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Old 17th Sep 2022, 1:08 pm   #1
Michael Maurice
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Default Decca SRG500

There is a problem with FM in that its totally quiet, MW and LW are all ok.

I've replaced the ECC85 with a NOS but thats not made any difference.

As MW and LW are working, can I assume the problem is around the EB91 valve?

I know that there is audio from pin the junction of R10 and R12, there is around -1.3V across C33 (I think the diagram shows C33's polarity incorrectly)

But that is it. I'm sceptical of it being the EB91 valve, could it be the valve or is it more likely something else?
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Old 17th Sep 2022, 1:40 pm   #2
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Default Re: Decca SRG500

I would inject a signal 10.7Mhz into the last IF amp, it should produce a voltage across C33 ,ref maker manual.
BTW the manual appears to have C33 connected the wrong way round.
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Old 17th Sep 2022, 5:39 pm   #3
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Default Re: Decca SRG500

Except I dont have a signal generator, well not one at RF/IF
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Old 17th Sep 2022, 6:34 pm   #4
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Default Re: Decca SRG500

There are quite a few things that can cause this problem. I you say that you can tune in VHF stations albeit with low volume.

The EB91 could be low emission. If you don't have one to hand, you could temporarily substitute silicon diodes, such as the 1N4148 to prove the point. If the audio at the junction of R10/R12 is good, but it is much lower at the junction of R12/C32, C32 could be leaky. Beyond that, you could be looking at a faulty capacitor in one of the IF transformers or an alignment issue. If that's the case, it will be difficult to track down without a signal generator of some sort.

If you have an audio generator with a square wave output, that will produce harmonics which go to well beyond 10MHz. You can use that as a signal source for basic fault finding around the detector. Couple into the radio via a low value (say 10-100pF) capacitor. You will hear be able to hear the signal as you tune the audio generator. Choose a frequency on the signal generator which gives a reasonable signal level on the radio. The exact frequency to use will rather depend on the frequency ranges on the signal generator. Use the highest frequency range it offers and you are sure to find a suitable harmonic. If the maximum frequency is 1MHz, 973kHz, using the 11th harmonic at 10.7MHz, would be a good starting point. It's not a perfect arrangement, but it will work.

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Old 17th Sep 2022, 7:37 pm   #5
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Default Re: Decca SRG500

By totally quiet I assumed no audio output, is that correct?
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Old 17th Sep 2022, 7:41 pm   #6
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Default Re: Decca SRG500

Quote:
Originally Posted by Nuvistor View Post
By totally quiet I assumed no audio output, is that correct?
Yes, no hiss, no nothing
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Old 17th Sep 2022, 7:55 pm   #7
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Default Re: Decca SRG500

Can you feed an AF signal in at L17, also pins 1 and 2 of the EF91, that would prove if those coils are not O/C.
Perhaps you have already tried this.
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Old 17th Sep 2022, 8:39 pm   #8
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Default Re: Decca SRG500

I misunderstood your first post. Now I know that there is no reception at all, I suggest you check that the local oscillator is running.

It will either be at 10.7MHz above or below the frequency radio is tuned to. Use another receiver to tune to the appropriate frequency. e.g. Decca tuned to 88MHz, the second receiver should hear the oscillator on 98.7MHz if the Decca's oscillator is running above the received frequency. If you don't hear anything, try the opposite combination, with the Decca on 98.7 and the other receiver on 88MHz.

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Old 24th Sep 2022, 2:54 pm   #9
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Default

Quote:
Originally Posted by Nuvistor View Post
Can you feed an AF signal in at L17, also pins 1 and 2 of the EF91, that would prove if those coils are not O/C.
Perhaps you have already tried this.
The coil is not o/c, putting a signal from a generator on either pins gives audio out. That would appear to exonerate the EB91.

I still dont have a rf signal generator.

The wiring from the output of the tuner to the switch is ok, but trying to put a signal admittedly from an audio generator results in click noises from the speakers.
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Old 26th Sep 2022, 1:40 pm   #10
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Default Re: Decca SRG500

No it doesn’t exonerate the EB91 as far as I can see, only the coil windings.

