Bogen CHB20A audio amplifier question

[VE2ITZ]

Hello all:

I got a question that needs to be clarified;

I am working on a Bogen amp and i was measuring the voltages at the power supply  as per the schematic diagram of which i include a link to it:

http://www.emailtom.org/images/bogen-chb20a.jpg



It says there should be 400 volts at the top of c18, then 375 volts after the 2.7 kohm 3 watt resistor (Top of c19b), and then 250 volts after the 22kohm 1 watt resistor (top of c19c).

I can see 450 volts in my voltmeter on the first point. measuring from chassis to that point. Then i go to the next point and see no voltage. Same happens with the third point.

If i go in between c18 and c19a, i see a voltage of 250 volts.

I am testing the voltages with no tubes in the sockets.

I am also making sure that i am not poking around with my two hands and making sure i am discharging the caps all the time.

The amp seems to be dead when i tried it out the first time. I can see that the filaments light up alright but there is absolutely no sound whatsoever.

Please help!!!!!!

[KA5N]

You need to check the resistance of R30 and R31 (with AC power unplugged and capacitors
c18 and c19 discharged (screwdriver to ground).  One or both of them may be open and will have to be replaced.  The voltage (250 volts) you are reading between c18 and c19 is just leakage and should be ignored.  One or both of the caps many be shorted (if so they probably
caused the resistors to fail).  If the amp is very old the caps probably need replacement anyway.
The two caps c18 and c19 and resistors R30 and R31 are a filter network to remove 120 Hertz
hum from the DC   otherwise the amp would have a loud hum from the speaker.
You really need to do some reading about audio amplifiers if you are going to work on them.

Allen

[AC5UP]

R30 is open for sure, R31 might be.

You're seeing 250 volts across C18 because the twin diode / C19A combo on the transformer B+ secondary look suspiciously like a voltage doubler circuit. Plan on replacing the electrolytics in the filter section and it wouldn't be a waste of money to replace the two rectifier diodes.

Epoxy 3 amp 1000 PIV silicon would be a good choice.

[VE2ITZ]

Got it! It is that 2.7 k 3 watt resistor that is bad.

Yes; I am planning on recapping the whole thing with new caps and as someone has suggested changing the rectifiers.

And yes I am planning on putting a bleeder resistor on the circuit to discharge them nasty caps!


Thank you all again and 73 de VE2land!

[N4NYY]

Got it! It is that 2.7 k 3 watt resistor that is bad.

That is a common value, but your will need to get Metal Oxide as a replacement. Your filter caps should be replaced with 47uf, 22uf, and 10uf, with the same corresponding voltages. Do not look for 40uf or 20uf because they are uncommon and will cost a ton. The other values will only cost a few bucks.

On C16 and C16, I would likely go with a 33uf for a common value. Those 25uf will definitely cost much more. Most of these supply companies really keep the prices down for the common values with quantity. But when I have done AA5 radios, some of those uncommon values gave me sticker shock.

[VE2ITZ]

Got it! It is that 2.7 k 3 watt resistor that is bad.

That is a common value, but your will need to get Metal Oxide as a replacement. Your filter caps should be replaced with 47uf, 22uf, and 10uf, with the same corresponding voltages. Do not look for 40uf or 20uf because they are uncommon and will cost a ton. The other values will only cost a few bucks.

On C16 and C16, I would likely go with a 33uf for a common value. Those 25uf will definitely cost much more. Most of these supply companies really keep the prices down for the common values with quantity. But when I have done AA5 radios, some of those uncommon values gave me sticker shock.

Thanks for all the valuable info.

I am off to the electronics store this weekend and see what i can get to fix this beast.



Cheers all and 73 de VE2LAnd!

[VE2ITZ]

All recapped and resistors changed.

I got it going again.

Thank you all for your valuable guidance!


Cheers and 73 de VE2ITZ,

Juan