Hi all,
I'm trying to learn to do some basic repair/up keep on my vintage gear and wanting to understand how to troubleshoot faulty parts. I've done a few recaps and have a basic understanding of things but still a greenhorn. Anyway, I have an old Mitsubishi DA-R20 receiver I'm using as my learning platform. When I got it it worked so-so. It would trip the speaker protection relays randomly and there was the usual scratchiness in some of the pots, etc. Didn't sound all that great either. I replaced the speaker protection relays and learned to set the bias and dc offset to spec which I did. I noticed some bulging caps so I did a full cap replacement. Everything seemed to be working and sounding good. Then out of the blue it went silent. Power was still on, relays went closed and I could smell heat so I turned it off and let it set. I powered it back up and took a quick measurement of the DC voltage. The right channel was still good at < 10mv. The left channel was showing 30v. Well, that's not good. I also noticed that transistor Q507 was super hot to the touch.
So I need some guidance on narrowing the issue down. I have a DMM and transistor tester and basic understanding of things but certainly understand the complexity involved so I really appreciate any assistance. I do have a service manual that someone scanned into pdf so it's a little awkward to read but it's there.
So what would be the recommendation of what to test and how? I was thinking of doing some voltage tests and reading up but need more guidance before poking around. What's the 1st thing you guys do in a situation like this?
Link to the SM. Use Chrome to view as adobe reader has an issue rendering some of the pages.
mitsubishi_da-r10-r20.pdf - Google Drive
Many thanks!
I'm trying to learn to do some basic repair/up keep on my vintage gear and wanting to understand how to troubleshoot faulty parts. I've done a few recaps and have a basic understanding of things but still a greenhorn. Anyway, I have an old Mitsubishi DA-R20 receiver I'm using as my learning platform. When I got it it worked so-so. It would trip the speaker protection relays randomly and there was the usual scratchiness in some of the pots, etc. Didn't sound all that great either. I replaced the speaker protection relays and learned to set the bias and dc offset to spec which I did. I noticed some bulging caps so I did a full cap replacement. Everything seemed to be working and sounding good. Then out of the blue it went silent. Power was still on, relays went closed and I could smell heat so I turned it off and let it set. I powered it back up and took a quick measurement of the DC voltage. The right channel was still good at < 10mv. The left channel was showing 30v. Well, that's not good. I also noticed that transistor Q507 was super hot to the touch.
So I need some guidance on narrowing the issue down. I have a DMM and transistor tester and basic understanding of things but certainly understand the complexity involved so I really appreciate any assistance. I do have a service manual that someone scanned into pdf so it's a little awkward to read but it's there.
So what would be the recommendation of what to test and how? I was thinking of doing some voltage tests and reading up but need more guidance before poking around. What's the 1st thing you guys do in a situation like this?
Link to the SM. Use Chrome to view as adobe reader has an issue rendering some of the pages.
mitsubishi_da-r10-r20.pdf - Google Drive
Many thanks!