Well fellas, looks like I've got it with your help. I went through all the suggestions and just couldn't find anything that seemed like a physical ground issue. New that is.
I went back to my original fix, the point where the power board grounded to the chassis. Seems what I did was an improvement, just not enough. i removed the screw and the brass nut i had put under it to try to bridge the gaps of poor contact. I fired up the soldering iron and flowed a perfect ring around the screw hole, put everything back together, and all seemed good.
No more crackles, cut outs or fuzz tone. I noted that one leg of the filter caps are in a wide path on the board very close to the ground point. Forgive my primitive terminology, anyway it works. I still cant believe how loud this little guy barks. with the 4 ohm Black Widow i replaced the original 8 ohm speaker with, I get all 160 watts and it is a pretty efficient speaker.

I tried to find some specs on the Fender/Celestion 8 ohm speaker that is original equipment. The most I could find is that it is sold as "Fender 15" replacement speaker" no specs. It was used in other amps and cabs of the era ~ 200 watts or so, so I guess it is a pretty typical middle of the road 15. 200, maybe 300 watts ? Anyway, I may put that back in the Peavey cab I stole the Widow out of and experiment with my slave amp idea with that.It will be plenty for that.
THANK YOU  !!!!

Thanks phatt, I'll keep picking around. your help is much appreciated.

Well, I tried all your suggestions phatt,

I re-touched all the solder connections on the fx loop jacks as well, nothing seemed to change, bridged them with a short stompbox connector, nope, still crackle.

I re-touched the terminals on the big filter caps, no real change after that. I did notice that wiggling them when poking around for loose components the did produce a static-y noise at a low level.

When I reinstalled everything and powered up, all was dead silent, even -0- hum.as I was putting knobs back on the chorus pots, the hum came back. Then I wiggled the chorus knob shafts and suddenly sounded like I was playing through a 60's fuzztone, all low frequencies were gone, I couldn't make it go away by more wiggling.

I came back a while later, powered up and all was well as before at least. seems as if something in the vicinity of chorus/FX loop is wonky.

When I cut power, it went off with the hiss decaying very quickly, and a sudden sharp pop. Isn't this typical of filter cap deterioration ? anyway, I'm having fun and really want to debug this critter.

Many thanks for any and all help !

P.S. just a curiosity, I know it was extreme, but as I poked around for noise sources, I had a small external speaker attached, the lead on the internal is too short. I had the volume cranked. As I worked I noticed when my  hand came into the vicinity of the input jacks, it would hum like crazy! Not touching anything, just my hand in the air. If I grounded my finger on the chassis, it would stop. Some sort of inductance/ microphonic thing ? Just curious.

Thanks phatt!
I'm going to put a pedal connector in the FX loop jacks per your suggestion. Actually, when I "aggrevate" the face plate, that seems to be the most sensitive area.

I learned exactly what you said about the solder joint on the input jacks. They looked sound at a glance, but when I got the new parts and was removing the old and had nothing to lose putting some minor pressure on them to desolder, a few of the legs came out with little to no effort.
I think pots are alright, so far anyway. If bridging the loop doesn't work, I'll do the poking around with a stick.

I really like this amp as combos go, I would like to get the bugs out. I don't need a big gigging amp anymore, but a still a power and tone junkie. I can make this sound huge with my Hartke bass attack warming up the input signal  :trouble

Since I can never leave well enough alone, I have a power amp from a 150watt Peavey keyboard amp, and may build a duplicate slave cabinet ( ala the old acoustic 361) of the same dimensions in case I need a monster some time. I find the smaller lighter size appealing and love to build and tinker with old stuff.

Update: Got the new input jacks installed, no more problem with them.
Replaced the 8 with a four ohm speaker. I can't believe how flipping LOUD this thing is for it's rating.

I still have an intermittent sizzling/crackling kind of sound in the background.

Even though my original ground repair on the main board lessened hum considerably there is still a lower but still annoying ground hum that is enhanced when I engage the deltacomp switch. The crackling is aggravated if I physically push on the front of the control panel anywhere.

Any clues where I might start to find the source of these symptoms ? Many thanks !

Yes interesting I read a bit about the how the alternating cycle does this. You'll probably laugh I was using this beast as a practice amp.Never was an issue at band levels of course.I may look again just for giggles to see if all those years of low frequency pounding  have loosed something. I read about potting just like pickups very interesting. I'll stick to my combos for practice :-)

I opened the critter up today and it was really clean, everything in nice shape. All solder joints and grounds seem to be good. I was able to determine that the hum is a mechanical hum from the power tranny. Does this indicate anything perhaps on the horizon or is this just the nature of the beast ?

Many Thanks.

Thanks G1. Think I'll just keep it as is, look for the obvious age issues, and not sweat the balance pot if it's not available. Many Thanks !

