Newb. Glad to find this forum ! Fender M80 Bass Combo

[dogface]

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

[dogface]

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

[phatt]

welcome Dogface. 
Obviously the input socket is in need of replacement,, well past it's use by date so start there as it might help with the hiss. The hum might be a separate problem but worry about that later.

Oh the fizzle you hear might be some crossover distortion but leave that for now,, input socket first.
Phil

[dogface]

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

[g1]

  You will have a hard time finding another jack that will fit properly in there.
The part # you need is 099-0912-000.  It should be available online or through anyone that sells Fender parts.
  While you have it open, resolder the main filter caps.  Then see if any of the faults have been cured.
Service manual attached:

[dogface]

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.

[Roly]

Hi dogface, welcome.

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.

Good observation, well spotted, well fixed.  Enjoy to the full (it doesn't always go that easily)   :dbtu:

For all the blather on the forums advising everybody to "change all the caps", whatever the problem, ...

"I broke a string!"
"Change all the caps".

... this is the kind of stupid fault that is the bread and butter for amp techs.  It's not all loud bangs and clouds of smoke ya know.   

I noticed that there was some residue around the cap leads ... does this shed any light ?

No.  Solder flux.  Quite normal.  Some companies wash it off, others don't.

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.

Roll 'em out, one per thread (and I'd suggest one at a time to avoid confusion).

[dogface]

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:

[DrGonz78]

Thoughts on analogy for the shotgun approach is: You would have about as good of a chance fixing the unit with that approach as Hunter Thompson did trying to fix a misbehaving typewriter.

That sure will fix something and it will never give you a problem ever again.  :lmao: :trouble :dbtu:

[Roly]

Good ol' Hunner - tripewriter, shotgun, joint...


HST's remains being blasted out of a cannon.

"I hate to advocate drugs, alcohol, violence or insanity to anyone, but they've always worked for me" - HST

They do not, however, work so well when fixing amps.   8|

[J M Fahey]

Well, there is **ONE** officially approved Guitar players use for shotgunning :

https://www.youtube.com/watch?v=ibUbIyska9k

although the idea is not exactly *repairing*  electronic stuff  :duh :loco

[dogface]

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 !

[phatt]

hi Dogface,, you are getting there :tu:

Crackles,, hum
Anywhere from a bad solder joint to a failing component but obviously mechanical more than an electrical issue.
If your Amp has an FX loop? jump it with a working guitar cord and see if that helps. A bridging cord bypasses the internal switch.

FX loop can be labeled under at least 3 names, pre output/pwramp input,, Send /Return but they all do much the same thing and the switch inside the socket often fails and cause all sorts of intermittent issues.
As you have already found the main input socket was stuffed so good chance the others are on the way out as well.

As Those sockets are pcb mounted they can be a pain to replace and IME tiss often more cost effective to buy a short lead and leave it permanently bridged. Unless you just love hard work don't bother trying to fix it.

If you still have crackle then might be time to pull the pcb and in very good light,, (Sun seems to work best) and even if you think your eyes are brilliant use a magnifying glass and check all the solder pads for hairline cracks. Also very fine tracks can often break right on the very edge of solder pads. they can be very hard to see.

Fire the amp up before you pull the pcb and while it's running use a wooden stick to gently press all around the board and see if you can narrow down the search,, can often save a lot of time. :tu:

Probe around all the pot legs they can be a problem also.

Q/ do any of the pot crackle when turned?  The centre wiper can often give trouble with intermittent crackles and may need replacement.

Phil.

[dogface]

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.

[dogface]

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.