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PostPosted: 24 Jun 2010, 13:26 

Joined: 18 Jun 2010, 09:31
Posts: 3
hi folks,
i am new to in depth circuit electronics. i have a limited knowledge of ac wiring but found that it doesn't translate too well into this. it all started when i was trying to wire up a couple of guitar pick ups with a dual fader. man was i confused. so i started reading. it got me interested in more than just passive guitar wiring. i have been trying to educate myself using online educational sources as well as some rather shallow books. it will take me a while to keep moving through the course i am now following. but in the meantime i am trying to gain practical knowledge of circuitry. i have no one to learn from so i'm putting it out there for some of you kind gentlemen to help(where are all the women anyway?). i have been acquiring free tube amps from old stereo components. a couple i have work to some degree. and have possibly narrowed down the culprits to the filter caps. i am now in the process of finding the electronics lists so i can replace them. more on that another time....

now here is where i need some advice......

i just got hold of a motorola 67hfk2 stereo. the chassis is a beauty(HS-544). i took everything out of the console and hooked it up on my work table as it was in the console. i plugged the power into a variac. i was bringing power up slowly as i did with the other units. about halfway through(and about 4 minutes or so into it) i started to get an awful hum. non of the potentiometers would affect it. i tapped on all the tubes. only one produced sound by tapping. i replaced it. same. the hum was constant. so i shut down. i went back and made sure everything was connected properly. i looked again underneath for signs of a damaged component or an unconnected wire. i changed out the speaker to one i know works. i tried it again. same results.
so now i'm not sure what the next step is. it didn't seem prudent to disconnect the speaker and continue to full power. so here i sit. all thoughts are appreciated in advance.

PostPosted: 25 Jun 2010, 23:31 

Joined: 26 May 2009, 18:51
Posts: 129
Location: Lilburn, Georgia USA
Hello, It would have to be the power supply filter caps causing the hum. They dry out and also end up shorted out and are a sure way to fry a power transformer. I would go online to Antiqueelectronicsupply and order some new filter caps as well as any other electrolytic caps in the unit. I have revived two old tube HI-Fi sets, one a Sears Silvertone from the mid sixties and a 1957 Stromberg Carlson console with AM-FM and phonograph. The Stromberg was a hummer. New caps and it works fine now. Good luck and welcome to the forum. Robert Parzych

PostPosted: 26 Jun 2010, 06:22 

Joined: 04 Oct 2008, 11:29
Posts: 375
Location: Chillicothe, Ohio
I have been doing some vintage restorations of my own, and I agree with rparsh and his assessment.

While other things can cause hum, in the described scenario of an amp some 40-50 years of age, undoubtedly this would be a leading cause. And, even if this doesn't take care of the hum issue, it is something that needs to be done anyway. Electrolytic capacitors really need to be given a hard look. Sometimes they can be successfully reformed, other times not. And with the cost of said capacitors, it's a small price to pay for the end results achieved. Replacement also makes for a safer amp to use.


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