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Old 18th May 2017, 4:35 pm   #281
ms660
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Default Re: Restore a Beau Decca

Might be worth measuring the values of the resistors while your in there....R4, R5, R6 & R8.

Lawrence.
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Old 18th May 2017, 4:37 pm   #282
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Default Re: Restore a Beau Decca

Check continuity of the MW coil (both sides)
Check the values of R6 and R5.
Check the vanes of the tuning capacitor are not shorting out and check earth continuity to the shaft.

In you first set of readings in post 262, the triode anode voltage was much higher suggesting it was oscillating. Can you connect one end of your DMM to the cathode and the other through a 100K series resistor to the triode grid. Then try switching between each waveband with the tuning capacitor fully open then fully closed. We are hoping to see a -ve voltage!

Last edited by PJL; 18th May 2017 at 4:43 pm.
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Old 20th May 2017, 9:38 am   #283
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Default Re: Restore a Beau Decca

Thanks ms660 and PJL. Having checked both silver micra caps are not shor circuit,i have re-connected them and not yet replaced them. Here are the results of the checks you suggested.

Resistor values (measured in situ with set off):
R4 789 ohms
R5 55.8Kohms
R6 124.9 ohms
R8 60.0Kohms

Check continuity of the MW coil (both sides). Continuity on both coils. Resistance of one is 10.0ohms, the other is 1.1 ohms

Check the vanes of the tuning capacitor are not shorting out and check earth continuity to the shaft.- I have checked that there is no continuity between A and B, and C and D (see photo) and that the shaft is grounded- I think this is what you meant?

Can you connect one end of your DMM to the cathode and the other through a 100K series resistor to the triode grid. Then try switching between each waveband with the tuning capacitor fully open then fully closed. We are hoping to see a -ve voltage! -With -ve MMR lead on Cathode (Pin8) and +ve on pin 5 through 100K resistor the following measurement are shown:
Flick to LW- a reading between -0.2 to -0.7 shows as you tue along the dial. Switching to MW a reading of -1.9V raising to -2.4V is shown. I did several switchovers between LW and MW moving dial between each time. No stations were tuned into but the variable noise of tuning was heard.
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Old 20th May 2017, 10:11 am   #284
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Default Re: Restore a Beau Decca

The oscillator is doing something then.

Do you get clicks when switching between channels?
Does the radio volume change the level of the tuning noises?
Do you get hum if you touch an insulated screwdriver on the volume control connections?
Have you got a suitable aerial wire connected?
What happens if you touch the aerial wire on the ECH35 top cap?

It would be worth measuring the anode and cathode voltages of V3 and the AGC voltage on C18.

After that you will need a signal generator.
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Old 23rd May 2017, 11:22 am   #285
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Default Re: Restore a Beau Decca

Thanks PJL

In response to your questions:
Do you get clicks when switching between channels? Yes there is a noticeable click when selecting from L to MW (but not the other way) Also no click either way from SW.
Does the radio volume change the level of the tuning noises? Yes. The volume control affects the tuning noise.
Do you get hum if you touch an insulated screwdriver on the volume control connections? Yes- is you use a mains tester screwdriver and touch the volume connection, you get a loudish hum when you touch the screwdriver earth/thumb point.
Have you got a suitable aerial wire connected? I believe so (see photo). There are 2 sockets labelled A and E (areal and earth?). I have a plug connected to a short insulated copper wire soldered to a long length of fine copper 'transformer coil' wire.
What happens if you touch the aerial wire on the ECH35 top cap? The tuning noise gets about twice as loud when you touch the aerial wire on the cap- but still can't find any stations

It would be worth measuring the anode and cathode voltages of V3 and the AGC voltage on C18. Access to V3 is covered by a plate and quite difficult. However I believe Anode is 206V and K is 3.63V. Voltage on top of C18 is 0.87V.
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Old 24th May 2017, 4:22 pm   #286
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Default Re: Restore a Beau Decca

You can try detecting the local oscillator on another MW radio. It should be picked up 465kHz below the dial position and will block out the background hiss as it is not modulated so there is no sound.

I would get out a signal generator and scope at this point!
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Old 26th May 2017, 7:46 pm   #287
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Default Re: Restore a Beau Decca

Thanks PJL
I've bought a little PC based oscilloscope, so when it arrives will try to have a look at the oscillator.
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Old 5th Jun 2017, 7:53 pm   #288
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Default Re: Restore a Beau Decca

Thanks PJL. My scope arrived yesterday - just a cheap USB one (Hantek 6022BE).

I attached the probe to pin 5 of V1 which is the grid of the triode (local oscillator?) to see if a wave with frequency around 500MHz from the tuned freq could be found by the scope. However- as you see from the 2 screenshots- nothing sensible is being detected by the scope- so I guess there is no local oscillator freq being generated. Is there another check I can do to be sure?

Some possible good news was that, with the probe connected, I did manage to pick up a strong local station- fairly quiet but definitely there. This station disappeared as soon as the probe is removed though.

ANother bit of good news is that I did check an old radio unit that was sitting in the garage and found it contained an ECH35 valve. However substituting it does not get the oscillator working as far as I can see.
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Old 5th Jun 2017, 8:24 pm   #289
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Default Re: Restore a Beau Decca

Try connecting to the triode anode (pin 6) but use a series capacitor. Does the probe have a x10 setting as this will put less capacitive load on the circuit.
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Old 6th Jun 2017, 7:28 pm   #290
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Default Re: Restore a Beau Decca

Quote:
Originally Posted by thejazzageuk View Post
to see if a wave with frequency around 500MHz from the tuned freq could be found by the scope.
500MHz? Is that a typo?
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Old 13th Jun 2017, 6:23 pm   #291
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Default Re: Restore a Beau Decca

Thanks PJL (and bluepilot).

