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Old 7th Oct 2018, 6:35 pm   #41
Mrgroovy
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Default Re: Need help identifying old capacitor from 1930s radio

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To help verify the bias arrangement maybe the OP can measure the resistance across the 500 ohm resistor without disconnecting anything.
Sorry for my dumb question, but to measure the resistance across, does that simply mean to measure the 500ohm resistor in a normal manner? The terminology is still a bit new too me. If I understood what you asked I measured the resistance which is 527ohm.
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Old 7th Oct 2018, 6:50 pm   #42
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Default Re: Need help identifying old capacitor from 1930s radio

I meant measure the resistance across the 500 ohm resistor without disconnecting the resistor from the solder tags, in other words the resistor remains connected as per your photos then measure the resistance between the two solder tags that the resistor is attached to. It's to get an idea of the actual bias arrangement that's configured in your receiver with the AL1 fitted.

On a separate note I wonder if the switch near the inductor that's fitted to the rear of the chassis is a tone switch, the inductor that's bolted to the chassis rear and the capacitors next to it being part of it? Could do with the control grid (g1) of the AL1 tracing back to get an idea what route the signal from the detector takes to get to the grid, I wonder if the other choke that's fixed to the underside of the chassis is part of the HT filtering or an anode load for the detector? Or if the inductor fixed to the back of the chassis is?

Just bunging a few ideas in.

Lawrence.

Last edited by ms660; 7th Oct 2018 at 7:10 pm. Reason: alteration
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Old 7th Oct 2018, 7:45 pm   #43
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Default Re: Need help identifying old capacitor from 1930s radio

Then it is definitely 527ohm. As for the inductor near the rear with the switch; I made a quick scetch of the lead coming from the AL1 going to the capacitors and the inductor. Probably not the right symbol for inductor in my scetch but maybe you can make something out of it nonetheless.
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Old 7th Oct 2018, 8:26 pm   #44
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Default Re: Need help identifying old capacitor from 1930s radio

Cheers for that latest drawing, that appears to sort the PU/gram input and detector grid leak out so far as I can make out.

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Old 7th Oct 2018, 8:43 pm   #45
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Default Re: Need help identifying old capacitor from 1930s radio

There is no assurance that the AL1 modification ever worked properly. If we could find a circuit diagram we could at least see what has been changed but it seems this is very unlikely. It would be helpful to see how the other valves are wired.

Based on the circuit by George, I would say it is wired incorrectly. The reservoir and smoothing capacitors should go to chassis and the centre tap should go to the 500 ohms. A resistor and capacitor smoothing would then be taken off the centre tap to provide the grid bias. The wiring for the other valves may give a further clue as they may also have -ve grid bias.

Last edited by PJL; 7th Oct 2018 at 8:49 pm.
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Old 7th Oct 2018, 9:04 pm   #46
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Default Re: Need help identifying old capacitor from 1930s radio

Do you mean that the centre tap should not go to ground? Did you have a look at the second scetch that I made? It is far from complete but maybe enough to tell if there are any obvious errors. The 500 ohm resistor is placed in parallel between the 0.1uF capacitor in the block and ground.
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Old 7th Oct 2018, 9:38 pm   #47
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Default Re: Need help identifying old capacitor from 1930s radio

Lets wait for input from Lawrence.

One of your connections to the mains transformer is probably a screen. This is a foil placed between the primary and secondary that is wired to chassis to reduce noise coming from the mains.

Keep up the good work with the circuit drawing...
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Old 7th Oct 2018, 10:32 pm   #48
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Default Re: Need help identifying old capacitor from 1930s radio

Is there a position on the band selector switch for gram input (GR) or are there only the two positions, one for the medium wave band and one for the long wave band ?

Lawrence.

Last edited by ms660; 7th Oct 2018 at 10:38 pm. Reason: clarification
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Old 7th Oct 2018, 11:51 pm   #49
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Default Re: Need help identifying old capacitor from 1930s radio

I am getting the impression that changing the output valve to an indirectly heated type has left the bias circuit floating on the heater supply.
It might well be an abandoned repair job that has never worked after the output valve was replaced.
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Old 8th Oct 2018, 10:17 am   #50
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Default Re: Need help identifying old capacitor from 1930s radio

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Is there a position on the band selector switch for gram input (GR) or are there only the two positions, one for the medium wave band and one for the long wave band ?
Hi Lawrence,

I'm not sure if I am entirely qualified to answer that question. As far as I can see there are three switches on this unit: One switch that cuts or connects the mains to the power transformer. This switch is actually controlled by the 1000K volume pot in that there is a "metal tag" at the far end of the potentiometer that opens or closes a toggle switch that goes to the mains/OT. Clever little device. I just left it out of the "caveman's schematics" to simplify things. Above the volume pot/power switch, there is another toggle switch, that connects something-something to one of the (I would presume) RF coils? From my overly primitive sketches, it seems like the switch simply connects one of the leads from the coil to ground, if that makes sense. The last and third switch is the one on rear end that you saw in my previous drawing. I'll try make a schematics for the entire receiver as soon as I'm able to.
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Old 8th Oct 2018, 10:19 am   #51
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Default Re: Need help identifying old capacitor from 1930s radio

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I am getting the impression that changing the output valve to an indirectly heated type has left the bias circuit floating on the heater supply.
It might well be an abandoned repair job that has never worked after the output valve was replaced.
The chassis potential is going to be the reference for the RF and detector but it is fed via the 500 ohm resistor and is not properly decoupled from HT.