Have you a wideband scope that can display 10.7Mhz, if so tune through the band looking for a 10.7Mhz signal on the IF amplifiers anodes.

Has Paula suggests look for the LO on another receiver.
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Old 2nd Oct 2022, 6:21 pm   #11
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Default Re: Decca SRG500

Michael brought the SRG500 over for me to look at the 'no FM' problem.

Injecting 10.7MHz FM into the ECH81 first IF amplifier grid produced a strong tone from the speakers, proving that the IF chain and FM detector were working. A few basic checks were done to see the problem could be pin-pointed before diving in to the tuner

With the ECC85 tuner valve removed, full HT was found on the two anode pins.

With a short 30AWG wire-wrap wire hooked round pin 7 of the tuner ECC85, an AVO8 showed a negative voltage on the mixer/oscillator grid. This indicated that the oscillator was running.

The tuner was removed by taking off the front panel, removing the tuner drum after taping the drive cord in place, unplugging the 5 pin plug and unsoldering the IF output co-ax cable and the aerial ribbon cable. Three bolts fixing the tuner (one was siezed) were unscrewed, releasing the tuner.

Inspection of the inside of the FM tuner, after removing the two end-plates showed that the secondary of the IF transformer was open-circuit - in fact it was disconnected from its feed-through bushing in the tuner case. The wire was re-connected and the tuner replaced in the chassis. The set then worked well on FM, producing strong output and 28V DC across the detector stabilising capacitor with a simple ribbon cable aerial.

I was extremely lucky to find a simple fault on this tuner, as the internal construction was unusual (to me at least), with various unidentified items fitted in slots in a pcb, and a cam moving a cantilevered component for tuning. The entire tuner would have needed to be dismantled to access the parts around the valve base, because of the wrap-around aluminium case and the way the pcb was mounted in it.

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Old 2nd Oct 2022, 7:16 pm   #12
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Default Re: Decca SRG500

That’s good news, glad it’s working.
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Old 3rd Oct 2022, 10:45 am   #13
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Default Re: Decca SRG500

Unidentified items? Possibly leadless disc capacitors? Dropping them into a slot was the usual way of connecting them back in the days before SMDs proper arrived.
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Old 3rd Oct 2022, 3:08 pm   #14
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Default Re: Decca SRG500

I'm sure you are correct, Chris. Thanks for the explanation.

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Old 3rd Oct 2022, 3:13 pm   #15
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Default Re: Decca SRG500

Yes, they're leadless ceramics.

And the cam arrangement for the FM tuner, it's 'unusual'. The cam moves a metal flap closer to two etched-onto-the-PCB spiral coils...

[I gutted the chassis to one of these a few years back... see https://vintage-radio.net/forum/showthread.php?t=100804 ]
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Old 4th Oct 2022, 9:42 pm   #16
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Default Re: Decca SRG500

This is the text from the Radio Components Show article from Wireless World page 216 of the May 1959 edition, identified by Synchrodyne in Tanuki's thread link above. It describes the tuner exactly as used in the Decca SRG500, made by Cyldon.

"An interesting f.m. tuner introduced by Cyldon is based on a printed circuit which includes the coils, and these are tuned by an eddy-current system. Rotation of the control shaft operates a cam which uses small metal plates to approach, or move away from, the printed coils - the cam being designed to give a linear tuning scale. A single ECC85 double triode is used, one half acting as an r.f. amplifier and the other as an oscillator/mixer. The gain is 60dB at 87Mc/sor 57dB at 101Mc/s. The printed circuit is made from glass fibre and the oscillator drift is 35kc/s for a 15°C temperature rise."

Can anyone explain how varying the eddy-currents in a tuned circuit can cause the resonant frequency to change? My limited understanding of eddy-currents is that they cause losses, dissipated as heat. What am I missing?

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Old 5th Oct 2022, 10:47 am   #17
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Default Re: Decca SRG500

Eaglesfield on eddy current tuning, Wireless Engineer, May 1942, mag. page 202:

https://worldradiohistory.com/UK/Exp...er-1942-05.pdf

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