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Old 17th Oct 2018, 3:40 pm   #121
Mrgroovy
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Default Re: Aga Baltic AH37. Need help identifying old capacitor from 1930s radio

Right, I think I understand. Have not tried to check the resistance of the OT. What is the easiest way of doing that? I only have a simple multimeter. Do I measure the resistance across the two wires of the OT?
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Old 17th Oct 2018, 3:48 pm   #122
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Default Re: Aga Baltic AH37. Need help identifying old capacitor from 1930s radio

Yes, use your meter to measure the resistance of the following:
The speaker field coil.
The output transformer primary winding (input to the transformer from the chassis).
All 3 of the chokes
The mains transformer primary (connection to mains supply) and the HT secondary (3 wires including the centre tap).
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Old 18th Oct 2018, 7:56 am   #123
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Default Re: Aga Baltic AH37. Need help identifying old capacitor from 1930s radio

Ok, I'm having a hard time getting any readings from the mains transformer. Don't know if I'm doing it all wrong or if there might in fact be something wrong with it. I found that there is continuity between both wires going into the primary. There is also continuity between both pins on the power cord. It is a brand new cord and I'm 100% sure the cord is ok. Furthermore, I've traced the continuity and it goes all the way into the primary. Based on my very limited knowledge, I would automatically assume there is a short, but then I am surprised that it wouldn't blow a mains fuse when powering it up and all the tubes are lighting up so there is definitely voltage on the secondary. Which means, it can't be a short, or can it? Have not tried measuring the HT voltage. However, I can't seem to get any ohm readings. When powering up the radio (not for long of course since there is a bad hum from the power transformer), I get some radio transmitting noise. If there was a short on the HT but not on the windings to the filament, if that is even possible, that shouldn't result in any noise or sound at all, right? Just thinking out loud...

As for the chokes the readings are:

Choke in the middle: 5.29K
Choke at the rear: 4.73K
Choke at the side: 1,58K

OT: 1.1K

Speaker field coil: 1.3M This sounds very high compared to the other coils, but that's what it says. Don't know it this is within normal readings for a speaker field coil.

Back to the PT: There is no physical connection between the primary and the secondary, right? So, I really shouldn't get any impedance readings (other than infinite) when it is not powered? If I've understood the science of power transformer it is the magnetic field created between the primary and the secondary when the primary is connected to the mains that in turn makes a "connection" between the primary and the secondary. So when I try to measure between the windings on the primary and the windings on the secondary with no voltage connected I shouldn't get any readings? Please correct me if I'm wrong.
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Old 18th Oct 2018, 8:42 am   #124
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Default Re: Aga Baltic AH37. Need help identifying old capacitor from 1930s radio

I get impedance readings between 15-20 ohm between the two (three?) wires of the primary. This at least results in continuity/beeping sound on my multimeter. To put it in perspective, the resistance between two random points on the chassis is approximately 3 ohm. I'm not sure what readings should be expected. I've read a couple of other places that a power transformer with low resistance usually results in continuity on certain multimeters when measuring between the primary. I hope this is the case here because if the PT really is bad then I'm afraid I have no choice but to abandon this project.
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Old 18th Oct 2018, 8:46 am   #125
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Default Re: Aga Baltic AH37. Need help identifying old capacitor from 1930s radio

The DC resistance of the primary winding of a mains transformer will be low. It's the inductance of the winding which impedes the flow of AC current through it. Were this not the case very large currents would flow and burn out the winding. That's what happens if an AC only set is connected to a DC supply.
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Old 18th Oct 2018, 9:16 am   #126
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Default Re: Aga Baltic AH37. Need help identifying old capacitor from 1930s radio

The chokes and output transformer are all good.

I read your post #1 again and it was working but the sound was very weak, so the mains transformer must be working, but the fault might be a problem with the speaker field coil.

You should be measuring the resistance of the field coil on the paxolin board connections between the chassis and the speaker (post #94 picture 3). It should be around 10K ohms. Corrosion on the terminals often makes it difficult to make proper contact with the meter probes.

