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Old 6th May 2019, 9:22 am   #1
squinancy
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Default Vidor CN429 Lady Margaret rattling sound

Hello Everyone,

I don't know how to put it in words, maybe I should upload an MP3 recording. My Lady Margaret has been working well for years. It underwent an o/p transformer replacement. All iffy caps changed, LT & HT batteries supplying the right current and voltage.

But, at slightly higher volumes (not yet full) and certain sound frequencies - perhaps the frequency range of an average female voice - the speaker begins to resonate and produce a sound that is best described as screws loudly rattling at the bottom of a tin bucket. As if something was loose, torn or broken in or around the speaker.
Only affects mid-range sounds, bass and high treble OK.

There is nothing wrong with the speaker by the look of it, the paper cone is intact, healthy, solid to touch. I wonder what this could be. Any suggestions, please? Changing the speaker is quite a hassle in a CN429.
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Old 6th May 2019, 10:28 am   #2
Mike. Watterson
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Default Re: Vidor CN429 Lady Margaret rattling sound

Try a separate speaker first.
Also check the main HT electrolytic. I forget if it's 4uF or 2uF. I've had strange issues because of that drying out.

I've a couple of CN429 as well as a few sets with a very similar chassis. Not heard what you describe though distortion or motorboating if that HT cap is dried out.

Also check the grid bias resistor between 0V and the -HT that gives the -ve bias for the DL96. The value can be +/- 20% without much change. If it's too high you'll get clipping (class B without the other channel) and if too low, there will be excessive DL96 anode current and transformer saturation then causes distortion.

I've used the mains transformer from cheap Argos clock radios etc as audio transformers for the DL91 / DL92 / DL96 series. The DL94 are often x2 output power so often the sets had a less fragile, larger transformer.
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Old 6th May 2019, 4:09 pm   #3
squinancy
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Default Re: Vidor CN429 Lady Margaret rattling sound

Thanks for all the above.
I checked service sheet 1153: the HT decoupling is 2uF.
The HT cap in this set is 2.2uF (400v) which I installed about 12 years ago. I don't think it has dried up.

However, I connected a different speaker. Being a small collector I don't have that many suitable ones readily available. I used a small 4Ω 5W speaker and none of the distortions I mentioned could be heard.

I listened to the "distortion" on the original speaker a bit longer and I can say that it only happens in the lower mid-range. General instrumental music, for instance, is always clear (if a bit flat, which is normal), free from this sound. That solo male tenor or low female voice produces it.

Further, I think that even when this failure of sound occurs, the sound that's meant to be produced is produced flawlessly, but an added high pitch rattling sound, as if a handful of screws (or broken glass) started to dance in or around the speaker, is also heard. When the volume is turned well down - but still audible, the added sound disappears, the same voice, same frequency is heard clearly, without interference.

Does this mean that the speaker is definitely responsible? If so, what kind of a flaw is this? Any hope of repair?

If beyond repair, how easily can a CN429 replacement speaker be obtained? This one says "Type MB5MO, Spec 2864" on the plate behind the speech coil. I googled these codes but not a single hit.
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Old 6th May 2019, 5:16 pm   #4
Simon Gittins
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Default Re: Vidor CN429 Lady Margaret rattling sound

It sounds likely that there is something sitting on the loudspeaker cone, rust for example, and it might be easily cleared with the speaker removed. Another test you could try is with the fault present, hold the set upside down and see if gravity reduces the distortion.

One way of extracting the speaker would obviously be to remove the chassis first (which is quite involved). If you did this you would want to test the speaker with another source before putting it all back. Looking at my example, two of the speaker nuts are easily accessible and the third isn't (I think there are only three). You might be able to get the third nut out by undoing the hinges and then using pliers but I have never tried this!

It's the same details on my speaker.

Last edited by Simon Gittins; 6th May 2019 at 5:24 pm. Reason: Clarification.
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Old 6th May 2019, 5:36 pm   #5
TonyDuell
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Default Re: Vidor CN429 Lady Margaret rattling sound

Having read the procedure to remove the chassis in the Trader Sheet, I think you don't need to do all that to reach the speaker nut.

I suspect (without doing it) you could:

Pull off the volume and tuning knobs. Be careful, these turn very brittle with age and you can easily break off the boss that goes onto the spindle.

Remove the 4 nuts (4BA, 1/4" nutdriver is near enough) that retain the chassis to the back of the panel.

Lift the chassis aside to get to the speaker nut. Do not lose the spacers on the chassis fixing studs.
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Old 6th May 2019, 7:49 pm   #6
Mike. Watterson
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Default Re: Vidor CN429 Lady Margaret rattling sound

There is nothing special about the speaker. Any that physically fits will likely do well enough, but it could be something trapped there. Since it sounds OK on an alternate speaker it's the most likely culprit.

I agree with Simon. Shake set hard upside down (with valves out) then operate upside down and see?


400V! It's only a 90V battery!
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Old 6th May 2019, 10:39 pm   #7
Herald1360
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Default Re: Vidor CN429 Lady Margaret rattling sound

Easier to find than a 2.2u 160V I expect.
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Old 7th May 2019, 4:19 pm   #8
squinancy
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Default Re: Vidor CN429 Lady Margaret rattling sound

And Simon was right!

I did not shake up the set. Just carefully turned it upside down at full volume when the rattle was loudest on a sharp medium wave station. ---->>>
Lo and behold, the whole noise racket stopped right when the set was totally flipped around. (No change at 90 degrees turn.) The speech sound came across so clear that I hadn't known this set to be able produce.

My guess is that they are flakey rust particles sitting on the cone because at one point I removed rust from the metal chassis.
This is a great relief because the problem is only cosmetic at the end of the day. The set is electrically sound. I will try to remove the particles, but not as a matter of urgency.

And..
yeah, I stacked up on high voltage caps when I was told that their time was up. Ordered them from the States about 15 years ago. I have a few 1-2000v ones that are still healthy. Can't do any harm I guess.

Thanks for all the good advice.

P.s.:
If anyone's interested, the fender coupling transformer (4Ω-14.5kΩ) capable of replacing the o/p transformer in the CN429 available from WES Components (recommended in an another thread)
https://www.wes.com.au/ConfirmMessag...no-back-button

Order Code NSC922921
See catalogue: https://www.wes.com.au/mediapub/eboo...p/1383/#zoom=z

costs $30 AUD + shipping but individual (non-corporate) buyers can only order from this site:
https://www.wagneronline.com.au/Login.aspx
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