McIntosh MC 2255 Power Guard Problem

[Matt's Dad]

My first post here after lurking for a couple of weeks!

I have been able to acquire a McIntosh MC 2255 amp a few weeks ago. It's in pretty good shape and when I turn it on with my Forte IIs hooked up it's DEAD quiet.

I wanted to test everything out on the unit so I put some Muse (Supermassive Black Hole) in and slowly cranked it up, this was to test the Power Guard circuit.

Well the red Power Guard(PG) Limit lights lit but there was also very audible sounds of distortion - Cracking/Crackling from the midrange on both channels, NOT good!!! My understanding of the PG circuit is that when it senses .5% distortion on the output it triggers not only the red Power Guard Limit lights on the front panel but also causes a reduction in the gain via LDR (Light Dependent Resistor) 201 (Left Channel) & 301 (Right Channel).

In looking at the 2255 schematic circuit for PG it appears that the circuit is working because it flashes the PG Limit lights so I would think that the PG Comparator chip (IC 401) is OK and the rectifier circuit is OK. My gut feeling is that the LDRs are shot and need replacing.

Has anyone had any prior experience with this problem? The amp was supposedly checked by a reputable McIntosh Tech but its obvious that this feature wasn't part of the diagnostic.

Thanks!

 

[c_dk]

PG will come on and limit the input signal, that is dump some signal to ground, anytime the distortion goes above a certain level. It can come on at 300 watts or 10 if your power supply is limited.

It needs to be put back on a test bench for a complete stress test. I am guessing it is not making rated power. If happening in both channels the power supply is the first suspect.

The failure mode we found back in 1982 with the Clarix LDRs was intemittantly dropping out a channel, Mac then changed vendors.

 

[damacman]

So - just how loud was this?

 

[Matt's Dad]

So - just how loud was this?

Very loud! Forte IIS are 99bd @ 1 watt 1 Meter.

I did pull the tweeter out today and check to see if it was fried... still measures and sounds good.

 

[Matt's Dad]

PG will come on and limit the input signal, that is dump some signal to ground, anytime the distortion goes above a certain level. It can come on at 300 watts or 10 if your power supply is limited.

It needs to be put back on a test bench for a complete stress test. I am guessing it is not making rated power. If happening in both channels the power supply is the first suspect.

The failure mode we found back in 1982 with the Clarix LDRs was intemittantly dropping out a channel, Mac then changed vendors.

I was watching the meters as I slowly increased the volume and they were indicating full power. There is NO hum on either channel and the voltages seem OK via a fluke meter.

I'd like to send the unit back but the shipping both ways is way too expensive. I had full confidence in the tech and expected him to give it a good going over.

My unit is a bit newer than 1982 and I believe that the LDRs are by Vactrol.

It's odd that the PG Limit lights work but it doesn't attenuate. I suppose I could pull the LDRs and check with a meter but then if they are out I might as well put new ones in.

 

[jlovda]

It's odd that the PG Limit lights work but it doesn't attenuate.

PG will just not allow the output signal to get any louder once it hits the 0.5% THD and the lights come on. It will not drop the level, just keep it from getting any louder regardless of any increase in the input level.

 

[clinic-audio]

Hello
Send a picture of your LDR

 

[c_dk]

Power Guard is not a govenor, it reacts with the dynamics of the music and distortion level. It can help protect you from yourself but mainly will try to protect your speakers from your overdriving exuberance.

A ten minute bench test by a tech will quickly confirm if the amp is working correctly vs. wild speculation about possible faults.

 

[Matt's Dad]

PG will just not allow the output signal to get any louder once it hits the 0.5% THD and the lights come on. It will not drop the level, just keep it from getting any louder regardless of any increase in the input level.

Right, that is what it is supposed to do. It regulates the input by shunting a bit of it to ground thereby not allowing it to push the amp into further distortion. The amp went into distortion so something is not working correctly. In reading the schematic the circuit is pretty simple. I'll open the amp up this evening and put a scope on the LDR.

 

[Matt's Dad]

Power Guard is not a govenor, it reacts with the dynamics of the music and distortion level. It can help protect you from yourself but mainly will try to protect your speakers from your overdriving exuberance.

A ten minute bench test by a tech will quickly confirm if the amp is working correctly vs. wild speculation about possible faults.

I can read a schematic, so it's not wild speculation. The amp distorted so Power Guard is not working.

No readily available bench tech in my area, the guy who supposedly checked it is a couple of thousand miles away.

 

[62caddy]

Countdown feature working properly?

 

[c_dk]

Read the original patent from 1977. To test the amp properly you need to drive it into a static load and observe the distortion on a scope and measure, typically with a distortion analyser. The amp will distort when driven beyond the power supply capability. I personally think you are exceeding your speakers dynamic range (bottoming out a voice coil or twisting it in the magnetic gap)but a amp test is the only way to remove the amp from the variables.

 

[whoaru99]

PG will just not allow the output signal to get any louder once it hits the 0.5% THD and the lights come on. It will not drop the level, just keep it from getting any louder regardless of any increase in the input level.

I always understood that Power Guard basically acts as a limiter. The peak level won't increase much, if at all, but average power/average level will increase if one were to keep pushing the input higher.

So, maybe it's a problem with the amp...or maybe the speakers were just receiving more than they can cleanly handle given the potential for this type increased average level with the PG indicators lit up.

 

[Matt's Dad]

Countdown feature working properly?

Yes. Works every time I power it on. the PG lights weren't on continuously, they just flickered and I heard the distortion. Speakers have a continuous rating of 100 WPC and a peak of 500 WPC. I have never pushed 250 watts into them before so it could be a speaker problem. Will put a scope on things tonight. Thanks!

 

[Matt's Dad]

Read the original patent from 1977. To test the amp properly you need to drive it into a static load and observe the distortion on a scope and measure, typically with a distortion analyser. The amp will distort when driven beyond the power supply capability. I personally think you are exceeding your speakers dynamic range (bottoming out a voice coil or twisting it in the magnetic gap)but a amp test is the only way to remove the amp from the variables.

Thanks It is possible that I hit the limit on the speakers. They have a continuous power rating of 100 WPC with a peak rating of 500 WPC.

 

[62caddy]

Yes. Works every time I power it on. the PG lights weren't on continuously, they just flickered and I heard the distortion. Speakers have a continuous rating of 100 WPC and a peak of 500 WPC. I have never pushed 250 watts into them before so it could be a speaker problem. Will put a scope on things tonight. Thanks!

I once had a problem with an MC2125 that, when used with one particular pair of speakers, in which PG would activate at far lower output than it should have. I can't recall the figure but it was far less than the 2125's 0dB reference of 120 watts.

When used with other speakers, PG engaged in the 120 - 140 watt range where it should. I never discovered the reason why.

 

[c_dk]

I would guess that the speakers were rated as 6 ohm......that is really mostly 4 ohms but the marketing department demanded a higher 6 ohm rating to not limit sales. Hooked up to the 8 ohm tap your power transfer would not be optimum and the amp distortion would rise higher before the meters would indicate max power. If you would factor in the discrepancy and multiply the meter indicated power by 1.3 or 1.5 or even 2.0 you would see your 140 watts or so.

 

[Corktowner]

Reviving an old thread here - Matt’s Dad did you ever figure out the issue? I have a MC2255 and it does the same thing - sends some distortion to the speaker when the power guard comes on. The power guard on my MC2120 did not do this - it was completely silent when activated.

Thanks It is possible that I hit the limit on the speakers. They have a continuous power rating of 100 WPC with a peak rating of 500 WPC.