Last edited by PJL; 18th Oct 2018 at 9:22 am.
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Old 18th Oct 2018, 9:27 am   #127
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Default Re: Aga Baltic AH37. Need help identifying old capacitor from 1930s radio

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Originally Posted by PJL View Post
The chokes and output transformer are all good.

I read your post #1 again and it was working but the sound was very weak, so the mains transformer must be working, but the fault might be a problem with the speaker field coil.

You should be measuring the resistance of the field coil on the paxolin board connections between the chassis and the speaker (post #94 picture 3). It should be around 10K ohms. Corrosion on the terminals often makes it difficult to make proper contact with the meter probes.
I unsoldered the wires going into the field coil and simply measured from the one wire to the other but I take it that was not the right way to do it?
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Old 18th Oct 2018, 10:21 am   #128
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Default Re: Aga Baltic AH37. Need help identifying old capacitor from 1930s radio

That works. Though don't confuse field coil with humbucking coil, if there is one. It's lower resistance. The polarity of that coil increases or decreases hum from field coil.
I've not looked at schematic.
Just make sure there are no leaking capacitors, usually every paper dielectric type. I usually check HT electrolytics for leakage by connecting 32V DC PSU in current limit mode to a set with it unplugged. That also reforms them. Excessive leakage pulls down HT. Lack of capacitance (dried out) creates hum, or on a cathode resistor reduces gain (loudness). A leaky AGC capacitor increases gain. If there are screen grids (Tetrodes and Pentodes etc, the g2), then a leaky capacitor to ground will dramatically reduce gain (little volume).
An open circuit IFT coil, or dry joint where wire of coil is on tag may still work due to capacitive coupling, but volume will be low.
Also check volume control from wiper and all the valve sockets.
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Old 18th Oct 2018, 10:27 am   #129
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Default Re: Aga Baltic AH37. Need help identifying old capacitor from 1930s radio

Re: Mains transformer:
Power set with no valves fitted. If the transformer gets hot (it can run cold or just get slightly warm) then there is a short. Check all secondary voltages, any missing mean an open circuit. The primary can seem to read infeasibly low, it's much higher impedance at 50Hz.
I've not looked at circuit. A heater / cathode leakage on rectifier can reduce HT if the rectifier doesn't use a separate heater winding.
Reduced HT or a worn out output valve gives lower volume.
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Old 18th Oct 2018, 10:48 am   #130
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Default Re: Aga Baltic AH37. Need help identifying old capacitor from 1930s radio

Quote:
Originally Posted by Mike. Watterson View Post
That works. Though don't confuse field coil with humbucking coil, if there is one. It's lower resistance.
So i measured the field coil the right way? Then why is it reading 1.3 Mega Ohm?
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Old 18th Oct 2018, 11:31 am   #131
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Default Re: Aga Baltic AH37. Need help identifying old capacitor from 1930s radio

Quote:
Originally Posted by Mrgroovy View Post
Quote:
Originally Posted by Mike. Watterson View Post
That works. Though don't confuse field coil with humbucking coil, if there is one. It's lower resistance.
So i measured the field coil the right way? Then why is it reading 1.3 Mega Ohm?
If that's the resistance of the field coil then it suggests that the field coil is shot, you can still use the receiver without it by disconnecting the loudspeakers speech coil from the secondary of the output transformer and connecting a low impedance (3 to 8 ohm) loudspeaker in its place, the field coil according to what we've worked out so far is connected across the HT, that means without it you will still have HT present for the receiver to operate, but with the field coil out of circuit the HT would rise above it's design voltage so in order to limit that rise you could connect a resistor in place of the field coil as a substitute load, a 12k 10watt might do the job.

Lawrence.

