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Rubin 2nd October 2010 01:07 AM

Musical Fidelity A1 Integrated repair/upgrages
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I am hoping to get some advice on whether and how to repair my dead MF A1, or whether I should perhaps build another, more reliable, amp instead. If this in not an appropriate forum for such questions, I apologize in advance and ask if someone can refer me to a different forum.

Note that I have limited experience with electronics, so at this point I would only be considering replacing parts on the current amp or building from a kit, rather than building something from scratch.

I have the original A1, the one with the two-part lid and the axial 10000uF caps. The caps do not have any apparent bulges in them.

Here is the problem with the amp. One day, it just failed to start up. So I replaced the slow-blow fuse in the back, thinking this would fix the problem. When I turned the amp back on (with the lid off), I noticed that after a few seconds some of the resistors (R1, R2, R30, & R31) started to glow and then smoke. Now they just smoke instantly when I power up the amp. Needless to say I cannot use the amp in this state.

What I am wondering specifically is whether the amp can be fixed by replacing all or some of the power supply parts (plus the R30 and R31 resistors), or whether I have to test each piece of the amp to determine if there are more extensive failures down the line. What is the best way to determine how to fix this? Or should I even bother? Thanks.

Bill Fuss 2nd October 2010 10:17 AM

If you can find a schematic and post it you will probably get some help.

It looks like R1 is part of the PS CRC so there is a fault pulling a large current from the PS. Probably in the output stage.

jaycee 2nd October 2010 11:02 AM

I can guarantee that the output transistors have failed. These things run stupidly hot and this drastically shortens the life of all of the parts in there. R30 and R31 are emitter resistors for one channel, so those smoking means that the output transistors on that channel have failed.

Mark Hennessy has a great page about these amps here

I note that it looks like someone has already replaced the volume control - Mark goes into why those fail on his site (poor preamp design).

To bring the unit back to full working order, you will need to:

* Replace all the electrolytic capacitors, especially the 4 main power supply ones. These simply fail due to the high heat.
* Replace the output transistors. They may appear "custom" but they are really MJ2955/2N3055. I'd use the much more rugged MJ21193/4 series.
* Replace the fried resistors.

Edit: I just noticed you said that you started up the amp with the lid off - don't ever do this on an A1. The Lid is the heat sink for the amplifier, and without it, it will quickly fail. This is probably what has caused the damage you have now. The fuse likely failed simply due to age. You will need to clean off the original heatsink paste and replace it with fresh.

I would also replace the preamp as detailed by Mark.

Rubin 2nd October 2010 03:17 PM

The power supply schematic is sketched out on Mark Hennessy's page:
Mark's Project Pages - Image Viewer

Thanks for the tips jaycee. I will put together a parts list of what I would need and see what the total cost would be.

I just have two questions before I shop for parts. The first concerns the 10000uf capacitors. These are axial in my amp, and large axial caps are apparently uncommon. Can I use any axial capacitor as long as it is 10000uf, and rated at least for 25V and 85C? Or can I use the radial caps Mark recommends by extending the leads? I could not find any axial caps with the specs he recommends for radial caps so far.

The second question concerns the failed resistors. They are a dull green unlike the other resistors on board. What kind of resistors should I use here?

Thanks again.

Pierluigi 21st November 2010 11:37 AM

Hello to all,
if i remenber correctly, on that PCB there are the footprints for radial caps in addition to the axials installed; if so i think you can use for example the Panasonic ECG 10000uF 35V 105įC. It's on Digi-Key, P/N: P6665-ND. The caps recommended by Mark seem out of stock.

jaycee 22nd November 2010 03:57 AM

The green resistors are probably just some kind of high temperature resistor. They look too small. The original specifies 2W resistors - I'd probably try and squeeze some 3W wirewound (or metal oxide if I could) devices in there.

I'll also be very surprised if only the output transistors have failed. Expect to have to replace the driver transistors - BD135/136 labelled as TR6/TR8 on Mark's schematic. BD139/140 will be a suitable replacement here.

spendorspain 3rd March 2011 07:43 PM

Musical Fidelity A1 restoration/upgrade
Hi all,

Iím restoring a Musical Fidelity A1 amplifier 20-years old. It is a MkII (pcb issue 8) in very good condition, fully functional and with no apparent damage from the heat except the bulging power capacitors. I need to change them, but after reading several websites (mostly the Mark Hennessy page) and threads here about the A1 and its relatives A100, MA50 or MA100, Iím also decided to make some improvements. These will be:

1) Direct connection from the RCA inputs to the power amp section through a 10 or 20k pot. I donít need the phono or line preamp as I listen only to CD or audio files from my PC. Is it possible to completely disable all the pre-amp circuit and even desolder the unused parts from the pcb (I probably need to do this to make room for a delay/DC-offset protection module) without affect the power amplifier section?

