17th Oct 2018, 3:40 pm | #121 |
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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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17th Oct 2018, 3:48 pm | #122 |
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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). |
18th Oct 2018, 7:56 am | #123 |
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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. |
18th Oct 2018, 8:42 am | #124 |
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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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18th Oct 2018, 8:46 am | #125 |
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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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Graham. Forum Moderator Reach for your meter before you reach for your soldering iron. |
18th Oct 2018, 9:16 am | #126 |
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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. |
18th Oct 2018, 9:27 am | #127 | |
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Re: Aga Baltic AH37. Need help identifying old capacitor from 1930s radio
Quote:
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18th Oct 2018, 10:21 am | #128 |
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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. |
18th Oct 2018, 10:27 am | #129 |
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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. |
18th Oct 2018, 10:48 am | #130 |
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Re: Aga Baltic AH37. Need help identifying old capacitor from 1930s radio
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18th Oct 2018, 11:31 am | #131 | |
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Re: Aga Baltic AH37. Need help identifying old capacitor from 1930s radio
Quote:
Lawrence. Last edited by ms660; 18th Oct 2018 at 11:38 am. Reason: clarification |
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18th Oct 2018, 11:48 am | #132 |
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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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18th Oct 2018, 6:04 pm | #133 |
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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. |
18th Oct 2018, 6:18 pm | #134 | |
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Re: Aga Baltic AH37. Need help identifying old capacitor from 1930s radio
Quote:
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18th Oct 2018, 6:50 pm | #135 |
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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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18th Oct 2018, 6:54 pm | #136 |
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Re: Aga Baltic AH37. Need help identifying old capacitor from 1930s radio
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18th Oct 2018, 7:07 pm | #137 |
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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? |
18th Oct 2018, 7:35 pm | #138 |
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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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18th Oct 2018, 9:31 pm | #139 |
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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. |