MF XPre troubleshooting

[kepa1]

Hello,

Well this is not tubes stricltly speaking, but more tube than SS anyhow, so let's get started!

I've got a Musical Fidelity XPre that I have tweaked a lot for 2 years. Now and then it would not work but it usually was easily fixed (bad soldering).

But this time I can't figure out what is happening...

3 weeks ago, my system was not emitting sound and I was 99% sure it was the pre's fault (and my tweaking it!). So I decided to take a look inside but didnd't notice anything special; in the meantime I had the idea to bypass the 2,2µF caps with styroflex caps. Plugged again, the speakers SCREAMED in my ears like never before with a totally unbearable sound = no music

Mesuring the DC current on the output gave a nice 10V a startup, diminishing to a few mV in a minute. All connection seemed ok with a continuity check. A French man on a forum told it could be the tubes oscillating but I can't verify it because I don't have access to the correct equipment nor the knowledge to use it.

So here is the circuit, I have mirrored the back of the PCB so that it can be read more easily:



The red lines are the connection of R107 / 207 that you can track ont the schematics of the X10D which is almost identical to the XPre:



Well, a big thanks to anyone with a view to solution this mess.

[SY]

Hard to tell from the pictures, but it looks like there might be some significant leakage path from the feedback components to nearby traces, especially the heater.

It would be helpful to CAREFULLY measure the DC voltages and put them on the schematic.

[kepa1]

Wow, already an answer, thank you!

I'll have to check voltage again but I can already post this zoom in of the board that is easier to look at...



As you can see soldering too much has caused some "fire" marks but I'm not sure about the drawbacks of such things.

[kepa1]

No need to go further guys, the SCR 2,2µF caps were dead.

I figured out in my sleep that these had suffered when being decoupled with styroflex caps and decided to replace them with MKC caps that I had mounted before and guess what, now the pre sings again...

Wonderful, almost 3 weeks without music

[Andy Bartha]

Hi Kepa 1,
Couple of suggestions on upgrading your MF pre amp. First replace C 101,102 and 104 with a good quality film cap. Auricap would be a good choice. Replace C105 with a 10uf film cap. A SCR 10uf film cap will be very much better than the electrolytic that is in circuit. Only replacing C105 will make a nice improvement.

[kepa1]

Thank you for suggesting Andy,

I'll definitely replace C105 but as you can see space is tight in this box and a 10µF SCR caps is huge, the ones I used for C104 is onl 2,2µF - imagine the size oa 10µF

So I'll stick to an elctrolytic, maybe an ELNA, but it will have to be 63V minimum because high voltage goes inside of them.

As for the other caps, they are all upgrades from the stock cheap ones. I chose Wima MKP4 because they fit nicely. C103 I think is already a styroflex, or at least it looks very much so.

But what about upgrading resistors, I mean only the ones that receive the signal, I'd say R102, 103, 104, 110.

Does that sound good to you?

By the way, I already changed all electrolytic, all diodes and the walmart for a toroid transformer, a lot of improvement indeed!

[Andy Bartha]

For C105, if there is some space, glue the cap or caps to the chassis and run wires over to the proper place. Also try to get the smallest voltage possible. Another trick is to use 2 5.1uf caps or 3 3.3uf in parallel. This will get smaller size plus better ESR.
I'm not much of a resistor person. The resistors look like like Roderstiens, so they are good pretty good. But if you want to change some out you can replace R102,103,105,106,107,108 and 109. To get any noticable improvement you will have to go to caddocks or vishays. These are pricey reisitors.

[henkel]

kepa1, hi!

Very helpfull pics you've got there. I hope you can further enlighten me about the MFX10D.

I intend to build one for use as a tube buffer in the car audio system. I see that the X10D runs on 12VAC power wallawart power supply.

At which point in the schematic where conversion of AC converts to DC? and after conversion, how many DC is it supposed to measure?

I look forward to your help.

Thanks.

[kepa1]

Hello henkel,

Well for this, you need the PS Schematics...

... and an expert to determine the exact AC/DC transformation point. Beware because even though the in wall trafo is rated at 12V, the X series uses voltage doublers.

[henkel]

kepa1,

Thanks.