Wierd behaviour-> Musical fidelity A100

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A 8

A 8

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Joined 2003
#1
I am trying to fix an old (1985)Musical Fidelity A100 integrated class A amp.

The symptom is as follows, for the first minutes or so (up to 5min) one channel distorts heaviliy and then there is a small click and it starts to sound ok.

It also seems to get better if I do not switch it on for a few weeks, it will then be ok for a few start ups and then get back to how it was.

I just have a voltmeter and measuring the voltage over the output resistors R31 and R30 it seems that one (or both?) of the output transistors do not draw current until after the click.
I initially figured there was a bad solderjoint somewhere and re-soldered the whole board but no change. :bawling:

Any ideas are much appriciated!
 
#2
Here are the schematics
 

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#4
A8,
Check the input electrolytics. Looks to me like if the leakage is
high it could defeat the biasing - but I could be wrong (very small diagram) :clown:

I would expect to see a small dc difference between TR1/3 bases
and a ratio of this accross R30/32.

Dave /looks like Sajti beat me to it !!
 
#5
Thx guys, I agree the other option would be the caps but can they really pass DC when faulty?

EDIT Just remembered the input caps C6 and C7 have actually been changed for film types.

And a larger pic
 

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#8
A 8, Change the electrolitics, after all they are almost 20 years old and one of the top two failure modes is high leakage (passing DC current). Its a good excuse to upgrade the caps any way. Remember to do both channels while your at it and think about the power supply caps too.

Good luck
BZ:geezer:
 
#9
HDTVman said:
A 8, Change the electrolitics, after all they are almost 20 years old and one of the top two failure modes is high leakage (passing DC current). Its a good excuse to upgrade the caps any way. Remember to do both channels while your at it and think about the power supply caps too.

Good luck
BZ:geezer:
Click to expand...


I agree with that! change all the caps!!
 
#12
A8, I would consider these to be in the signal path (even though they are AC shunts) so use good quality types. Do you have the room for film types (?) as there is not so much voltage keeping the electros polarised. Don't ask me about sonics :dead: !!!

:att'n:
Having once come very close to having my face 'reworked' by an exploding PSU electro (despite it having a vent) I will advise caution working near the amp when powered up.

Dave
 
#13
A 8 said:
BTW, am I right to assume that the quality/type of C10,C13 and C14 will not affect the the sonics of the amp?
Click to expand...

Hi A8,

C14 and C13 could well affect the sound quality because they are effectivly in the NFB loop - I would use good quality components rated at 105C here - Panasonic FC's, for example...

As to C10, the jury is out here. You may have already seen my Musical Fidelity A1 page - over the years I've had conversations with many people about the design. I've not found an absolutely convincing explanation for the C10/R17 series combination. I'd perhaps guess that it was determined empirically to ensure stability or perhaps improve distortion performance, but I'd welcome comments from people here.

The message from my page, based on my experience with these things is simple - change every cap! I can understand you wanting to "fix" the amp first, but it's important to realise that you can not attempt any form of logical fault-finding when you've got an amp with dried-up old capacitors! I know that others have already said this, but trust me - it's important 😉

Out of interest, how did you get the diagram? I know that it can be tough getting info from Musical Fidelity. Would you be willing to scan the whole schematic - I'd love to add it to my collection. Another reason for asking - I get a lot of email about MF amps , so I'm interested to learn anything I can about them!

Does it only have one pair of output devices per channel? Can you tell what they are, or have they been re-labelled?

Does the preamp look anything like the A1 preamp? If you've seen my page, you'll know what I think of it 😉

Do you know what the supply rails are?

Sorry to ask more questions than I've answered - good luck with the repair!

Cheers,

Mark 😉
 
#14
Out of interest, how did you get the diagram? I know that it can be tough getting info from Musical Fidelity. Would you be willing to scan the whole schematic - I'd love to add it to my collection. Another reason for asking - I get a lot of email about MF amps , so I'm interested to learn anything I can about them!
Click to expand...

I got them from the local reseller, it was easy I just asked.

wrt to your other issue we should take it offline..send me a message.

