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lowpoke 31st January 2013 12:07 PM

Power amp just buzzing! Need help with diagnosis.
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The past couple of days I noticed a loud buzzing coming from my Australian Monitor 1K2 power amp. It interrupted the audio, but was intermittent and didn't continue long enough for me to work out exactly where it was coming from.

Tonight however, on power up it just buzzed loudly and no audio. I had a look tonight and have marked where the buzzing is coming from. It seems to be the group of small caps I've indicated. This thing is a monster and getting under that board (where there a couple of giant capacitors) to change those smaller caps out looks like a pain.

Can anybody shed any light on what this problem is likely to be? I'm no expert and am in a little over my head

Ian Finch 31st January 2013 06:51 PM

A big amp and inexperience as a DIY repairer is not a good combination for repairs, if needed.

Amplifiers are not all identical so the first thing anyone contemplating troubleshooting needs to know is the schematic of
the whole amplifier and then the problem symptoms. In this case, it may be a fault common to a lot of similar amplifiers and
not too hard to trace, if you have the details to refer to and test instruments like a DMM and hopefully, access to a 'scope.

The capacitors you have indicated appear to be the main power supply smoothing caps. If they are really what seems to be
buzzing, you need to check the operation of the rectifier bridge diodes and the DC voltages of the power supply rails to the
amplifier. Obviously, you need to know the correct design voltages to check against so that will be difficult if you don't know
what is correct. I guess this isn't a DIY design, and not as your signature suggests, so any schematic should be believable.

It needs to be said that a 1kW amplifier will have lethal rail voltages - don't try to touch, push or prod any part of the
exposed electronics whilst powered or for some time after, until the rails fall to a safe level. Though with just 4 pairs
of output transistors, the 1kW power claimed probably refers to the 2 channels bridged.

indianajo 31st January 2013 06:56 PM

If this is fairly new, it is likely a bad solder joint is at fault. Or a broken circuit trace. It can't be that old, toroid power transformers have been all the rage about 15 years.
First thing to do is get a DVM or VOM and check all metal at under 25 VDC to analog ground before you touch it. Clip lead the negative meter probe to the speaker ground while you are at it, so that you only use one hand to measure. One hand at a time. No jewelry, 3 VDC will burn your finger off with a ring. If <25 v, you can unplug and resolder everything. If there are tin plated or brass connectors, they can also oxidize and create gratuitous hum. You clean them by taking them off and putting back again. Ribbon connectors are particularly cheap and suspect, in my experience.
If that doesn't help, get a schematic diagram and check the DC voltages powered up against those shown on the print. Don't work alone with the power on. Often you can find a transistor with b to e greater than 0.7 v, on the pwb, or a collector and emitter not spread apart sufficiently with b to e at ~.6 v. Or a diode not showing a .6 voltage drop forwards or the proper zener voltage backwards. Zener diodes tend to be brown or grey, marked with a number R number which is the voltage point voltage (comma when you speak Euro english).If you want readings on both sides of a part you take two readings to analog ground and subtract them, you don't use 2 probes and two hands. Not on an amp with rails over 25 VDC. That voltage across your heart can stop it.
If this amp has surface mount parts, throw the PWB away, the soldering iron to resolder those joints is >$600. Buy some diy PWB designed for leaded parts. Farnell and mouser sell the parts, and have warehouses in Anzac. The big money in an amp is the transformer, heat sinks, case, filter capacitors, and output transistors. You may be able to salvage all those in a new design. I mount odd sized PWB's on spacers I cut out of Nema C laminate, drill and tap small holes as necessary with a hand crank drill, usually.

Ian Finch 31st January 2013 07:36 PM

Error above - As you point out, the main electrolytics are beneath the small board. In a 2U case, I wouldn't have thought there was enough room for it!

lowpoke 31st January 2013 08:18 PM

Thanks for your input so far! To answer a few questions, this is not a DIY amp. It was made by Australian Monitor in the early 90's and the schematic is available here:

As has already been pointed out, it's a big, powerful and complex amp and I am way too much of a newb to attempt a repair (or even diagnosis) myself. I was however, hoping to try and work out what may be wrong with it to help me decide how to move forward. This is a great amp and I want to try and repair it (or have it repaired).

indianajo 31st January 2013 08:39 PM

If this amp/speaker is band property that gets moved a lot, the broken solder joint/pwb trace theory is quite likely. Also since it is a discontinued product from the nineties, the corrosion in plug connectors theory.Or a bad joint in a push on ribbon connector. But follow the safety rules before you touch it. both are simple fixes, but if you don't read and heed the rules this amp could electrocute you.
I just spent a year fixing (on & mostly off) a burnt up 1.3 kw amp, with one other transistor amp under my belt. (that one had only 14 transistors). So don't underestimate what you can do. Just work safely. Another safety rule is safety glasses with the power on or unsoldering, there is enough energy to shoot electrolytic caps or transistor dies through your eye.
At about 20 years age I replace all electrolytic caps in an amp before spending much time diagnosing what is wrong, but they don't suddenly start humming. Nor do they suddenly stop. That sounds more like a connection problem. The salas tutorial on bringing an amp back up after repair is on but he doesn't discuss finding the bad parts very much. A surplus community college text like Thomas Floyd, Electronic Devices, the Electron Flow Version is more useful for how transistors and diodes should behave and how to use a meter. I paid $2 for mine, a lot of things have changed since I studied tube circuits for scientists and dogs in college.

Ian Finch 1st February 2013 05:56 AM

Those are good schematics - well organized and easy to follow. Having looked at them; it's a pair of standard design Hitachi
Lateral Mosfet amplifiers but the area you marked is actually called "Slo-Start board" which handles the 240VAC into the
transformer, fans, the two low-power supplies of regulated DC to the preamp. and the amplifier driver stages.

The "slow start" mechanism is a relay driven triac and 50W power resistor that is switched out of circuit immediately after
the initial start-up surge current has passed. That protects components from the high starting current peak, preventing
early failure. 'Sounds good but that part, including several other components, are at LIVE MAINS POTENTIAL and dangerous.

If the buzz is constant, even after starting normally, it could be that the relay is damaged, having succumbed to many
cycles of operation. After all, it makes no difference if the amplifier is used for high or low power, concerning the number
cycles it has already gone through.

The marked electrolytics are shown as 220uF 200V, and are the smoothing caps for the driver stages.
I don't think it will be them buzzing mechanically but if so, don't attempt to repair. Get a repairer of pro.
audio equipment to give an opinion and do any work. Expensive? likely, but is that a cheap amplifier?

lowpoke 1st February 2013 06:09 AM

Ian, thanks for taking a look at this for me. I really appreciate it.
This morning I dropped the amp off to 'Justware' here in Adelaide. They were recommended for this type of work. This was a very expensive amplifier when new and is very well put together (unlike more recent equipment from the same manufacturer). I actually have a couple of these … and just love them. Two versions were made, one for sound reinforcement applications and one for studio use. I have the studio versions with quiet Pabst fans for home use. I'm happy to pay for the repairs as these are quality amps, but just hope it doesn't get too expensive!!!

And thank you for your input also indianajo!

Ian Finch 1st February 2013 06:35 AM

Note that the alternate design Slow-start board has just a 30A relay to switch the resistor. I don't see it on the PCB, though.

megajocke 1st February 2013 04:36 PM

Isn't it the one right there inside the red rectangle, next to the aluminium clad resistor?

The failure might very well be in the soft start circuit, causing the relay to buzz.

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