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Musical Fidelity P170 repair

Discussion in 'd.i.y.' started by Gervais Cote, Aug 30, 2021.

  1. Gervais Cote

    Gervais Cote Predator

    Hi gents,

    As mentioned in the title, I'm on my way to refurbish a nice example of a Musical Fidelity dual mono power amplifier P170, serial no. 3466.

    So far, in good shape, no major trace of overheating but all the electrolytic capacitors are original so they are due for replacement with next higher voltage available as always with that brand.

    My main concern is more about the output transistors. They are in a good condition electrically speaking but some of them have broken leads due to an impact that occurred on the cooling fins to which they are attached with screws.

    These output transistors are not available as they are from Hitachi and they no longer make them.

    I would like to see if any of you would know what would be the exact replacement electrically speaking, same center distance for the 3 leads although the shape would be slightly different.

    I am comfortable with the fact I will have to drill and tap new holes to hold the new transistors to the fins as long as the function and sound signature remains the same, all good for me.

    There are 2 Hitachi transistors no. J83 6C2 (green) and 2 others are K227 6B2 as per picture below.

    Also would like to know how to adjust the bias and what value should it be if any of you has the info.


    Thanks in advance.
  2. russel

    russel ./_dazed_and_confused

    BUZ900P/BUZ905P from a reputable supplier, bias 100-150mA, Jez will be along shortly since musical fidelity is his manor for more tips and tricks.
  3. Gervais Cote

    Gervais Cote Predator

    The BUZ900P would be equivalent to the J83, right ?
    I may have to look for some NOS as it seems some manufacturers stopped making them......
  4. canonman

    canonman pfm Member

    Maybe I shouldn't have ditched a few hundred of these
  5. russel

    russel ./_dazed_and_confused

    J is p channel 905 is p channel iirc, just check the data sheets, which you will do as soon as you see the price for NOS devices.
  6. Arfer

    Arfer pfm Member

    Alternatively, if they all have as much lead length as those in the pics, use the transistors you already have if they are electrically OK. Use a suitably thick wire to make the connections and make a good mechanical joint by wrapping before you solder, there is plenty of space between those pins, just check everything with a meter before switch on. Maybe put some insulation on the centre pin to be safe.
    Might not look the best but it should do the job
    earlofsodbury likes this.
  7. Gervais Cote

    Gervais Cote Predator

    This was my plan B ! :) Trickier but fine if not parts are available.
  8. davidsrsb

    davidsrsb pfm Member

    Modern parts are far less robust with chronic SOAR problems, so recovering the original parts would be my first choice if they have not been damaged.
    Genuine NOS are expensive and too often turn out to be fakes
  9. russel

    russel ./_dazed_and_confused

    Do the Semelab parts have problems?
  10. Gervais Cote

    Gervais Cote Predator

    Wow, thanks for the heads up !
  11. davidsrsb

    davidsrsb pfm Member

    Actually I did not notice that these are laterals, so modern Semelab/Profusion parts are fine if you can get them.
    It is the verticals that have gone downhill in linear operation robustness

    Given the price of modern laterals I would still try to salvage
    Gervais Cote likes this.
  12. russel

    russel ./_dazed_and_confused

    Semelab/Exicon are a reasonable price, in fairness to verticals they are not designed to be linear and as they don’t go negative tempco until they are passing quite a few amps in some cases tens of amps the later ones also have very high gains as well which can make thermal stabilization as bad or worse than bipolars. Having said that low current/low gain ones like IRF520 irf9520 devices can make decent amplifiers as can ones based on the 2sj200/2sk1529 but the latter are quite difficult to get hold of.
  13. Gervais Cote

    Gervais Cote Predator

    Update for anyone interested :

    The P170 is repaired and improved and now playing music in the basement.

    Here are the things that have been done :

    -all electrolytic capacitors replaced with higher voltage and 105C instead of 85C temperature rating

    -some 5 watts resistors that showed some traces of heat have been replaced with 12 watts resistors allowing a running temperature of 83C instead of 115C on the hottest ones

    -same update with some little 1/2 watts that went up to 1 watt

    -some aluminum heat sinks added to the medium size pre-stage transistors

    -some single strand wire and solder added where boards were about to peel off or where some large resistors are installed

    -contact cleaner where needed (Deoxit)

    -as recommended by Arfer, I have been able to re-use the power transistors by wrapping them with a single strand wire and solder

    Oh, and this little monster runs at 60 VDC so better not let fall any screwdriver when the lid is removed !

    So far sounding very good, lot of PRAT as mentioned by Jezz in another thread, very natural human voices and a near perfect treble although a bit harsh but this may get better over time as it played music for a few hours only.

    The first picture is before and the other two are after the upgrade :




    And again thanks to all of you for your comments and advice.
    Stuart Frazer likes this.
  14. Gervais Cote

    Gervais Cote Predator

    Latest update : all of the harshness I heard is now gone as the amplifier has been running for more than 48 hours. Very astonished by the dynamic sound and tuneful notes of bass guitar and drums but overall, the human voices are simply breathtaking.
  15. davidsrsb

    davidsrsb pfm Member

    A comment about the last photos, the wires to the diode bridge should not be laced up with the wires from the big capacitors to the main pcbs
  16. Gervais Cote

    Gervais Cote Predator

    You’re 100% right as we can see on the picture below, thanks for the heads up !

    chiily and 337alant like this.

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