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tenderland 21st June 2006 03:59 AM

Magic Capacitor

I am repairing an old Marantz reciever. I have no gain coming out of the rt ch
1 st preamp xistor. I have determined that when I swap the cap from the other chanel (coming off the emiter47uf@50 v) I have gain. Here is the voodo. I replaced the cap and viola no gain. I replaced the same cap in the other ch with a new one -no gain. I only get gain when I use the original old raty cap. Any thoughts. I made certain the polarity is corect


bikehorn 21st June 2006 03:21 PM

what model is the receiver?

anatech 21st June 2006 03:39 PM

Hi tenderland,
Is the transistor a 2SC458? If so, replace with a 2SC1815. Those early 2SC458's are known to go intermittent.


tenderland 21st June 2006 08:29 PM

The model is 2230 the cap in question is c 754. The circuit will lose gain in either ch unless the original cap is in circuit. I replaced the xistor in one ch but no difference, same same. I checked the print and it definately calls for a 47uf@50. I checked the polarity and it is as in the print.


Netlist 21st June 2006 08:47 PM

Here's the schematic.
At least you can measure some DC values to see which are incorrect.

/Hugo :)

teemuk 21st June 2006 10:43 PM

I have a theory: Are the original capacitors in open circuit? As far as I see it the sole purpose of those caps is to lower the gain and distortion (I guess). AC bypass to same phase signal instead to the ground attenuates, right? I might be totally wrong.

anatech 21st June 2006 10:53 PM

Hi tenderland,
That cap is in a low impedance position. It would have to be awfully leaky to cause a problem, it is in your feedback loop as a coupling cap. If it were to open, your gain should go up.

Look at R754 and the foil around that area. I would install new caps in both positions if you don't have a cap checker and troubleshoot from there. I'm wondering if the high gain is a fault condition.

C753 and it's mate on the other channel should be checked. I have more trouble with the small 1 uF types. You may be further ahead to just simply replace all the electrolytic caps in the amplifier boards to be honest with you.


excetara2 22nd June 2006 12:13 AM

An update where you at in repairing the marantz. I have a similar Marantz and thought about reparing it myself. Let me know. Thanks.

anatech 22nd June 2006 12:24 AM

Hi excetara2,
It's a nice little set. Should last years after you are done.

What's wrong with yours?


tenderland 22nd June 2006 02:32 AM

Thanks all who contributed to this thread.
Thanks Anatech and teemuk whos resonse put me on the corect path. Anatech , right on .The old cap created a fault condition. With all new caps gain ON THE OUTPUT is equal. I am a bit embarased I didnt figure it out.

Thanks again

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