Magic Capacitor

tenderland · 21st June 2006, 03:59 AM

Hello

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

conforzaled

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.

-Chris

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.

Thanks
clueless

Netlist · 21st June 2006, 08:47 PM

Here's the schematic.
Homepage
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.

-Chris

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?

-Chris

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
Jeff

21st June 2006, 03:59 AM	#1
tenderland diyAudio Member Join Date: Jul 2004 Location: us	Magic Capacitor Hello 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 conforzaled __________________ "If every one is making wheels , why make wheels ? " Steve Jobs

21st June 2006, 08:29 PM	#4
tenderland diyAudio Member Join Date: Jul 2004 Location: us	? 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. Thanks clueless __________________ "If every one is making wheels , why make wheels ? " Steve Jobs

22nd June 2006, 02:32 AM	#10
tenderland diyAudio Member Join Date: Jul 2004 Location: us	victory 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 Jeff __________________ "If every one is making wheels , why make wheels ? " Steve Jobs

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21st June 2006, 03:21 PM	#2
bikehorn diyAudio Member Join Date: Aug 2004 Location: toronto	what model is the receiver?

21st June 2006, 03:39 PM	#3
anatech diyAudio Moderator Join Date: Jun 2004 Location: Georgetown, On	Hi tenderland, Is the transistor a 2SC458? If so, replace with a 2SC1815. Those early 2SC458's are known to go intermittent. -Chris

21st June 2006, 08:47 PM	#5
Netlist diyAudio Moderator Join Date: Jan 2003	Here's the schematic. Homepage At least you can measure some DC values to see which are incorrect. /Hugo

21st June 2006, 10:43 PM	#6
teemuk diyAudio Member Join Date: Nov 2004 Location: Suomi, Finland	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.

21st June 2006, 10:53 PM	#7
anatech diyAudio Moderator Join Date: Jun 2004 Location: Georgetown, On	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. -Chris

22nd June 2006, 12:13 AM	#8
excetara2 diyAudio Member Join Date: Jun 2006	An update where you at in repairing the marantz. I have a similar Marantz and thought about reparing it myself. Let me know. Thanks.

22nd June 2006, 12:24 AM	#9
anatech diyAudio Moderator Join Date: Jun 2004 Location: Georgetown, On	Hi excetara2, It's a nice little set. Should last years after you are done. What's wrong with yours? -Chris

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