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Old 29th Aug 2020, 7:18 pm   #1
Gabe001
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Smile Ultra twin R766, few questions

Hello,

I've spent the best part of the day today bringing a lovely ultra twin radio to life.

It is now working well with good reception on both bands, good volume, no hum or any other pathology obvious to me.

Electrolytics were all fine, including the 12v ones, and they don't appear to be getting hot. I've replaced all the waxies, the caps that look like resistors (took me a while to work that out) and one of the hunts capacitors. 2 resistors were out of the 10% range and were replaced. The high value resistors were fine, measured using the resistor in parallel trick learnt from here

So my questions are:

1. Should I proactively replace the electrolytic caps to future proof the set, or is it a case of "if it ain't broke don't fix it"?

2. There are 2 hunts caps in the Pico range (100pf and 500pf, C3 and C6). You can see them in the picture on the left. Are these an obligatory replace on sight or can they be left if set works ok?

Finally, I'm attempting a sympathetic Bluetooth integration by using a tiny encapsulated transformer (240v to 9v ac) followed by half wave rectification, smoothing and a 5v regulator. The signal out from Bluetooth goes to the volume control. It works really well on preliminary testing, but the signal ground and power ground are commoned on the Bluetooth module. Hence, do I need to be concerned if someone reverses L and N on the plug and the case carries 240v?

I tried using a 10nf type y cap initially on the earth signal line but it creates terrible hum, so although it appears on the prototyping board it's not part of the circuit anymore.

Many thanks for for help
Gabriel
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Old 29th Aug 2020, 7:41 pm   #2
cathoderay57
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Default Re: Ultra twin r766, few questions

Hi Gabriel, re: additional caps replacement, no, quit while you're ahead!
Re: Bluetooth power supply, providing neither the encapsulated transformer shield/frame nor the negative supply output are connected to mains earth you should be OK. In other words, the radio and the Bluetooth supply transformer must both be powered from a 2-wire mains supply. Neither will care if the chassis is at mains live or mains neutral since the power supply negative and receiver chassis are both at the same potential. Different story of course if you are plugging in external earthed audio sources from wires but, as I understand it, the Bluetooth adapter connects via wires to the audio input and receives its audio signals on-air? That being the case you should be OK. Clearly, under normal circumstances, the receiver chassis must always be wired to mains neutral. If you are selling it on then I would recommend putting a warning notice to that effect somewhere prominent. Cheers, Jerry
P.S. Gosh, my 1000th post...........

Last edited by cathoderay57; 29th Aug 2020 at 7:47 pm.
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Old 29th Aug 2020, 7:54 pm   #3
Gabe001
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Default Re: Ultra twin r766, few questions

Thanks Jerry. A work colleague has got her eye on this one so I'll be around for emergency repairs!
I'll use a mains cable with a moulded plug just to be sure.
Thanks for your help, as always.

Last edited by Gabe001; 29th Aug 2020 at 8:03 pm.
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Old 30th Aug 2020, 6:17 pm   #4
Gabe001
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Default Re: Ultra twin R766, few questions

Jerry I'm afraid I spoke too soon. I continued testing today and there was a continuous oscillation/motorboating type sound present constantly. It was quite faint but audible. This was not present when i used the lamp limiter which dropped the voltage by around 15v.

This prompted me to re check the voltages on the valve pins and whilst HT was relatively good (89v instead of 85v) the voltage on pin 3 of V2 was 72 instead of 55. Similarly the voltages on pins 2 and 3 of V1 were too high. LT was spot on.

There was a 20v drop across R10 which equates to a current draw is 3mA for a 6.8k resistor. This was dropping the voltage to around 70v instead of 55v. The resistance value is spot on according to the trader sheet (infact it was 8k initially and I replaced it with 6.8k so I may have inadvertently caused the problem myself).

I replaced this resistor with a 12k one and am now getting appropriate voltages (56) and the oscillation/motorboating stopped.

I am posting the relevant part of the service sheet for reference.

Any insight on why the problem occurred? Is it because the valves are a bit tired and therefore the valve current draw is about half what it should be? The filament voltage is fine.

