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McIntosh MC502 No Sound At All. No Normal lights.

Johnny_Law

Johnny_Law

o.O
Subscriber
I acquired some Mac gear in poor shape (see http://audiokarma.org/forums/index.php?threads/price-check-on-jbl-mcintosh-gear.698168/) I managed to save the C24 with some new transistors! However, it seems to have very weak output - unimpressive, at least - and research has led me to believe the old electrolytic caps may have gone bad.

Now I'm trying to diagnose the MC502. I'm getting no sound from the speakers whatsoever, no humming, no crackling or popping, just nothing. The transformer hums pretty good when on, which I believe is a normal issue from research I've been doing.
  • The lights all light up on the front, except for the Power Guard lights.
    • EDIT: The bulbs ARE blown; the Limit bulbs are fine
  • I believe it's a power supply issue
  • I am not getting any voltage at the power supply, AC IN, pin 1 on the power supply from the transformer.
    • The manual says I should be seeing 5.3V at pin 1, "AC IN", but I see 0V
  • Pin 1 on the power supply leads directly back to the transformer according to the service manual.
So I'm afraid my transformer is blown. I'm making this thread in the hope that someone knowledgeable can recommend another procedure to disprove that hypothesis. I really hope the transformer isn't blown!

Thank you,
Ryan

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I believe you are using your multimeter incorrectly, which scares me as you attempt a repair........
 
c_dk said:
If the panel lights are lit your measurement can not be zero........check the AC path as shown in the schematic.
Click to expand...

Thanks again. I did re-check, and my dumb self was measuring the AC voltage from the transformer in DC mode. You're right - rookie move.

Good news is I do see 5.3VAC at pin 1 on the power supply. So no more bad transformer theory! WOO!!

PowerSupplyBlock.jpg


However, I did verify measured 0VDC at pins 17 and 19, for the Normal lights. So of course it seems like it's time to re-cap the power supply board, I'm suspecting C205 and C206, the big 2200uF caps. Verified with 5.3VAC measured on the pin 1 side of C205 and 0V measured on the other side.

PowerSupplyDetail.jpg


Thanks again for the help. I'll update this as soon as I've re-capped.
 
Hello
I always say : "fix the problem first before to think to recap" . Recap is not the cause of your problem and this "habit" to make "recap" is every where on the web and it is stupid
Your problem is coming from the contacts of the small flat cable coming from the power supply board going to the "lamp board" .

Ihave refixed many MC502 more than 80 and I saw that many times
 
No dice with the ribbon contacts. :( Will follow the signal backwards through the transistors and see where it falls off.
 
OK , let make a try
Can you measure DC Voltage between one end of the coil (you can see it on the driver board ) and ground . Do it on left and right channel and come back to say what .
 
I just think it's overblown. 50 year old stuff likely need PS caps
Some pieces have notoriously leaky coupling caps.
Some Suis have black flag death caps that take out unobtanium outputs.
I do agree that it's not the answer to everything. :thumbsup:
 
There are caps, used in 70s Mac units that are failing on a regular basis, that are 10-15 years past their expected life span and a responsible tech should replace them to minimize callbacks. You can not expect replacing them will solve every problem, good tech practices should be employed to find out IF they are a problem first.

Has anybody other than myself and some of my local clients bothered to listen to a recapped compared to a non recapped unit. We have done A/B comparisons between recapped and stock MAC4100s, C33s and MC2205s. A recapped C35 made a stock C30 just sound sick driving a MC7270 and Wilson WATT/Puppies. I have regularly loaned out recapped units for locals to take home to make their own call.

There is a consensus: the recapped units sound better and it is worth the $40-60 worth of parts and 3-6 hours of labor at your local rates to go thru these units. If you have not made the time or been given the chance to listen to a recapped unit compared to your stock unit you are missing out.
 
clinic-audio said:
OK , let make a try
Can you measure DC Voltage between one end of the coil (you can see it on the driver board ) and ground . Do it on left and right channel and come back to say what .
Click to expand...

Grounding at pin 19 on each driver board and the coil, I am seeing 0V DC on both the left and the right.
 
OV DC very good news . Output left and right working . Now we will see why your speaker do not work . Have you checked F201 ?
I think that there is something wrong with C205/C206. Replace them both . I see very often these Roderstein in bad conditions with the time and in some case shorted with damaging diode D203/204. Same trouble on MC2120 and MC2125
Waiting your confirmation

MC502-power-supply-pcb-045396.JPG
 
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I will place caps on order. I have found that this Nichion is the component recommended by another thread on the topic of MC502.

I've now pulled the board. F201 is fine. However, D203/204 both test in the same direction even though they are marked and placed reverse from one another. (700mV with positive probe on upper lead and negative probe on lower lead of each, 333kOhm the other way on each). I happen to have a few spare 1N4004's around, which seem to be a fitting replacement, so I will swap them out.

Also noticed black burn marks on the underside of R203 and pin 1 as you can see in the pic. The two are not connected so I'm not sure how that would have happened. R203 seems to be fine, it measures 6.67k.

And finally, I pulled the lamp board too, and both Normal bulbs are in fact open, which might be the cause of the whole mess in the first place ... (but open bulbs couldn't possibly cause no sound ...) I'm adding two 7382's to my Mouser order.

IMG_20160625_104645213.jpg


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(but open bulbs couldn't possibly cause no sound ...) no you should get sound whatever your lamps are open or not
I think that one cap get shorted and distroy one diode . After replacement of both diodes and both capacitors , everything should go back to normal function .
The capacitor you found is OK .
Diode 1N4004 would work fine .
 
Thanks again for the help. I have sound! However it is weak sound ... don't get to normal listening levels until the gain is all the way up, using my cell phone's Tidal app as the source, the volume on the cell phone also turned all the way up. It's almost like it only goes to 10 although I wish it could go to 11. So now I need to diagnose to confirm there's an issue, then figure out why the speaker output is low.

Progress report is that the caps and bulbs went in without issue. The factory manual, however, has an error regarding the diodes. It shows them in opposite orientation in one layout (the lower one on this page). I learned this the hard way and got a nice shock before switching them.

Here is the correct configuration:

IMG_20160702_085434204.jpg


Also discovered some damage to the board itself, the lead lifted off with the capacitor. This one must have gotten hot.

IMG_20160702_080426483.jpg


So I gave it the 'ol razzle dazzle with the new lead.

IMG_20160702_081306631.jpg


My phone's camera makes the Normal lights look red somehow but they are in fact a nice green.

IMG_20160702_091129457.jpg


IMG_20160702_091042372.jpg
 
I would not trust your cell phone or for that matter any portable device to output 2.5 volts RMS that these high end amps expect.

My IPads or Itouchs barely drive them......tis' but a coveinence.......not a reference for proper tesimg.
 
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