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 Post subject: Re: Telefunken Gavotte 5253W - Diode question
PostPosted: Dec Fri 22, 2017 2:03 am 
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Joined: Nov Wed 14, 2007 11:37 pm
Posts: 1600
Maybe you can stuff that AEG can to save room?


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 Post subject: Re: Telefunken Gavotte 5253W - Diode question
PostPosted: Dec Fri 22, 2017 2:31 am 
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Joined: Aug Thu 20, 2015 3:09 pm
Posts: 1176
Location: Albion, CA, USA, 95410
Metzman, very nice stuff job! Mine is similar in that I used a piece of perf board to mount the diodes but is not stuffed in the old AEG can. Yours is much nicer for sure. I see you have some ceramic caps, what is the purpose and values of them? I have room to add them if needed. Did you add a dropping resistor? I found that with the silicon diodes and an input voltage of 125VAC I needed a 300 ohm resistor but I got 274VDC on B+, perfect. The transformer is running much cooler now.

Turns out the hum is barely noticeable, you can only hear it faintly during moments of silence. I'm pretty sure it was not there before though. I have tried getting the wiring back where it was and I'll investigate further.

Over all, things are going well so far. This radio has great sound on FM. What kind of antenna does it need for AM?


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 Post subject: Re: Telefunken Gavotte 5253W - Diode question
PostPosted: Dec Fri 22, 2017 2:48 am 
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Joined: Jan Mon 16, 2012 4:15 pm
Posts: 5597
Location: Near Brandon, Iowa
madlabs wrote:
(snip)
What kind of antenna does it need for AM?

It uses the ferrite loopstick antenna that you mentioned earlier as having fallen out of its holder during transit.
You can also attach a longwire antenna (and ground) to the antenna ports on the back of the set. The internal loopstick should work satisfactorily for broadcast band (550-1700 kHz) but if you want to pick up much on the shortwave band an external longwire antenna will be needed.


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 Post subject: Re: Telefunken Gavotte 5253W - Diode question
PostPosted: Dec Fri 22, 2017 3:29 am 
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Joined: Nov Wed 14, 2007 11:37 pm
Posts: 1600
Quote:
I see you have some ceramic caps, what is the purpose and values of them?


Not sure the values. This is just an example I found to show you that it can be done.


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 Post subject: Re: Telefunken Gavotte 5253W - Diode question
PostPosted: Dec Sun 24, 2017 9:08 am 
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Joined: Jan Tue 16, 2007 7:02 am
Posts: 3031
Location: Lexington, KY USA
I expect that the old 50uF capacitor on the output tube cathode was dried up and much reduced in capacitance. The new 47uF capacitor restores the low frequency gain to its normal value. Any hum on your audio signal will be louder in the speaker with a good capacitor. Touching something that's grounded with your hand may somewhat shield whatever is picking up the hum.

I'm a bit concerned about your added series resistor that reduces B+. Where is this resistor connected? If it is ahead of the first filter capacitor, the waveform across the resistor will make measuring volts, amps and watts for the resistor non-trivial.

Ahead of the filter is functionally the best place for the resistor. This location will minimize strain on the power transformer, rectifier, and filter capacitor. However it makes determining the power dissipated in the resistor difficult.

The resistor is heated by the RMS current through, or the RMS voltage across it, while the B+ current is an average value. The RMS value can be two to three times the average. So the resistor can get hotter than you might expect.

We might go through how your various measurements were carried out, but often the simplest solution is trial and error. Find a resistance that gives the B+ voltage you want, then a power rating that allows the resistor to run reasonably cool.

Ted


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 Post subject: Re: Telefunken Gavotte 5253W - Diode question
PostPosted: Dec Sun 24, 2017 4:37 pm 
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Joined: Aug Thu 20, 2015 3:09 pm
Posts: 1176
Location: Albion, CA, USA, 95410
Metz, thanks for finding an example to show me. Lorenz, the ferrite doesn't get me much but I live out in the middle of nowhere. However, I could hear WWV on both 5 and 10mHz! With no antenna that is not bad. I'll have to get a long wire up for it. I did briefly touch the antenna input with a 40 meter dipole but not much improvement, impedance mismatch I'm guessing.

Ted, you have me worried now. The dropping resistor is before the filter cap, right after the 1n4007 full wave bridge. Voltage on B+ is perfect on 125VAC mains. For measurements I just read current through and voltage across the resistor using a Fluke 117.With the calculated dissipation of 2.3 watts, even if it was doubled that would be 4.6 watts through a 10 watt resistor. It does seem warmer than it should be but within safe limits.

Merry Christmas eve to all!


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 Post subject: Re: Telefunken Gavotte 5253W - Diode question
PostPosted: Dec Sun 24, 2017 10:05 pm 
Member

Joined: Jan Tue 16, 2007 7:02 am
Posts: 3031
Location: Lexington, KY USA
The resistor is in the best place, electrically. And likely your 10W resistor is OK, but will run pretty hot. The problem is figuring-out the power. The current through the resistor is not at all a sinewave, but instead, it's broken into rounded pulses near the AC line voltage peaks.

I think the AC ranges of your Fluke 117 can be used to accurately measure RMS values for the AC component of non-sinusoidal waveforms (within limits).

For either volts or amps you can measure the AC and DC components separately, and then combine them with a little math.

If the meter was set for DC when you made your measurements, the AC component will be in additon to the DC amount you found.

Probably, you can just take an AC measurement with the Fluke, calculate the resulting power in the resistor, and add this AC power to the DC power you have already calculated.

Whatever wattage rating resistor you use, it will generate the same heat. A larger one just spreads it around better. Wirewound resistors are made pretty much like heating elements, and can run very hot before the resistor suffers immediate harm. However, you need to be sure it's not cooking something else that's nearby.

Ted


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 Post subject: Re: Telefunken Gavotte 5253W - Diode question
PostPosted: Dec Mon 25, 2017 6:25 pm 
Member

Joined: Aug Thu 20, 2015 3:09 pm
Posts: 1176
Location: Albion, CA, USA, 95410
Ted, thanks for the information. I was indeed checking DC voltage/amperage across the resistor, not AC. So I will measure the AC component as soon as the Christmas madness has passed. If I understand correctly I need to measure AC volts AND current across the resistor and add that to the 2.3 watts I measured on DC.

Right now the set is playing very well and I am looking forward to completing the re-cap and getting this set ready for the next 60 years of its life.

Merry Christmas to all!


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