Musical Fidelity X-A1 amp servicing - Page 1
Forum section: electronics / projects

author: Richa
author_rank: Newbie
timestamp: September 10, 2025, 03:38:32 pm
content: Hi all at eevblog,,, hoping someone can help with my tech issue,,,I'm currently servicing my Musical Fidelity X-A1 amp, (50w per channel) ,, going very well but i want to measure bias shortly. I'm getting zero help from the Musical fidelity service department,, and they won't supply schematic,, so have no idea what bias voltage should be or where measuring point is, I can clearly see variable resistors (I'm guessing for setting bias?), but there are no actual test points,, Hoping someone can put me right on expected voltages and where to test.? I've attached pcb photo.Thanks for reading and thanks in advance for your help.Richard

author: Gyro
author_rank: Super Contributor
timestamp: September 10, 2025, 04:11:28 pm
content: Hi,I'm wondering what you mean by servicing in this context. Have you been doing any repairs to the board or anything that would require subsequent resetting of the output transistor biassing? You haven't posted an image of the heatsink side of the board but I'm guessing that it uses those unobtainium Sanken transistors with integral emitter resistors.The normal method of bias setting is to measure the voltage (mVs) across the emitter resistors and use them as current shunts to set the specified (warmed up) bias current using the pots. Unfortunately trying to probe around the output transistor wiring in this case is likely to lead to accidental shorts and destruction of the transistors, drivers etc. The design in that area is downright nasty.If you haven't done anything to the board, particularly anything that would affect the output transistor bias, I would leave well alone. If Musical Fidelity won't tell you the specified bias current anyway, you have no way of knowing what to set it to. The best you could do is to check that the bias currents on the two channels match, but as I said, with that layout you're more likely to blow something non-replaceable. Even if they don't match, you won't know which is right and, of course MF will want you to send it back for service.P.S. I would visually check the output transistor wires under a magnifier. I downloaded the image and a couple of them (second transistor from the right) look uncomfortably close.Ah...EDIT: I found the image you posted, it's from Stereonet forum rather than your own unit. Yes - from the other images, Sanken SAP-15 transistors. If you blow them, the best you will do will be dubious pulls or fakes from China...https://www.stereonet.com/forums/topic/288336-musical-fidelity-x-a2-amp/You appear to have asked the same question there, back in 2023.

author: Richa
author_rank: Newbie
timestamp: September 12, 2025, 12:19:40 pm
content: Hi Gyro,, thanks for your input,,I am a noob,,, and learning all the time,, I have just repaired my Cyrus 7,, replacing parts that were missing when i bought it,,, It had incredibly high DC offset which i have fixed by replacing input transistors,,,. I have recapped the unit also with new audio grade capacitors,, It is now running sweetly.My MF X-A1 ( it works ok) but is now on my bench,,, and i'm replacing capacitors,, slightly high DC offset on one channel,, at 20mv, (not the end of the world i know) and just 5mv on the other, I'm servicing it as further learning,,, with the knowledge gained from my foray with the Cyrus. Just wondered if anyone knew what sort of bias i should expect,, and point me to test points,, my goal really is just to lower the offset,,, replace capacitors,, e.t.c ,,, and learn more.Thanks again for your help... i may have put up the wrong phot ,,,, doh!!!, it's likely from my searches for X-A1 schematics,,,.. cheers,

author: Gyro
author_rank: Super Contributor
timestamp: September 12, 2025, 12:57:05 pm
content: You're welcome.I wouldn't worry about the offset voltage, 20mV isn't going to hurt anything and something that the bias adjustment won't affect. As I mentioned, the bias adjustment is normally done by measuring the low voltage across the output transistor emitter resistors, which, in the case of SPA-15s are integrated into the transistor package (the datasheet should give the resistance value). I suppose you could break into the positive or negative supply leads and measure the current there - this will give you output bias + all the preceding stages current. Again, as I mentioned, the output transistor wiring makes a slipped probe and destruction quite likely. It would be safer to tack a couple of wires to the emitter resistor pins (either one, the NPN or PNP) and measure the voltage externally. Again, it is a man with 2 clocks situation, if the two channels differ significantly, you won't know which is correct.With regard to the schematic, it is only a single sided PCB with discrete components so it would be an easy (though tedious) task to draw your own. The two channels are identical, which immediately halves the effort. There is the phono pre-amp too but you may well not be interested in this section, so can probably just start at the line level input selector.*Before somebody else says it, wholesale replacement of electrolytics is unlikely to make any significant difference but it's your money and inclination.Just one other thought. Looking at the small Aluminium block that the output transistors are mounted to and the subsequent heatsinking just to the bottom of the thin extruded or sheet metal case, I wouldn't risk running the amplifier at full output for any significant time (I noted your function generator posts), I don't think it will take it, regardless of what the spec says - more concern about the unobtainium SAP-15 transistors, I've seen examples where the integrated emitter resistors have blown up too...https://www.eevblog.com/forum/repair/repair-of-audio-amp-using-sanken-sap15n-sap15p-output-transistors-fixed/Edit:*The simplest way to do this is to take two identically sized images of the board (directly from above), one of the component side and the other of the track side. Mirror image the track side one vertically. You can then either, look at them side by side or, with a multi layer graphics package like Gimp, superimpose them with transparency. Either way, it will save you the handling stress of repeatedly flipping the board.

author: Uunoctium
author_rank: Frequent Contributor
timestamp: September 12, 2025, 06:20:06 pm
content: Sanken datasheet gives all information needed. I assume, the amp don't run at class A, so Sanken recommends an idle current of 40mA.The integrated emmitter resistors having a value of 0.22 Ohms and as a nice gestus, Sanken spended them an own pin, marked S.Either you measue at NPN between pin 4 and 5 (PNP differ, here 2 and 1), or you measure over both resistors at NPN pin 4 plus PNP pin 2 at doubling the resistor value for calculation.

author: Richa
author_rank: Newbie
timestamp: September 15, 2025, 11:09:44 am
content: Hi Uunoctium, and thanks so much for your help,,, between the replies received i have learned tons !,,, cheers