[ARC5] Capacitor C-5 in BC-454

[Brian Clarke]

Hello Chris,

 

There was no attachment.

 

C5 in the original specifications (TO 08-10-50) is an electrolytic capacitor. After 80 years, whether in service or on the shelf, it will have dried out and lost capacitance. It is there to suppress noise that might have been generated by the wiping action of the SENSITIVITY control, a wire-wound resistor and noise pick-up on long remote control leads between the receiver rack and the remote control. The original specification for C5 was not more than 1750 Ohm ESR at 60 Hz – today, that’s a pretty ho-hum specification – but it is NOT a leakage resistance. The Maintenance Manual only specifies DC resistance; it does NOT mention leakage resistance; I think that is your interpretation.

 

The only reason you are reading any B+ Voltage is because you applied it. There is no large HT storage capacitor in the BC-45N series of receivers.

 

In the Systeme Internationale (SI) standard for units and measures, lower case k refers to 1000; upper case K refers to Kelvin, a measure of temperature.

 

73 de Brian, VK2GCE

 

From: Chris Hu [mailto:lu1eom.ch at gmail.com] 
Sent: Saturday, 5 February 2022 3:46 AM
To: Brian Clarke
Subject: Re: [ARC5] Capacitor C-5 in BC-454

 

Helo Brian, 

you are right my mistake in between "." and ","

If you read in Resistances attachment herein, you will read about C-5 leakage resistance 300,000 ohms.

I tried to say 300,000 ohms or 300K. I wrote down what is my confusion getting B+ at C5,Why is there  B+.

Any comments will appreciated.

I'll study you answer and make some tests.

Best Regards,

Christian

 

 

 

 

El vie, 4 feb 2022 a las 2:17, Brian Clarke (<brianclarke01 at optusnet.com.au>) escribió:

Hello Christian, 

Be careful in your use of units. A full-stop in a string of numerals is usually taken as a decimal point. If you want to express higher values, use the comma to separate numerals into groups of 10^3. Your statement of 300.000 Ohm is always interpreted as 300 Ohm. To express 300 k Ohm in full, you should type 300,000 Ohm. Further, it is customary to have a single space between the last numeral and the unit, whether as an abbreviation / symbol (eg, Ω) or in full. 

The whole purpose of the line to which C5 is connected is to alter the grid-to-cathode bias on the RF and IF amplifiers. That bias is varied via the SENSITIVITY control which, operated in a normal installation, is directly across C5. 

The nominal value of 300 k Ohm is NOT the leakage resistance of C5. That resistance to ground at the ‘hot’ end of C5 is via R10 and assumes that:

·         the dynamotor is in place to provide a low resistance to chassis ground

·         the dynamotor is not running, and 

·         no SENSITIVITY control is connected. 

However, C5 is across the SENSITIVITY line. If you have the normal 50 k SENSITIVITY control pot at full resistance with the dynamotor in place but not running, the reading could be 43 k. Any other setting of the SENSITIVITY control and the resistance will fall. With 250 Vdc applied to the receiver, the maximum should be about 36 Vdc. At maximum SENSITIVITY, the resistance and Voltage should be 0. If you have a lower resistance without the SENSITIVITY control in circuit, you have a circuit fault, eg, leakage of the cathode bias capacitors, such as C6, C7 and so on. I usually test those potted capacitors by undoing the 2 mounting screws to leave the capacitor body swinging freely. When I re-assemble, I remove any corrosion on the inside of the chassis and use a zinc-rich grease on the screws and chassis. 

Bearing in mind that your BC-454 is probably 80 years old, the nominal 300 k of  R10 could easily have increased by 20% or more, to perhaps as high as 500 k. So long as there is still some resistance in R10, ie, it is not open circuit (most unlikely), the circuit will work properly. 

73 de Brian, VK2GCE

 

From: arc5-bounces at mailman.qth.net [mailto:arc5-bounces at mailman.qth.net] On Behalf Of Chris Hu
Sent: Friday, 4 February 2022 9:59 AM
To: ARC5 at mailman.qth.net
Subject: [ARC5] Capacitor C-5 in BC-454

 

Hello,

I'm trying to solve  a problem with the C-5 capacitor in my receiver BC-454

According to the "Resistance to ground from receivers terminals" 300.000 Ohms. is the leaking resistance of C-5. 

In my case I have 400.000ohms without C-5.And voltage of 230V on terminals 3 & 5 of V3, 8 of V4 and 3 & 5 of V5.

Once C-5  is connected, I get 3500.000ohms and 110v voltage.I'm sure C-5 is not working properly.

I replaced C-5 for a cap 3uF 300V and not solution.Voltage do not drop to 4V as it should be.

Resistance is still higher, it's around 400K ohms when it should be around 300K.

Please any idea or help to get good resistance and voltage.??

All the test are done without  Dyna or tubes.

Regards,

Christian

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