MC452-Power Guard Circuit Light On

[Capitalcityguy]

Amp won't work. Why would the Power Guard light be on and what the heck should I do?

[Masterlu]

Cap... did anything happen prior to prompt this behavior?

[Capitalcityguy]

no, nothing. For some reason the orange lights came on. Strange. I have it plug into a Burmester 948 like I always have. I wonder if my son changed the settings on the 948 or something.

[jdandy]

Capitalcityguy.......Have you tried unplugging it from the Burmester 948 and plugging it directly into the wall outlet? Let it sit unplugged for a few minutes after you unplug it before plugging it into the wall outlet.

[djwhog]

Is or are speaker wires somehow shorted etc?

I would do as mentioned Dan said, unhook all and then one by one reconnect.

[c_dk]

PG reacts to distotion at the outputs, lowering the input signal level.

If BOTH PG lights are lit the amp is reacting to some major distortion issues, most likely a catastrophic failure in the power supply which is one the few parts of the amp common to both channels.

Disconnect your speakers and get it into your local repair shop. Do not put your speakers at risk or possibly cause additional internal amp issues by turning it on and off in non controlled conditions.

[Cohibaman]

http://www.audioaficionado.org/mcint...-question.html

[c_dk]

The PG circuit lowers the input signal as a reaction to a certain distortion level. That distortion threshold has been refined since PG was introduced back in 1976 with the MC2205 amp. It at first was sampled via the negative feedback loop, then feeds we're added from the taps of the auto transformers in later models. The threshold has also been lowered from 1% back in the 70s, to 1/2% in the 80s to .25 % in the newest amps.

If the distortion is great enough it will really drop the signal level until the sentry monitor kicks in to minimize the release of smoke and ultimately pop the fusing.

Replacing burned out speaker drivers gets to be needlessly expensive, especially while trying to test.

I seem to remember over dinner one night Gordon Gow claimed that implementing the PG circuit in all of Mac's amps in the 70s lowered his warranty mid dome replacement cost by more than 70%. Most of that continued cost was the earlier non PG units made in the late 60s and early 70s.

[damacman]

Hmmm - although this is not spelled out in the manual, is it possible that when one or both of the PG lamps is lit solid this simply indicates that one of the amplifiers protection circuits is engaged? If that is the case, then several above have spelled out methodical ways to attempt to resolve the problem.

[c_dk]

The correct procedure would be on a test bench, hooked up to a load box, and being able to bring the amp up to power with a variac while monitoring the output with an oscilloscope. In most cases I monitor the AC current draw to look for impending doom.

Hypothetically, if a voltage regulator fails the distortion could go over 50 %( missing half the waveform) causing both PG circuits to drop the input signal 40-50 dB. The output stage still will want to amplify the remnant. A auto transformer will not pass DC but will get close enough to possibly use tweeter and mid driver voice coils as fuses.

Now I need some more coffee.