Troubleshooting: amplifier Musical Fidelity A1

S

Sala Massimo

Guest
Hi

I am the happy owner of one A1 amplifier, bought in 1991.
It is 20 Watt class A amplifier, build around four MOSFET:

In the last months the A1 started to malfunction, the volume knob works very
bad: when I turn the knob I hear strange noise ("shhh shhh" as there is sand
inside the knob, and also some "crack") and the volume output isn't in
accordance with the knob position.

I took the unit to repair, but they said me they cannot because they haven't
the potentiometer.

I ask you:
1) is really the pot the culprit ?

2) where can I found one (or two) spare parts?
It is a Alps 0071 50KAX2 (I cannot measure the value because it is
soldered).

3) by the way, do you know which are the exact final transistors?
I found on newsgroups many codes:
original: A1 N8915 and A1 P8919
poor quality? 2N3442 and 2N5876
good ? Motorola MJ15003 & MJ15004

My unit (model A1 X) has these transistors:
A1N9020 A1N9034
A1P9022 A1P9018
I don't understand why are there four different codes instead of two exact
couples!

ciao, Massimo
Italy
 
Sala Massimo <saltlakeCHIOCCIOLA@tiscalipunto.it> kenjka:
I am the happy owner of one A1 amplifier, bought in 1991.
It is 20 Watt class A amplifier, build around four MOSFET:
Pricy, huh? :))

In the last months the A1 started to malfunction, the volume knob works very
bad: when I turn the knob I hear strange noise ("shhh shhh" as there is sand
inside the knob, and also some "crack") and the volume output isn't in
accordance with the knob position.
Sounds like bad contacts in that knob...

I took the unit to repair, but they said me they cannot because they haven't
the potentiometer.
I guess that isn't just a simple pot, more like a shunt... Musical Fidelity
is high-class audiophile company, so I guess they use high-quality shunts
for volume knobs...

2) where can I found one (or two) spare parts?
It is a Alps 0071 50KAX2 (I cannot measure the value because it is
soldered).
Don't know... You could contact Musical Fidelity and ask them to help you
find the right service-shop where you can repair your unit... Or simply find
some service shop from some audiophile genious... Local audiophile stores
are best thing to start... :)

ciao, Massimo
Italy
Oh, neighbour! :)))) Greetings from Fiume, Croatia... ;)


--
"Sravnjens li bananau crtu ?" upita Milosevica podjebava cizmao zmice.
"Nisam ja nikog bombardiro !" rece cizma gladi "Ja samo stolo njise humanitaranm !" By runf

Damir Lukic, calypso@fly.srk.fer.hr
a member of hr.comp.hardver FAQ-team
 
Stephen, thank you for the posting.

I never stressed the A1, I used it in a "religious" way, always wired to my
Celestion 5 speakers.
I didn't use the A1 in the last 3 years, after the pot failure (I married,
two daughters and a lot of job...)

The resistors and the opamps seem fine.

At a first glance, one of the big power caps (10000 uF 25 V) is strange: its
plastic dome is going yellow (the other three caps have a transparent dome).
Is this a clue for a incoming failure ?


About the op-amp: I look carefully, my A1 X has two TL084CN, not a TL074.
Is the AD713 a raw replacement for the TL084 - without tweaking ?
FYI: see Mark Hennessy op-amp replacement,
http://www.mhennessy.f9.co.uk/mf_a1/


The Alps pot is not available in Italy.
I searched "stereo" pots, also third parties, on local shops and
surplus. They haven't.

Perhaps I found the pot at Farnell UK, ALPS RK27111250K (former 0071
50KAX2), Farnell code 697928.
Good news: it is Ł 16.33 plus shipping cost to ITALY (about Ł 8), because
Farnell Italy doen't sell to end-users... :-(

If anybody can help me to get the pot at a cheaper cost...

ciao, Massimo
ITALY
(to reply: remove CHIOCCIOLA and PUNTO)
 
The AD713 will sub for the TL084 as well, yes. As memory serves, the TL084 is actually a
cheaper & noisier version of the TL074. As for the resistors in the amp circuits, you don't
need to use the amp heavily to have them burn up. Even at idle, many of those small resistors
are running at or a bit above their rated heat dissipation. I would advise replacing all of
them in the amp channels, with 1/2w or even 1w metal films.
If you remove the 10,000u/25v cap & see any white or brown leakage under it, especially around
the cap's leads, then it's going bad. Or if the bulging top is actually the metal can inside
bulging.

--
Stephen Sank, Owner & Ribbon Mic Restorer
Talking Dog Transducer Company
http://stephensank.com
5517 Carmelita Drive N.E.
Albuquerque, New Mexico [87111]
505-332-0336
Auth. Nakamichi & McIntosh servicer
Payments preferred through Paypal.com
"Sala Massimo" <saltlakeCHIOCCIOLA@tiscaliPUNTO.it> wrote in message
news:brv0nq$afn$4@lacerta.tiscalinet.it...
Stephen, thank you for the posting.

I never stressed the A1, I used it in a "religious" way, always wired to my
Celestion 5 speakers.
I didn't use the A1 in the last 3 years, after the pot failure (I married,
two daughters and a lot of job...)

The resistors and the opamps seem fine.

At a first glance, one of the big power caps (10000 uF 25 V) is strange: its
plastic dome is going yellow (the other three caps have a transparent dome).
Is this a clue for a incoming failure ?


About the op-amp: I look carefully, my A1 X has two TL084CN, not a TL074.
Is the AD713 a raw replacement for the TL084 - without tweaking ?
FYI: see Mark Hennessy op-amp replacement,
http://www.mhennessy.f9.co.uk/mf_a1/


The Alps pot is not available in Italy.
I searched "stereo" pots, also third parties, on local shops and
surplus. They haven't.

Perhaps I found the pot at Farnell UK, ALPS RK27111250K (former 0071
50KAX2), Farnell code 697928.
Good news: it is Ł 16.33 plus shipping cost to ITALY (about Ł 8), because
Farnell Italy doen't sell to end-users... :-(

If anybody can help me to get the pot at a cheaper cost...

ciao, Massimo
ITALY
(to reply: remove CHIOCCIOLA and PUNTO)
 
Lubricate the pot with a vegetable based sewing machine oil.
Dont use mineral oils, they dry out and give the problem you describe.
 

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