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 Post subject: Re: Just a hum......
PostPosted: Mar Mon 05, 2018 9:31 pm 
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AJJ wrote:
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" :)


Copy that, will go over them again......

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 Post subject: Re: Just a hum......
PostPosted: Mar Mon 05, 2018 10:17 pm 
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Dutch Rabbit wrote:
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.


So my next task is to replace these......Mica's right?


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 Post subject: Re: Just a hum......
PostPosted: Mar Tue 06, 2018 1:09 am 
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Managing expectations:

I think some low level hum is inherent in some sets, but should be at a very low audio level when everything is properly installed in the cabinet.

Ceramic and mica capacitors have a very low failure rate. I've seen some reports on this forum (like Dutch Rabbit's comment) that certain disk ceramic capacitors in some 1950's sets have gone bad. Keep in mind that not all "bad" capacitors result in hum.

Perhaps similar note was in your set's original owner's manual. Here's are notes from the Eico HF-20 and HF-60 manuals.

https://www.hifiengine.com/manual_libra ... f-20.shtml
https://www.hifiengine.com/manual_libra ... f-60.shtml


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 Post subject: Re: Just a hum......
PostPosted: Mar Tue 06, 2018 2:38 am 
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if those ceramic disc capacitors are not causing you any problems, you *could* let them alone ;

buuuutttt, when my set messed the bed, the whole set was "restored" and all assembled for useage and display.

when i was enjoying it in my shop for a few days, all was well. when i put it in the radio room and was enjoying it even more, that's when the problem started.

soooo, i had to take it all apart again and whilst i was in there :!: THE SECOND TIME :!: , i replaced all those discs.

:evil:

i avoid "repairing the restoration" over a few 99 cent capacitors.

at least this time, a set did not fail when i was demonstrating it for friends and family.

i've been down that road, sailed that ship, visited that venue and have the hat/t-shirt combo for the souvenir.

it is your set and it is all up to you, but if it were mine, they would be gone.

some may disagree :|

steve

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 Post subject: Re: Just a hum......
PostPosted: Mar Tue 06, 2018 5:30 am 
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AJJ wrote:
I think some low level hum is inherent in some sets, but should be at a very low audio level when everything is properly installed in the cabinet.
Ceramic and mica capacitors have a very low failure rate. I've seen some reports on this forum (like Dutch Rabbit's comment) that certain disk ceramic capacitors in some 1950's sets have gone bad. Keep in mind that not all "bad" capacitors result in hum.


Thanks AJ, as always I look forward to your insight and knowledge.......I isolated the power input from the turntable, as it is switched from there (and I needed to rule that out) and hooked it up to a desk-top speaker. With not having the big bass hum from the 14" speaker, I did have the hiss/crackle/pop that Dutch Rabbit talked about.......so I guess I need to rule out the ceramics?

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 Post subject: Re: Just a hum......
PostPosted: Mar Tue 06, 2018 7:30 am 
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Yep. Noises like that shouldn't be there for a fully functional set. My only experience with these noises were due to failing tubes and arcing in damage switch. If you can't find another cause quickly, it's not unreasonable to spend a couple of hours to swap out those ceramic capacitors.

I vaguely remember reading about problems with those Zenith disc capacitors. A quick search turned up this thread:

http://www.antiqueradios.com/forums/vie ... 9&t=283669

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 Post subject: Re: Just a hum......
PostPosted: Mar Tue 06, 2018 3:11 pm 
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AJJ wrote:
I vaguely remember reading about problems with those Zenith disc capacitors. A quick search turned up this thread:
viewtopic.php?f=19&t=283669


Well that was interesting!

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 Post subject: Re: Just a hum......
PostPosted: Mar Tue 06, 2018 4:39 pm 
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Disregard.......found solution.


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 Post subject: Re: Just a hum......
PostPosted: Mar Tue 06, 2018 11:05 pm 
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And the answer was?

Even with good disk ceramics, I have, in an Oscilloscope, had to remove them for picking up noise. They may have to be treated as crossing wires, or did you just eliminate them.

There is often potential to get hum modulation at the Mixer where there are AC heaters, dirty DC feeding filaments, or leakage at the cathode.

