7055, this thingy?

scoloheros

Super Member
What is this thing I circled on the STR-7055? So what is it, is it important and do I have to fix in order for this receiver to work? I haven't plugged it in yet, I was just cleaning it up and saw a few probs. I picked it up locally. It looks like it was sitting in a damp garage but it doesn't look too bad.
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The wiper in that trimpot got snagged and pulled up off the carbon track.
It may be important to keep things from going "ka-blooy!" when you turn it on if a bias pot.
I wouldn't plug it in until this is worked out, and be sure it didn't happen somewhere else as well.
 
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I saw that, I want to know if it will go "ka-blooy" if I can't fix it. It was obviously adjusted and fixed there with the white glue. It looks too spindly and weak even when working to be a fire preventer but I could be just as wrong. Anybody know what this thing does?
 
It sets a voltage, current, or signal level, depending on application and wiring.
You could try carefully retensioning it firmly against the carbon track at the point it originally contacted. Then it may be safe to plug in and switch on.
 
OK, I just looked at it again it looks like it was one piece and that the other part is making contact, need a dental mirror, I can't see back there. btw the guy I bought it from said it worked even though it probably shouldn't have been plugged in. I have one that works and I like it so I was going to buy this for parts but now I think I may just try to get it going, it's not as bad as I thought it was.
 
OK, I just looked at it again it looks like it was one piece and that the other part is making contact, need a dental mirror, I can't see back there.
I think I see the contact button on the lifted bit, try to get a good look at the other part with good strong light.
 
It's a trimmer as already pointed out. The unit should not be powered
up until it's sorted. It may smoke otherwise.

Service manual shows it as RT-701 10K(B) on the power amp board, used for biasing.

Replace with bourns 3296P series, double check pinout and adjuster orientation from
datasheets.

652-3296P-1-103LF, Mouser $3.27-
 
Alright, I think I fixed it, a pic just for the heck of it, computer scope comes in handy. But better if I just buy what mbz found, thanks I'm going to do that. I wonder if I messed that up, I must have hung it up on the long sleeves I have on because I was down there around the volume knob with deoxit. I really appreciate the help Pio and mbz, I'd be completely lost!
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A braided wire wasn't soldered to the tuner cover anymore also, going to fix that. What kind of ka-blooy thing could happen if that wasn't attached?
 
Oh OK then it's stable now, it's not going to come off like that again. Due to the little kid in me that runs my life sometimes along with being confident that trimmer was making contact where it's supposed to, I turned my head away and turned it on. By the way I could tell exactly were that trimmer used to be and moved it there. I hooked it up to my CS-A500 speakers that were previously hooked up to my professionally serviced HK730. It sounds just as good as the 730, maybe better but it does have a little more power. It's not perfect though, "maybe" the left channel is a little weak and when I switch it to L+R speakers on the Sony, it's noticeably quieter. Not that I expected everything to be perfect, original capacitors and all. It cleaned up pretty well, candle wax, dust, webs, some dead bugs.
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Check the relative temperatures of the output heat sinks between channels.

I turned it on for a while, the ones circled in blue are barely warm but the ones circled in red are noticeably warmer. I'm going to remember that idea, does that confirm a weak channel? Only makes sense to me that it does.

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As mbz mentioned the pot that was broken is used for biasing one of the amplifier channels. The one you fixed is probably either set too high or too low causing the difference in heat between channels you're feeling.
I wouldn't use it anymore until you repair it. Some amps go all explody when those pots open up. Either way, you're not going to be able to properly set the bias without a functioning pot.

Order up some mini grabbers for your multi-meter before attempting the set the bias. Lots of smoke has been released on this forum due to a slipped probe.
 
OK. I don't want to go too tech on such a specific problem but I have a question having to do with that bias trimmer. So is it standard that one channel is generally hardwired and that bias pot is used to match the values of the other to it? I know it's such a simple question but I don't know the answer to it. I can take things out and replace them but I haven't learned very well just what everything is doing in these.
 
it sets the current through the output transistors as a basic starting point so they run correctly throughout the normal operating range .
its best to replace the trimmer and adjusting it properly .
 
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