I'm not sure how to approach this one. This amp is distorting at low volumes. It is roughly equal in both channels. It is at a third of a watt for the picture of the scope.
I can't find any obvious issues. Someone has been in here before and done not particularly neat work. C15 and C16 had been increased to 33pf. But swapping them back to 22pf in one channel didn't do anything.
The bias is about what the schematic said it should be in one channel, but much lower in the other channel.
I can't find any obvious issues. Someone has been in here before and done not particularly neat work. C15 and C16 had been increased to 33pf. But swapping them back to 22pf in one channel didn't do anything.
The bias is about what the schematic said it should be in one channel, but much lower in the other channel.
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Pff I used to repair these but I can not say directly why your issue is. Have the electrolytic caps been replaced?!
Did you measure the volume circuit around the TL084? BTW One of the best mods is to remove that circuit as the volume potentiometer is in the feedback loop causing the odd power on phenomenons.
http://www.markhennessy.co.uk/mf_a1/mods.htm
Did you measure the volume circuit around the TL084? BTW One of the best mods is to remove that circuit as the volume potentiometer is in the feedback loop causing the odd power on phenomenons.
http://www.markhennessy.co.uk/mf_a1/mods.htm
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I haven't replaced any caps.
I didn't measure the volume control circuit but I can check the signal at the input to the power amp to see if it is good there. But I suspect a power amp issue.
I didn't measure the volume control circuit but I can check the signal at the input to the power amp to see if it is good there. But I suspect a power amp issue.
First thing with any A-1 or A1-X is replacing the electrolytic caps. These are almost always gone as it get really hot internally. The input caps 1 µF can all 4 be film caps so 4 caps less that will wear out. The volume control is peculiar and prone to defects/annoyances. Th first A1 without the ventilation slots had the potentiometer going kaputt because of the heat 🙂 Quite dangerous device, it nearly cost me an eye when measuring one and a filter cap was launched.
As usual it is a question of pragmatic working (and I hope it is not the semis). As I see the output transistors have solder lugs so they are probably replaced. Can you see what types they are? I am sure I still have a complete set for repair.
As usual it is a question of pragmatic working (and I hope it is not the semis). As I see the output transistors have solder lugs so they are probably replaced. Can you see what types they are? I am sure I still have a complete set for repair.
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I think I'll go with that too and the 22 to 33pF mod is a common mod.
On the other hand it would be interesting to see if you manage to adjust the bias in the right value in the channel set lower and at the same time, look at the underside of the PCB
On the other hand it would be interesting to see if you manage to adjust the bias in the right value in the channel set lower and at the same time, look at the underside of the PCB
The ouputs are original by the looks of them. I did take them out and regrease them.
I've pulled it apart to change caps and the ESR meter is indeed telling me these are toast.
I've pulled it apart to change caps and the ESR meter is indeed telling me these are toast.
They’re always toast in these toasters. Choosing 105 degrees rated caps is not luxury in A1-X. Panasonic FC will last a long time.
As much as I despise fans in any audio device … this probably is the only one I would recommend installing a small fan in.
As much as I despise fans in any audio device … this probably is the only one I would recommend installing a small fan in.
I changed all the caps except the mains and one channel is good, but the left channel clips the top of the wave around the same power. It is a more normal clipping instead of the waveform on the scope in the first post. This is the channel that has the low bias. I don't see a way to adjust bias.
I can change the mains too but they seem okay with the ESR meter and I don't have replacements on hand.
The client doesn't want to put more into it than he has to unfortunately.
I can change the mains too but they seem okay with the ESR meter and I don't have replacements on hand.
The client doesn't want to put more into it than he has to unfortunately.
If you mean the bulk filter caps, yes you can wait for it. I have had internally shorted ones and defective rectifiers in these. As said even one cooking bulk cap that had liftoff and touched the ceiling. If you have seen a few you replace them all by habit. Otherwise you’ll see it again 😉
The client thinks it is OK!? Well please notice these DC coupled amplifiers have no speaker protection whatsoever. Failure because of unsolved issues always cost a woofer.
Half repair = no repair. The sweet after the sour is however a very nice sounding amplifier. The semis don’t cost an arm and a leg either and it seems one or more are shot.
You did measure the Zener diodes did you!?
The client thinks it is OK!? Well please notice these DC coupled amplifiers have no speaker protection whatsoever. Failure because of unsolved issues always cost a woofer.
Half repair = no repair. The sweet after the sour is however a very nice sounding amplifier. The semis don’t cost an arm and a leg either and it seems one or more are shot.
You did measure the Zener diodes did you!?
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The zeners seem okay, 12V across all of them.
I can do the mains. I quoted enough that I can do what is needed.
I just need to bite the bullet and pull and check the entire channel I guess.
I can do the mains. I quoted enough that I can do what is needed.
I just need to bite the bullet and pull and check the entire channel I guess.
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Mmmm, unfulfilling situation. 12 PNP/NPN transistors and 2 sets of BD's don't cost much. I think I would draw the line at the power transistors. Better have a shot at it than give it to the client like it is now.
As it has been repaired before by a nono that only fiddled in 1 channel (resistors in L and R are clearly different) it may be that the guy misread one of those resistors. You could check all the resistor values. To those reading: always replace all the semis in both channels of old amplifiers to prevent uneven channels. To diminish risk first repair the defective channel and then (after success) the still working channel. It goes against the deep desire for cheap but you won't regret it.
Seen through the eyes of the client the device has been repaired twice and still those darn repair men haven't been able to make it functioning correct again.
As it has been repaired before by a nono that only fiddled in 1 channel (resistors in L and R are clearly different) it may be that the guy misread one of those resistors. You could check all the resistor values. To those reading: always replace all the semis in both channels of old amplifiers to prevent uneven channels. To diminish risk first repair the defective channel and then (after success) the still working channel. It goes against the deep desire for cheap but you won't regret it.
Seen through the eyes of the client the device has been repaired twice and still those darn repair men haven't been able to make it functioning correct again.
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What transistors would make good subs for the To-92?
BC184 and BC214
I have a good selection of the Fairchild transistors. Maybe the KSC1008?
Or maybe BC546/556 if I should stick to BC parts?
BC184 and BC214
I have a good selection of the Fairchild transistors. Maybe the KSC1008?
Or maybe BC546/556 if I should stick to BC parts?
R1 and R2 look like they have seen better days but I'm not quite sure what their value is as the bands are a bit discoloured
I can replace the rectifier diodes as well, what current rating should they be? Looks like 2 of them have been replaced before.
I'd quoted $200 (canadian) for the job. Probably a bit low but decent if it works well after changing the rest of these parts.
I can replace the rectifier diodes as well, what current rating should they be? Looks like 2 of them have been replaced before.
I'd quoted $200 (canadian) for the job. Probably a bit low but decent if it works well after changing the rest of these parts.
0.47 ohm for R1 and 2.
I'll use the BC556/546 if no one suggests anything else, they look like a good fit
I'll use the BC556/546 if no one suggests anything else, they look like a good fit
I got it working. One of the outputs had a missing junction. Not sure how I didn't catch that earlier. I changed the transistors as well. And the outputs, rectifiers, zeners. So it should be reliable.
thanks!
thanks!