Proton 930 pre-amp stage not working

GarageSale · 23rd July 2010, 07:32 PM

I have a Proton 930 whose amp works (the RCA jacks labeled "MAIN" bypass the preamp and work fine) but whose preamp/control stage (switching between inputs, AM/FM, tone and volume controls etc) won't pass or create any sound but for an occasional buzz or crackle as I fiddle with buttons and rheostats. I can tune and save FM and AM stations, so the preamp stage does at least have power.

Is anyone willing to share the schematics/repair manual with me?
I found the schematics to the Proton D940 in another thread here and will try to use those if I need to.

Also, can anyone familiar with Proton or NAD amps tell me where I might begin in troubleshooting this?

I posted recently this in a two-year-old thread, but I'm creating a new thread as the old one will likely go unseen.

Really looking forward to getting this classic amp working again and pairing it with some nice bookshelf speakers. Also, I'm new to diyAudio and am pretty excited about the community here.

Thanks!
GarageSale

sakis · 24th July 2010, 10:19 AM

obviously your fault lays in the pre .. since i have no sch. i can only guess that fault lays in the secondary psu that works for the aux circuits of your amp

check this and also take a look at this vintage amplifier repair/upgrade manual.... its made for amps ... but procedure is more or less the same for pre amps

regards sakis

GarageSale · 24th July 2010, 03:18 PM

Sakis, I'll start with your tips.
Thank you very much.

Request for schematics/service manual still open for contribution. Thanks!

mjf · 24th July 2010, 04:06 PM

hello.
i would begin to clean all contacts (switches,fuses,potentiometer......),perhaps with a contact cleaner (spray).

dshambala · 25th July 2010, 12:57 AM

Hi,
I believe that I have manual for this. I do not think I can get to this tonight if you still need this send me a pm.

dshambala

GarageSale · 25th August 2010, 07:48 PM

I disassembled the amp and saw that the regulators are fine, and used a scope to trace the audio signal all the way through the end of the pre-amp stage and past the FM muting circuit. I was stumped! Are both of the traces on the board that take the pre-amp signal into the amp severed? Oh, there ARE no such traces on the board! An older friend of mine told me that I need to use external jumpers to connect the pre-amp out and main in. Hah!! BLAH!!!

At least now that I've got it apart I can replace filter capacitors in its tuner that I assume are responsible for a pretty bad hum.

Thanks for the tips guys!

24th July 2010, 10:19 AM	#2
sakis diyAudio Member Join Date: Jan 2006 Location: nea makri athens greece	obviously your fault lays in the pre .. since i have no sch. i can only guess that fault lays in the secondary psu that works for the aux circuits of your amp check this and also take a look at this vintage amplifier repair/upgrade manual.... its made for amps ... but procedure is more or less the same for pre amps regards sakis __________________ SERVICE ΙΑΠΩΝΙΚΩΝ ΜΗΧΑΝΗΜΑΤΩΝ ΗΧΟΥ www.eastelectronics.gr

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24th July 2010, 03:18 PM	#3
GarageSale diyAudio Member Join Date: Jul 2010	Sakis, I'll start with your tips. Thank you very much. Request for schematics/service manual still open for contribution. Thanks!

24th July 2010, 04:06 PM	#4
mjf diyAudio Member Join Date: Jan 2009 Location: upper austria/near linz	hello. i would begin to clean all contacts (switches,fuses,potentiometer......),perhaps with a contact cleaner (spray).

25th July 2010, 12:57 AM	#5
dshambala diyAudio Member Join Date: Mar 2008	Hi, I believe that I have manual for this. I do not think I can get to this tonight if you still need this send me a pm. dshambala

25th August 2010, 07:48 PM	#6
GarageSale diyAudio Member Join Date: Jul 2010	I disassembled the amp and saw that the regulators are fine, and used a scope to trace the audio signal all the way through the end of the pre-amp stage and past the FM muting circuit. I was stumped! Are both of the traces on the board that take the pre-amp signal into the amp severed? Oh, there ARE no such traces on the board! An older friend of mine told me that I need to use external jumpers to connect the pre-amp out and main in. Hah!! BLAH!!! At least now that I've got it apart I can replace filter capacitors in its tuner that I assume are responsible for a pretty bad hum. Thanks for the tips guys!

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