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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

Last edited by crackle; 21st Oct 2020 at 8:37 am.
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Old 21st Oct 2020, 8:31 am   #23
crackle
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Default Re: HMV 2152 Restoration

Quote:
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
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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
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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.

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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.
Mike
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Old 26th Oct 2020, 6:05 pm   #27
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Default Re: HMV 2152 Restoration

Speaker Autopsy:

I've been able to remove the voice coil to try and figure out what is rubbing on the pole piece.

It looks like the adhesive around the rear quarter of the voice coil failed, causing the coil wire to embed itself in the gap between the pole piece and the magnet. I had to pull a lot of wire out.

The 'crackling' sound would have been the dislodged coil wire moving about. A replacement speaker will be needed.

The original speakers have an impedance of 10Ω. For further testing, would it be OK to use 8Ω speakers for the mean time?
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Old 26th Oct 2020, 6:14 pm   #28
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Default Re: HMV 2152 Restoration

Wow that looks like it has been getting hot. I take it the C83 capacitor was very leaky allowing DC to pass through the speech coil.
I would think 8 ohms should be OK, I would change the speakers in both channels.
I think I would also consider replacing the 1 ohm resistors R80 & R81.
Mike
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Old 27th Oct 2020, 8:56 pm   #29
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Default Re: HMV 2152 Restoration

C83, C81 and their LHC counterparts leaked about 2.5mA of current at 18VDC. The capacitance of all the capacitors had more than doubled. It will be prudent to change all of the Callins electrolytics and any high stability resistors.

I've attached a pair of 3" 8Ω speakers and things sound good. The stereo FM decoder is working fine, although the 36V indicator lamp has burnt out.

Once I replace all of the electrolytics, I will do more testing.
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Old 30th Oct 2020, 9:30 pm   #30
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Default Re: HMV 2152 Restoration

I've repaired the aerial and reattached it to the circuit. Now MW and LW are working correctly. LW is a bit poor, probably needs the coil readjusting. Locktite super glue was used to bond the three bits of ferrite together.

Would anybody be able to suggest a replacement pair of 5" 8ohm full range speakers? I don't know how well this 7W P/C amplifier would drive modern speakers rated for huge amount of watts.

I have a list of capacitors and resistors to buy. I will add in R114 and R115 as suggested earlier in the thread. They are described as 'output transistor protection loads'. Seems odd to delete them from the circuit?

The teak cabinet has responded very well to some elbow grease with TCut and wax polish. I've used some brown wood filler to fill some missing veneer and wood worm holes.
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Old 1st Nov 2020, 8:18 pm   #31
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Default Re: HMV 2152 Restoration

I've noticed that the only excessive current draw protection the HMV has is the 3A fuse in the plug!

Would it be worth adding a fuse to the power supply, say 250mA?

The bridge rectifier, although working correctly, does get a bit warm. Would a modern replacement be worth while?

When did FM stereo start broadcasting in the UK? I can't find an exact date online.
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Old 1st Nov 2020, 9:13 pm   #32
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Default Re: HMV 2152 Restoration

Hi, a chassis fuse is always a good idea on any restored set, possibly 2. One on the pri side and the other in the HT feed.
The older bridge rects did run a bit warm and were often chassis (contact) cooled. They can stink when they die. Replace with a modern Si bridge and about 200R 10W in series to get the Ht voltage correct


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Old 2nd Nov 2020, 4:00 pm   #33
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Default Re: HMV 2152 Restoration

Quote:
Originally Posted by OldTechFan96 View Post
I've noticed that the only excessive current draw protection the HMV has is the 3A fuse in the plug! Would it be worth adding a fuse to the power supply, say 250mA?
In my post (#12), the attached Word document includes this text:


The final incarnation of this chassis looks to be fitted in the Ferguson Unit Audio model 3409.

A 500mA(T) fuse is fitted in the mains feed from the on-off switch, to T1.
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Old 3rd Dec 2020, 7:36 pm   #34
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Default Re: HMV 2152 Restoration

I've replaced all of the leaky Callins electrolytic capacitors and changed six high tolerance resistors.

I've added two 100ohm 'output transistor protection load' resistors which were omitted on my example. As a result, the audio output is much lower than it should be. They will be removed.
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