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Old 24th Jun 2017, 5:41 pm   #1
ian rose
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Default RA17L HT current

Hello all
what is the specified total HT current draw for the above, please?
At present my set is working but below par. My HT current is approx. 70mA.
Regards
Ian Rose
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Old 24th Jun 2017, 11:52 pm   #2
Silicon
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Default Re: RA17L HT current

The circuit generates the 25V AGC voltage by passing the negative HT current through a 165 Ohm resistor.

This implies a 151mA HT current. It does not say if the calibrator is turned on.
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Old 25th Jun 2017, 11:37 am   #3
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Default Re: RA17L HT current

Hi Ian,
As Silicon says, the HT current should be about 150mA in a working set.
Where did you measure 70mA?
What is the heater voltage measured at the transformer - should be 6.4v AC - adjust the mains taps if not.
What is the HT voltage measured at the HT1-HT2 strap on the rear panel - should be 250v or thereabouts.
With the Service switch in the Standby position what is the HT voltage measured across the 10k resistor in the power supply compartment? -should be 280v or more.
Is this a set that was working normally and failed or are you doing a restoration?
You say the set is working below par, what other symptoms do you have?
cheers
Peter G8BBZ
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Old 25th Jun 2017, 12:50 pm   #4
ian rose
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Default Re: RA17L HT current

Thank you Silicon and Peter
Heater voltage at the transformer is; 6.43.
HT at the rear strap is 224.
HT across the 10K resistor is 318
There are no other untoward symptoms. As I say the set has been very usable following a part restoration a few years ago. [a few capacitors and R's were replaced, alignments checked and parameters at the various test points checked to be reasonable.
Ian
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Old 25th Jun 2017, 9:02 pm   #5
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Default Re: RA17L HT current

Hi Ian,
Ok so it looks like it's not a power supply fault.
I should have asked what is the voltage across the 165ohm resistor mentioned by Silicon?
You didn't say where you measured 70mA.
Is the problem low audio? lack of stations? Does the calibrator work?
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Old 25th Jun 2017, 10:35 pm   #6
ian rose
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Default Re: RA17L HT current

Hi Peter
HT current was measured at the HT fuse position.[70mA]
I improved audio some years ago by paralleling V22 and V23
for more power which worked well. No lack of stations and the calibrator works well. I find that I am advancing the gain controls more than usual. I am about to go through all the test points again. Without a valve voltmeter I am not quite sure how to measure accurately the voltage of the 37.5 MHz signal at the L42 end of the 37.5 band pass filter. I only have a [good] digital meter and an Avo Mk 8 [6] and a 'scope [up to 55 MHz]. will the 'scope do? I understand from the manual this voltage is critical. I am looking for, or am going to construct valve extenders in order to check voltages and currents for the valves in the 1st and 2nd VFO's. Just trying to avoid removing them and setting up 'flying leads'!!! With the antenna attenuator at maximum I can just about peak up the noise at 14 MHz with no aerial attached but not as well as before. It would appear that a stage somewhere is not drawing enough current, but, now that I know that total current should be around 150mA I can 'dig' around a bit.
Regards
Ian
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Old 26th Jun 2017, 3:55 am   #7
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Default Re: RA17L HT current

If you try to measure the current going through the HT fuse you will be measuring a pulsating DC current.

This can confuse some digital meters and possibly give erroneous readings on moving coil meters.

It might be better to measure the voltage generated across the 165R resistor and calculate the HT current because it will be smoothed DC at that point.
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Old 27th Jun 2017, 5:34 pm   #8
ian rose
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Default Re: RA17L HT current

Thank you Silicon and Peter.
I have now taken a reading from R124 the AVC voltage generator as you suggest.
R124 measures 143 ohms and voltage across is 26.
Giving me a current flow of 181.8mA. So, all seems to be well with current draw.
However, my 'scope tells me that I only have 1 volt of 37.5Mhz drive at test point 3 so perhaps I can squeeze out a bit more there. Will also do a few valve substitutions.
regards and thanks
Ian
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Old 28th Jun 2017, 6:58 pm   #9
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Default Re: RA17L HT current

You can't accurately measure 37 MHz across a tuned circuit, which that is, with
an ordinary scope probe which has a capacitance of at least 7 or so pF.
Note that C113 is only 27 pF.

You need to get an accurate 1 pF cap and put it in series with the probe tip.
The you need to know the probe capacitance to calculate the real voltage.
My Racal measure much lower than expected with a scope and regular (12 pf) probe
but OK with 1 pF.
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Old 29th Jun 2017, 9:36 am   #10
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Default Re: RA17L HT current

Hi Ian,

I also had a problem with low output at 37MHz. One of the causes was mistuning of the 1MHz crystal oscilator anode circuit. Peaking this up made a big difference.
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Old 1st Jul 2017, 5:31 pm   #11
ian rose
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Default Re: RA17L HT current

Thank you Keith and Donald
Have taken new readings according to your advice.
Am now happy with results.
Best regards
ian
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