Author Topic: B&O Audio Monitor AM1 restoration  (Read 839 times)

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Offline cheeseit

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B&O Audio Monitor AM1 restoration
« on: August 30, 2016, 08:14:26 pm »
Got this audio monitor as part of a score and it works. Cosmetically it is in kinda bad condition with the paint flaking off but the meter looks as new, perhaps it has been replaced during the units life. Judging from date codes on two LM301's it is from 1980.

It is a combined stereo wattmeter, AC voltmeter and measuring amplifier with 9 ranges. As a wattmeter from 10nW to 140W, with 2x100W/1x200W continuous and 2x140W/1x280W for up to 5 minutes. Selectable impedance is 2x8R, 2x4R, 1x2R and 1x16R. On the back is a DIN connector for an external 2x1KW/1x2KW load. I had a hard time believing that when I read it, seeing those puny wires leading to the connector. That seems a bit.. ambitious. I don't see how those thin wires can carry up to 2KW on a DIN connector where only two of the pins are used for the signal?

Full specs and schematic here (in danish).

Inside I found the aluminium heat sink for the LM386 had fallen of but had luckily wedged itself under a few wires. A few of the Roederstein electrolytics looks bad with a black smudge near the '+' sign on top of them, and one of them even has small bulge on the top. I think I'll replace those that looks bad, or should I just replace all of them? I don't have an ESR meter yet but I would like to keep it as original as possible so I will keep them and test them when I get one, and/or try to find some NOS at some point in the future.
Those blue Philips has some oxidation on the positive terminal too so I'll replace them also.

I plan to restore the case by sandblasting and painting the case, in as close as possible to the original colors but I'm not quite sure how to restore the front. I'm thinking that I have to measure everything, or perhaps scan, the front and create an image from which I can have a stencil cut? Toner transfer maybe, how would enamal paint react to the required heat? I would imagine that I have to apply lacquer on top of the toner if that is an option? Advice and ideas are appreciated.

This will take some time and it is not on top of my pile so don't expect updates anytime soon. ;)

Last image shows a 1kHz sine on a Yamaha M-35 amp specced at 57W dynamic power into 8R, which is what the meter shows precisely, right before clipping.
« Last Edit: September 04, 2016, 06:47:16 pm by cheeseit »

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