I have carried out some preliminary measurements with the scope as you suggested. With a 47nF cap in series onto pin 6 I get the following traces (see photos).

On MW I just see 50Hz mains hum, but switching to LW brings up an oscillation modulated onto the 50Hz (I think). On reducing the timebase to investigate further there appears to be a frequency of around 100KHz. I checked this at both ends of the LW tuning dial, but the variation of frequency is minimal (although amplitude does increase!).
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Old 14th Jun 2017, 5:31 pm   #292
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Default Re: Restore a Beau Decca

I would agree, 100kHz at about 40V PP. Frequency too low but amplitude OK and tuning has only minor affect.

As the DC conditions seem OK, resistors checked and capacitors replaced, I would recommend you first do a very careful check of your work. Too low a frequency could mean you have wired a capacitor to the wrong place.
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Old 15th Jun 2017, 5:36 pm   #293
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Default Re: Restore a Beau Decca

Thanks again PJL. I guess it looks like I may have to start undoing each capacitor replacement to try to return the radio to original condition. However I'd prefer to do this in a logical sequence.

The oscillator key components appear to be (for LW) L11, C7, C8 and C12). And for MW C9 C10, L9 and C12. Of these the only one I've replaced is C12 as the others are pretty in-accessible on the underside of the 'pcb' covering S1-S4. As they are silver micra's I think it best not to disturb them at this stage?

The ones I've replaced are highlighted in blue. I guess a good place to start is the middle block- C4, C5, C54, C15, C53 and C16 unless you can recommend anything better?
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Old 18th Jun 2017, 11:21 am   #294
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Default Re: Restore a Beau Decca

I've now replaced all the original paper caps (see photo). I also checked the resistors etc as I performed each re-fit. All appeared OK within about 10%.
The traces from the triode annode still show much the same picture.
On SW there is a clean-ish 50Hz trace and very little background noise
switch to MW the 50Hz trace is slightly modulated and there is a slight background white noise
switch to LW white noise increases significantly and there is a 100MHz(?) modulation on top of the 50Hz.
Does this mean the local oscillator is functioning on LW at least, but at 100MHz instead of around 450MHz?
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Old 18th Jun 2017, 12:08 pm   #295
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Default Re: Restore a Beau Decca

There's a bit of confusion in your report? MHz instead of kHz?

The oscillator frequency should be 465 kHz (the IF frequency) higher than the frequency the receiver is tuned to, eg: if the receiver is tuned to 200kHz then the frequency of the oscillator should be 665kHz.

To convert wavelength in Metres to frequency in MHz divide 300 by the wavelength, to convert frequency in Mhz to wavelength in Metres then divide 300 by the frequency.

Eg: 1500 Metres to Mhz: 300/1500 = 0.2MHz (200kHz)

Eg: 0.25MHz (250kHz) to Metres: 300/0.25 = 1200 Meters.

Lawrence.

Last edited by ms660; 18th Jun 2017 at 12:29 pm. Reason: correction
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Old 18th Jun 2017, 12:50 pm   #296
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Default Re: Restore a Beau Decca

The frequency of the waveform in the 4th picture looks to be somewhere around 870kHz if I've figured it out right, based on 1us per major division.

Lawrence.
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Old 25th Jun 2017, 8:45 pm   #297
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Default Re: Restore a Beau Decca

thanks ms660. You are quite right- it is kHz not Mhz! I will take some traces of the frequency waveform at both ends of the tuning range to check if oscillator frequency is around 465khz above dial frequency and post my findings soon.

The traces at 10ms/div appear to show the oscillator frequency to be modulated onto a 50Hz carrier wave- is this correct or expected?
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Old 25th Jun 2017, 9:02 pm   #298
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Default Re: Restore a Beau Decca

The 50Hz could be stray pickup by the scope probe or it's coupling arrangement to the circuit, try moving it around a bit with it still connected while still observing the waveform, if the 50Hz varies in amplitude then that would suggest stray pickup.

Lawrence.
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Old 14th Aug 2017, 9:48 am   #299
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Default Re: Restore a Beau Decca

Thanks ms660. I think I will put the radio on hold for a bit and have a think!

Instead I would like some advice on the record deck. I'm not sure if this would be better in the thread 'RC80 advice' but as that thread is closed I posted it here.

I have now acquired an RC80 deck following the advice of TECHMAN way back in post 19.

I would like to use in in place of the RC80M deck I'm currently using. However I want to play stereo records without damaging them so would like to swap the heads (and cartridges) over. The RC80M that I was using is fitted with a much lighter head and cartridge which I'd like to fit to the RC80. However not sure if this is even possible. The arm on the RC80 is shorter and the head is longer- the heads do not appear to be interchangeable. Can anyone advise if there is an adaptor to enable me to swap the heads over- or would I need to swap over the arms as well? I've attached some photos to illustrate what I mean.
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Old 16th Aug 2017, 5:08 pm   #300
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Default Re: Restore a Beau Decca

The earlier, longer Garrard headshell contains a rare BSR TC8H high-output mono cartridge which is not suitable for your DECCA - but - these cartridges are worth a small fortune if still working. The headshell on the left contains what looks like a BSR X5 series stereo-compatible cartridge. If it's a BSR X5M, this will be fine to get you going. If it's a X5H, then read as for TC8H above.
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