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 Post subject: Intractable hum problem solved
PostPosted: Oct Sat 15, 2016 5:27 pm 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 302
Location: Windsor, CT USA
After restoring my Grommes 61pg 6L6-based mono integrated amp, I had a hum problem with the volume at minimum. Not a loud hum, but irritating nonetheless. I finally traced it to an unexpected source. The Grommes uses PECs underneath the bass, treble, and loudness controls. My other Grommes 61pg uses discrete capacitors and resistors. Anyway, the twisted pair of wires leading to the on-off switch passed directly over one of the PECs. I happened to move the wires, and the hum level changed! Apparently, the PEC was acting like an inductive pickup which absorbed 60 cycle AC waves from the twisted pair. Moving the PEC and wires away from each other gradually reduced the hum. I eventually added wire to the twisted pair and completely rerouted the wires away from the PECs. Result: hum completely eliminated.

I guess I'm just going to have to replace the PECs with discrete components. No telling how off-spec the PECs are by now.


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 Post subject: Re: Intractable hum problem solved
PostPosted: Oct Sat 15, 2016 6:45 pm 
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Location: Boston, MA USA
I think PECs have indefinite life as long as they are not exposed to excessive light, which can burn them.

-David


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 Post subject: Re: Intractable hum problem solved
PostPosted: Oct Sun 16, 2016 1:57 am 
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Location: Tucson, Arizona U.S.A.
dberman51 wrote:
I think PECs have indefinite life as long as they are not exposed to excessive light, which can burn them.

-David

?
I have never seen a "burned" one but I have seen several that have failed. How would it get exposed to excessive light under the chassis?

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Jim Mueller


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 Post subject: Re: Intractable hum problem solved
PostPosted: Oct Sun 16, 2016 6:53 pm 
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Location: Boston, MA USA
They don't -- they get burned from careless handling when exposed.

-David


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 Post subject: Re: Intractable hum problem solved
PostPosted: Oct Mon 17, 2016 10:09 pm 
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Location: Chicago IL
Anyone chime me in as to what a PEC is?

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-Carlos


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 Post subject: Re: Intractable hum problem solved
PostPosted: Oct Mon 17, 2016 11:30 pm 
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I'm with you Carlos??????????????????? :D

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 Post subject: Re: Intractable hum problem solved
PostPosted: Oct Tue 18, 2016 12:13 am 
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PEC is a brand name that means Packaged Electronic Circuit. Another brand name is Couplate. It is a ceramic wafer with resistors and capacitors screened on and then conformal coated. They're pretty reliable but they have been known to fail. If you know what is inside, you can replace it with discrete components.

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Jim Mueller


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 Post subject: Re: Intractable hum problem solved
PostPosted: Oct Tue 18, 2016 5:44 pm 
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Location: Boston, MA USA
Oops, I assumed incorrectly that this amplifier had some sort of lamp-driven audio controls using Photo Electric Cells (known as PECs).

The other usage for PEC is Packaged Electronic Circuit as was noted.

Never mind.....

-David


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 Post subject: Re: Intractable hum problem solved
PostPosted: Mar Fri 15, 2019 11:34 pm 
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Joined: Mar Mon 26, 2018 9:20 pm
Posts: 6
Location: Calgary,Alberta T2R0S9
Hello Mstamper, could you get back in touch with me in regards to restoration of the Grommes 61PG, I'd appreciate any help you could offer in doing this resto?
Ken
gold@nucleus.com


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 Post subject: Re: Intractable hum problem solved
PostPosted: Mar Sat 16, 2019 3:13 pm 
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Joined: Aug Sat 01, 2009 2:23 pm
Posts: 1656
You find PEC's in a lot of hi fi audio equipment, some of it considered quite "high end". For example, McIntosh used them as well as Fisher and quite a few others. My last restoration of a Fisher 800 series receiver presented me with a PEC related problem. Measuring the phono section I found a huge difference between left and right channels. The problem was the PEC in the feedback loop of the phono preamp. Thankfully it only required 4 parts to replace it.

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John Stone
Antique Radio Club of Illinois


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