Son of Ampzilla problems - diyAudio
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 1st May 2010, 09:05 PM   #1
diyAudio Member
 
Join Date: Apr 2010
Default Son of Ampzilla problems

Ok, my Son of Ampzilla, which is used to drive two 12" subwoofers, is giving me some weird readings. At idle with the amp hooked up to speakers and to my preamp with no load i'm getting -.6 amps at -2.3V from the right channel and -2.3A at 8.76V from the left channel. The left channel heatsinks are significantly hotter than the right, so much so you cant keep your hand on them for more than a few seconds. I also checked the B+ and B- voltage from the power supply board and both are at +/-43V which is where it should be. Its obvious as well that the left channel sub cone is being greatly pushed out while the right is slightly pulled in.
So it seems there are polarity issues as well as power issues. I was thinking possible the bias IC was busted on the left channel, but am unsure what else may be wrong. Any help or advice would be greatly appreciated, thanks! -Ryan
  Reply With Quote
Old 1st May 2010, 10:36 PM   #2
llwhtt is offline llwhtt  United States
diyAudio Member
 
Join Date: Jun 2008
Location: SoCal
Have you checked and set the bias(idle current)? What was your initial problem? You were asking about capacitors in a previous thread, why? Start at the beginning, the problems and all that you have done.

Craig
  Reply With Quote
Old 1st May 2010, 10:47 PM   #3
diyAudio Member
 
Join Date: Apr 2010
First, i haven't made any modifications, so at the very least i haven't screwed anything up worse than it was when i got it. The reason i was asking about the caps was that i wanted a project to keep me busy and thought replacing the original caps would work. Also, the PDF file of the schematic i have is pretty blurry and i wanted to be clear that i had all the values correct.
My initial problem was that the meters showed two different wattage outputs the left is at 3.2W while the right is .8W. I adjusted the pots on the meter but this was as low as i could get the meter to go. The person i got the amp from also told me he had trouble with one channel when he used it, 10 years ago, but couldn't remember which channel it was.
I know how to set the bias on my Dynaco ST-70's but have had little luck locating the directions on how to set the SOA. If thats my first step i would greatly appreciate any direction you may have. Thank you.-Ryan
  Reply With Quote
Old 1st May 2010, 11:34 PM   #4
llwhtt is offline llwhtt  United States
diyAudio Member
 
Join Date: Jun 2008
Location: SoCal
There are 4 adjustment pots for each channel. The pot on the bottom PCB (where output transistors are) is for bias (idle current). The upper most pot on the top PCB is for DC offset. The other two (one in front and one in back) on the top PCB are for the VI limiters. How were you getting the measurements you posted previously? What test equipment do you have? With the input grounded measure the DC on the output (no speakers or load) and adjust for 0VDC. The bias is set by measuring across one of the 4 emitter resistors (.2/5W cement boats) on the bottom board, .35VDC, I think. Search for other posts for the exact voltage. The VI limiters are set using a 2 Ohm load and is somewhat explained by djk on another post. I have NOT done this adjustment as my Son is still in pieces so I cannot comment on that.

Craig
  Reply With Quote
Old 2nd May 2010, 12:19 AM   #5
diyAudio Member
 
Join Date: Apr 2010
Currently i am using a standard multimeter. If needed though i am lucky enough to have friends in the electronic repair shop at my work, but im hoping i can do it with my multimeter. When adjusting the DC output to 0VDC i am adjusting the bottom PCB pot while measuring across the speaker posts, correct? I did this and saw no change in voltage across the posts or the emitter resistors in the left channel. I measured the right channel and am getting .34VDC at the emitters and the posts so that channel seems fine. The voltage for the left channel, however, was around 8V at both the posts and emitter resistors.
Could it be possible that somewhere on the left channel board a resistor is out of whack or that the Bias IC is broken?
  Reply With Quote
Old 2nd May 2010, 12:32 AM   #6
llwhtt is offline llwhtt  United States
diyAudio Member
 
Join Date: Jun 2008
Location: SoCal
The upper most pot on the top PCB is for DC offset, the pot on the lower PCB is for idle current. Connect your meter to the output binding posts and adjust the top pot for 0VDC. Connect your meter across one of the emitter resistors and adjust the pot on the lower board for .35VDC. Forget about the meters for now, the readings you are seeing are related to the DC offset. If the bias IC was bad you'd see much more serious problems than what you have.

Craig

Last edited by llwhtt; 2nd May 2010 at 12:35 AM. Reason: forgot something
  Reply With Quote
Old 3rd May 2010, 12:34 AM   #7
diyAudio Member
 
BAUHAUSLER's Avatar
 
Join Date: Jul 2007
Location: Detroit, more or less
.35V across a .2 Ohm resistor is 1.75A of idle current. That's wayway too high. With a Son you're looking for more like 0.05A of idle current (0.01v across 0.2 Ohms). There are schems floating around the intertubes that should clarify this.
__________________
Fumbling toward entropy.
  Reply With Quote
Old 3rd May 2010, 12:59 AM   #8
llwhtt is offline llwhtt  United States
diyAudio Member
 
Join Date: Jun 2008
Location: SoCal
Just went back thru some previous posts and 30mv(.03) across the emitter resistors seems to be the number.

Craig
  Reply With Quote
Old 3rd May 2010, 01:27 AM   #9
diyAudio Member
 
BAUHAUSLER's Avatar
 
Join Date: Jul 2007
Location: Detroit, more or less
I'll buy that. I've repaired 3 or 4 SOA amps and I always have to look up the spec. That's my favorite of the GAS amps. I always keep 1 or 2 around. While you're in there, consider replacing electrolytic caps. They're old, it's cheap to do and nothing lasts forever. It's a VERY nice sounding vintage amp when running right.
__________________
Fumbling toward entropy.
  Reply With Quote
Old 3rd May 2010, 01:41 AM   #10
llwhtt is offline llwhtt  United States
diyAudio Member
 
Join Date: Jun 2008
Location: SoCal
From what I gather on the various forums the SOA is the best one of the lot. Picked it up broken along with a Thaedra II for 200 bones. Already replaced all of the electrolytics, just need to finish it. Also have Ampzilla II and Thalia.

Craig
  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
Amp Repair - GAS Son of Ampzilla Horizons Solid State 55 31st August 2010 07:49 PM
My Son of Ampzilla Dappel Solid State 12 18th December 2008 07:21 PM
Son of Ampzilla meters not lit? orbsandoz Solid State 5 3rd November 2008 11:29 PM
GAS son of ampzilla BIAS problem help needid JDRvintage Solid State 22 15th May 2008 12:12 AM
G.A.S. Son of Ampzilla help needed Cambe Solid State 1 25th September 2001 03:38 PM


New To Site? Need Help?

All times are GMT. The time now is 06:57 PM.


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