2) Iím not going to re-use the original, beautiful but small, A1 box. Iíll install the pcb vertically in a new and better ventilated tower-like case (32x25x10cm) with a much bigger heatsink attached to the original U-shaped aluminium piece where the output transistors are. In the place of the original finned ďlid-heatsinkĒ, Iíll use the Aavid EK/B/300 heatsink (30x30x4cm), rated at 0.30ļC/W, with its fins vertically oriented for better dissipation, so I hope there will be no heat-related problems in the restored unit.

3) Iíll use a new fully-encapsulated power transformer, mounted on the rear plate of the box (but outside). It's a 2x18Vac dual secondaries type, wired to obtain the required 18v-0v-18v by joining the two 0v output cables in one. Here I have a doubt regarding its VA size. The 120VA Nuvotem 0120P1-2-018K is a perfect fit (104 mm diameter) and the 160VA is too wide for the new box. The original A1 transformer is around 100VA, so I think the 120VA will suffice. I will use the amplifier with a pair of small loudspeakers (nominal 4 ohms impedance) and I listen usually at low volume, so the limited A1 power is no problem. Do you think is there any advantage in going to the 160VA transformer? Perhaps its higher rating will stress too much the PSU caps and diodes.

4) Iíll use new Panasonic FC 105ļC-rated for all the electrolytic capacitors. The new four big PSU capacitors will be Evox-Rifa PEH536 10000uF/35V (same value but not rated at only 25V). They are 30x40mm (same diameter and a little taller than the originals), but with much better specs (ripple 4600mA, ESR 0.022Ω, 105ļC rated, 6000h life), and Iíll probably bypass them under the pcb with film types.

5) New diodes (at least the D1-D4 BY255 in the PSU), new small transistors (BC550C/560C and BD139-16/140-16) and, more important, new Onsemi MJ15003G/15004G TO-3 output transistors. From what I have read these are better sounding and more rugged that the originals 2N3055/MJ2955.

6) Iíll use new better power resistors (those on the pcb rated from 1 to 5W, like R1-2, R13-16, R30-31 and R39). Iím not sure about changing the other, smaller, metal film resistors (they look fine). My plan is to maintain all the original values, although Iíve read that it is possible to modify several of them if it is needed for decreasing a little the quiescent current, like R6/R11 or the output resistors R30/R31.

7) And, lastly, Iíll probably fit at the A1 output some on/off-thump and DC-offset protection for the loudspeakers, like the Velleman K4700 kit, the Dada protection-delay circuit, or other similar. If Iím not wrong, the ďthumpĒ at turn-on/off is avoided by disconnecting during about 5 seconds the amplifier from the loudspeakers, but in the A1 userís manual there is a warning specifically against this: ďYou MUST connect the A1 to a pair of loudspeakers before you switch it on. If this is not done, it is relatively easy to damage the output stageĒ. Does this mean that these output protection circuits are not recommended? If the temporally disconnected A1 output is directed to a power resistor (like 8ohms/50W) instead of the loudspeaker binding posts this problem is solved?

Do you have any recommendations? Is there any mistake in the plan Iíve outlined? Please, Iíd like to know your comments before start to buying the parts. Thanks in advance for your help

Best regards

Zen Mod 3rd March 2011 08:16 PM

too much work

recap it , sell it and then make F5 with rest of hardware and money

spendorspain 5th March 2011 09:38 AM

Thank you for your answer, Zen.

I've read about the Pass F5 in Diyaudio and it's very interesting. If some day I build a power amplifier for my main system, I'll probably use it. Anyway, I'd like to upgrade my MF A1, now it's opened. I use it as second audio system in another small room and I like very much its sound. I don't think it would be too much work (it's just swapping of several parts and audio is a hobby, isn't it?) and, regarding cost, the only really expensive part is the huge heatsink, but using it will kill any A1 reliability trouble from the beginning. My plan could be also simplified if necessary.

Please, have you (or somebody else) any specific comments to the upgrades 1 to 7 I've outlined in my previous post? I'd like read your knowledgeable opinions before ordering the components

Thanks in advance

BTW, your avatar brings back childhood memories to me... Pippi Langstrumpf ("Pippi Calzaslargas" here in Spain) and its monkey, "Mr. Nilsson"

Zen Mod 5th March 2011 09:57 AM

if you are patient enough - I'll do these days one A1 ;

recapping , selector repair , tossing all unnecessary parts out etc.

I'll make photo-strip , so it will be easier to understand

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