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Old 14th May 2021, 11:47 am   #1
rojosuiza
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Default R746 Ultra Carnival Twin - does not sing

I am trying to get my R746 to sing again, but it doesn't want to sing.

My radio does oscillate, the amplifier works fine from the volume pot to the speaker, but no reception. it is quiet, except at one spot on mediumwave, where it makes a terrible screech!

This was already so at stage one, trying it out for the first time on batteries. Since then I changed the Hunts capacitors hoping this would fix the problem. No luck. I tried new valves DF91 and DAF91, no luck either.

Since my predecessors have worked extensively on this radio I am looking for photos of the underside of the chassis, in the original state, also the connections to the antenna-coils on the back cover.

As far as documentation is concerned I have only a simple schematic (without voltages) and no wiring diagram, exploded view, etc. That information would be very helpful.

The radio is a beauty to look at, but looks are not everything, right?
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Old 14th May 2021, 1:56 pm   #2
Gabe001
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Default Re: R746 Ultra Carnival Twin - does not sing

I've got a good hi-rez picture of the 766 under-chassis -partially recapped but otherwise original.

I'm sure others will be able to provide you with what you need, but if you think this would help you pm me your email address.

Mine needed 2 valves, some resistors had gone high and the usual capacitors replaced.

Gabriel
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Old 15th May 2021, 6:15 pm   #3
rojosuiza
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Default Re: R746 Ultra Carnival Twin - does not sing

I have now got a picture of the chassis of the R766, and also one of the R656. The R656 is nearer to my radio than the R766, but even so, there are a lot of differences. I will have to wait for another R746 to come along, I am afraid.
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Old 15th May 2021, 6:36 pm   #4
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Default Re: R746 Ultra Carnival Twin - does not sing

Hi Rojosuiza

I would check the HT voltage which would be 90V and if the heaters are series / parallel this would be probably 7.5V but it may want the I F's re-aligned, also has the smoothing cap been replaced and any other electrolytic ones

Ken
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Old 15th May 2021, 6:51 pm   #5
rojosuiza
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Default Re: R746 Ultra Carnival Twin - does not sing

LT and HT are okay, Hunts and TCC caps are changed, radio oscillates and amplifier runs.

Resistors are spot on, except one; if I take that one out and exchange it, however, the radio behaves the same.
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Old 15th May 2021, 6:55 pm   #6
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Default Re: R746 Ultra Carnival Twin - does not sing

Have you tried injecting a signal into the IF strip to see whether that's working?
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Old 15th May 2021, 7:03 pm   #7
rojosuiza
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Default Re: R746 Ultra Carnival Twin - does not sing

I have nothing here to inject a HF signal
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Old 15th May 2021, 7:09 pm   #8
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Default Re: R746 Ultra Carnival Twin - does not sing

You say the set oscillates. By that do you mean that the Local Oscillator is running? If so is it running at the correct frequency?

You say that at one tuning point the set makes a terrible screech. Is it possible that the vanes of the tuning capacitor's RF section are touching?
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Old 15th May 2021, 7:24 pm   #9
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Default Re: R746 Ultra Carnival Twin - does not sing

the tuning capacitor is o.k. The oscillation I can observe in a radio next to the R746, it produces whistles at about the same spots an third radio will produce them.

I do have some doubts about the antenna arrangement. Those leads have been removed several times, it seems, and the color scheme is different to that of other models, even the similair R656.
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Old 15th May 2021, 8:28 pm   #10
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Default Re: R746 Ultra Carnival Twin - does not sing

The service data for the R746 can be found here https://www.service-data.com/section...-carnival-twin. This helps fund the forum. The service sheet is from the manufacturer and shows the underside layout as well as the anode and screen voltages.
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Old 16th May 2021, 5:50 am   #11
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Default Re: R746 Ultra Carnival Twin - does not sing

Thank you, that seems the way to proceed.
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Old 17th May 2021, 5:11 pm   #12
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Default Re: R746 Ultra Carnival Twin - does not sing

I have two of this model I have worked long and hard on but I also have been unable to make them sing so they are back on the to do list for now.
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Old 18th May 2021, 6:51 am   #13
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Default Re: R746 Ultra Carnival Twin - does not sing

Ah, dear clockman, the to do list threatens here too... However, I put the antenna coils in direct physical contact with the output of my ITAX active antenna. With that help there is weak reception on longwave, i can get in BBC4 and Luxemburg. On Mediumwave there is just the terrible screech near the higher end of de band.
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Old 18th May 2021, 11:08 am   #14
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Default Re: R746 Ultra Carnival Twin - does not sing

First thing, looking at your pictures of the aerial assembly it looks like the aerial wire is missing on the green tag have you checked continuity? The other thing I would check is the continuity of the wave change switch contacts. As the radio has had previous work done I would presume it was left in working order, so the only other thing I would suggest is cold continuity resistance tests on all of the coils in the aerial circuit.

John
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Old 18th May 2021, 6:41 pm   #15
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Default Re: R746 Ultra Carnival Twin - does not sing

Aerial assembly seems to be correct after all, the measurement look good. Reception with the ITAX antenna is now possible on LW and on MW, but only if everything is perfectly adjusted. The sound the radio produces is still weak. When not perfectly adjusted LW is completely quiet, MW as well, except for de squeal.
The high frequency amplification is somehow missing, no idea why... Without the ITAX there would be no audible reception.
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Old 18th May 2021, 6:55 pm   #16
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Default Re: R746 Ultra Carnival Twin - does not sing

I think you're going to have to get yourself a signal generator and check out the IF stages.
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Old 18th May 2021, 7:29 pm   #17
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Default Re: R746 Ultra Carnival Twin - does not sing

Were I wise I would...
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