Thanks again
Gabriel
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Last edited by Gabe001; 30th Aug 2020 at 6:46 pm.
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Old 30th Aug 2020, 7:13 pm   #5
cathoderay57
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Default Re: Ultra twin R766, few questions

Hi Gabriel, motorboating is caused by positive feedback typically via the HT line or the AGC line. Unfortunately, the Ultra Service Sheet does not give valve currents for comparison. Comparing the Ultra Sheet with Trader Sheet #1100 for the R786, there are quite a lot of circuit differences around the frequency changer, including the use of a 1C2 frequency changer whereas yours uses a 1C1, therefore the total current drain through your R10 cannot be compared directly. I would guess that 3mA is reasonable, possibly a little bit low owing to reduced emission of one or more of the valves. My best guess is that C9 (2uF, 150v) is leaky and maybe has lost capacity, hence compromising decoupling between V3, V2 and V1. So change C9 and restore R10 to its intended value. It sounds like you've already replaced C12 (0.04uF) on the AGC line but if not try that as second culprit. Hope that helps. If not I'm sure you will let us know! Cheers, Jerry P.S. The only explanation I can think of for the higher than expected anode and screen voltages on V1 is a low emission valve. I have a DK91 (direct equivalent) in my spares box but I don't know if it is any good so don't get your hopes up! I'll test it either later tonight or tomorrow.

Last edited by cathoderay57; 30th Aug 2020 at 7:28 pm.
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Old 30th Aug 2020, 7:46 pm   #6
Gabe001
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Default Re: Ultra twin R766, few questions

Thanks jerry. I'll give it a go tomorrow and report back
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Old 30th Aug 2020, 9:56 pm   #7
cathoderay57
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Default Re: Ultra twin R766, few questions

Hi Gabriel, sorry to report my DK91 had an o/c filament so is now in the bin. However, there are plenty about inc. supposedly new old stock on eBay. Direct equivalents are 1C1, DK91, 1R5, CV782, X17. Cheers, Jerry
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Old 31st Aug 2020, 7:20 am   #8
Gabe001
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Default Re: Ultra twin R766, few questions

Thanks for checking Jerry

As per my post above, the problem with the sound is cured by applying the correct screen/anode voltages on v1/V2 with the higher value resistor. I'll see if I can post videos later to show you.

Given HT is 89v, from my interpretation of the circuit, the main determinants of the 55v at v1/v2 are c9, r10 and the valves themselves (v1,V2 or even V3). I'll replace c9 today (although on testing yesterday there was no voltage on the negative end on c9) and see whether that helps

My money is however on low emission valves. I need to find an extra 2mA from somewhere, which would lead to an appropriate drop at R10 and some global HT drop, and bring the voltages in line. From what I've read low emission valves are common if this set has been driven from mains.

As either of V1 V2 or even V3 can be culprits (or all of them), I suspect I'll either need to get them tested, or purchase and replace them in turn. I've had a bad run with valves lately.

Gabriel
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Old 31st Aug 2020, 9:04 am   #9
cathoderay57
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Default Re: Ultra twin R766, few questions

Hi Gabriel, yes, I agree. I've had another dig in the spares box and I have got a used DF91 (=1F3) and DAF91 (=1FD9). The DF91 tests excellent (Ia=5.4mA (3.5); gm=0.9mA/v (0.9). However the DAF91 pentode section is very poor (Ia=0.9mA (2.0); gm=0mA/v (0.7) although the diode section is good (82%)). You have a PM. Also, if you are willing to trust the postman, and you want to send me any of the valves, I can test them on my VCM163 for cost of postage. Cheers, Jerry
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Old 31st Aug 2020, 9:57 am   #10
Gabe001
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Default Re: Ultra twin R766, few questions

Thanks Jerry. Ive pm'd you and will post on the wanted section for v1 and a good v3
Thanks for your kind offer of testing the valves. I'd be interested to know how bad they were so I'll drop them in the post once I've got the the radio up and running correctly, hopefully next weekend.
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Old 4th Sep 2020, 9:59 pm   #11
Gabe001
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Default Re: Ultra twin R766, few questions

Final update:

Put the valves in place today and restored resistor R10 to original value.

So new x17 (v1) df91 (V2) and currently Jerry I'm using your very poor DAF91

Voltages much better, down to 61 after resistor (should be 58) so I have no doubt that with a better daf91 it will be spot on. Put everything back together and sound is great!

Thank you Peter and Jerry for the valves.

Ps: getting stations in foreign languages on medium wave, it's a first for me and I'm really impressed!

Gabriel
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Old 9th Sep 2020, 6:09 pm   #12
Gabe001
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Default Re: Ultra twin R766, few questions

DAF91 replaced with a tested good one today, which completes this project.

Performance is pretty good with the internal aerial only. Video below for your interest:
https://youtu.be/ko1RE5IX-Jo

I'm really pleased with how this has turned out. Many thanks to everyone who advised.

Gabriel
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