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Old 2nd Mar 2021, 11:55 pm   #21
Diniz Diniz
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Default Re: AZ41 supply AC voltage?

Hello.
I have constructed the lamp limiter with a 40w bulb (the one I have at the moment).
The lamp runs bright, and the voltage at the positive lead of the rectifier voltage is 57vdc. Is it normal!?!
I now know that it is not normal, because I assumed that the low voltage was caused by the lamp limiter and I ended plugging the set directly to the 230v and I burned my last AZ41.... I know, I know, I am a dummy and I now know that I haven’t correctly understand the usefulness of the lamp limiter!
One important questiono is, Until I get one AZ41, is the valve replaceable by one common rectifier bridge and a dissipative resistance to bring the voltage to its supposed value? I just don’t want to wait for a new valve to keep trying fix the set...
Thanks in advance for all the help and support
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Old 3rd Mar 2021, 7:55 am   #22
Gabe001
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Default Re: AZ41 supply AC voltage?

Until the lamp limiter bulb lights dim, only switch on the set via the lamp.

Yes, you can use rectifier diodes + resistor if you feel competent doing that. Bear in mind that the HT voltage will need lowering quite a bit and the resistor needs to be of suitable wattage.

A full wave rectifier would be a better replacement than a bridge rectifier imo, I'm happy to be corrected if wrong. If you're connecting diodes to the rectifier valve base, remove the valve from the socket.

As mentioned by Alan, you may have a very leaky capacitor or two causing this, but also do some basic checks to ensure any wires aren't shorting, the resistors are ok and the valve's don't have any internal shorts. Measuring voltage drop across anode resitors of known values may also give you an indication of whether any of the valves is drawing excess current.
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Old 3rd Mar 2021, 5:25 pm   #23
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Default Re: AZ41 supply AC voltage?

Look at the diagram below for diode placement with regard to centre tap of tranaformer
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Old 3rd Mar 2021, 5:26 pm   #24
Diniz Diniz
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Default Re: AZ41 supply AC voltage?

Thank you for your advice. As I mentioned before, I have replaced the electrolytic capacitors as well as all the other capacitors (except for 2 of them that I cannot identify in the circuit.... probably I will replace them with 10nF capacitors). But I will do a re-check on the resistors and look for some continuity where it is not supposed to be any.

Just to be confident on the rectifier bridge "implantation", I am planning to use one of the 295Vac leads of the main transformer and the "0" lead (I mean use 295-0 from the 295-0-295 previously described) as AC input to the rectifier and connect the rectifier outputs to C1 (the positive DC lead of the rectifier) and to chassis (the negative lead of the rectifier). Then I will measure the current in the wire connecting the positive lead of the rectifier and C1 to determine the value (ohms) and wattage of the resistor to be installed between these two points (to bring HT++ to an acceptable value). Do I plan to do something wrong?

Thanks again for all your help and support.
Diniz from Portugal
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Old 3rd Mar 2021, 5:29 pm   #25
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Default Re: AZ41 supply AC voltage?

Look at the diagram below for diode placement with regard to centre tap of transformer

Posted twice in error sorry
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Old 3rd Mar 2021, 5:44 pm   #26
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Default Re: AZ41 supply AC voltage?

Quote:
Originally Posted by Diniz Diniz View Post
Thank you for your advice. As I mentioned before, I have replaced the electrolytic capacitors as well as all the other capacitors (except for 2 of them that I cannot identify in the circuit.... probably I will replace them with 10nF capacitors). But I will do a re-check on the resistors and look for some continuity where it is not supposed to be any.

Just to be confident on the rectifier bridge "implantation", I am planning to use one of the 295Vac leads of the main transformer and the "0" lead (I mean use 295-0 from the 295-0-295 previously described) as AC input to the rectifier and connect the rectifier outputs to C1 (the positive DC lead of the rectifier) and to chassis (the negative lead of the rectifier). Then I will measure the current in the wire connecting the positive lead of the rectifier and C1 to determine the value (ohms) and wattage of the resistor to be installed between these two points (to bring HT++ to an acceptable value). Do I plan to do something wrong?
No No No.
That will short the transformer. You cannot implement a 'bridge' rectifier (four diodes) here. You have to use 2 diodes (just like the AX41 has). See the picture Gabe has posted. You need 1N4007 or UF4007 types here.

