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Old 22nd Feb 2016, 7:44 pm   #41
sobell1980
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Default Re: HMV 653 Broadcaster all wave.

My meter reads 0.00 ohms with the leads touched together. The service data specifies that the secondary should test at 0.5 ohms. If you look at the photo, two leads come our either side of the secondary as normal then on one side you have an additional two like they are paired together as well as the one for the speaker connection. I'm afraid Chris I have no means of powering thus up with low voltage ac unless I can use my sig gen and my scope? This set is a real mess. Snipped wires all over and stubs of wire sticking out of tag boards. I'm just wiring back as per circuit diagram. I am finding the Marconi version of the diagram easier to follow. However, as per my previous post one diagram shows an earth connection to the opt and one doesn't. This could explain the extra wires. But why would the secondary need an earth, would it be giving it a ground 0 volt reference?
Thanks for your reply.
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Old 22nd Feb 2016, 10:02 pm   #42
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Default Re: HMV 653 Broadcaster all wave.

I have completely removed the opt from the set to get a better idea and make it easier to test it . Ok, on the secondary there are 4 wires . 2 pairs give readings and checking any other combinations of pairs gives no reading / open circuit. One pair gives a reading of a rock steady 0.7 ohms which I would imagine conects to the speaker as per the secondary should measure in the service data . The other pair of wires measures 0.00 to 0.1 fluctuating. I'm totally baffled. I first though it was to connect an additional speaker . But if you want to do this you connect a speaker to the tag strip at the speaker terminals. Hope someone can shed some light. Many thanks.
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Old 22nd Feb 2016, 10:10 pm   #43
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Default Re: HMV 653 Broadcaster all wave.

The 'paired' wires would appear to be a tapping on the secondary, perhaps to enable a better match when an external speaker is permanently connected, as this reduces the load impedance. The flexible wires shown on the circuit suggest that the best tapping could originally be selected by plug and socket.

I'd suggest using the outer wires and leaving the paired wires free and insulated from earth.
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Old 22nd Feb 2016, 10:14 pm   #44
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Default Re: HMV 653 Broadcaster all wave.

The paired wires are probably a tapping quite near one end of an 0.7 ohm total winding. Your ohmmeter may be getting confused and it shouldn't be fluctuating. Have you got an analogue meter?
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Old 22nd Feb 2016, 10:24 pm   #45
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Default Re: HMV 653 Broadcaster all wave.

Phil, thankyou for your reply. Yes, I have an analogue meter at work. I will take the OPT to work tomorrow and test it. When I say it fluctuates, if I test it once I get 0.00 ohms. Then if I test it again I may get 0.10 ohms. This of course could be how well the crop clips make a connection to the tested wire. Why would there be this extra tapping? Why does one diagram in my earlier post today show an earth connection on the secondary? Many thanks.
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Old 22nd Feb 2016, 10:51 pm   #46
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Default Re: HMV 653 Broadcaster all wave.

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Originally Posted by sobell1980 View Post
I have two wiring diagrams for this set. One is of the marconi, an identical set and one of the HMV. One diagram shows an earth connection to the transformer and the marconi diagram does not. Seeing how this opt was not connected it's not obvious to me how this should be wired to chassis.
Looking at the two schematics it looks like different loudspeakers were fitted, In both cases one side of the speaker is connected to chassis so far as I can make out.

Lawrence.
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Old 22nd Feb 2016, 10:51 pm   #47
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Default Re: HMV 653 Broadcaster all wave.

My guess is to match the reduced 'speaker impedance when an EXT LS is connected. Earthing one side of the EXT LS was probably just a safety measure. Don't assume that the Marconi and HMV were identical in every circuit detail, there may have been subtle differences for marketing purposes - weren't Maroni-badged sets offered at a slightly higher price than the HMV equivalent?
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Old 22nd Feb 2016, 11:10 pm   #48
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Default Re: HMV 653 Broadcaster all wave.

Just a note, the difference in loudspeakers referred to in post#46 is highlighted in the trader sheet under "Internal Speaker"

Lawrence.
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Old 23rd Feb 2016, 12:06 am   #49
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Default Re: HMV 653 Broadcaster all wave.

Are you quite sure this might not be a special NFB winding? Edward
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Old 23rd Feb 2016, 6:36 am   #50
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Default Re: HMV 653 Broadcaster all wave.

C25 is the reservoir cap,V5 cathode (filament, its directly heated) to chassis.
L16 is the speaker energising coil used as a choke for HT.
C24 and C28 in parallel are the smoothing cap. Presumably they had the box cap in stock and just used it, paralleling 2 caps to get the smoother.

So far that is normal. If L16 is OK, just put it back to the original. All the rest is just propaganda.

