I would say that your multimeter is in working order - it is kind of difficult to drive current though the plastic housing:
Regarding power mosfets (I have not hendled them before):
a) how do you test their working condition and state (out of circuit)?
b) when buying replacement ones - are there any "grades" that should be considered?
c) is this amplifier safe to test with DBT?
Next thing to do is to unsolder them and check out what is physically undeneath the heatsink,
Also tracks need to be repaired:
That is small. Kind of reminds me of the old Rotel's like the RA820. I almost don't like to ask what that thing is in it... bit of tree, dog chew 😀 😀
According to Martin MS it is a hum reduction device 🙂
Seems like there is some contact problem with the shielding of the source selector switch so the shield needs to be pressed down. Needs to be sorted out.
See if they read short D to S. The high gate impedance means you might be able to get them to turn on and off just by poking a finger on the gate to zap it with a bit of charge. If they are not short chances are good they are fine.
I wouldn't think so.
Absolutely 🙂
The biasing is similar to a vbe multiplier. Turn the preset to give minimum bias on both channels first. Wiper toward R16 to turn Q14 on.
Capacitors in signal path need replacement and MC/MM switch probably needs cleaning.
Since this amp is currently on my desk then if anybody has a surplus original remote control unit then I could consider buying one.
Question:
The "emitter resistors" seem to be present only in the lower leg - upper leg output transistor is directly between voltage rail and speaker output.
Is this common for this type of amplifiers?
The "emitter resistors" seem to be present only in the lower leg - upper leg output transistor is directly between voltage rail and speaker output.
Is this common for this type of amplifiers?
There is nothing common about that design 😀
It is a quasi complementary design (two N channels FET) and will rely on tight thermal feedback of the vgs generator (like a vbe multiplier) to prevent thermal runaway and stable bias current. How well it pulls that off I don't know. You would normally expect source resistors for bias stability but if the thermal management is good enough without then they are not essential.
It is a quasi complementary design (two N channels FET) and will rely on tight thermal feedback of the vgs generator (like a vbe multiplier) to prevent thermal runaway and stable bias current. How well it pulls that off I don't know. You would normally expect source resistors for bias stability but if the thermal management is good enough without then they are not essential.
After adding some nuts, washers and lugs:
it powers up, relay clicks, led is green and bias is adjustable and rather stable:
Next thing to do - measurments.
it powers up, relay clicks, led is green and bias is adjustable and rather stable:
Next thing to do - measurments.
OK, first measurement results (with RMAA) - covers open, output signal from speaker terminals with 8R load in parallel:
Spectrums with Direct ON
and OFF:
Measuring results:
Not too bad as a starting point baseline. The difference in levels seems to com from volume pot - when turned up to ca 13:00 the difference becomes substantially smaller.
One more glitch surfaced - heaphones socket is giving out only left channel.
And one peculiarity - with Direct set to ON the balance control is also bypassed.
Spectrums with Direct ON
and OFF:
Measuring results:
Not too bad as a starting point baseline. The difference in levels seems to com from volume pot - when turned up to ca 13:00 the difference becomes substantially smaller.
One more glitch surfaced - heaphones socket is giving out only left channel.
And one peculiarity - with Direct set to ON the balance control is also bypassed.
That's why the phones output needs to be fixed, I cannot make too much noise in house at free will 🙂
But judging from the first measurement results it has potential to sound pretty good (for my humble taste). After recap (at least signal path) and switch cleaning it should improve.
And should I decide to keep it then definitely I would rewire the balance pot to be effective also in direct mode.
Edit:
There is another peculiarity - mains switch both contact pairs are in parallel and acting only on one line. So it is essential to connect the mains cable to te wall socket in a correct manner. If it were my amp then I would rewire the switch to interrupt both mains lines.
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That is a bit unusual with the switch although it doesn't really matter at all operationally or even safety wise for a user. I wonder if there were concerns over switch failure if L and N were both switched independently and the fact interrupting a large inductive load which can cause a large voltage spike. Maybe concerns over internal tracking and conduction within the switch.
OK, missing headphone channel problem solved - it is bad practice to leave one's gear behind...
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I swear the headphone thing wasn't me ....
Wow folks, I'm learning quite a lot by following this. What an amazing forum.
Wow folks, I'm learning quite a lot by following this. What an amazing forum.
Weekend, time for some hobby stuff...
We have agreed meanwhile with Martin MS thet he will get his amplifier back and he will have all the recapping and modding fun 😉
So the following comments are all in context "what would I d with this piece if I were to keep it for myself".
So starting up - hum reduction device (a wooden one:
A closer look reveals the problem it is fixing - the back part of the selector shield is somewhat bent so the supporting leg is hanging (perhaps even making contact) with one off the traces.
Wooden stick pushes that leg towards the front panel (to its designed location):
Proper solution would be to unsolder the shield and bend its back part to the correct angle.
Quick and dirty solution would be to bend the support leg towards the front panel.
We have agreed meanwhile with Martin MS thet he will get his amplifier back and he will have all the recapping and modding fun 😉
So the following comments are all in context "what would I d with this piece if I were to keep it for myself".
So starting up - hum reduction device (a wooden one:
A closer look reveals the problem it is fixing - the back part of the selector shield is somewhat bent so the supporting leg is hanging (perhaps even making contact) with one off the traces.
Wooden stick pushes that leg towards the front panel (to its designed location):
Proper solution would be to unsolder the shield and bend its back part to the correct angle.
Quick and dirty solution would be to bend the support leg towards the front panel.
👍 that's nice.
Maybe you 'have to be there' to see it for real but from the picture I see an old pair of cutters being used to bend that bit of leg up an dout of the way. 1mm or so would be enough, slip a bit of mica or plastic under it to stop it ever thinking of doing the same again.
🙂