After some years, back to trying to fix the MAC-4100 again

[mrz80]

Well hello again everyone. Almost five years ago (to judge from the date on the last post) I set my dead MAC-4100 aside in frustration and put the MX-112 and MC-275 into the main system (after determining that the 275's only problem was a tube that had cracked glass letting go). Here's the thread from that little excursion:

https://audiokarma.org/forums/index.php?threads/minor-surgery-on-the-mac-4100.795276/

Now I'm getting tired of having this wonderful receiver on the dead-line, and dug it out from under the desk and tried to refresh my memory on what the HECK was going on with it. No input, great big 8ohm dummy loads on the output. After a brief power up (and a flash-bang that scared the living daylights out of me but turns out to have been an insect sitting on the terminals of one of the big filter cans!) and some poking and prodding, here's what I've got:

Unit powers up
NO arcing or sparking, no smoke, no hot spots, no weird smells
Dial lights ok
Output meter LEDs go full-scale and stay there
None of the input indicator or dial pointer LEDs light up no matter what input I select

Looking around the power supply board with a meter shows me:

48VDC input ok (pin 1)
-48VDC input ok (pin 3)
5.6VAC input ok (pin 5)

no 18VDC output
no -18VDC output


On Q601 I should see: C 48V, B 20.6V, E 20V
I see: C 48V, B 3V, E 2V

On Q602 I should see: C -48V, B -18V, E -17.4V
I see: C -48V, B -10V, E -9V

I'm guessing at the very least those transistors are cooked. Any suggestions on what else may have headed south on the power supply (or further into the unit for that matter)?

 

[w3jn]

Another member recently went thru a similar issue, the thread is in this forum somewhere - I think we finished it up around the end of August. edit - here it is https://audiokarma.org/forums/index.php?threads/mac-4100-power-supply-issue.1003537/

To your problem, if the -18V is incorrect, the +18V will not be correct either. First, are the two resistors circled OK? If there's any overcurrent on the + or - 18V either or both may have overheated.

Second, disconnect all the plugs assosciated with the + and - 18V rails from the power supply board (the terminals indicated by the red brackets below). Check resistance from pin 34 to ground, and pin 22 to ground. If you have a low resistance then I would suspect the capacitors circled. Otherwise you can check the resistance of each one of the plugs (still disconnected from the power supply) to ground, and this will tell us if there's a short in one of the other boards. IE check the plug pins 22, 23, 24, 25, 26, 32, 33 to ground (-18V supply) and pins 17, 18, 19, 20, 21, 34, and 35 to ground (+18V supply)

Report back and let's see if we can't narrow down the problem.