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 Post subject: Re: Just a hum......
PostPosted: Feb Mon 19, 2018 1:01 pm 
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R5 sets the DC balance for the output tubes....does not affect hum.

Note what appears to be an error on the schematic: The note on adjusting "R6" presumably refers to R5.

I assume R5 is the one previously shown on the back of the chassis

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 Post subject: Re: Just a hum......
PostPosted: Feb Tue 20, 2018 3:58 am 
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The schematic tells to adjust it using a VTVM. The control is a potentiometer, adjusted with a screwdriver.

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 Post subject: Re: Just a hum......
PostPosted: Feb Tue 27, 2018 10:53 pm 
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Well unfortunately the chassis re-cap did not remove the hum, so I think I now need to replace the large canned cap.......whats the best way to do this? Never done one before........


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 Post subject: Re: Just a hum......
PostPosted: Feb Tue 27, 2018 11:10 pm 
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addyboy wrote:
Well unfortunately the chassis re-cap did not remove the hum, so I think I now need to replace the large canned cap.......whats the best way to do this? Never done one before........

Replacing that type of electrolytic capacitor is the primary reason to replace old capacitors. They cause hum when old and not working. There are three electrolytic capacitors inside. Most straight forward is to replace them as individual capacitors.

Modern ones look this this:
Attachment:
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Most of the time the square/triangle/half circle symbols indicate the positive leads; they share a common negative.
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Replace them with a pair of 47mfd and a 22mfd (modern values).

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Last edited by AJJ on Feb Tue 27, 2018 11:19 pm, edited 2 times in total.

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 Post subject: Re: Just a hum......
PostPosted: Feb Tue 27, 2018 11:18 pm 
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AJJ wrote:
addyboy wrote:
Well unfortunately the chassis re-cap did not remove the hum, so I think I now need to replace the large canned cap.......whats the best way to do this? Never done one before........

Replacing that type of electrolytic capacitor is the primary reason to replace old capacitors. They cause hum when old and not working. There are three electrolytic capacitors inside. Most straight forward is to replace them as individual capacitors.

Modern ones look this this:
Attachment:
imgres.jpg


Most of the time the square/triangle/half circle symbols indicate the positive leads; they share a common negative.
Attachment:
Screen shot 2018-02-27 at 2.10.24 PM.jpg


Replace them with a pair of 47mfd and a 22mfd (modern values).



Nice......will do!

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 Post subject: Re: Just a hum......
PostPosted: Feb Tue 27, 2018 11:21 pm 
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Many of us leave the old one on the chassis (but disconnected). Make sure new ones' voltage ratings meet/exceed original values (450V).

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Last edited by AJJ on Feb Tue 27, 2018 11:29 pm, edited 1 time in total.

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 Post subject: Re: Just a hum......
PostPosted: Feb Tue 27, 2018 11:26 pm 
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AJJ wrote:
Many of us leave the old one on the chassis (but disconnected). Make sure new ones have voltage ratings above the original values (450V).


Will do......I guess I am going to have to build some kind of support so they are not floating around.

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 Post subject: Re: Just a hum......
PostPosted: Feb Tue 27, 2018 11:30 pm 
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addyboy wrote:
AJJ wrote:
Many of us leave the old one on the chassis (but disconnected). Make sure new ones have voltage ratings above the original values (450V).


Will do......I guess I am going to have to build some kind of support so they are not floating around.


Folks usually add a terminal strip to mount the new capacitors; or there may be unused terminals already in the radio.

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Last edited by AJJ on Feb Tue 27, 2018 11:35 pm, edited 1 time in total.

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 Post subject: Re: Just a hum......
PostPosted: Feb Tue 27, 2018 11:31 pm 
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Copy that.......

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 Post subject: Re: Just a hum......
PostPosted: Feb Tue 27, 2018 11:43 pm 
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If you leave the old chassis cap in situ, often you can solder the mounting lug/s of a tag strip to it's terminals. You do not wire to those mounting lugs unless it is actually a ground. Watch the voltage rating on any set with a filament rectifier, or anything that behaves like one, when mixed with heater tubes,

Marc


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 Post subject: Re: Just a hum......
PostPosted: Feb Wed 28, 2018 12:27 am 
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Marcc wrote:
If you leave the old chassis cap in situ, often you can solder the mounting lug/s of a tag strip to it's terminals. You do not wire to those mounting lugs unless it is actually a ground. Watch the voltage rating on any set with a filament rectifier, or anything that behaves like one, when mixed with heater tubes,