Last edited by ms660; 18th Oct 2018 at 11:38 am. Reason: clarification
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Old 18th Oct 2018, 11:48 am   #132
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Default Re: Aga Baltic AH37. Need help identifying old capacitor from 1930s radio

That's just my luck... Thanks Lawrence, it's good to know I could make a workaround using a resistor, but I would really like to get the field coil back in working order. I guess I would have to send it somewhere to get it fixed but then again this is starting to getting pretty expensive.
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Old 18th Oct 2018, 6:04 pm   #133
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Default Re: Aga Baltic AH37. Need help identifying old capacitor from 1930s radio

Your luck was good as all the other parts are OK!

Sometimes the break is where the lead out wires are connected to the winding. Take the speaker out and do some investigation, you have nothing to lose. The coil can be rewound and you could do this yourself but it will have many turns.
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Old 18th Oct 2018, 6:18 pm   #134
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Default Re: Aga Baltic AH37. Need help identifying old capacitor from 1930s radio

Quote:
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Your luck was good as all the other parts are OK!

Sometimes the break is where the lead out wires are connected to the winding. Take the speaker out and do some investigation, you have nothing to lose. The coil can be rewound and you could do this yourself but it will have many turns.
I guess you're right. As for rewinding the coil myself, I've actually began the prosess of trying to get to the coil. It is protected behind a cup of metal and I can't seem to get it off. I've tried using brute force and after getting it a few mm off I'm starting to think I'm at the point of no return here. Any tips on how to get off that lid/cup over the coil?
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Old 18th Oct 2018, 6:50 pm   #135
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Default Re: Aga Baltic AH37. Need help identifying old capacitor from 1930s radio

Add some photographs and maybe we can make some suggestions..
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Old 18th Oct 2018, 6:54 pm   #136
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Default Re: Aga Baltic AH37. Need help identifying old capacitor from 1930s radio

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Add some photographs and maybe we can make some suggestions..
Will do that, later or maybe tomorrow. It's not safe for me to go near that speaker now, or I'll surely throw it in the trashcan!
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Old 18th Oct 2018, 7:07 pm   #137
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Default Re: Aga Baltic AH37. Need help identifying old capacitor from 1930s radio

It is definitely the original speaker. The outside looks very solid and there is a circle in the top which is probably where the centre pole attaches. See: https://en.wikibooks.org/wiki/Engine..._-_Loudspeaker

The whole magnet assembly with coil will be very firmly fixed to the frame. Are there any bolts underneath?
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Old 18th Oct 2018, 7:35 pm   #138
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Default Re: Aga Baltic AH37. Need help identifying old capacitor from 1930s radio

Yes, 3 bolts underneath the frame, and I removed them before I started pulling it off. I inserted a flathead screwdrive in a small crevice between the lid and the speaker frame. I worked my way around in circles, but when i got it off between 0,5 and 1mm, the speaker frame started to bend slightly - probably due to the metal getting hot from pulling with the screwdriver. At that point I realised that I probably wouldn't be able to pull it off without braking the speaker entirely.
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Old 18th Oct 2018, 9:31 pm   #139
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Default Re: Aga Baltic AH37. Need help identifying old capacitor from 1930s radio

Update,

The speaker got a taste of my wrath (yeah, I know I shouldn't have gone back to it tonight). Anyways, it is totally ruined now, beyond any chance of ever being restored.
I figured I should at least see if I could get the field coil off the damn thing; if anything to see how it is constructed. Well, the irony of fate is quite clear in that matter. When I've finally gone to the point and broken the thing to pieces, the frame is still stuck to the three screws which still holds tight onto the field coil. They seem to be die-cast into the socket. I'm sure I can hear the screws laugh at me.

With the speaker frame warped into oblivion and practically useless I cut out the speaker cone as well and unsoldered the wires to the secondary of the OT. If anything I did a measure across the secondary and the impedance is 2.3 Ohm.

I would need to find a new speaker now. Should I go with a 4 or even an 8 ohm speaker? Lawrence suggests 3-8 Ohm. Sounds good to me if the 2.3 ohm DC resistance on the secondary doesn't change anything on that part.
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