That comment reeks of being timid.
If in high school, and later schooling, my instructor was that way, I'd have never gotten anywhere in a successful career in electronics servicing.
In today's world anything said can be and usually is taken incorrectly. If I had been blunt and simply stated that I've seen better soldering work from a 5 year old the forum would have blown up and I would have correctly been called an a**hole. Instead I sought to minimize the possibility of immediately offending the individual in question and the rest of the forum.
Myself I had to learn to solder correctly because I was a repair and instillation tech for over 25 years. Having done many large sound systems in College and University Football Stadiums and other large format systems my work was subject to the inspection of the sound consultant that was responsible for that particular design and that saw to it that every aspect was done exactly to the specification book. You talk about anal? Wires were stripped to exacting lengths, wires tinned, heat shrink cut to exact lengths, XLR solder cups filled perfectly, heat shrink tubing heated ONLY by a heat gun and not the end of a soldering iron or a cigarette lighter. Every wire numbered and labeled with the label carefully covered with clear heat shrink tubing. I'm not ever going to mention the books that I wrote outlining systems operations or video's I made in the way of system operation and training.
Any deviation from the standards and all the work in question was done over again and re-inspected by the sound consultant and or sound consultant firm. So, yes I can solder with the best and I am the most unforgiving SOB when it comes to work being done incorrectly and it unfortunately doesn't stop with soldering or wiring. I am very anal when it comes to the fabrication of something as simple as a chassis or the simple usage of tools involved in the manufacturing process. I guess once an A hole always a A hole.
And that's the trouble with today's society.... worrying about "offending" someone, and instead using "soft" terminology which lacks the punch to make one take note.
You are one step above most in the ability to admit you need to work on your skill. Soldering correctly does require some practice and with practice your joints will improve. My suggestion is to find an old circuit board and practice removing parts and then re-soldering them. Compare your work to an area of the board that you haven't removed any parts from.
Finally, I found where the problem is:
it’s because the Emitter pin of TR10 touched the heatsink which is grounded. The Emitter goes through the hold on the heatsink And it’s covered by the transisto, so it’s very hard to spot. The dim bulb tester helped a lot. Thanks a lot, guys.
now the amp worked, though the right channel still have some noise ocationally, I suspect that’s the soldering problem some where. But not related to this short issue.
Be careful where yo get your caps. A lot of counterfeits. You marginal 25V, which should be 35V at least , may be 15V.
If increasing the bank, you must do the calculations to see if you need to increase the rectifier current rating.
I take it this amp does not have fuses on the rails? Your output short is likely the cause of the resistor burning up, but it should be protected by a fuse.
This "purity" garbage that got popular a few years ago, no fuses, no protection, no tone, loudness, pre-out-main in, etc has really done music a dis-service.
A pi-filter on the rails before the outputs. I wonder what they were thinking. It sure was not resistance to rail droop. Lower ripple sure, but you feed the VAS and IPS with a PI, not the outputs.
When replacing transistors with similar, but not identical, you run the risk of HF instability due to a change in phase margin. Just a possibility.
If increasing the bank, you must do the calculations to see if you need to increase the rectifier current rating.
I take it this amp does not have fuses on the rails? Your output short is likely the cause of the resistor burning up, but it should be protected by a fuse.
This "purity" garbage that got popular a few years ago, no fuses, no protection, no tone, loudness, pre-out-main in, etc has really done music a dis-service.
A pi-filter on the rails before the outputs. I wonder what they were thinking. It sure was not resistance to rail droop. Lower ripple sure, but you feed the VAS and IPS with a PI, not the outputs.
When replacing transistors with similar, but not identical, you run the risk of HF instability due to a change in phase margin. Just a possibility.
Unless the holes are greatly oversize, there should be high temperature rated insulation (sleeving) over wires and component leads that pass through the heatsink. This goes double for amateur construction and repairs because we usually have no idea of specific material qualities or what's going to be safe over time, tinkering or other abuse.
At one time, woven fiberglass sleeving was usually specified, followed later by high temp. synthetic fibre because typical cheap PVC and heat-shrink sleeving will literally melt and become useless in many fault conditions with class A or any high current output stages. This is general assembly practice but I expect MF owners will be aware that the manufacturer was notorious for production shortcuts and pragmatism at a level that left repair folk with their heads shaking, amazed that anyone would build and sell retail products like they did. Just ask "Jez" Arkless, once active as a member here and former MF engineer.
