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Old 23rd January 2012, 11:54 AM   #211
mkusan is offline mkusan  Croatia
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Quote:
Originally Posted by AndrewT View Post
Do you have a highish value plastic cap you can use instead of the electrolytic 4u7F?
Clean the PCB top and bottom around the cap pins. Old crud may be creating a high resistance circuit. Toothbrush and dishwashing liquid and warm water. Dry thoroughly before powering up.

I correct myself, I just found this at work place. If this doesn't block DC, nothing will.
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Old 23rd January 2012, 11:59 AM   #212
AndrewT is offline AndrewT  Scotland
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1uF plastic anything would do. It may reduce bass response slightly, but prove where your problem is and how to solve it.

Any plastic anything will perform better than any electrolytic, except size.
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Old 23rd January 2012, 12:13 PM   #213
Mooly is offline Mooly  United Kingdom
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Quote:
Originally Posted by mkusan View Post
Will swap it then, I have some no name (KSC-KINGSUN ELECTRONIC) low-esr caps in 4.7uF value.
They sound like "flying bomb" batteries

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Originally Posted by mkusan View Post
Or if I may suggest to perhaps bypass it with film or ceramic cap, that way we can be sure that no DC passes?
Don't bypass it, it won't achieve anything. If a cap is leaky it will still be leaky with another across it.

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Originally Posted by mkusan View Post
I did not stopped thinking as promised, I just can not , so I if you can look at the picture below, with green are marked the points I can normally touch with the red probe of DMM set to DC with ground probe to chassis, but if I touch the red point amp goes into the protection with a sound from the speaker very similar to what I experienced yesterday, while I repaired that soldering blob I completely unsoldered that side of R522 and checked on ohmmeter, it was good at 14.98K, that leaves C512 30pF ceramic cap. I know ceramic caps are very reliable, but what if this one has gone wrong, it is certainly in the right place as I see it?
I'll have to study what you mean with the picture Touching any points in a high gain amp will cause all sorts of noises/oscillation etc so the protection relay clicking out is normal.

Ceramic caps in reality are very very unreliable (relatively speaking), particularly in older equipment. As a service tech many many faults were caused by the small "compressed disc" type ceramics.

That big cap looks like a "motor start" type, don't use that.
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Old 23rd January 2012, 12:18 PM   #214
Mooly is offline Mooly  United Kingdom
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You can always take a cap from an unimportant part of the circuit to try.

C628,629,630 etc in the VU meter amp all look to be 10uf 25 volt caps. Take one of those out to use if in doubt.
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Old 23rd January 2012, 12:34 PM   #215
mkusan is offline mkusan  Croatia
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Originally Posted by Mooly View Post
They sound like "flying bomb" batteries



Don't bypass it, it won't achieve anything. If a cap is leaky it will still be leaky with another across it.



I'll have to study what you mean with the picture Touching any points in a high gain amp will cause all sorts of noises/oscillation etc so the protection relay clicking out is normal.

Ceramic caps in reality are very very unreliable (relatively speaking), particularly in older equipment. As a service tech many many faults were caused by the small "compressed disc" type ceramics.

That big cap looks like a "motor start" type, don't use that.
Hi Mooly,
you really do know everything, that is indeed a motor start cap. It was meant as a joke, after that "incident" with collector to the ground I am very careful with what I put where.

I will switch positions of C517 and C518, if the same channel stays noisy then we know it is not the cap causing it, if it moves to the other channel then we know the cap was leaky. Is that OK, good thinking?

If noise stays in the same channel I can put 1uF film cap in place of C518 for testing purposes, as AndrewT kindly suggested.

That point was the only point that ever made my amplifier go into protection while checking voltages, even its LEFT counterpart does not produce same behavior.
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Old 23rd January 2012, 05:24 PM   #216
Mooly is offline Mooly  United Kingdom
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If you have a 1uf film handy then you can just fit that, otherwise swap the caps over.