Does it only have one pair of output devices per channel? Can you tell what they are, or have they been re-labelled?
Click to expand...

Yes one pair only and I cant make out the marking, they've soldered all over it.
I've put it back togeather but think the rails were +- 35v, I'll check again when I open it up to change the caps.

Does the preamp look anything like the A1 preamp? If you've seen my page, you'll know what I think of it
Click to expand...

It looks pretty similar and the pot was changed 14 years ago to a more exotic one that actually has worked well since.

I found the output resistors beneficial to change for less inductive types..eased up on the ssss sounds that were all over the place.
 
#15
DRC said:
A8, I would consider these to be in the signal path (even though they are AC shunts) so use good quality types. Do you have the room for film types (?) as there is not so much voltage keeping the electros polarised. Don't ask me about sonics :dead: !!!

:att'n:
Having once come very close to having my face 'reworked' by an exploding PSU electro (despite it having a vent) I will advise caution working near the amp when powered up.

Dave
Click to expand...

Exactly my experience with an A100 ! I was measuring supply voltages as I noticed one supply was 15 V intermittently.
As my hand was close to the supply caps I noticed that one of them was very hot. When I dived to pull the mains plug the cap exploded and covered my room with particles of paper and a lot of smoke and smell... Although it sounds silly I use a safety goggle since then when testing MF class A products as my eyes are too valuable.

The cap was shorted and gave up. Please replace all caps as suggested by several members and use 105 degree types instead of the standard types. Check the fans as they fail sometimes too. I am not sure now if the A100 has a volume pot as there exist some monoblocks too with that name but anyhow; replace the potentiometer and input switch as well as they mostly fail a day after you've replaced the caps 😉

I want to warn any potential new owner of second hand A1/A100 amps: these amps are real dangerous as when they fail they take your loudspeakers with them. There is not a single protection used and these products become much too hot internally. I repaired quite some of them that were bought second hand that failed in a short time. Some but not all of them were repaired before in a non optimal way leaving the caps where they were.

The output transistors are relabelled types but using the MJ1500x types gave good results without nasty side-effects.
Please use solder lugs at both sides connected with eachother. The original way ( soldering the wires to the case directly ) is not correct and prone to failure.
 
#16
Hi guys,

So...I changed all the caps but guess what the problem still persists!!!

Now, by chance(I got fed up with it) I hit the shell (collector) on the PNP TO3 power transistor with a screwdriver and it immediately started to work. I repeated this a few times with the same result.

So now I am starting to think that the connection between the shell and the silicon inside is weak and/or faulty, perhaps because they soldered directly on to the shell?

Two questions,
*Is there anywhere else but on the shell I could get to the collector?
*The power transistors are Motorola MJ15003 and MJ15004, can I just replace one of them without any matching?

Any suggestions/advice!?

Thx/Michael

This is how it looks and it the right one that seems to be the problem.
 

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#17
Could it be that the soldering on the TO3 case is not OK ? Please use solder lugs with a wire at both 3 mm holes. Much safer !
Directly soldering on steel TO3 cases is plain unsafe and technically not correct.

If the transistors are broken you better change them alltogether ( the good channel too ) for avoiding channel differences, no real need for matching. I never bothered anyway.

BTW I found the schematic of MA100 ( very similar ). Must have A100 too on paper somewhere but I forgot my promise to post it completely so I did not look for it actually.
 
#19
OK, replace all 4 power transistors and use solder lugs at both sides. If they aren't used already you can use cut-off pieces Teflon insulation sleeve on the pins of the TO3's ( no PVC, no explanation needed I hope ). Use good insulation pads. Carefully check for shorts before powering the amp.

Aargh, the picture you attached with that white s**t on it remembered me of why I don't want to repair these kamikaze amps anymore !
 
#20
I am pretty fed up with it.....a complete mess to work on.

It's for my father in law (I don't think it is worth all the effort) but I am not sure I'll do all the transistors.

I mean I've never seen or heard of transistors behaving like these, they work or they don't...right!?

I am thinking it is more likely that there is still a semi-broken solder joint or cable somewhere?

Happy new year guys!:drink: :drink: :drink:
 
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