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Old 16th September 2013, 08:52 AM   #21
Leeds is offline Leeds  Singapore
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Quote:
Originally Posted by Mooly View Post
The phono preamp is really a separate item. It is a high gain preamp in its own right. A failure of one cap on one channel on that phono board would mean that the other channel still played OK. A problem with the PSU (meaning the PSU local to the phono board, in other words the phono boards supply derived from the main PSU) would affect both channels of just the phono stage but would most likely just cause distortion rather than a change in response.

A starting point might be to measure the voltage across that large 10 volt cap. That looks like the supply (although a 10 volt supply is really low by todays standards). Could it be a shared feedback return cap for both channels for economy ? That's a strange possibility. Measure the voltage as a starting point.

Do you have a scope ????
Do you mean to measure the DC voltage across the 1000 uf capacitor while the amplifier is power on and playing the turntable? How would the reading tell me about the problem?

I only have a analogue multimeter for the job.
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Old 16th September 2013, 09:47 AM   #22
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Originally Posted by Leeds View Post
Do you mean to measure the DC voltage across the 1000 uf capacitor while the amplifier is power on and playing the turntable? How would the reading tell me about the problem?

I only have a analogue multimeter for the job.
Well you have to start somewhere And measuring supplies is the first rule of any faultfinding. Just looking at it and wondering about caps and stuff isn't going to fix it...

You saying The problem occurs gradually over a period of time. implies it starts off OK and then deteriorates. Is that correct ?

If so then a good start would be to compare the supplies on the phono board with it working OK and then after a while when it is showing the problem.

Realistically, you could replace all the electroylitics and tantalums (using modern electros for those) in minutes... but its nice to find and prove a fault if possible. With only a meter, no scope and lack of experience that may not be possible though.
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Old 16th September 2013, 10:40 AM   #23
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Quote:
Originally Posted by Mooly View Post
Well you have to start somewhere And measuring supplies is the first rule of any faultfinding. Just looking at it and wondering about caps and stuff isn't going to fix it...

You saying The problem occurs gradually over a period of time. implies it starts off OK and then deteriorates. Is that correct ?

If so then a good start would be to compare the supplies on the phono board with it working OK and then after a while when it is showing the problem.

Realistically, you could replace all the electroylitics and tantalums (using modern electros for those) in minutes... but its nice to find and prove a fault if possible. With only a meter, no scope and lack of experience that may not be possible though.
Sorry I did not make it clear. What I meant was the phono stage was good until recently. Now, the high is missing, sounded soft and muddy each time I play the turntable and not gradually over an extended period of listening.

There are only 3 electrolytic capacitors in the phono card. That is the reason why I am attempting to just change the 3 capacitors and hopefully the problem solved.

What is tantalums capacitors that you suggest I should also replace? Are you referring to the two red capacitors, or the two smaller brown capacitors with black insulation on their leads? I have no schematic so have difficulties finding the specifications of the capacitors.
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Old 16th September 2013, 11:14 AM   #24
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The tantalums are the four (as you say red and brown) caps. The values will be printed on them along with the voltage. Logically, if both channels exhibit this soft muddy sound then I wouldn't immediately suspect those caps because a failure of one would only affect the channel that particular cap were in. There is nothing lost in replacing though as its literally a few minutes work.

Make sure you make a note of the polarity of the caps before removing them.
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Old 16th September 2013, 11:25 AM   #25
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Quote:
Originally Posted by Mooly View Post
The tantalums are the four (as you say red and brown) caps. The values will be printed on them along with the voltage. Logically, if both channels exhibit this soft muddy sound then I wouldn't immediately suspect those caps because a failure of one would only affect the channel that particular cap were in. There is nothing lost in replacing though as its literally a few minutes work.

Make sure you make a note of the polarity of the caps before removing them.
May I know is the single (the bigger of the 3 caps) electrolytic capacitor (1000uf 10 volts) affects the PSU of the phono stage? If so, this cap should affect both channels if it is the problem. In another word, this cap is more likely the cause of the problem as compared with other caps.
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Old 16th September 2013, 11:34 AM   #26
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The 1000uf is almost certainly just across the voltage supply to the phono board and so is common to both channels.

When you get new caps you can use a higher voltage rating than the originals. For example 16 or 25 volt for the 10 volt cap (as long as they physically fit)
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Old 16th September 2013, 11:39 AM   #27
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Quote:
Originally Posted by Mooly View Post
The 1000uf is almost certainly just across the voltage supply to the phono board and so is common to both channels.

When you get new caps you can use a higher voltage rating than the originals. For example 16 or 25 volt for the 10 volt cap (as long as they physically fit)
Thanks Mooly, you have been wonderful.
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Old 16th September 2013, 03:14 PM   #28
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Quote:
Originally Posted by Mooly View Post
The 1000uf is almost certainly just across the voltage supply to the phono board and so is common to both channels.

When you get new caps you can use a higher voltage rating than the originals. For example 16 or 25 volt for the 10 volt cap (as long as they physically fit)
Hello Mooly,

Could I use a 2200 uf 63v in place of the 1000 uf 10v cap for the PSU phono stage? I happen to have spare for this cap. How will it affects the phono stage?
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Old 16th September 2013, 03:54 PM   #29
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Default HOLD ON A MINUTE!

There is something very wrong here!

Before you go swapping caps willy-nilly, never a good idea, further info is needed.

The photos of that pcb do not show any RIAA shaping network. Apart from the tantalum beads and electros, which are NOT used for FR shaping, there do not seem to be any other caps, apart for a few pF ceramic across the inputs.

As the more experienced of us know, caps on the rail do not have a first order effect on the frequency response so I strongly suggest you do not get diverted swapping these about.

Are you sure this is an RIAA MM pcb? How do you know?
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Old 16th September 2013, 04:32 PM   #30
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More pics of the inside please, That looks like a buffer stage or a gain stage. There will be additional circuitry.
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