I never really gave this a lot of thought because I ran it with the original bi-amp spectrum bottom, or just one side into a 1x15 later. I have since gone to a more conventional head as my main rig.

Do I understand correctly that in mono mode, into two cabinets, I'm getting the same wattage and frequencies into the two cabs ? I just never did it that way. In that case, I just forget my brainstorm.   :duh

If that's the case, I'll just leave it alone and keep it for a back up. I'll get a couple of fridges for it hehe.   :lmao:

It does have hum issues but I'm going to do the solder joint ground connection filter cap inspection to see to that first .
There is a balance pot on it that got the shaft and knob broken off by some knucklehead little brothers of a band member who "borrowed" our gear one day. Can someone reference me a part number for that ? , It's about the only thing really broken on the amp.

Many thanks !

Thanks Roly, and the info on my other post as well.
I hear where you're coming from, I have a pretty good instinct for troubleshooting once I know what I'm into.
What I hear you describing people saying is what I call scripts, it's just a meaningless cliche that may or may not apply.
I like to find the real problem and not use the shotgun or "spray and pray" approach to fixing things.  :lmao:

 Hey really enjoying this forum, already got some great advice and can't believe what a great place this is.

Here's my latest brainstorm. I have  BXR 400 I used as a gigging amp for several years. I've always wondered if this two power amp Bi-amp for two different speakers setup could be readily bridged so it is a sho-nuff 400 watt amp feeding a single cabinet with all the watts.

Is this possible or just a crazy idea ?  :duh   It's a really well made and powerful amp, I hate to just let it slip away. I would like to make it into a more useable configuration. Thanks for any info.

Thanks G1, man this is incredible ! PDF saved.

In the meantime, I had taken some cues from other posts, and pulled the power amp board to check the cap connections and look for anything loose. The main caps were tight as could be, however :

There is one of the screws that holds the board to the heat sink which was an obvious ground point. all the others are isolated from the circuit, anyway, it looked suspicious like it wasn't making good solid contact with the bare tinning (?) whadda you call it anyway ? on the board. I found a thin brass mini switch nut of bigger diameter than the screw head, put everything back together, fired that beggar up and the ground hum was gone. To say I was tickled doesn't quite get it.

I noticed that there was some residue around the cap leads where they soldered in, kind of like rosin when you solder, and the same thing in the vicinity of the power transistor closest to the caps. there was bare tinning and the same residue in that vicinity around the middle leg of the transistor, does this shed any light ?

Anyway, it worked great, and I'm off to find that jack. I'll see what happens when I replace that.
I still get a similar hum when I engage the compressor, and the hiss gets bad when I crank up the treble, maybe just the nature of the beast ? However I no longer get the pop when I plug or unplug the worn jack, it functions but obviously is worn out.

Many thanks to everybody so far, Looking good.

I have a Fender BXR 400, a Kasino Concert, and a Laney Linebacker 100 watt chorus combo waiting in the wings all with issues I'd like to solve.

Thanks for the welcome and info phatt.
The socket is in a clear plastic housing, unlike ones I have seen before. Do I need to find this particular part, where might I source it ?

Or, do these have a standard pin config between different makes, I have plenty that are are the black poly type where the curved contacts are visible, such as on a peavey, are they interchangeable ? Forgive my crude terminology... :duh

A couple of more things noticed:

When I sustain a not for a long time, a sizzle develops in the background, not loud but present and annoying.
When I engage the compressor, the constant hum is noticeably louder, more than I noticed before.
On the noisy #1 LH input jack, the metal sleeve liner is noticeable missing, as opposed to the RH jack which seems to have no issues.

All controls work, the amp is loud, tight good tone, just noise issues so far..

Thanks again

Hey all,
Decided to try to learn something about SS amp repair and troubleshooting, what a resource this is. I have several older amps with issues, hope to have then all up to par with the help I can get. Many thanks.

My M80 bass chorus combo has a hum when powered up, and a noticeable hiss as well. I have read several threads about the filter caps being a common problem, as well as other loose solder joints. Should I try this first , and then go from there ? Any advice greatly appreciated. I don't know of any schematic out there in the cloud, I have searched but don't necessarily know, nothing jumped out at me in searches plainly labeled as such.

I suspect perhaps more than one problem, the hum stays constant, while the hiss is affected by preamp controls, and the first (passive? ) input jack is noisy, it looks really worn, will emit a loud pop when a plug is inserted or removed, and really ups the noise game if I crank up the bass frequencies. The second input ( rh ) is quiet.

When I power up the speaker cone jumps back a little but then seems to slowly come back. The volume tone and chorus controls work normally and fully, I do notice a change in the noise when engaging the chorus or compressor switches.

Hope this helps to get started. It's incredible that there are folks here who share their expertise just for the love of the craft. Many thanks again.   :dbtu:

I have a basic meter, and have a good knack for troubleshooting, developed in other trades and endeavors. Any help greatly appreciated