Go Back   Home > Forums > Amplifiers > Solid State

Solid State Talk all about solid state amplification.

Please consider donating to help us continue to serve you.

Ads on/off / Custom Title / More PMs / More album space / Advanced printing & mass image saving
Reply
 
Thread Tools Search this Thread
Old 21st July 2007, 05:26 AM   #1
timbert is offline timbert  United States
diyAudio Member
 
Join Date: Jul 2007
Default Newbie's NAD 701 receiver hums, clicks, shuts down

You will all laugh at my ignorance but here goes. I did STF. When the unit powers up, it puts out a major hum through both the headphone jack and/or the speakers, then clicks into some kind of protection mode. I heard replacing the big things that look like C batteries could fix it, so I matched the numbers on the two capacitors to $4 parts on Digikey and soldered in these replacements, but with the same result. I read about shorting the cap before working on it and that was pretty neat. What do I do next? My options are to actually get an EE degree, mooch knowledge off a distant friend who builds spy satellites, or just chuck this thing I bought for $40 knowing it was broken and that I was stupid to take on an insurmountable challenge and I should just get back to my real job. Or, I could embarass myself in front of you all.

I am a rock-ribbed DIYer. I fabbed and installed my own Corian countertops, I change my own oil, I make my own CAT5 and AV interconnects. I once salvaged important hard drive data by buying the same model drive and swapping the controller boards. Am I wrong to attempt this new hobby of audio equipment repair?
  Reply With Quote
Old 21st July 2007, 07:52 AM   #2
timbert is offline timbert  United States
diyAudio Member
 
Join Date: Jul 2007
I have also reheated a bunch of the solder joints starting at the power supply, but no difference. There are two similar transistors for what I think are the two channels, based on the layout and they are connected to a huge heatsink, and when I put the meter on each of three legs of the first transistor and power up, I get current on all three. On the other transistor, I power up and get nothing. Mind you, the current only lasts a split second before the protection kicks in, but it's enough to show the difference between the two channels. With the current off, I get 2 different resistance values between the three legs, and both transistors measure the same. I did all of this with the pre-power jumpers out, but it made no difference if I put them in.

I have checked the speaker terminals for loose wire threads, and put an RCA in each input to clear any gunk. I still get the loud hum at poweron and then it kicks into protection. After ten seconds it goes into standby and if I hit the power switch it does the same thing again. I am using some cheap computer speakers connected to the headphone jack to prevent harm to real speakers, but it makes no difference to the symptoms.

Thanks in advance.
  Reply With Quote
Old 21st July 2007, 08:19 AM   #3
diyAudio Member
 
Craig405's Avatar
 
Join Date: Jul 2005
Location: Sussex
HI,

Usually among the first things you try with this kind of problem is to disconnect the power supply rails from the power amplifier section and leave the pre-amp section connected.

If the unit is fine without power section connected the chances are it has blown its output devices.
Get the data sheet for the transistors mounted on the big heatsink and check they arent short circuit between;

C-E/E-C/E-B/C-B

(E=emitter, B=base,C=collector)

I.E. the only significant conduction should be from base to emitter (B-E).

If they are short circuit then they are probably dead.
See what you get and post back, good luck!
  Reply With Quote
Old 21st July 2007, 08:20 AM   #4
Netlist is offline Netlist  Belgium
diyAudio Moderator Emeritus
 
Netlist's Avatar
 
Join Date: Jan 2003
Most likely, one or more power transistors are broken.
Can you take a few clear pictures? I presume you have no schematic.

/Hugo
  Reply With Quote
Old 21st July 2007, 07:48 PM   #5
timbert is offline timbert  United States
diyAudio Member
 
Join Date: Jul 2007
Many thanks for the help. Disregard my previous transistor test. I downloaded the datasheet.

On one pair there's resistance all around, but for the other pair, one is shorted completely (zero resistance, base to emitter) and in that pair the other transistor has resistance that changes as I hold the probes on (increases).

I have no schematic, and probably couldn't read one anyway. So I removed the four 4A fuses and left the small pair of 1A fuses and while the unit powers on and does not move into protection mode, the tuner's station search light comes on although there's no display.

Sooooo, if I order a new set of all 4 transistors and use new thermal grease and solder them in, what are my chances of success? Digikey has the 2N3055, but not the MJ2955 in stock. Heck even my local radio shack has the 2N3055. Is there a substitute or upgrade I should use? Should I avoid buying a set of five of both on eBay from a guy in Hong Kong for $20?
Attached Images
File Type: jpg nad701rcvrpowertransistors.jpg (94.1 KB, 247 views)
  Reply With Quote
Old 21st July 2007, 07:48 PM   #6
timbert is offline timbert  United States
diyAudio Member
 
Join Date: Jul 2007
Top view
Attached Images
File Type: jpg nad701rcvrtopview.jpg (98.4 KB, 265 views)
  Reply With Quote
Old 21st July 2007, 08:01 PM   #7
Netlist is offline Netlist  Belgium
diyAudio Moderator Emeritus
 
Netlist's Avatar
 
Join Date: Jan 2003
I would recommend MJ21193 or MJ21195. The 2N3055 could be replaced by MJ21194 or MJ21196. Buy from a good source.
You will also want to check the smaller transistors between the big TO3 cans.

/Hugo
  Reply With Quote
Old 21st July 2007, 08:52 PM   #8
Netlist is offline Netlist  Belgium
diyAudio Moderator Emeritus
 
Netlist's Avatar
 
Join Date: Jan 2003
I should add that measurements are best done with devices out of circuit.

/Hugo
  Reply With Quote
Old 22nd July 2007, 04:36 AM   #9
Bigred is offline Bigred  Canada
diyAudio Member
 
Join Date: Jul 2007
I would just like to chime in about those big things that look like C batteries as you called them (actually look REALLY small to me ). I'll refer to them as the filter caps and I would advise not to short them in the future. I would recommend a bleeder resistor instead.
  Reply With Quote
Old 26th July 2007, 07:03 PM   #10
timbert is offline timbert  United States
diyAudio Member
 
Join Date: Jul 2007
Failure. As suggested I replaced the four T03's with the MJ21195 and MJ21196, as well as the two smaller transistors between them. I measured the old T03's, now out of circuit, and got shorts between the pins on at least one of them. Now instead of hum and then protection mode, it will blow two of the four 4A fuses immediately upon plugging it in. I guess I will go back and first check my solder work, and then try to take that whole channel offline. I have not been able to find the schematic for this model. But I did get one for my NAD 2200 so that's nice to have. Is there a likelihood that the power supply is causing this? The power switch? The startup relay? I'll measure the voltage coming off the transformer at the point it connects to each channel and see if there's a difference. Thanks again for any and all help.

Tim
  Reply With Quote

Reply


Hide this!Advertise here!
Thread Tools Search this Thread
Search this Thread:

Advanced Search

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are Off
Pingbacks are Off
Refbacks are Off


Similar Threads
Thread Thread Starter Forum Replies Last Post
Help a newbie out with a busted up receiver - HK AVR 335 utjay2008 Solid State 6 15th December 2007 03:17 AM
ML #23 shuts itself off after 5 seconds. Howard Lan Power Supplies 5 8th December 2007 07:17 AM
Amp Shuts off or blows fuses Oddy88 Car Audio 6 2nd August 2007 08:33 PM


New To Site? Need Help?

All times are GMT. The time now is 10:07 AM.


vBulletin Optimisation provided by vB Optimise (Pro) - vBulletin Mods & Addons Copyright © 2014 DragonByte Technologies Ltd.
Copyright 1999-2014 diyAudio

Content Relevant URLs by vBSEO 3.3.2