At one time, woven fiberglass sleeving was usually specified, followed later by high temp. synthetic fibre because typical cheap PVC and heat-shrink sleeving will literally melt and become useless in many fault conditions with class A or any high current output stages. This is general assembly practice but I expect MF owners will be aware that the manufacturer was notorious for production shortcuts and pragmatism at a level that left repair folk with their heads shaking, amazed that anyone would build and sell retail products like they did. Just ask "Jez" Arkless, once active as a member here and former MF engineer.
I used 35v filtering capacitors, so no worry. The original ones are 25v, which are very close to the rail voltage at 24v, but surprisingly they were still working when I replaced them. I didn’t increase the capacitance.
it doesn’t have fuse on the power rail, the resistors act as kind of fuse..lol
The amp is poorly made for sure, very simple, not reliable. But it sounds very good to me. The best I have heard at this price range.
I would say the holes are big enough, but the installation had problem, I installed it the same way as the original one was installed. BUT, apparently it was upside down, so the pins dont match the positions of the holes very well. I noticed it right after I installed it, but didn’t bother to chance it as it was how it was installed originally from manufacturer. Yeah, that’s how this product was made, amazing!! lol.It turns out that I didnt have the luck they have, so it get shorted.
I think the noise comes from the degraded input selector (not 100% sure, but definitely its in the line preamp section). So I removed R4 and feed the input directly to the right hand side of R4, in this way the amp acts as a power amp only. The crackling sound are now completely eliminated.
there are so many things can go wrong in the preamp section, including potentiometer and selector, so its best to use it as power amp only I think, as the power amp section of the amp is very good.
hope it helps to someone else work on this amp too
While this result lead me thinking that musical fidelity MA50 might be the best version of A1 as it doesnt have preamp section, no potentiometer, no selector, but boosted voltage.
Perhaps you should read Mark Hennessy's pages on all A1 known versions with comparative comments: https://www.markhennessy.co.uk/mf_a1/others.htm
Yes, it’s very very good website, I read it before, it’s where I started my recap project. But still only realise how much trouble these bad circuits caused after my own experincense.
re: the noisy preamp, #53
Because the active volume control circuit has to be operational to have the preamp work at all, you may need to prove to yourself that it's not just the volume pot and usual source of the noise. "Proof" can be necessary because active circuits conceal the source of problems, much as feedback conceals the source of faults in a power amp. Our everyday logical thought processes become confused by control loops, so we wind up chasing shadows while the real problem is right in front of our nose. I think the intrinsic design fault explained by M.Hennessy is what's in play here and its only exacerbated by the effects of wear and grime on the contact surfaces over how many years? The selector switch though, should still have enough contact pressure to clear its contacts with regular use. A good clean with IPA and a stiff brush followed by a light smear of (non silicone) contact lube in the right places wouldn't hurt though.
It may not be necessary to replace the pot yet, if you can find some standard 1/4W resistors that approximate (+/- 10%) the measured end to wiper resistances of the pot at a sensible volume setting, where the noise is still obvious. Disconnect the noisier channel pot. leads (assuming your version has them) from the pot. and solder the resistors to them accordingly. If your model has a standard pot. which fits directly to the board, it will actually, be necessary to remove it and fit said resistors to both channels in place of it. This removes any way to adjust volume but as you don't need a signal source for this, simply switching out or disconnecting all sources should silence it as necessary, short of just powering off.
Because the active volume control circuit has to be operational to have the preamp work at all, you may need to prove to yourself that it's not just the volume pot and usual source of the noise. "Proof" can be necessary because active circuits conceal the source of problems, much as feedback conceals the source of faults in a power amp. Our everyday logical thought processes become confused by control loops, so we wind up chasing shadows while the real problem is right in front of our nose. I think the intrinsic design fault explained by M.Hennessy is what's in play here and its only exacerbated by the effects of wear and grime on the contact surfaces over how many years? The selector switch though, should still have enough contact pressure to clear its contacts with regular use. A good clean with IPA and a stiff brush followed by a light smear of (non silicone) contact lube in the right places wouldn't hurt though.
It may not be necessary to replace the pot yet, if you can find some standard 1/4W resistors that approximate (+/- 10%) the measured end to wiper resistances of the pot at a sensible volume setting, where the noise is still obvious. Disconnect the noisier channel pot. leads (assuming your version has them) from the pot. and solder the resistors to them accordingly. If your model has a standard pot. which fits directly to the board, it will actually, be necessary to remove it and fit said resistors to both channels in place of it. This removes any way to adjust volume but as you don't need a signal source for this, simply switching out or disconnecting all sources should silence it as necessary, short of just powering off.
Last edited: