M-500 Refurb.

Marshmallows? Please do tell.

If you've got the marshmallows, I could use a PM-1.5 and an under-sized power resistor to boil some water for hot chocolate...

I can't vouch for it; I've only read about it, but always wanted to try it.

The oven works by creating a standing wave inside the oven. I believe the frequency is about optimum to excite water molecules, and the friction as they move causes the heating (another story, but the hershey bar in the pocket of a guy on a flightline standing in front of a jet fighter's radome is pretty cool, but probably fabricated ((at some flightline's they have to drink a qt of water every 20 min to stay hydrated; I doubt his hershey bar would have survuved, but anyway)))

So you take the tray out, so it doesn't rotate, and put a plate of marshmallows in. 'Nuke' it until they just start to melt, and measure the distance between the melted pits. On average (or more likely rms?) they should equal the half wavelength of the mocrowave frequency printed on the back.

Every time I remember this, I don't have any marshmallows around to actually try it.

[edit: I just looked this up, and evidently you're calculating the speed of light (3.0 X 10^8 m/sec, so if velocity=wavelength X frequency, you take the frequency printed on the back (it says usually 2450MHz) and multiply by twice the distance measured (the half wavelength) in meters and you should get close to the speed of light.]

The saga continues. I had taken my problem off-line and worked with RichP with advice from bob p and jvandyke_tx on how to proceed. My amp would run beautifully until it blew a fuse. The transformer would get way hot and blowing a fuse meant that it was pulling more than a sustained 10 amps. On voltage checking, the high rail would climb from its normal 74v to over a 100v. I really don't know how high it got when it blew the fuse (and I don't want to). I'd just shut it down when it hit 100v and wait to do more testing until it cooled down. So, we knew it was heat related and the feedback to limit triac firing wasn't working. After much noodling about exactly how the amp worked, the problem had to be between the optocoupler and the triac. Fortunately, it was the first - the optocoupler, as verified by a good shot of freeze spray.

Opto.JPG (151.06 KiB) Viewed 904 times

PC713 is the offending part. It's an 84¢ part available HERE

I'm waiting for delivery. I'm going to put a socket on the board to make soldering easier. I'll let y'all know how this turns out.

Next up - new binding posts.

Man am I loving all of the M-500 techs we're gaining around here !
Good deal BillD. Hope you get that baby back in service.

OMG -- this has to be the first time that a Carver guy has tried to fix his amp with a spray can ... and it worked!!!

Congrats, Bill!

The optocoupler's are known to be a week link in electronic. That and crystal oscillators. Congrats on the troubleshooting.

engtaz

The saga continues, but may be complete. Only time will tell. I replaced the opto by installing a chip carrier socket.

New M-500 Opto.JPG (192.2 KiB) Viewed 786 times

As jvandyke_texas told me, he didn't think the opto was at fault, and he was right. After testing the opto was no longer thermosensitive, but the rail voltage was continuing to go up. I figured it had to be a nearby part in the triac firing circuit, so I started trying adjacent parts. If you see the part that is below and just to the left the opto (directly left of the trimmer - not the resistor, stupid), that's the offending part - a bridge rectifier. It was a DBA-10C (1A, 200v), which I couldn't find, so I had to substitute a NTE167 (2A, 200v). I've run the amp for about 5 hours continuously at a shot with no apparent rail voltage drift.

While I was in there, I took RichP714's advice and replaced the binding posts with THESE. Unfortunately, they were too long and ran into the transformer housing. So I had to cut two of them off and attach th wires with these.

M-500 Solder Lug Washer.JPG (315.61 KiB) Viewed 786 times

The resulting installation looks like this (it doesn't touch - honest).

M-500 Binding Posts.JPG (144.02 KiB) Viewed 784 times

Anyway, it's all done. Here's what those posts look like from the back.

M-500 Posts.JPG (161.67 KiB) Viewed 783 times

That's all for now!

Very nice binding post. You da Man.

Months later, I got around to doing something I should have done while I was in there — Blue LED lamps. The current lamps in there are incandescent and run off a little transformer that can barely put out 700ma (the bulbs are 800 ma). The transformer has two taps, 6v and 8v. For LEDs, you need DC, so after some discussion with RichP and Ruskall, I came up with the following circuit.

M-500 LED 2.jpg (32.64 KiB) Viewed 628 times

It works by rectifying the 8v tap from the transformer and delivering it through the 3R6 resistors to the lamps. When you want it bright, it drops in a set of 3R6 resistors in parallel to halve the resistance (and therefore double the current — supposedly, more later).

So I built a little board with the components on it (except the transformer and the lights). They didn't have 3R6 resistors at Fry's electronics, so I got a pair of 3R3s and a pair of 3R9s. The 3R3s are in-circuit all the time, and the 3R9s are dropped in for brightness.

M-500 Lamp Board.JPG (96.31 KiB) Viewed 627 times

Underneath it's wired like so.

M-500 LED Circuit.jpg (23.97 KiB) Viewed 625 times

There is really only one spot in the M-500 where this can fit, and it's over the power supply next to the barrier between it and the amplifier board.

M-500 LED lamp board.JPG (330.18 KiB) Viewed 626 times

And that's all there is to it. Some rewiring of the switch and the lamp board was all that was necessary.

Of course, things don't always go perfectly. I found that dim is bright enough, and bright makes hardly any difference. It seems that wimpy transformer is maxed out, current wise, with the 3R3 resistors in line, and lowering the resistance just makes the voltage sag so you don't get anymore current to the LEDs.

Anyway, here's what it looks like now.

Family Room System.JPG (46.12 KiB) Viewed 626 times

I'm building another little board with 6R6 resistors on it to replace the one in there now, so the dimming will work better. When I'm in there, I'll adjust the lights as one is a little low and has some "shine through".

bill, did you use the 1W or the 2W luxeons?

Those blue bulbs are gorgeous.

I used a pair of THESE in series for each bulb. So, the bulbs are approximately 2½ Watts apiece.

I have been reading the post with great interest. I have an M500t that I bought new in 1986 and I have never had a problem with it yet. I was wondering if it would be wise to replace the Caps in this unit before it gives me any problems. Also are there any other parts that one should replace while I'm in there. Thanks in advance for any info you guys can give me.

The only true consumables in amplifiers are electrolytic caps. Heat takes its toll on lots of things, but 'lytics wear out (dry up) whether or not they are in use. If you keep an amp within its operating parameters, the other components should last a lifetime, and maybe a few generations. The things I've seen fail (diodes and diode devices, like rectifiers) were probably overheating do to failing caps. Of course, any component could be badly made and fail. But as far as wholesale replacement, I can't think of anything other than electrolytics.

Thanks for the reply BillD. Would recomend I either do this myself or get it done before I start having problems?

If nothing has ever been done to it before, it's time. Your skill will dictate how.