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Vintage Radio (domestic) Domestic vintage radio (wireless) receivers only.

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Old 20th Oct 2020, 4:42 pm   #21
OldTechFan96
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Default Re: HMV 2152 Restoration

I removed the dust cap and noticed what I thought was corrosion around the pole piece. Using a small bit of 400 grit sand paper I rubbed down the inside as much as I could. I'm not sure if this improved things as the coil was still rubbing. After testing the speaker with a small mono amplifier i noticed a bit of wire protruding from the gap between the voice coil and the pole piece. This may be the culprit. The speaker still ohms out OK.

Does anybody know if the speaker can be disassembled futher?
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Old 21st Oct 2020, 8:18 am   #22
crackle
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Default Re: HMV 2152 Restoration

I can't see any rust there, Looks like the yellow anti rust treatment is still there.
These types of speakers with glued on magnets are almost impossible to disassemble successfully.

I would never use any abrasive paper on the magnetic poles anything that is dislodged will be pulled into the gap.

One thing I would mention, have you checked the inner suspension, has it become unstuck through surface rust there, If it has then the speech coil will not be held securely and central in the gap.

I have glued the inner suspension back using PVA, after first centralising the speech coil with 3 shims, and them and placing a weight on the diaphragm to hold the suspension down whilst the glue dries.
Again when I have done this sort of repair I have not scraped the rust off because of the risk it will get into the speech coil gap, so this is not a long lasting repair.
Mike
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Old 21st Oct 2020, 8:31 am   #23
crackle
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Default Re: HMV 2152 Restoration

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Originally Posted by OldTechFan96 View Post
160mV is measured across each speaker terminal.
I am surprised at that, would anyone else agree that 16mA DC leakage is unreasonable for a new electrolytic capacitor.
Have you checked the DC voltage across the C83 blocking capacitor with the speaker connected and no signal going through the amp, and confirmed the meter polarity corresponds to the capacitor polarity.
Not wishing to sound too condescending but it is normal for the negative wire of a electrolytic to be the one that is identified.
Mike
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Old 21st Oct 2020, 10:53 am   #24
Nickthedentist
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Default Re: HMV 2152 Restoration

The only other thing I'd try with the speaker is driving it from a signal generator so that you can hear it at its worst, then flexing the chassis slightly to see if you can find a "sweet spot" where the rubbing is minimised. Tightening the mounting nuts unevenly, or adding shims (washers) can sometimes make things better.
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Old 21st Oct 2020, 12:31 pm   #25
frsimen
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Default Re: HMV 2152 Restoration

The DC voltage measure across the speaker terminals is what I would expect. It is caused by the bootstrap connection formed by R79 and the loudspeaker.

R79 is connected to the base of Q13 which, given the mid rail voltage of 11.5V, will be at a potential of 11.4v, give or take a bit. R79 is 390 ohms, an 8 ohm impedance loudspeaker will have a resistance of about 6 ohms.

V across the loudspeaker = 11.4 x 6/(390+6) = 172mV, which is pretty close to the 172mV measured. That seems to be working correctly and isn't an indication of leakage in C83.

Paula
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Old 22nd Oct 2020, 7:56 am   #26
crackle
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Default Re: HMV 2152 Restoration

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The DC voltage measure across the speaker terminals is what I would expect. It is caused by the bootstrap connection formed by R79 and the loudspeaker.

R79 is connected to the base of Q13 which, given the mid rail voltage of 11.5V, will be at a potential of 11.4v, give or take a bit. R79 is 390 ohms, an 8 ohm impedance loudspeaker will have a resistance of about 6 ohms.

V across the loudspeaker = 11.4 x 6/(390+6) = 172mV, which is pretty close to the 172mV measured. That seems to be working correctly and isn't an indication of leakage in C83.

Paula
Thanks for clarifying that, I missed that resistor.
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