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Need help diagnosing hum/output issue on Onkyo TX1500

 
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PostPosted: Wed Oct 13, 2010 1:54 pm    Post subject: Need help diagnosing hum/output issue on Onkyo TX1500 Reply with quote

Here's hoping someone can help...

I just picked up a vintage Onkyo TX-1500 receiver from the Goodwill store and at first test I loved it. It sounded great. I then decided it needed to be cleaned up etc, so I took it to work and got it up on the bench, immediately noticing it had two blown fuses, which turn out to be fuse lamps for the front panel. I also did not power in on a second time before taking apart at the shop, so I'm not sure if the following problem existed in transport etc but at any rate, I replaced the fuses with radioshack equivalents which turned out to be wrong because they're fuse lamps. I powered it up and found that there were no front panel lights and that made me think there was no power to the unit so I also changed the power fuse. Still nothing. I ended up putting all the original fuses back in place and plugging in headphones to test. I got lights, but there is now a nasty hum in the right channel and the audio on that side is gone. I sprayed deoxit in the headphone jack and cleaned it. I checked the balance knob and it doesn't seem to be working properly, but I have not sprayed it out yet. I didn't feel like this was a balance issue at first because it worked fine at home. I did pull the knobs off to clean them but that shouldn't have affected it....

I also broke off a small black wire that I believe went from the LED board on the front panel (still connected there) to i think the fuse lamp PCB, but I really am not sure and can't remember where it went. Probing it around to the obvious places doesn't solve anything or hurt anything so it's hard to figure out where it went though it couldn't have been far away as the wire length is short.

I tested the unit at home with a turntable plugged in and speakers, and I confirmed that I do get audio on the left channel and the tone controls and volume work, but the right side still gets that hum and no audio. The hum is present even with the volume down all the way. It almost feels like a grounding issue but I am having trouble figuring out what went wrong...

If anyone has any info on this I'm pretty desperate to get it going... I got it for real cheap but I loved the sound of it and I'd hate to dump it or pay out the rear-end for service on it....
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vtech



Joined: 08 May 2006
Posts: 1264
Location: USA

PostPosted: Sun Oct 17, 2010 3:58 am    Post subject: Reply with quote

Here is a couple of schematics.
The yellow is the pcb for the lamps and all you need is two wires direct from txformer. Second one is the main & part of preamp. perhaps a quick dc voltage check may reveal something. Sounds like the hum may be either a bad connection issue or more like a "man made" problem. Either way it should be easy to find out.




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PostPosted: Wed Oct 20, 2010 2:38 pm    Post subject: Reply with quote

Thanks for that- I did download the manual but I was having difficulty trying to find a place to start...

Someone suggested I may have blown the power amp, STK075 on the right channel. I figured if it was that, I'm not going to repair it because 1) I already have another working receiver, and 2) it would not be cost effective for what it is..

What I did do, if it helps diagnose and since i'm testing with phones (but speakers revealed the same issue), is unscrew the headphone jack from the chassis, and a mono signal returned to both sides. I confirmed that it was mono by playing a known stereo album through it and it played in mono. I eliminated the hum by grounding one of the wires from the headphone jack (yellow) to the chassis. I don't know what this really reveals, plus it didn't actually "fix" anything, but I know it's not a solution since I am now hearing everything as mono and obviously re-routing the signal path somehow by jumping it...
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minnie



Joined: 18 Aug 2005
Posts: 2877
Location: Hell

PostPosted: Wed Oct 20, 2010 11:20 pm    Post subject: Reply with quote

The amp should be less than $20. What's the markings on it?
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