Both 4 ohm speaker outputs not working? 8 ohm is OK

[kdove]

I just did some work changing the filter caps on the '78 mesa/boogie and the 8 ohm output that I usually use is working fine. But neither 4 ohm speaker output is working ... and they were before I did the work. I can hear sound but it is really low ... same for both outputs (used individually or together). Plug back in to 8 ohm and she's fine.

Any ideas ... obviously something I did ... looked OK when I reviewed the work.

:?:

 

[stokes]

Dont see how changing the caps would affect the output jacks,thet arent even near each other.I would think that maybe you knocked one of the connections loose somehow.Since the 8 ohm tap is at the end of the winding inside the OT,if it were a problem with the tranny itself it would be likely the 8 ohm tap wouldnt work,its gotta be something with the connection at the jack itself.

 

[kdove]

My mistake ... thanks

 

[kdove]

Dont see how changing the caps would affect the output jacks,thet arent even near each other.I would think that maybe you knocked one of the connections loose somehow.Since the 8 ohm tap is at the end of the winding inside the OT,if it were a problem with the tranny itself it would be likely the 8 ohm tap wouldnt work,its gotta be something with the connection at the jack itself.

OK I checked the connections ... rock solid. Is there a chance that the transformer that feeds the output jacks could be failing. I notice there are 3 leads to the transformer. I had just used the 4 ohm jacks right before I did the filter caps work.

BLACK - To Ground
YELLOW - to 8ohm jack
GREEN - to both 4 ohm jacks

Is there a possibility that the transformer could be failing (on only the GREEN lead)? When I plug into either 4-ohm jack the volume is very low and is wavering. The 8-ohm sounds fine.

Also the 8 ohm jack has a GREEN wire going to the effects loop while the 4-ohm connections have a YELLOW wire going to the circuit board ... anything to consider (look at) there? Could something else be wrong on the circuit board oof teh 4-ohn YELLOW lead?

Man this is wearing me out.

 

[stokes]

Its not likely that the OT would be bad,since the 4 and 8 ohm taps are on the same winding inside the tranny,one end of the winding is connected to ground and the other end is the 8 ohm tap,the 4 ohm tap is between ground and the 8 ohm,so if the tranny were bad at the 4 ohm tap it would affect the 8 ohm.Check the resistance from the yellow wire to ground and from green wire to ground and from green to yellow.It is possible the 4 ohm lead became disconnected inside the tranny.Also be sure the jack makes contact when you plug into the output.

 

[kdove]

Its not likely that the OT would be bad,since the 4 and 8 ohm taps are on the same winding inside the tranny,one end of the winding is connected to ground and the other end is the 8 ohm tap,the 4 ohm tap is between ground and the 8 ohm,so if the tranny were bad at the 4 ohm tap it would affect the 8 ohm.Check the resistance from the yellow wire to ground and from green wire to ground and from green to yellow.It is possible the 4 ohm lead became disconnected inside the tranny.Also be sure the jack makes contact when you plug into the output.

When I check the resistanc between GREEN-to-GROUND and YELLOW-to-GROUND ... what value am I looking for?

I think the jack is OK ... both 4 ohm jacks do exactly the same thing.

The yellow wire that goes from the 4 ohm jacks back to the circuit board ... what does that do?

 

[stokes]

If,as I suspect,the 4 ohm lead has come disconnected inside the tranny,you will see no reading on the DMM when checking that tap.Otherwise you will see a low resistance from the 8 ohm tap to ground(maybe 1.2 ohm or so) and approximately half the resistance from the 4 ohm tap.Like I said,since the 8ohm tap is working normal it is not likely the problem is a bad tranny,if your 4 ohm lead has come loose inside the tranny there is not much you can do about it except to use the 8 ohm tap exclusively.The yellow wire from the output jack to the board is the feedback lead going to your presence control.The green one from the 8 ohm out is the same.

 

[kdove]

8 ohm (yellow to ground) = 1.1 ohm

4 ohm (green to ground) = .6 ohm

(green to yellow) = .6 ohm

That sounds like my transformer is working and connected doesn't it?

I measured these by plugging in a cord and measuring the center and ground connections ... so I think the jacks are working correctly too.

 

[NoGlassNoClass]

Let me start by saying I am certainly NOT an amp tech, but it appears to me in the last pic you posted, that there is no ground wire attached to the middle jack. Is it supposed to be like that? Ground to amp chassis via the nut on the outside? The other two jacks have a black wire from the sleeve connection to a trace on the board. Ignore if I am an idiot, but that just doesn't look right.