Have you put the replacement 50uF caps in the right way round? Negative to chassis?
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Old 3rd Mar 2021, 5:45 pm   #27
Gabe001
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Default Re: AZ41 supply AC voltage?

I'm slightly out of my comfort zone here advising about rectifiers, but I don't thing a bridge would work as you'd be shorting your centre tap from transformer to the -ve of the bridge output (there is a connection between centre tap and chassis on the diagram). Use a full wave rectifier as in the diagram using two diodes (one on each of the AC lines. See my sketch. Remove the az41 from the socket whilst you have this setup in place.
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Old 4th Mar 2021, 12:42 am   #28
Diniz Diniz
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Default Re: AZ41 supply AC voltage?

@Snowman and Gabe: Thank you so much for the advices and guidelines.
@Gabe: Thank you Gabe for the sketch! I will follow it.
The Az41 is not anymore in place and yes, the negative leads of both 50uF capacitors are connected to the chassis (both positive leads are connected through a 1kohm/5W resistor - the close I have to 1.2kohm).
I don't have those diodes, and we are currently under severe lockdown here in Portugal... I have here the following diodes (several of each model): philco 210-1N48, philco 210-1N64, sylvania 1N34A... does any of these suits this setup for the purpose of testing? (I was not able to find the maximum allowed voltage for this models). Later I will buy the recommended ones.

Thanks in advance, you're help is being much appreciated.
Diniz from Portugal.
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Old 4th Mar 2021, 7:15 am   #29
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Default Re: AZ41 supply AC voltage?

Hi diniz

None of your diodes are suitable, you need ones rated at 1000v

295v will be rectified and smoothed to around 410-415v so you probably need to drop the voltage by around 80v, judging by the characteristics of the valve. I'm not sure what your HT is meant to be exactly. For a set consuming 50mA, that's at leat 1600ohm, and 4W, or whereabouts, depending on the health of your other valves. You may need to experiment and you may be better off, say, with 2x 800ohm 5w resistors in series than one 1600ohm 5w resistor.

If you have to wait for the diodes, why don't you just wait for the new valve instead?

Ps: the chassis and centre tap are already connected, so all you need to do is put a diode in each of the 295v AC lines, connect them together, add your resistor and join this to c1 + (or cathode pin of az41 valve base)

Obviously treat the high voltages with respect.
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Old 4th Mar 2021, 10:50 am   #30
Diniz Diniz
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Default Re: AZ41 supply AC voltage?

Quote:
Originally Posted by Gabe001 View Post
Hi diniz

None of your diodes are suitable, you need ones rated at 1000v

295v will be rectified and smoothed to around 410-415v so you probably need to drop the voltage by around 80v, judging by the characteristics of the valve. I'm not sure what your HT is meant to be exactly. For a set consuming 50mA, that's at leat 1600ohm, and 4W, or whereabouts, depending on the health of your other valves. You may need to experiment and you may be better off, say, with 2x 800ohm 5w resistors in series than one 1600ohm 5w resistor.

If you have to wait for the diodes, why don't you just wait for the new valve instead?

Ps: the chassis and centre tap are already connected, so all you need to do is put a diode in each of the 295v AC lines, connect them together, add your resistor and join this to c1 + (or cathode pin of az41 valve base)

Obviously treat the high voltages with respect.
Thank you for all your inputs.
To be honest, for the purpose of testing if I am able to fix the set, the diodes are much more affordable. Also, valves are not easy to find in Portugal and are quite expensive. If I fix the set, I'll dig around my other radios (at my parents home far away from Lisbon) to see if some uses a AZ41.

Best regards,
Diniz.
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