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Onkyo M-508. A couple of questions for the experts

S

Skywatcher

AK Subscriber
Subscriber
I recently got one of these in for repair. It was in orange protection. This is my first M-5xx amp I have had on my bench and am a bit cautious.
I found the 2A bridge in the servo was caput. I changed that and installed a 4A I had on hand, and replaced the 2200uf cap with a PW. I then repaired a boat load of bad and iffy solder joints on all the boards.
Checked the outputs for shorts, and any signs of the main caps physically leaking. They seem fine and do seem hold a charge. But that doesn't mean they are "fine" with internal leakage.
Next I pulled the relays and found the main relay had 2 sets of points with high resistance no matter what. And the A speaker relay with one bad set of points. No amount of cleaning changed a thing. The bond of the points to the metal has poor contact. Both replaced with new Omron. The B speaker relay is fine and was probably never used.
Now, the questions..
Which direction do the BIAS pots turn to decrease current? The SM doesn't say. CW or CCW? I can then drop the BIAS and go from there without risking anything to see if it comes out pf protection. If not, then more digging will be necessary.
What wattage of DBT is recommended? I used a 100w which dimmed somewhat after the caps charged, but remained too bright for my liking so I stopped there. It was about half brightness compared to the inrush brightness. I am hoping it is because the BIAS drifted high because of aging pots, or just needs to be a higher wattage bulb, which I have. This looks to be a "virgin" and I found no signs of any previous work.
I want to determine if anything "unusual" is going on before I place a Mouser order for the 22k BIAS pots and get anything else to fill an order.
 
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You'll want minimum resistance between the anode of D421 and the cathode of D423, that will very little bias.

Craig
 
llwhtt said:
You'll want minimum resistance between the anode of D421 and the cathode of D423, that will very little bias.

Craig
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I just did a quick check, and CCW decreases resistance for these particular pots. So I should find the Bourns that turn the same way to keep it "normal". But I gotta run so I won't get back to it until later.
 
What Omron relays did you use? I read through every thread on here about the M-508 and many stated the Omron MY4-02-DC12 was the correct one, so I bought three. Turns out they are nothing like the ones that are actually in my M-508 (the new ones have a whole extra row of pins and the spacing on the other pins is off).

I used a 300 Watt bulb in my dim bulb tester and it dimmed quite a bit, much more than the 150 Watt bulb which only dimmed a bit more than 1/2 way.
 
krautmaster said:
What Omron relays did you use? I read through every thread on here about the M-508 and many stated the Omron MY4-02-DC12 was the correct one, so I bought three. Turns out they are nothing like the ones that are actually in my M-508 (the new ones have a whole extra row of pins and the spacing on the other pins is off).

I used a 300 Watt bulb in my dim bulb tester and it dimmed quite a bit, much more than the 150 Watt bulb which only dimmed a bit more than 1/2 way.
Click to expand...
Snip off the 4 unused pins on the MY4-02-DC12 and it fits like a glove. :D Look at the original Fijitsu relay and you'll see they just didn't put the pins on, but still have the points there as a rest stop.
Thanks for the DBT wattage. I do have both 150w and 300w bulbs on hand. :beerchug:
 
Skywatcher said:
Snip off the 4 unused pins on the MY4-02-DC12 and it fits like a glove. :D Look at the original Fijitsu relay and you'll see they just didn't put the pins on, but still have the points there as a rest stop.
Click to expand...

Ok, that sure makes it easy...now I have a project for this weekend !
 
I have not been able to figure out how to remove that "68 Chrysler 440 Magnum". I sold it several years ago because I couldn't take the 8 mpg it got (on a good day) and it was in need of new paint and upholstery that would have cost more than the car was worth. If it was a "B" body instead of the bigger "C" body I would have probably had it done since B body cars are worth 5 times as much as C bodies even though mechanically they are all the same.

Newport BIG 001.jpg DSCN0920.JPG
 
krautmaster said:
I have not been able to figure out how to remove that "68 Chrysler 440 Magnum". I sold it several years ago because I couldn't take the 8 mpg it got (on a good day) and it was in need of new paint and upholstery that would have cost more than the car was worth. If it was a "B" body instead of the bigger "C" body I would have probably had it done since B body cars are worth 5 times as much as C bodies even though mechanically they are all the same.

View attachment 2550738 View attachment 2550741
Click to expand...
I always loved those factory "mag" wheels.

In the upper right, click on your name.
Immediately below your Avatar, you will see "Custom Title"
Make it blank, or change it to something else.
Then go to the bottom and click on "save changes".

I've never tried it, but I assume it should do the trick. Mine is left blank so it says "AK Subscriber" by default.

Now get that M-508 up and running this weekend. :rockon:
 
krautmaster said:
I don't get the "Custom Title" thing...maybe if I log off then back in again it will show up...
Click to expand...
I use a PC so it may look different if you use a mobile device. So for me, if I click on my name at the top right of the page (where your notifications show up), it takes me to my profile. On the right side of the profile just below your avatar picture, is a line labeled "custom title".
 
Skywatcher said:
I just did a quick check, and CCW decreases resistance for these particular pots. So I should find the Bourns that turn the same way to keep it "normal". But I gotta run so I won't get back to it until later.
Click to expand...
My trick is I set the Bourns pots to the same resistance as the originals when I install them and use that as a starting point. Then I don't have to worry about which direction is minimum and risk getting it wrong. Sometimes the direction ends up reversed when I install the Bourns.
 
Skywatcher said:
I use a PC so it may look different if you use a mobile device. So for me, if I click on my name at the top right of the page (where your notifications show up), it takes me to my profile. On the right side of the profile just below your avatar picture, is a line labeled "custom title".
Click to expand...
I'm using a PC with Google chrome, but it shows no such option... I can change my avatar all day long but there is nowhere that I can find where it shows "68 Chrysler" to get rid of it... guess I'll have to keep living with it :dunno:
 
Skywatcher said:
Snip off the 4 unused pins on the MY4-02-DC12 and it fits like a glove. :D Look at the original Fijitsu relay and you'll see they just didn't put the pins on, but still have the points there as a rest stop.
Thanks for the DBT wattage. I do have both 150w and 300w bulbs on hand. :beerchug:
Click to expand...
Which pins did you remove, the ones closest or furthest away from the "top" (where there are only two pins)?

s-l400.jpg
 
The ones in the red rectangle at the side of the relay closest to the top of your picture. For the pins that are engaged when the relay is in the open/unpowered state.
 
krautmaster said:
I'm using a PC with Google chrome, but it shows no such option... I can change my avatar all day long but there is nowhere that I can find where it shows "68 Chrysler" to get rid of it... guess I'll have to keep living with it :dunno:
Click to expand...
That's weird as I can see mine using FireFox.

Screenshot 2022-04-28 at 10-59-06.png
 
Nope, I don't have that, but above the Avatar I have a block for "status message" but it didn't change anything. Mine also also has the gender / DOB blocks, just no "Custom Title".
 
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