I would try to contact the owner of the set on the radiomuseum site.

The alternative is to finish off the circuit diagram and redesign the AL1 modification. It looks like the modifications were made a long time ago so it is difficult to tell how extensive the alterations are.
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Old 8th Oct 2018, 10:22 am   #52
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Default Re: Need help identifying old capacitor from 1930s radio

So far as I know this is a two wave band receiver, there must be a control on the front to switch between the two wave bands, what I was asking was does that control have a gram position as well as a position for medium wave and a position for long wave ?

Lawrence.
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Old 8th Oct 2018, 10:36 am   #53
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Default Re: Need help identifying old capacitor from 1930s radio

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The chassis potential is going to be the reference for the RF and detector but it is fed via the 500 ohm resistor and is not properly decoupled from HT.

I would try to contact the owner of the set on the radiomuseum site.

The alternative is to finish off the circuit diagram and redesign the AL1 modification. It looks like the modifications were made a long time ago so it is difficult to tell how extensive the alterations are.
You may both well be right in your assumptions that the modifications done have some shortcomings. I would not know this myself, but if I can manage to make a fully functioning schematics or at least a faithful reproduction of how things are connected, it would probably be easier to help me out.

The most confusing part, for me at least, is the power transformer. There are 8 leads coming out from the secondary, one of which goes to ground, which I would assume is the centre tap? Then there are four that goes to the rectifier tube, two that goes to the power tube, and a last one that goes to the 2,4,2 lugs on the capacitor block.

Sorry for my ignorance, but the two leads that goes to the power tube, for the filament, are those the same that goes to the filament on the rectifier tube? In which case there are not 8 but 6 leads from the secondary?

I'm sure you all understand that this is probably way beyond my current knowledge. I can only hope that I ask the right questions so that I can learn how this works.

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So far as I know this is a two wave band receiver, there must be a control on the front to switch between the two wave bands, what I was asking was does that control have a gram position as well as a position for medium wave and a position for long wave ?
The switch in front has two positions. As have all the three switches in this unit. I'll try make that schematics so you can have a look.
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Old 8th Oct 2018, 11:21 am   #54
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Default Re: Need help identifying old capacitor from 1930s radio

Ref. post 53: The transformer will have three secondary windings, those windings are isolated from each other inside the transformer, one winding (two wires) will be for the rectifier heater, another winding, a centre tapped winding (three wires) will be for the HT feed to the rectifier, one wire to one anode, one wire to the other anode, the other wire (the centre tap) will be connected to the chassis via the 500 ohm resistor.

The other winding (two wires) is for the heaters for the rest of the valves and the dial lamp.

The wire that's left over is probably connected to an internal electrical screen and would normally be connected directly to chassis.

Lawrence.
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Old 8th Oct 2018, 12:18 pm   #55
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Default Re: Need help identifying old capacitor from 1930s radio

Thanks Lawrence, well explained, still a bit technical but I think I understood. Here's a new caveman's schematic. I'm not sure how to present the (what I call) RF Coils, nor the tubes for that matter, so I've just drawn a circle with connecting dots showing where the different components are connected. "Caveman's Vintage Radio Service, how may I help you!?"

Edit: There are still some stuff that have not been accounted for; for instance there are 5 or 6 connectors on the RF Coil 1, whereas I only drew in three. Also the pilot lamp is missing here, which connects with the filament(?).
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Old 8th Oct 2018, 12:23 pm   #56
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Default Re: Need help identifying old capacitor from 1930s radio

Circuit updated with extra components.

EDIT: - oops post crossed - later...
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Old 8th Oct 2018, 12:37 pm   #57
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Default Re: Need help identifying old capacitor from 1930s radio

Your schematic looks very neat. You may want to take a look at the last one, as I think that I found some errors in that first (or second) schematic I did. Can't guarantee that the last one I did is correct either. I'll have a look at what you did, and see if I can make something similar to your. It would be easier to spot any mistakes if the symbols are nicely organized like yours here.
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Old 8th Oct 2018, 1:00 pm   #58
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Default Re: Need help identifying old capacitor from 1930s radio

OK - I will hang fire for a bit. My original post was to help identify the capacitor connections and the negative grid bias arrangement used. I found a good explanation on this site which shows that the HT secondary centre tap is not connected to chassis ground:-
http://www.aikenamps.com/index.php/what-is-back-biasing

I looked up the E452 on the radiomuseum and it appears to have a top cap anode which I can't see on your circuit. Also the two E452 bases in the sketch have different pins wired to the filament supply so would be worth re-checking the pinouts.
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Old 8th Oct 2018, 1:10 pm   #59
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Default Re: Need help identifying old capacitor from 1930s radio

The cathode tag for the E452 is the one towards the corner of the valve socket.

The one for the detector/gram amp is connected to chassis.

The one for the RF amp has a de-coupled resistance in series which appears to be connected to the vol/gain control.

Lawrence.
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Old 8th Oct 2018, 1:15 pm   #60
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Default Re: Need help identifying old capacitor from 1930s radio

Struggling with it now. Both sides of tuning cap connected to chassis and four out of five pins of second coil? Will leave you guys to it for a bit as the rain has stopped (temporarily) here.
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