Marc


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 Post subject: Re: Just a hum......
PostPosted: Mar Wed 07, 2018 12:46 am 
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Ok, this is just getting plain weird.
Started to replace the ceramics (except two, more coming on that) and the parts list didn't have a voltage rating for C5, C15 and C16....thats what the disregard, found solution post was about.
C18 also did not have a voltage rating, and after coming up zero with a web search, decided to go with a 1000v as that seems to be the default......I know, a bold assumption.
When all was installed, I hooked up and turned on......wow!
Hum was noticeably better, but when I increased the bass it sounded like a machine gun and the presence would squeal when adjusted. I shut down and went back to the bench.
C18 is the first in line from the presence pot, so I removed it and put the old one back....I don't have another cap to put in it's place.
Powered up and all is good, but the hum is back.......

Why is C18 double the size of what it's replacing and all the others are 5 times smaller? All the old Zenith caps are uniform in size.....
And I don't get the major audio disruption......thats a brand new cap.


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 Post subject: Re: Just a hum......
PostPosted: Mar Wed 07, 2018 1:01 am 
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According to the documentation, C18 is 100pf (.0000000001); the unit for that capacitor on that chart is "pico" (micro-micro), not "nano." Your written notation says ".1μ" (.0000001).


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Last edited by AJJ on Mar Wed 07, 2018 1:22 am, edited 2 times in total.
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 Post subject: Re: Just a hum......
PostPosted: Mar Wed 07, 2018 1:09 am 
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Damn!
I noted the nf not the uf!

Thanks AJ!

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 Post subject: Re: Just a hum......
PostPosted: Mar Wed 07, 2018 1:15 am 
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Well I guess I now know how to introduce machine gun bass!

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 Post subject: Re: Just a hum......
PostPosted: Mar Wed 07, 2018 1:31 am 
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:)

That c18 is in an interesting circuit labeled "Presence." It takes audio output and feeds back to one side of the push-pull audio.

I'm curious, if you disconnect that circuit completely (disconnect C18 and R27), would the hum go away?


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 Post subject: Re: Just a hum......
PostPosted: Mar Wed 07, 2018 1:38 am 
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AJJ wrote:
:)
That c18 is in an interesting circuit labeled "Presence." It takes audio output and feeds back to one side of the push-pull audio.
I'm curious, if you disconnect that circuit completely (disconnect C18 and R27), would the hum go away?


Thats a great observation, will try it out tomorrow while enjoying a snow day........

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 Post subject: Re: Just a hum......
PostPosted: Mar Wed 07, 2018 5:58 pm 
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AJJ wrote:
:)
That c18 is in an interesting circuit labeled "Presence." It takes audio output and feeds back to one side of the push-pull audio.
I'm curious, if you disconnect that circuit completely (disconnect C18 and R27), would the hum go away?


Well I tried your suggestion but no change, actually a worse hum.
Put the correct new cap back in and the original hum is still present.....gotta admit, this one is a mystery.

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 Post subject: Re: Just a hum......
PostPosted: Mar Wed 07, 2018 6:01 pm 
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Marcc wrote:
There is often potential to get hum modulation at the Mixer where there are AC heaters, dirty DC feeding filaments, or leakage at the cathode.


You mean the stereo/balance mixer?

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 Post subject: Re: Just a hum......
PostPosted: Mar Wed 07, 2018 8:24 pm 
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Assuming there are otherwise no faults or deviations from the schematic:

The schematic shows two different configurations for the speakers. One with no crossover, the other with just a 3mfd capacitor to block bass from the tweeters. If you connect a more modern stereo speaker there (with a more sophisticated crossover), see if there's hum.

Does adjusting the "Presence" control change the volume of the hum?

One last idea. The diagram is from "Elements of Radio Servicing." "C-12" is the same capacitor as C25 and C26 (47pf) on your set. They're probably ceramic or mica. Double check for the correct value.


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 Post subject: Re: Just a hum......
PostPosted: Mar Wed 07, 2018 9:38 pm 
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AJJ wrote:
The schematic shows two different configurations for the speakers. One with no crossover, the other with just a 3mfd capacitor to block bass from the tweeters. If you connect a more modern stereo speaker there (with a more sophisticated crossover), see if there's hum. Does adjusting the "Presence" control change the volume of the hum?
One last idea. The diagram is from "Elements of Radio Servicing." "C-12" is the same capacitor as C25 and C26 (47pf) on your set. They're probably ceramic or mica. Double check for the correct value.


So, connected a modern speaker and still hum, mind you, it wasn't as big as the 12" in the cabinet.
Adjusting the Presence just changes clarity, no effect on the hum.
C25 and C26 were swapped out this morning....no change.

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 Post subject: Re: Just a hum......
PostPosted: Mar Fri 09, 2018 5:38 pm 
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Well after some more research, trying different preamp tubes was the best result.
The 12AY7/6072 is an excellent sub for the 12AX7 and the hum has been reduced by half........

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