Posts: 3
Threads: 1
Joined: Jun 2015
Reputation:
0
Could someone please help me with the proper wiring of the OT, jacks and speaker of a 70's pcb SB-12. The way i got it there are only two wires coming from the main speaker. And the black is ungrounded It's soldered to the est. amp jack's switching lug which has the tip terminal snapped off. this looks fatory but i'm having trouble with oscillation turning the treble control ccw. and lifting the negative feedback doesn't raise the volume it actually lowers and distorts more.
The OT IS the replacement from fliptops.com
Any help from someone who has an original like this one or can help from the schem is much appreciated. i've been stuck on this project for years
thanks
i tried to attach a pic of the wiring as is but file too large. tHis is the 6l6 version btw
Posts: 10
Threads: 2
Joined: Jun 2014
Reputation:
0
I suspect the amp is oscillating. Swap the the brown and blue wires from the OT at Pins 3 on the power tube sockets. Volume drops off alot cuz most of the power is being used to amplify the oscillation. Reattach the negative fb wire.
Posts: 3
Threads: 1
Joined: Jun 2015
Reputation:
0
I"ve tried reversing the plate leads already but nothing on the secondary side. Which Im just wondering if I have right. and with the NFB disconnected even there is a lower volume distortion. Should I troubleshoot this with the NFB disconnected
I've suspected a bad tone module since most of the symptoms coincide with the treble control pot being turned from full CW to CCW.
thanks
Posts: 10
Threads: 2
Joined: Jun 2014
Reputation:
0
As much as we'd all like to help, sussing out amp problems is nearly impossible without having possession of the amp. I seriously recommend bringing the amp to a repair tech. That said, check to make sure C6 in the tone circuit has one end grounded. If it is not referenced to ground in that part of the circuit, it could cause the problem that I assume I understand you described.
Posts: 3
Threads: 1
Joined: Jun 2015
Reputation:
0
unfortunately : ) I am the amp tech in my immediate and pretty distant area. This is one of those I've been stuck on for a long time. Using the scope to chase the oscill/dist., I found something strange that with my probe to pin 6 V2 the amp all but sounded normal. So I took a 390pf cap i had and ran it from pin 6 to ground and there it sits. A band aid yes but now a working amp at least. Just waiting on new chassis/cage bolts to have it together for the first time in quite awhile. thanks for any help still.
