GAS Thaedra - Page 3 - 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 12th December 2007, 10:49 AM   #21
d3imlay is offline d3imlay  United States
diyAudio Member
 
Join Date: Sep 2004
Location: Ohio
OK, operator error on my part. Relay doesn't pick up if line voltage under 110 vac. Found an open(!) pass transistor in a power supply. Was stamped GAS117. From What I could figure out it was a TIP117. Replaced it with a TIP126 (I think) and PS is good.

New problem is on the tone plugin board. Input OK, ouput distorted on one channel. Tried replacing the opamp with a 4558 since it appeared to be a dual. No luck. Two of the fets on the channel have been previously replaced by someone else. Don't know what they are as they're privately labeled.

Any ideas? Could sure use a schematic.
  Reply With Quote
Old 12th December 2007, 11:08 AM   #22
diyAudio Member
 
Join Date: Nov 2002
Quote:
. Was stamped GAS117. From What I could figure out it was a TIP117. Replaced it with a TIP126 (I think) and PS is good.
Its not a Tip117
  Reply With Quote
Old 12th December 2007, 04:47 PM   #23
diyAudio Member
 
Join Date: Jan 2006
Location: Virginia
Quote:
Originally posted by d3imlay
OK, operator error on my part. Relay doesn't pick up if line voltage under 110 vac. Found an open(!) pass transistor in a power supply. Was stamped GAS117. From What I could figure out it was a TIP117. Replaced it with a TIP126 (I think) and PS is good.

New problem is on the tone plugin board. Input OK, ouput distorted on one channel. Tried replacing the opamp with a 4558 since it appeared to be a dual. No luck. Two of the fets on the channel have been previously replaced by someone else. Don't know what they are as they're privately labeled.

Any ideas? Could sure use a schematic.
You probably want to put something closer to the original transistor in the supply. It's dropping approximately 20 volts at a hefty (over 200ma) current. I'll post the generic numbers for the original transistors when I get home.

As for the line amp, any board id numbers? What do the FETs look like? Metal cans, small plastic bead case, T0-92 molded plastic? I can supply you with a set if needed and, more importantly, the pinouts.

As for the problem, check the idle current (heh, heh) I believe it's a 100ma = to 3 volts acrossed each of the 30 ohm power resistors on the board. The offset on the output (which is taken from the collectors of the output transistors, both collectors tied together) should be below 25mv. The dual op-amp (MC1458) is for the offset servos for both channels.

My guess is, since you did not say you got a nasty pop when the relay closed and the fact it is making noise (although distorted), most likely an open output transistor, which the current and offset checks should point to. If the power transistors have an F printed on them I'd start betting money... For a while they used Fairchild TO220 devices that were notorious for opening up and doing exactly what you are seeing. This is most likely the problem with the supply transistor as well.

And then I could be wrong.

I can post a schematic of the line amp but I thought there was one out on the SAE_Talk site already. It's actually a fairly simple circuit. Let me know and I'll fight with my scanner and the file size limitations if there isn't one elsewhere.

Have fun, Mike.
__________________
"We are all agreed that your theory is crazy. The question which divides us is whether it is crazy enough to have a chance of being correct. " Niels Bohr
  Reply With Quote
Old 12th December 2007, 11:32 PM   #24
d3imlay is offline d3imlay  United States
diyAudio Member
 
Join Date: Sep 2004
Location: Ohio
The schematic for the preamp online is just a signal flowchart. It treats the boards as a box.

Would really appreciate your effort scanning the schematic. The output of the tone board is clipped on one half . I'm thinking the servo is working as the output shows some DC offset when driven further into clipping but then returns to near zero.
  Reply With Quote
Old 13th December 2007, 03:22 AM   #25
diyAudio Member
 
Join Date: Jan 2006
Location: Virginia
Quote:
Originally posted by d3imlay
The schematic for the preamp online is just a signal flowchart. It treats the boards as a box.

Would really appreciate your effort scanning the schematic. The output of the tone board is clipped on one half . I'm thinking the servo is working as the output shows some DC offset when driven further into clipping but then returns to near zero.
I'll give it a try in the morning. Think of the the tone controls as part of the feedback loop; From the line stage output- through the control network- back into the negative side of the differentials. The whole network can be bypassed with a 15K/1K to ground feedback network (bypassing the controls was popular back in the day ).

Mike.
__________________
"We are all agreed that your theory is crazy. The question which divides us is whether it is crazy enough to have a chance of being correct. " Niels Bohr
  Reply With Quote
Old 13th December 2007, 11:16 AM   #26
diyAudio Member
 
Join Date: Jan 2006
Location: Virginia
I've tried to attach it and even though it's a JPEG and only 90K it says it's to large. Nothing I can do in the simple editer I have seems to allow me to post it. I can e-mail it and the setup instructions.

Mike.
__________________
"We are all agreed that your theory is crazy. The question which divides us is whether it is crazy enough to have a chance of being correct. " Niels Bohr
  Reply With Quote
Old 13th December 2007, 11:47 AM   #27
diyAudio Member
 
Join Date: Nov 2002
Quote:
New problem is on the tone plugin board. Input OK, ouput distorted on one channel. Tried replacing the opamp with a 4558 since it appeared to be a dual.
The 4558 is overkill and you are probably much better using the 1458 in the servo.

To be honest ...if you have a scope your problem is at best a 15 minute repair.

Given the age of the piece and its history the smart thing to do would be to replace all semi's and caps on the board. Use good caps and bypass them and you will be surprised at the performance.
  Reply With Quote
Old 13th December 2007, 01:32 PM   #28
d3imlay is offline d3imlay  United States
diyAudio Member
 
Join Date: Sep 2004
Location: Ohio
Sure, I've got a scope. I pulled the board and used a power supply for the V+/-. I ran a signal into each channel one was good the other clipped. Respectfully, I'm reluctant to do wholesale transistor changes. I'd prefer to positivly ID failed components. I probably will do the cap replacement. The board has been damaged by previouse replacement of two of the fets so I'd rather not pull those two if I don't have to.
  Reply With Quote
Old 13th December 2007, 09:42 PM   #29
diyAudio Member
 
Join Date: Nov 2002
Quote:
Respectfully, I'm reluctant to do wholesale transistor changes.
I can see where you are coming from. It took a long time for me to get into the habbit of changing transistors in the older equipment. When one knows what to use it is very easy and certainly gets rid of the down time associated with changing one transistor at a time when failure occurs and they do fail when they get this old.

Quote:
Sure, I've got a scope. I pulled the board and used a power supply for the V+/-. I ran a signal into each channel one was good the other clipped.
And at what point in the circuit did it clip?
  Reply With Quote
Old 13th December 2007, 09:57 PM   #30
diyAudio Member
 
Join Date: Jan 2006
Location: Virginia
Try this attachment.

One thing. If the board was fired up on the bench it had no feedback; it would be clipped on the good channel; the background behind my comments on the tone controls.
Attached Files
File Type: zip theadra schematic.zip (60.6 KB, 245 views)
__________________
"We are all agreed that your theory is crazy. The question which divides us is whether it is crazy enough to have a chance of being correct. " Niels Bohr
  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
Thaedra I GAS Pre Amp robb christian Vendor's Bazaar 2 23rd February 2013 04:57 PM


New To Site? Need Help?

All times are GMT. The time now is 07:01 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