phono stage broken help!

[at68]

wondering if any of you people can help.

My Musical fidelity x-lps v3 phono amp was working fine then suddenly the gain is down. Turning up the volume to full tilt on my amplifier I can get a sound which sounds quite normal, no distortion or other nasties. But of course it is not loud enough for normal listening, and I shouldnt have to turn it up to full volume to start with. Here is what has happened recently:

1. I had one of the caps in the power supply pop two weeks back so I replaced it. i replaced all 8 power supply caps with equivalents from Jaycar (slightly higher rated voltage, but same capacitance). The unit worked fine thereafter.

2. On inspection of the circuit board, no visible signs of damage or capacitor leakage. No blown resistors

3. I tried another interconnect cable and another input on my main integrated amp and the sound is the same. So the cable and amp are not to blame.

4. the mc/mm switch seems to work fine. I am using the mc setting with a 0.4mv cart. The phono amp gain is 58db so it should be enough. I had it running fine just last week with plenty of volume headroom.

5. power light is fine, and power supply box (seperate unit) is supplying the required 12v ac. (checked with multimeter)

6. Checked my amp with another source (cd player) and the unit is working fine with plenty of volume headroom.

I dont have the tools to check much else.

Any ideas what it could be??

cheers Adam

[LogicprObe]

If you could dredge up a circuit diagram..........we can start guessing where to start!

[at68]

Sorry, no luck finding a circuit diagram. Should I replace every capacitor and then the ic's and regulators etc until it works? I suppose I could replace all the components, but I was hoping to hone it down a bit to see if there are any usual suspects.

I find it strange that it still works through both channels, but the gain is well down...

thanks.

[RoHo]

With both channels affected the power supply is where the problem most likely is

[LogicprObe]

With both channels affected the power supply is where the problem most likely is

I agree.

The power supply is the place to start.

[Guest Muon]

I don't like those caps from jaycar, best to get Panasonic ect' from somewhere like RS if replacing any more.

[at68]

Thanks everyone. I might order up some new caps and replace all the power supply ones. Give me a few weeks and I will let you know how it went.

Adam.

[andyr]

I don't like those caps from jaycar, best to get Panasonic ect' from somewhere like RS if replacing any more.

C'mon, DF ... the poor f&*^cker's volume is down. Given Adam changed all 8 PS caps, now changing these to a more esoteric brand is not going to do anything ... except make his volume-lacking MF phono stage sound better! (And BTW, I would recommend BG - if you can still get them (Michael Percy and Parts Connexion still have some) - or Nichicon Gold Tune.)

I agree with RoHo and LogicProbe - there is something wrong with the PS. Adam, you need a multimeter at the very least, so you can check the DC rail voltage ... and other things. If this is not a tool which you have ... you shouldn't be doing mods.

BTW, if you can read the secondary voltage on the power trannie, you can calculate what the DC voltage should be.

Regards,

Andy

Edited September 17, 2012 by andyr

[Guest Muon]

LOL

I'm a little one sided after having jaycar caps fail soon after installing them

I guess I would have been better to suggest checking those caps to make sure they are OK still

[at68]

I did check the power supply which supplies 12v ac and it is working fine. I have not checked the dc voltage in the unit - not sure what voltage it should be given i have no circuit diagram.

There are two ic's that I might also check. Would seem strange though that both channels are working, just not enough gain...

thanks Adam.

[LogicprObe]

LOL

I'm a little one sided after having jaycar caps fail soon after installing them

I guess I would have been better to suggest checking those caps to make sure they are OK still

Putting them in backwards doesn't count!

[skippy124]

Hi Adam,

Looking a some on line photos, it looks like there are two voltage regulators (TO220 package see pic below) that are situated in between the power supply caps. Can you post up what is written on them

Cheers

John

[Guest Muon]

Putting them in backwards doesn't count!

That was once only

And wasn't a jaycar cap

[Vintage_Hi_Fi]

check your soldering, could be a dry joint or broken track - agree with others go with panasonic or nichicon caps for longevity and quality.

[at68]

Hi John

the regulators are marked KA7815 they have a number 350 at the top.

Any further ideas and thanks for your support.

adam

[skippy124]

These are 15V regulators, and are set up to provide +15V and -15V rails. If you have a multi meter, you can measure the regulated voltage output on pin three of each regulator (measure from pin 3 to ground). Pin three is the right hand side pin when looking at the face with the writing on it. Be careful when doing this, it is very easy for the meter probe to slip and short out the regulator. If you measure +15V on one and -15V on the other, then the power supply is working.

Cheers

John