|
The Fisher 700-T Receiver
02-13-2015, 07:53 PM
Headphones..or speakers...unless you tell me it was not there before....all transistor amps do that. In fact one of the way to counter that was using a relay with a delay, it would enage about 2 sec after power up when the transition is over.
02-13-2015, 07:55 PM
The noise was not there before I reinstalled the front, top and bottom covers.
Running on headphones the entire time, no speakers connected - just the headphones. Speaker switch in the OFF position. Oh, and the metallic "thump" sound is coming from the unit itself, not through the headphones. It only does it once, as soon as it is powered up. -- Ron Ramirez Ferdinand IN
02-13-2015, 09:37 PM
Well, I hooked up a pair of speakers to it and tried it again.
Other than the metallic thump when first turned on, it sounds great. Nice and clear in both channels and the Stereo Beacon works as it should. Later on, I'll try removing the covers and then try it again to see if the thump goes away. As I said before, the thump wasn't there with the covers off. (Strange thing - now the on-off switch is working again.) 
-- Ron Ramirez Ferdinand IN
02-13-2015, 09:41 PM
Transformer magnetized by inrush current? Makes covers thump?
02-13-2015, 10:02 PM
Yes - I think this is what it is. I placed my hand on the top cover, turned it on, and could feel a slight, very brief vibration in the top cover when I turned the unit on.
Is this something correctable? Or is it nothing to worry about? It's weird...none of my other working receivers do this. And, sometimes the thump is louder than others. It is loudest when first turned on from a cold start, after sitting (off) for awhile. After it is on, I can turn it off and immediately back on and it doesn't make the thump noise. -- Ron Ramirez Ferdinand IN
02-13-2015, 10:09 PM
Yes that points to inrush.
When amp is cold and the caps are fully discharged the inrush current is the highest and limited by only parasitic transformer's resitance and inductance. When turning on fast after turn off, the caps are partly charged and the inrush is smaller. If it does not bother you, simply put some foam rubber or something soft and not flammable between the cover and what it hits aginst or something....
02-13-2015, 10:12 PM
Okay, so it's nothing to worry about, then? That's a relief...I'll see what I can find. There is very little space between the top cover and the top of the power transformer, and I think that is where the thump is coming from.
Hey - how about some nice asbestos?  Just kidding! -- Ron Ramirez Ferdinand IN
02-13-2015, 10:25 PM
Hey I can send you one of the two asbesos mats from one of my to Zenith 7G605s.
 Just kidding. I want to keep them for myself.
02-13-2015, 10:27 PM
There is a thing called NTC thermistor, Negative Thermal Coefficent one, some of them are large enough and are used as inrush suppressors. I used them.
http://www.mouser.com/ProductDetail/Amet...gnCA%3d%3d http://www.ametherm.com/inrush-current/i...heets.html You need to pick one with cold resistance so it limits the current to say 1/10 of what it is and when hot does not drop much. Or you could put one on the MAINs side. A different model.
02-13-2015, 10:33 PM
You mean like the CL-90 inrush current limiter?
I was just sitting here thinking that something like that might be a good thing for this receiver. I would sure like to finish it up...it keeps mocking me, as if it is saying, "G'ahead, fix me...I dare you..." Regarding the off-on switch: I do have one more junk Fisher to possibly draw a off-on-volume control from if I have to replace it (again). Or I could try opening this one up (again) and blast the heck out of it with DeoxIT (again). -- Ron Ramirez Ferdinand IN
02-13-2015, 10:36 PM
Do not use Deoxit, it protects but does not clean what is there.
But alcohol does. Theeeen (as chinese girls say it...very melodically) you can se Deoxit, to keep the oxidation out.
02-13-2015, 10:39 PM
What is your NAINs max drawn current?
I just remember I have some from one of the pojects I did, I had to limit inrush in the piwer supply I developed for Iindia. I reduced it from measured 40A to 5A. I might have something that would fit the application.
02-13-2015, 10:43 PM
(02-13-2015, 10:39 PM)morzh Wrote: What is your NAINs max drawn current? What is my what?!? Oh, I think you meant to type mains, yes? I'll check it later this weekend and get back to you. Getting late here  ...of course, I am on the same time zone as you but still...
-- Ron Ramirez Ferdinand IN
02-13-2015, 10:49 PM
I have just pulled out what I have....wait a few minutes.
« Next Oldest | Next Newest »
Users browsing this thread: 2 Guest(s) |