 

[stokes]

It seems that the OT connections are okay.Doesnt leave much else but the cable to the speaker.I cant imagine anything having to do with those feedback (yellow and green) wires going from the output jacks to the board having anything to do with any problem,you could actually remove them and the amp will still work.If you are sure there isnt an intermitent problem with the speaker cable itself,try cleaning the 4 ohm outputs with some contact cleaner or some 0000 steel wool.If those jacks arent used much it could be some oxidation build up.

 

[stokes]

Let me start by saying I am certainly NOT an amp tech, but it appears to me in the last pic you posted, that there is no ground wire attached to the middle jack. Is it supposed to be like that? Ground to amp chassis via the nut on the outside? The other two jacks have a black wire from the sleeve connection to a trace on the board. Ignore if I am an idiot, but that just doesn't look right.

The jacks are grounded at the connection to the chassis,those two wires you see going to the board are to supply a ground to part of the circuit on the board.What you see is normal.

 

[kdove]

It seems that the OT connections are okay.Doesnt leave much else but the cable to the speaker.I cant imagine anything having to do with those feedback (yellow and green) wires going from the output jacks to the board having anything to do with any problem,you could actually remove them and the amp will still work.If you are sure there isnt an intermitent problem with the speaker cable itself,try cleaning the 4 ohm outputs with some contact cleaner or some 0000 steel wool.If those jacks arent used much it could be some oxidation build up.

When I plug either cabinet/cable into the 8 ohm output they both work. So both the speakers and the cables seem OK. I was just using the pair of 4 ohm outputs last week (before I did the filter caps) with both speakers/cables and they did fine.

So I'll clean the outputs just to make sure. I might disconnect the lead going back to the board if nothing else works. This is kind of perplexing.

 

[kdove]

OK ...

Good News: It's working now.

Bad News: Not sure why?
- Cable options and working with the output jacks didn't help.

- But then I played around with the knobs in the back Presence and Effects Loop knobs and it broke loose. Now maybe I jiggled a output cable somehow or something. But it was almost like a "ground out" from one of those controls. Maybe I should clean the controls ... not what I was expecting though. Make any sense?

 

[stokes]

This is not a reissue we are talking about,right?The schem I have for the original doesnt even show the two 4 ohm jacks or the effects loop,are you refering to the slave as the effects loop?.But anyway it appears you have found the problem,check all the connections at the presence and effects loop.Sometimes the shorting jack in these amps in the effects loop goes and needs to be changed,clean the pots.Not having a schem to match what you are describing makes it hard to see from here,but you are definately in the right place with these controls and the associated connections.

 

[kdove]

No ... it's an original ... but you may recall it appears to be in the seam between the original and Mark II. So many things on the drawing are not exact. The power supply appears to be more like Mark II.

The loop I'm talking about is the SLAVE pictured above. When I began turning the PRESENCE and SLAVE knobs is when it broke loose and began working.

 

[stokes]

Oh,yeah I remember now.I would clean out those pots with some spray.If that doesnt cure it,see if turning one of the knobs to either extreme causes the problem,if so I would change the pot.These pots are not used often so it is common for them to get dirty from sitting in the same position and not rotated to keep the dirt moving.

 

[kdove]

This is an old problem ... I was never really able to fix it but I have a few more clues. Any adice would be appreciated.

Since this post ... 4 ohm output jacks have not worked. I just figured maybe the 4 ohm connection from the output transformer had failed as was suggested earlier.

A couple of days ago ... I had a jam session. It played OK at the session. But when I got the amp home ... it began losing volume again. I'm thinking that whatever is happening may be related to moving the amp ... shaking something loose or reconnecting an intermittent problem.

Yesterday ... I checked them and both 4 ohm output jacks were working. Later in the day, it began losing volume again.

Today ... With 8 ohm EVM cabinet plugged into 8 ohm output, the amp came on BUT the output was barely audible for about 30 seconds ... then it came to full volume gradually over the course of a few seconds.

I unplugged the EVM cab and tried another cab ... worked fine. Now it's working and not losing volume BUT the 4 ohm output jacks are not working again.

Any thoughts?

 

[kdove]

bump

 

[kdove]

It appears to have been the master volume all along! I cleaned all the pots 3 times ... the third time it seemed to be the master volume still doing it.

I took it out ... intending to replace today ... but could not find a 1-meg audio pot anywhere. I'm ordering from Acme Guitar Works tonight.

Anyway ... cleaned it really good while I had it out ... put it back in temporarily but it appears the master volume was the culprit. The 4 ohm outputs now work too.

I can make it fail by playing with the master volume - pushing and pulling the master lightly.