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Old 21st Aug 2019, 11:58 pm   #1
regenfreak
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Default Understanding Ekco U332 schematic.

I bought a working Ekco U332 for 17 a while ago. I have no interest in restoring it and I am going to strip the RF, LO coils, tuning caps, IF transformers and rebuild a new two-valves basic superhet experimental test bed using completely different converter and IF amp and demodulator, replacing triode-heptode by a pentagrid.

Ok, it is a bit of long story....I want to compare the actual working superhet component values to the mathematical solutions of different superhet tracking calculators that I created using different radio engineering textbooks, comparing the theoretical tracking errors with the measured errors.

I have problems with understanding the functions of the following attached schematic:

1. Assuming the schematic shows the position of the switches S1 and S2 are set for MW, why there is a need for a large valve of capacitor 3300pF C3 in series with the RF signal tank coil L2? Does C3 change the natural frequency of the RF tank?

I am aware of that C14 is the padder for the local oscillator. C6 and C9 are trimmers


2. This radio works without external antenna. In the case of an external antenna is attached, what are the functions of C1 (1800p) and R1 (1.5M)?

Is C1 for decoupling the antenna RF signal in case it may overload and detune the RF tank?


3. Which capacitor and resistor components are for the automatic volume control in the schematic? R3? C16 is the DC blocking capacitor?
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Old 22nd Aug 2019, 7:28 am   #2
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Default Re: understanding Ekco U332 schematic

C1 protects the radio from electrostatic charges that can build up on an aerial.

R1 discharges them to chassis/earth.

R9 and C17 are AGC components.
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Old 22nd Aug 2019, 8:38 am   #3
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Default Re: Understanding Ekco U332 schematic.

C1 is part of the isolation required due to this being an AC/DC chassis, must be a Y type capacitor, you will probably find the original is a wax paper type with 300VAC working. It also with C3 provide a capacitive potential divider with C1 to bottom end couple the external aerial to the aerial tuning coils.

R1 has been noted for static discharge, I am not sure what R2 is for unless the aerial tuned circuit requires a DC connection to chassis.

It is standard circuit that Ekco used in a number of their radios. The Trader series of circuits very often have a brief circuit description unlike the Ekco Manual, look at those for the above description.

A word of caution, this radio uses a live chassis technique, use an isolation transformer in the power line, this will give you some, not all, protection and allow you to connect test equipment for any measurements you take.
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Old 22nd Aug 2019, 9:29 am   #4
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Default Re: Understanding Ekco U332 schematic.

I think R2 might be for hum reduction.

Lawrence.
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Old 22nd Aug 2019, 10:01 am   #5
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Default Re: Understanding Ekco U332 schematic.

Thanks Lawrence.

The IF amplifier AGC circuit is interesting, well to me any way, the second diode in the UBF89 is connected across the AGC line, in earlier circuits see U195, there was a resistor (10M ohm) from HT to the anode of that diode to allow delayed AGC.

Perhaps Ekco found that although not quite a good a delay circuit they could save the cost of a resistor.
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Old 22nd Aug 2019, 10:07 am   #6
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Default Re: Understanding Ekco U332 schematic.

Quote:
Originally Posted by Nuvistor View Post
Thanks Lawrence.

The IF amplifier AGC circuit is interesting, well to me any way, the second diode in the UBF89 is connected across the AGC line, in earlier circuits see U195, there was a resistor (10M ohm) from HT to the anode of that diode to allow delayed AGC.

Perhaps Ekco found that although not quite a good a delay circuit they could save the cost of a resistor.
See here:

https://www.vintage-radio.net/forum/...d.php?t=159091

Lawrence.
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Old 22nd Aug 2019, 10:38 am   #7
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Default Re: Understanding Ekco U332 schematic.

Missed that thread, will have a look later, not that I will be able to sort it out.
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Old 22nd Aug 2019, 1:09 pm   #8
regenfreak
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Default Re: Understanding Ekco U332 schematic.

Many thanks gentlemen for the answers.

Quote:
It also with C3 provide a capacitive potential divider with C1 to bottom end couple the external aerial to the aerial tuning coils.
Is C3 only required for AC set with a live chasis?

If we put a large capacitor 3300pF in series with the RF tank LC circuit, it has no effect on its tuning natural frequency because of its low impedance to RF signal?

The reason I asked because I don't always see C3 equivalence in some other superhet circuits.

I am reconstructing a battery superhet using Ekco U332's matched tracking components. I built a battery superhet from scratch using DIY RF coils and matched components based on the tracking calculator solutions. The DIY superhet battery set works but there are still some significant tracking errors.
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Old 22nd Aug 2019, 1:18 pm   #9
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Default Re: Understanding Ekco U332 schematic.

Quote:
Originally Posted by regenfreak View Post
Many thanks gentlemen for the answers.

Quote:
It also with C3 provide a capacitive potential divider with C1 to bottom end couple the external aerial to the aerial tuning coils.
Is C3 only required for AC set with a live chasis?

If we put a large capacitor 3300pF in series with the RF tank LC circuit, it has no effect on its tuning natural frequency because of its low impedance to RF signal?

The reason I asked because I don't always see C3 equivalence in some other superhet circuits.

I am reconstructing a battery superhet using Ekco U332's matched tracking components. I built a battery superhet from scratch using DIY RF coils and matched components based on the tracking calculator solutions. The DIY superhet battery set works but there are still some significant tracking errors.
Bottom end antenna coupling is to be found in AC only (transformer isolated) receivers and live chassis receivers.

C3..Bottom end coupling article, W.W, Aug. 1975 mag page 389:

https://www.americanradiohistory.com...ld-1975-08.pdf

Lawrence.
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Old 22nd Aug 2019, 1:47 pm   #10
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Default Re: Understanding Ekco U332 schematic.

great many thanks
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