Hi, I love looking at descriptions and photos of repairs other people have done.
So I thought to give back to people also enjoying that.
A little while ago, I bought a MF A1X, being in good optical condition, but the owner said it had suddenly stopped working.
He also said the source selector was not working well anymore.
When I had it on the bench, I saw that in the past someone had fiddled with it, and not too expertly.
The power cable had been replaced by one with a ground wire, although it originally was sold with "Schutzklasse 2" (Germany / EU), meaning no ground and internal high voltage being completely isolated.
The ground wire of that "new" cable was just screwed to the painted case. Not sure what the intention behind all that "modification" was.
Power supply capacitors had visibly suffered. Other caps as well, see shrunken isolation on small electrolytic bottom right. I am not sure if these were the original caps or if they had been replaced before.
There were also other creative changes, like this strange wire, which was for fixing a broken PCB track (the PCBs of old MF equipment are real sh*t and tracks come off really easily).
Also not original was this "thing", maybe it was intended as part of some temperature circuit? It was not connected anywhere, though.
The sudden stopping to work was indicating the fuse was blown and it was. Testing after replacing it with a dim bulb tester showed a nice glowing lamp, so there was more.
I got out the board and tested the rectifiers on the PS first and indeed one of them had a short.
So I thought to give back to people also enjoying that.
A little while ago, I bought a MF A1X, being in good optical condition, but the owner said it had suddenly stopped working.
He also said the source selector was not working well anymore.
When I had it on the bench, I saw that in the past someone had fiddled with it, and not too expertly.
The power cable had been replaced by one with a ground wire, although it originally was sold with "Schutzklasse 2" (Germany / EU), meaning no ground and internal high voltage being completely isolated.
The ground wire of that "new" cable was just screwed to the painted case. Not sure what the intention behind all that "modification" was.
Power supply capacitors had visibly suffered. Other caps as well, see shrunken isolation on small electrolytic bottom right. I am not sure if these were the original caps or if they had been replaced before.
There were also other creative changes, like this strange wire, which was for fixing a broken PCB track (the PCBs of old MF equipment are real sh*t and tracks come off really easily).
Also not original was this "thing", maybe it was intended as part of some temperature circuit? It was not connected anywhere, though.
The sudden stopping to work was indicating the fuse was blown and it was. Testing after replacing it with a dim bulb tester showed a nice glowing lamp, so there was more.
I got out the board and tested the rectifiers on the PS first and indeed one of them had a short.
When disassembling I ruined the source switch. Good it was broken anyway... 😆
At first, I was not sure, what this blob was, but then later read somewhere on
the net, that it was a spacer, which is known to become "fluid".
This is where it was. Not much left of that strange material.
At first, I was not sure, what this blob was, but then later read somewhere on
This is where it was. Not much left of that strange material.
Adding some cooling during testing by putting heat conductive foil and some heat sinks on top of the tunnel.
While working on it and reading about it, I found Mark Hennessy's excellent page, where he suggest a better pre-amp (the original has a strange arrangement with the volume pot being in the feedback loop of an opamp).
I decided to build that and replace the original with it. In KiCAD I did the layout and got the board made by JLCPCB.
After the replacement. New Alps pot as well, BTW.
I decided to build that and replace the original with it. In KiCAD I did the layout and got the board made by JLCPCB.
After the replacement. New Alps pot as well, BTW.
I did not do anything regarding the bias after seeing the amp using exactly the amount of power it should (85W).
I repaired one of these for a friend when I lived in Taiwan. In my case it was a simple repair - just 2 resistors in the PSU. He had bought the amplifier from a dealer in Hong Kong the 1980’s and it hadn’t been working for 4 or 5 yrs when I looked at it for him in 2012.
Anyway, after repairing it, I connected it to my B&W 703’s not expecting much - 15 W, not particularly low distortion, very idiosyncratic circuit.
Fantastic sound and that’s what got be going down the class A road. Soon after that I did the sx-Amp and that recently evolved into the kx2-Amp.
Anyway, a seriously nice sounding little amp! Enjoy 👍👍👍
Anyway, after repairing it, I connected it to my B&W 703’s not expecting much - 15 W, not particularly low distortion, very idiosyncratic circuit.
Fantastic sound and that’s what got be going down the class A road. Soon after that I did the sx-Amp and that recently evolved into the kx2-Amp.
Anyway, a seriously nice sounding little amp! Enjoy 👍👍👍
85W is generally OK. Seeing further is for channels bias matching curiosity.
For the reader's reference here's A1's schematic as I had once simulated it.
*Power amp section only. You may meet different enough R6 R11 R30 R31 C15 C16 values in other schematic editions.
For the reader's reference here's A1's schematic as I had once simulated it.
*Power amp section only. You may meet different enough R6 R11 R30 R31 C15 C16 values in other schematic editions.
I have played with it in LTSpice as well while working on it.
Since the A1(X) only has one NPN/PNP pair per channel, matching of pairs is not possible. Maybe you mean positive and negative to have as little DC as possible? I actually had planned to measure the DC and forgot 😆.
Will do the next days.
But I will not open the beast and reapply the thermal grease again, unless the offset is really high.
Since the A1(X) only has one NPN/PNP pair per channel, matching of pairs is not possible. Maybe you mean positive and negative to have as little DC as possible? I actually had planned to measure the DC and forgot 😆.
Will do the next days.
But I will not open the beast and reapply the thermal grease again, unless the offset is really high.
BTW... the bias resistors are 1.5Meg in your spice file. The actual are much larger. You can see them in the pics, they are the two resistors twisted together to get a higher serial value.
I think they are 3.5Meg or 3.7Meg. Still the idle power is 20W per BJT.
I think they are 3.5Meg or 3.7Meg. Still the idle power is 20W per BJT.
I tweaked them values to get proper bias current with the LT Spice models. In actual amps they are higher but also also possibly differ between editions?
Yeah, I saw in my simulations, that the amp is not ideal for 4 Ohm speakers. I am using it with 90dB 4Ohm speakers at the moment and it can play pretty load and still sounds fine to my ears.