The diagram does show 2 output windings on the output transformer secondary, in series with the speech coil though there appears to be some alterations to the diagram. Matters little at this stage, put it as the drawing, all in series.
It is normal to connect one side of the speaker to chassis.

Plenty of time to tidy up the loose ends when you have it up and running, won't be long now!
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Old 23rd Feb 2016, 6:47 am   #51
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Default Re: HMV 653 Broadcaster all wave.

I have had a look at the diagram on electrotanya, it has the same alterations as yours on the speaker to transformer connections. Wonder why?

Replace C20 before powering up, its the grid coupling cap to the output valve, yes, THAT one!

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Old 23rd Feb 2016, 8:36 am   #52
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Default Re: HMV 653 Broadcaster all wave.

Those 'wavy lines' on the circuit diagrams - I think they represent flying leads rather than alterations.
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Old 23rd Feb 2016, 9:34 am   #53
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Default Re: HMV 653 Broadcaster all wave.

Good thinking Phil, but one seems to go to the output winding in a very odd place. The connections to the 2 coils seems not to make sense, but as I said, its really unimportant at the moment as long as the speaker is connected to a winding.

Terrible diagram, not the best. Will find out when he gets some power on it.
There is a wavy line across the left hand winding, its not a resistor but a crossing out!

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Old 23rd Feb 2016, 9:36 am   #54
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Default Re: HMV 653 Broadcaster all wave.

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C25 is the reservoir cap,V5 cathode (filament, its directly heated) to chassis
The -ve side of the reservoir capacitor (C25) doesn't connect to chassis, it connects to the back biasing arrangement.

Lawrence.
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Old 23rd Feb 2016, 10:35 am   #55
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Default Re: HMV 653 Broadcaster all wave.

Checked on the Marconi 857 diagram, its completely different!
The reservoir cap goes to ground but it is connected to the other side of the speaker energising coil! Weird.

Correct, Laurence, I was looking at the thumbnail only. It has a -HT rail for bias on the valves. But it should make no difference to the OP putting the +HT back together.

Looking at the 2 diagrams, it will be preferable to use the HMV one, the Marconi one has several differences. Maybe this is why the original repairer confused the issue?
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Old 23rd Feb 2016, 10:55 am   #56
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Default Re: HMV 653 Broadcaster all wave.

Ok, had a good look at the HMV sheet, Broadcaster service man's, July 1938. Its drawn in a different way. Got this from Electrotanya.

C26 is a decoupling cap.
C25 is the bias rail reservoir- -HT
Speaker coil is smoothing choke
C24 & C28 are the smoothing cap

There appears to be no specific +HT reservoir cap, perhaps because the rectifier was rated for a very low value anyway and there are several smoothers and decouplers on the HT rail.

It is a somewhat unusual circuit and layout, got me confused The Marconi diagram is one first look the same but drawn upside down to my way.
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Old 23rd Feb 2016, 11:36 am   #57
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Default Re: HMV 653 Broadcaster all wave.

The HT reservoir capacitor is C25 on both schematics it's -ve connection is referenced to the rectifier via the HT transformer center tap as it should be.

The receivers current draw sets the receivers bias line voltage which is developed across the 270 ohm resistor (R21 trader, R17 service mans) the current draw for practical purposes will be not far off constant in this type of receiver, hence the bias line voltage will be more or less constant. The value of the resistor that develops the bias line needs to be small compared to the value of the HT filter resistors which it is.

If there was no reservoir capacitor the rectifier would be looking at a choke input filter and the HT voltage would be considerably less than what is quoted.

Lawrence.
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Old 23rd Feb 2016, 12:55 pm   #58
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Default Re: HMV 653 Broadcaster all wave.

Appreciate all the many replies and interest from you all on my query. I took some photos before work this morning to make the speaker connections clearer to you as they are on a tag board and numbered with 8 possible connections to two different tag strips which may explain the 4 wires from the secondary of the OPT. I will try and get these photos uploaded throughout today.
Many thanks.
Dave.
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Old 23rd Feb 2016, 1:32 pm   #59
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Default Re: HMV 653 Broadcaster all wave.

If not a NFB winding, is it a Humbucking coil? Edward
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Old 23rd Feb 2016, 1:37 pm   #60
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Default Re: HMV 653 Broadcaster all wave.

Silly old me Its been a long day, we are 8 hours ahead of GMT here.

C25 is of course the reservoir cap.

You will need to get the polarities on all these new electrolytics correct, -ve ends to all the 'bottom' ends in the diagram. I'm surprised that they are not marked on the diagram, but as it was a box cap perhaps it was originally obvious?
Are you going to put the new ones in a box? It would look more original.
Hope that the speaker energising coil is OK.

Sam.
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