 8:34 pm   #15
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Default Re: Dansette Chorister Repair advice

0.32V would seem within tolerance to me, so long of course as if only reads one way.
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Old 19th Jan 2022, 9:31 pm   #16
Paul_A_R
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Default Re: Dansette Chorister Repair advice

Yes it is reading as a diode.

I will replace it.

I will try tacking the module to the radio with wires. I suspect there is more fiddling to be done and that the tabs on the module can will not take repeated bending and unbending.
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