It does sound a little strange that the amp goes unstable touching the point you show, and you say the other channel does not show this instability.

It's important that C516 is isolated after isolating the transistors following the order in post #204. C516 is part of the feedback network and it might object if the transistors were still in place.
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Old 24th January 2012, 07:28 AM   #217
mkusan is offline mkusan  Croatia
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If you have a 1uf film handy then you can just fit that, otherwise swap the caps over.

It does sound a little strange that the amp goes unstable touching the point you show, and you say the other channel does not show this instability.

It's important that C516 is isolated after isolating the transistors following the order in post #204. C516 is part of the feedback network and it might object if the transistors were still in place.
It turned out the highest value I have in film is 0.22uF, so I swapped the caps over. Unfortunately the noise remained in the same channel.

I left the transistors isolated when testing the day before so I removed C516, no change.

Then I removed C510, no change, right channel still noisy.

I just want to check something, in current setup, is it normal to measure 25V of DC across C518 (Q502- emitter and base desoldered, Q504-emitter and collector desoldered). And also the voltage on collector of Q503 risen to 14.6V.
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Old 24th January 2012, 07:51 AM   #218
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Originally Posted by mkusan View Post
I just want to check something, in current setup, is it normal to measure 25V of DC across C518 (Q502- emitter and base desoldered, Q504-emitter and collector desoldered). And also the voltage on collector of Q503 risen to 14.6V.
That sounds about right. The voltage across C518 (with transistors disconnected) should be the same as the supply to the preamp which is the voltage across C547.

So we still haven't cracked this. I'll have a think
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Old 24th January 2012, 08:24 AM   #219
mkusan is offline mkusan  Croatia
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That sounds about right. The voltage across C518 (with transistors disconnected) should be the same as the supply to the preamp which is the voltage across C547.

So we still haven't cracked this. I'll have a think
Can I ask you to think about this as well?

Sorry if my questions appear stupid, if I understand correctly increased noise in the current setup is in direct correlation with increased voltage across C518 as a result of removal of Q504 and Q502?

Can it be then that transistor change stopped popping in the right channel because new transistors are of lower gain and thus of lower voltage?

I have a feeling that if we replace C518 with 4.7uF film cap, or other non leaking type of cap that the popping would stop, but is that solving the root of the problem or just its side-effect? What is the negative side of keeping a cap across R530, which we proved is a method of bringing silence?

Thanks
Marko
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Old 24th January 2012, 08:39 AM   #220
Mooly is offline Mooly  United Kingdom
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Originally Posted by mkusan View Post
Can I ask you to think about this as well?

Sorry if my questions appear stupid, if I understand correctly increased noise in the current setup is in direct correlation with increased voltage across C518 as a result of removal of Q504 and Q502?
Questions are no problem...

The increased voltage across C518 and increased noise appears to be the case and in a way doesn't make sense.

I'll try and explain it better a little later

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Can it be then that transistor change stopped popping in the right channel because new transistors are of lower gain and thus of lower voltage?
The gain of the transistors shouldn't have any effect. That is because the DC conditions and the AC gain is set entirely by the passive components. And with the transistors isolated I would say it really should be silent... and that it isn't is a real puzzle. This is where rigging a pot up to bias C518 comes in. We have to do that to see just what the effect is.

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Originally Posted by mkusan View Post
I have a feeling that if we replace C518 with 4.7uF film cap, or other non leaking type of cap that the popping would stop, but is that solving the root of the problem or just its side-effect? What is the negative side of keeping a cap across R530, which we proved is a method of bringing silence?
The actual value of C518 isn't very critical. It does two things,

1. It prevents and DC from passing from Q504 to the next stage. The value of the cap has no effect on that property apart from how long it takes to initially charge. Big cap = long time

2. The value of the capacitor determines how well it passes low frequencies in the audio. If it's not big enough then the bass will be cut off.

If we just placed a cap across R530 then the amp would be very quiet... and all the time. It would just "short out" all the audio signal to ground.
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