Update....

Finally back on this amp - I tried a simple experiment where I connected channel A input (lifted R132 and 134) into B and vice versa. Now the A channel remains on for 15-20 sec at turn on and when ever there is an input connector removed from the input jack. This seems to point to an issue with the -15v which supplies R111/R134. The emitter of Q102 and pin 8 of IC106 are the -15v node. I can't find any issue with Q102 so I'm baffled. What's causing the voltage a R111 to slowly rise to a negative voltage (-7) after 15-20 sec?

Here's the issue - when you turn the amp on there is -13V at channel B stereo jack terminal - over a time constant of about 20 sec the voltage decreases to about -8 V and then the channel is off. The -15V line to the preamp board is stable. There isn't much between the input of IC106 and the channel B jack. Bad resistor?

I did put in a socket for IC103 and tried several different versions of the 74C04 - problem doesn't track replacing different inverters. Always use a properly grounded wrist strap when working around these older CMOS parts.

The board I have doesn't have C164/5 across pins 1/2 and 5/6 on IC103 so I'll try adding these.

I like your "replacement" for IC106.

TDA2007A discontinued July 29, 2009. Hope that isn't the issue.

I have a Sunn Beta Lead that originally had an issue where the "B" channel would stop working (pulsing on and off) when the footswitch was connected - worked fine without FS.

I replaced IC103 (74C04 HEX Inverters) and now the amp powers up with "B" channel LED on (NO INPUT). The amp stays in this state for 20 sec and then turns the green LED (B channel) off. If you plug a jack into the B input the amp works BUT when unplugging from the B input the LED stays on for 20 sec. The "A" channel and "both" work as they should (LEDS go of immediately after removing input). Now when you hook up the footswitch to the amp,all inputs behave as normal (i.e. fixed original issue!!). This is a digital switch/control issue only since the amp passes an input (sig gen, guitar, etc) without issue. All voltage supplies look normal.

I'm assuming that IC106 (MC14506 expandable AND-OR-INVERT gate) is the issue: B channel LED is on without input because Q102 is getting turned on by PIN7 of IC106. Any other ideas?

Anyone have a copy?

Found this too looks like it has some shielding between the pri/sec - not sure what the purple wire is on the primary - shield connection?

http://www.antekinc.com/content/AS-1220.pdf

Laney HC50 that had a history of cutting out. The issue turned out to be the PT. With the PT on the bench (secondary disconnected from PCB) and the primary connected to a VARIAC there is no secondary voltage present at any primary voltage applied. Primary windings also measure open with DVM.

The transformer in the amp has markings that identify the secondary as 2 x 21VAC (120VAC primary) - the schematic for the amp is attached for reference.

Looking for a low cost PT for this repair - found this http://www.antekinc.com/content/AN-1222.pdf but I'm wondering if it is too low of a VA rating?

Soooo embarrassing 

There are 4 screws that hold the preamp board to the chassis -  2/4 the screws I did use had a clad pad that made contact, via the screw head, between the top of the board to  chassis connection so I thought I had a chassis ground. What I didn't know was the 2 unused screws and their clad pads were the screws that supply the connection between the center taps, PA ground, etc. etc.   Live and learn.

Yes I will check this out when I get back to this amp - I'm assuming that the center taps of B+-, grounds on connectors  CN501A/B, AND grounds on connectors AN101/101-1 should all be connected to chassis? BTW this rev of the PA PCBs do not have a connection (ground bus) between AN101 and CN501 and ref designator R351 is loaded with a jumper and connects R321 to the emitter of Q310.

Yup there is no continuity between ground on AN101 and CON501. I traced out the circuit and it doesn't match the schematic even though the board I have is 4005197001. Nothing has been changed on this board (actually both PA boards). Finding the correct schematic for this amp has been a real PINTA.

But how did this amp ever work?

Starting at the +-15 supply it looks like the center tap is ground connection which is carried to both amps via connectors AN101/101-1. I'll check these out. My PA boards don't have resistor 351 just a jumper and I will see where ground on AN101 travels on the board. I don't have the amps in front of me right now but it seems like the issue is either in the ground connection two each amp - connectors AN105, 101 and 101-1 or an issue with C406/407 both pos/neg voltage regulators were replaced. Hopefull

Okay back at this again - I think I have found a problem that is triggering the protection circuit. When I scoped the node between R334/336 I didn't see DC voltage since the scope was AC coupled  :grr With a DMM I have -15V at that point and the reason the circuit triggers. Where is the DC voltage coming from is the next question? BTW BOTH amps have this issue........

What makes the noise change - controls on amp, pounding on cabinet?

Measuring from ground I have -48 v on the cathode of D309. I hate shot gunning boards but I did change out C314-18 and both the 470uF and 100uF caps were bad but the amp still isn't working.

Using ground for ref - assuming +-VCC are pins 5 and 9.

One side of R342 has -67V and the other side has -.7V.

Pin 9 of the TA7313 has +3.2V.