[ARC5] Capacitor C-5 in BC-454

Fri Feb 4 19:38:16 EST 2022

 You may be chasing a "non issue". Even if c5 was dead shorted, all that would happen is the gain control would not reduce the gain. The 50K pot is an important part of the circuit. At this point , just jumper c5 to ground, put a 600 ohm load on the audio out put ( speaker or phones) and power it up with an antenna . It will probably work.

    On Friday, February 4, 2022, 06:40:05 PM EST, Brian Clarke <brianclarke01 at optusnet.com.au> wrote:  

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Hello Chris,

There was no attachment.

C5 in the original specifications (TO 08-10-50) is anelectrolytic capacitor. After 80 years, whether in service or on the shelf, itwill have dried out and lost capacitance. It is there to suppress noise thatmight have been generated by the wiping action of the SENSITIVITY control, awire-wound resistor and noise pick-up on long remote control leads between thereceiver rack and the remote control. The original specification for C5 was notmore 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 DCresistance; it does NOT mention leakage resistance; I think that is yourinterpretation.

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

In the Systeme Internationale (SI) standard for units andmeasures, lower case k refers to 1000; upper case K refers to Kelvin, a measureof 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 readabout C-5 leakage resistance 300,000 ohms.

I tried to say 300,000 ohms or 300K. I wrote down what is myconfusion 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:

HelloChristian, 

Becareful in your use of units. A full-stop in a string of numerals is usuallytaken as a decimal point. If you want to express higher values, use the commato separate numerals into groups of 10^3. Your statement of 300.000 Ohm isalways interpreted as 300 Ohm. To express 300 k Ohm in full, you should type300,000 Ohm. Further, it is customary to have a single space between the lastnumeral and the unit, whether as an abbreviation / symbol (eg, Ω) or in full. 

Thewhole purpose of the line to which C5 is connected is to alter thegrid-to-cathode bias on the RF and IF amplifiers. That bias is varied via theSENSITIVITY control which, operated in a normal installation, is directlyacross C5. 

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

·         thedynamotor is in place to provide a low resistance to chassis ground

·         thedynamotor is not running, and 

·         noSENSITIVITY control is connected. 

However,C5 is across the SENSITIVITY line. If you have the normal 50 k SENSITIVITYcontrol pot at full resistance with the dynamotor in place but not running, thereading could be 43 k. Any other setting of the SENSITIVITY control and theresistance will fall. With 250 Vdc applied to the receiver, the maximum shouldbe about 36 Vdc. At maximum SENSITIVITY, the resistance and Voltage should be0. 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 asC6, C7 and so on. I usually test those potted capacitors by undoing the 2mounting screws to leave the capacitor body swinging freely. When Ire-assemble, I remove any corrosion on the inside of the chassis and use azinc-rich grease on the screws and chassis. 

Bearingin 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 500k. So long as there is still some resistance in R10, ie, it is not open circuit(most unlikely), the circuit will work properly. 

73 deBrian, 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'mtrying to solve  a problem with the C-5 capacitor in my receiver BC-454

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

Inmy 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.

OnceC-5  is connected, I get 3500.000ohms and 110v voltage.I'm sure C-5 is notworking properly.

Ireplaced C-5 for a cap 3uF 300V and not solution.Voltage do not drop to 4V asit should be.

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

Pleaseany idea or help to get good resistance and voltage.??

Allthe test are done without  Dyna or tubes.

Regards,

Christian

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