2205 power guard need help

[T3GTerry]

I have 2 MC 2205 amps. One is fine. The other not so. The amp appears to work find BUT when I turn it on the power guard NORMAL lights do not flash like my other amp does. Once the relay clicks the audio comes on and sounds great BUT the POWER GUARD LIGHTS do not light. Well sort of. As I said none of the PG lights flash at start up however the NORMAL lights do light up but very dimly compared to the other bulbs and compared to my other 2205. Obviously a very low voltage. The NORMAL switch IS in the NORMAL position. The LIMIT lights DO NOT light up either momentarily or otherwise. ALL bulbs are new. The sound does not appear to be limited or distorted. The amp sounds great. No distortion. The big problem is the NORMAL lights failing to light when everything is normal. Anybody here understand how to diagnose or repair this circuit? I make repairs all the time on other audio issues myself but not familiar with the McIntosh POWER GUARD system. I could use some help.

 

[kevzep]

Time to get the schematic out and have a look and how it works, should be no problem if you make repairs all the time.
Power Supply would be a good place to start.
Where did you get the lamps from? Are they the right ones? (Have to ask, wrong lamps wont work, see this a lot).

 

[Nickit]

I replaced the Normal lights on my 2120 recently and reading up found that often the lights are out due to bad/failing caps on the power board. On the 2120 that board is directly behind the front panel board (where the bulbs are). My Normal ights came back with new bulbs soldered in, but guess if the caps had been bad they wouldn't have worked- at least from what I read.

 

[T3GTerry]

Everything I have read points to the power supply board ESPECIALLY since both sides are acting exactly the same and the sound has no problem. I just want to make sure I am not diving into something without even having a clue. I do have another issue but that is only when the amp is off so that is for another day.

 

[IvoCRO]

If you have the right lamps (14 V, 80 mA), then check power supply board, as kevzep says. Two 2200 uF capacitors are known to fail, and sometimes two diodes in their circuit too. Diodes are placed between capacitors and revealed when capacitors are lifted from board. There are many posts about power supply boards in these amps.

You can switch power supply board from good amp and quickly diagnose that problem is indeed in PS board, but check that board version numbers are the same.

 

[T3GTerry]

Well it appears that I was thinking in the right direction and you all confirmed it. I don't feel so stupid now. Sooooo with that I took a good look at the schematic and decided to try the two 2200uF caps that IvoCRO mentioned above. Gave me a chance to try out my new LCR meter and my new Fluke 179. I removed both and when I tested them with the LCR all it showed was infinity as opposed to showing 2169 on the new caps however the Fluke 179 showed both of the old caps to be around 3800 and we all know that ain't right. So I put in two new Nichicons and bingo. All is well again. It amazes me that both caps failed in the exact same way. No clue how long ago they failed as these were brought to me by a friend. But they have both been tuned up and are ready to return home now. Thanks folks and hopefully I can return the favor some day.

 

[clinic-audio]

Roederstein capacitors used in MC2120/2125/2200/2205 are know to fail and if you let them in place they will become shorted with much more dammage on the power supply (diodes going dead shorted)
You can replace these cap without loosing time to measure them . As IvoCRO remind you , follow this advise : replace them when you see them . If your other amplifier still working , I will recommend you to replace these capacitors in the power supply .
You can make some research , I have posted many messages about this on AK

 

[T3GTerry]

The caps that were in both amps were Elko.

 

[c_dk]

Same issues with the Elko brand. Mac sourced from both.