circuit Debug help on Technics SU-7100 amp - diyAudio
Go Back   Home > Forums > Amplifiers > Solid State
Home Forums Rules Articles diyAudio Store Gallery Wiki Blogs Register Donations FAQ Calendar Search Today's Posts Mark Forums Read

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 2nd December 2015, 11:28 PM   #1
diyAudio Member
 
Join Date: Jul 2014
Default circuit Debug help on Technics SU-7100 amp

Have a Technics amp with left channel hum (only) and no audio on right. Audio is good from the preamp on both channels right up to the stablizer input (Test pins 18 & 19) See attached link for a schematic and my readings in red vs expected. Upload files feature isn't working at the moment and providing this google link seems to be the only way I can get a file up here.

Anyway - Getting very strange voltage readings around TR201/202 as well as the main amp IC201/202. TR201/202 are known for trouble based on some googling I've done already, including replacement solutions, but need help confirming, based on the voltage reading I've taken (noted in red), if one of both of them is actually the cause, or if one or both of the IC amps is blown, or something else is going on. Also did a cursory ESR meter check on the caps - all good unless I overlooked one by accident. Nothing else obviously burned etc.

https://drive.google.com/file/d/0B_Y...ew?usp=sharing

Last edited by jimpkarl; 2nd December 2015 at 11:53 PM.
  Reply With Quote
Old 3rd December 2015, 02:27 AM   #2
sregor is offline sregor  United States
diyAudio Member
 
Join Date: Jan 2006
Location: massachusetts
Based on your measured voltages, looks like tr205 is open. The input pair (201 and 202) look like they are performing but probably will need replacement. Good luck.
__________________
Steve
  Reply With Quote
Old 3rd December 2015, 02:54 AM   #3
diyAudio Member
 
Join Date: Jul 2014
Steve - thanks for the reply. I can certainly pull that TR205 and put it on a tester.
As I want to learn the whats/whys - what specifically do you see in those voltages on the B C and/or E that indicates it might be open? What should be the expected values(s)?
It's sibling on the other channel, TR206, looks somewhat equally odd (but different), but you don't see any issues with that one? Especially the 18v base? (note - I may have written it down wrong, perhaps it was -18 - I have to double check)
  Reply With Quote
Old 3rd December 2015, 03:09 AM   #4
diyAudio Member
 
Join Date: Jul 2014
Another 'why' question. You say 201/202 look like they are performing. Why is there 11.5v on b2 of 201 and 8v on b1 of 202 then the schematic shows they should be 0? Or am I reading the schematic incorrectly where it shows zeros by b1/b2 of 201? (they are sideways, which I thought was weird).
  Reply With Quote
Old 3rd December 2015, 03:31 AM   #5
sregor is offline sregor  United States
diyAudio Member
 
Join Date: Jan 2006
Location: massachusetts
Quote:
Originally Posted by jimpkarl View Post
Another 'why' question. You say 201/202 look like they are performing. Why is there 11.5v on b2 of 201 and 8v on b1 of 202 then the schematic shows they should be 0? Or am I reading the schematic incorrectly where it shows zeros by b1/b2 of 201? (they are sideways, which I thought was weird).
Consider the pair as balancing act. When the output goes positive, tr202 base goes positive, until the voltage reaches the breakdown voltage of the B-E junction. the transistor is "shut off", letting all the current supplied to the emitter go through 201, trying to make the output more negative, which doesn't happen because of 205 being open. If you changed 201/202, you would still get similar voltages. (However, They still may be bad or not up to spec. )
__________________
Steve
  Reply With Quote
Old 3rd December 2015, 04:25 AM   #6
diyAudio Member
 
Join Date: Jul 2014
Pulled 205 - tested bad on a transistor tester just as you predicted. I still need to understand the why though - how did you know it was bad/open from those voltages readings? Specifically from the readings at it's B/C/E?

I need to order replacements. Do you recommend a specific site for identifying good matches and perhaps one that allows spec to be compared side by side? (these may be different sites)
  Reply With Quote
Old 3rd December 2015, 06:08 PM   #7
sregor is offline sregor  United States
diyAudio Member
 
Join Date: Jan 2006
Location: massachusetts
KSC2383 is readily available (mouser and other Fairchild distributors) and a pretty close match.
I do most of my substitutions based on a combination of datasheets, looking at how the transistor is used, and other posts as to suitability and availability. There are substitution lists around, and if I can find one, I use that as a starting point.
__________________
Steve
  Reply With Quote
Old 3rd December 2015, 08:45 PM   #8
diyAudio Member
 
Join Date: Jul 2014
Steve, I was doing some research last night as well. 2SC2911 seem to be mentioned as a suitable replacement too. As I understand it, as long as most of the data value are equal to or greater they should work. I compared this to the 1855 and they seem to be a bit closer across the board, including one factor (HFE) which I think in layman's terms is effective gain. The spec hfe on the 1855 is 150, the 2911 is 100 and the HFE on the KSC is far lower at 60. In your opinion, so you think the 2911 would be a bit be better match (albiet in a diiferent package type)?

And maybe even more important - HOW COULD YOU TELL 205 WAS BAD FROM THOSE VOLTAGE READINGS?? I'm in a major learning mode and want to understand that. Please explain it to me. Thanks!
  Reply With Quote
Old 3rd December 2015, 09:29 PM   #9
kct is offline kct  Canada
diyAudio Member
 
kct's Avatar
 
Join Date: Dec 2006
Location: KW, Ontario
Hi Jimpkarl,

a transistor is bad if the voltage differential between Basis and Emitter is greater than about 0.7V. Effectively the PN (NP) junction is destroyed if the reading is greater than that of a normal PN junction like a diode is. Read some book on BJT.

This is a very easy way of telling.
  Reply With Quote
Old 3rd December 2015, 09:45 PM   #10
diyAudio Member
 
Join Date: Jul 2014
OK, given that, why isn't TR206 toast for the same reason?
  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
technics SU-7100 help nikp Solid State 4 29th July 2015 05:27 PM
Technics SU-7100 branet Solid State 11 5th August 2014 06:57 AM
Technics Flat Pre-amp SU-9070 circuit design marc brown Solid State 6 10th January 2014 07:19 PM
Panasonic SU-2700 Repair (Otherwise known as the Technics SU Z1) MChandler Solid State 22 11th April 2013 08:25 AM


New To Site? Need Help?

All times are GMT. The time now is 02:20 PM.


vBulletin Optimisation provided by vB Optimise (Pro) - vBulletin Mods & Addons Copyright © 2016 DragonByte Technologies Ltd.
Copyright 1999-2016 diyAudio

Content Relevant URLs by vBSEO 3.3.2
Wiki