Marc


Thank you Marc.... 8)

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 Post subject: Re: Just a hum......
PostPosted: Feb Wed 28, 2018 3:38 am 
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replace each wire that goes from the balance control to the cathode of each 6BQ5 with a precision 1 watt 1 ohm resistor.

record the voltage reading in mV across each resistor, adjust pot until they are equal. they should equal out somewhere in the 30 mV range.

this is not a "hum balance" control that can be adjusted to eliminate an AC hum as some amplifiers have.

this is using the "cathode resistor method" to adjust/match the cathode current draw between the two tubes.

you can not get this adjustment right "by ear".

proper adjustment of this control will allow the tubes to be 'matched' if they are not matched from the factory.

I've worked on quite a few of these amplifiers over the years. also, you might want to replace the ceramic disk capacitors as well.

I've had a number of them that were defective causing "airy" and "wind" noise in the preamp section ; hiss/crackle/pop as well.

also, I would install 500 volt electrolytics. this amplifier, since it uses a direct heated 5y3, it will have a turn on surge of current inrush, causing the voltage across (especially the) first filter to see well over 450 volts for a few seconds.

I don't trust even name brand good quality electrolytics (panasonic, nichion) to see a surge over their working voltage.

antique electronic supply sells Illinois capacitors at 10, 22, 33, and 47 mfd @ 500v.

steve

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 Post subject: Re: Just a hum......
PostPosted: Feb Wed 28, 2018 5:17 am 
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...that was a long detour back to the initial post asking about what to do with hum. The first two replies to the initial post 11 days ago recommended replacing the electrolytic capacitors... :)

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 Post subject: Re: Just a hum......
PostPosted: Feb Wed 28, 2018 3:25 pm 
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AJJ wrote:
...that was a long detour back to the initial post asking about what to do with hum. The first two replies to the initial post 11 days ago recommended replacing the electrolytic capacitors... :)


Yeah, I didn't realize it meant the big can as well........ :oops:

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 Post subject: Re: Just a hum......
PostPosted: Feb Wed 28, 2018 5:04 pm 
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Dutch Rabbit wrote:
also, I would install 500 volt electrolytics. this amplifier, since it uses a direct heated 5y3, it will have a turn on surge of current inrush, causing the voltage across (especially the) first filter to see well over 450 volts for a few seconds.
I don't trust even name brand good quality electrolytics (panasonic, nichion) to see a surge over their working voltage.
antique electronic supply sells Illinois capacitors at 10, 22, 33, and 47 mfd @ 500v.
steve


Great advice, thank you........I think I will have to go with radials, there's no room for axials.

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 Post subject: Re: Just a hum......
PostPosted: Mar Thu 01, 2018 3:01 am 
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you are in luck. the capacitors i recommend come in axial.

see the 500v ones here:

https://www.tubesandmore.com/products/c ... 3DIllinois

steve

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 Post subject: Re: Just a hum......
PostPosted: Mar Thu 01, 2018 3:04 am 
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you also could increase that 135 ohm common cathode wirewound resistor to 150 ohms to take the edge of how zenith runs those tubes so hot (magnavox and fisher do the same). the power output difference will be hardly noticed, if even noticed at all. those poor little tubes will be happier. the B+ will increase a tiny amount as well.

steve

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 Post subject: Re: Just a hum......
PostPosted: Mar Thu 01, 2018 12:05 pm 
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+1 on 500V caps minimum. One often forgets that the first filter cap could be (generalising) running 50V higher than the load side of the choke / field. But anything on B+ will cop the whole surge voltage prior to heater tubes conducting.

Marc


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 Post subject: Re: Just a hum......
PostPosted: Mar Mon 05, 2018 7:34 pm 
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Update:
So marginally better but hum still there, it's more pronounced at the low and midrange.....at full volume it disappears.


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 Post subject: Re: Just a hum......
PostPosted: Mar Mon 05, 2018 9:09 pm 
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Steve,

It may not be a major contributor to the hum you're hearing, but I think these solder joints can use more heat to ensure the joints are not "cold" :)


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Screen shot 2018-03-05 at 11.49.55 AM.jpg [ 121.41 KiB | Viewed 255 times ]

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Last edited by AJJ on Mar Mon 05, 2018 9:48 pm, edited 5 times in total.
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