18 watt mojo kit tremolo woes

[phsyconoodler]

Just putting together  mojo 18 watt trem kit for a friend and it works great except the tremolo doesn't. The layout has been altered significantly from the original 18 watt and it has a two button footswitch.One button shorts the trem to ground and the other switches in a 1meg resistor on the intensity control.
  Strange to say the least.
I have been over the wiring several times and it's wired exactly as the layout diagram shows.What it does is drop the volume when the intensity control is turned up.no trem,no noises,no ticking.Nothing but the volume drop.
  They have the layout and schematic on their site. Any insights would be appreciated!
Voltages are all in the reasonable range.

[g-man]

Deleted

[phsyconoodler]

The amp works flawlessly except for the damn trem.It still won't oscillate.I've been over this damn thing all day now and no dice.
 Tubes,lead dress,voltages,grounds,tubes.
I am getting rather tiffed! I even talked to Mojotone and he talked me through the circuit and still nothing but volume drop when you turn the intensity control.
  It has the two button footswitch which is kinda cool actually,if I could actually get it to oscillate!
Moved the cap twice now.Nothing. No changes at all.
   

[ajeffcote]

I was looking over a Mojo schematic for their low power tweed twin kit yesterday when I noticed some coupling caps missing. They were in their lay-out diagram. Who knows, maybe there was a mistake on yours too.

[phsyconoodler]

Ok.I'm definitely at my wit's end here.
  I have totally rebuilt this trem circuit from top to bottom and still no trem!!!!
Mojotone simply says it works for them.They did give me a few hints on how it works and what to look for but still no dice.
  One and only one difference from the stock circuit:
 I did not have a 6CA4 rectifier tube on hand so i used diodes.Can this increase in voltage be the reason this trem won't oscillate? I don't see why but who knows?
  Anyone have any insight here it would be appreciated!!!

[g-man]

Deleted

[phsyconoodler]

Ok,got the right rectifier tube.Still no-go.

 What the heck!??

[Willabe]

The LFO on the 5G9 I built was the hardest, most confusing thing to draw (schemo and layout) and solder up I've ever done on an amp.

I had to number the R's and C's that go from the LFO plate to the grid and K on the schemo before I was able to finely get the layout drawing, including all the grounds in it, right and soldered up right. That LFO ring is the ring of death.   

I could not get it to work even after checking it over and over again. I finely rebuilt it and it worked. But I still don't know what I had wrong on it. The LFO on my Warbler from Sluckey worked 1st time.  

(I know you rebuilt yours already.)  


      Brad        

[Willabe]

I looked at the Mojo schemo and this 18 watter from Doug's library and their very close. The LFO K is wired up a little different but I don't think it matters.    

Maybe you'll find something that's wrong on the Mojo layout if you compare it with this other layout?

http://www.el34world.com/charts/Schematics/files/marshall/marshall_18watt_layout.pdf

http://www.el34world.com/charts/Schematics/files/marshall/marshall_18watt_schem.pdf


            Brad    

[phsyconoodler]

So the differences I see are mainly the pot values,the way the intensity control is wired and the one extra capacitor in parallel in the oscillator circuit.
  I did try wiring the intensity pot like the original 18 watter but still no-go.I might try and remove one of the paralleled caps but I can't see that being an issue either.
  In the Mojo layout they run the ground all the way over to the cap can ground and the original is right on the pot ground.
So essentially it is either not oscillating or the intensity control is just shutting the tube off.
  Either way it does not make any sense to me.

[sluckey]

See the red jumper? Do you have that on your board?

[phsyconoodler]

Yes it's there.I have 291v at that point.

 By all rights this thing should be warbling away.Instead it just drops volume when you turn the intensity control.All the way up and there is no sound at all.
 6 different tubes in V3 so far.

[Willabe]

So the differences I see are mainly the pot values,the way the intensity control is wired and the one extra capacitor in parallel in the oscillator circuit.

Yeah there very close. The extra cap is a 2nd .01 to get .02 in total there to slow it down.

I was hoping it was something in the Mojo layout that you'ld see was wrong by looking at the other layout.   

If anybody can find it and get it going you will.


            Brad     

[sluckey]

How about some voltage readings for pins 1,2,3,6,7,8. Got a jumper between 4 and 5?

[phsyconoodler]

Voltages:
1)198v
3&8) 1.97v
4&5) 3.3vac
6) 290v
9) 3.3vac

 New developement: I moved the .047 cap again and the amp now motorboats,no difference when turning trem controls.Moved the cap back to the tube socket and back to where I was before.
  I didn't mention how crappy these Chinese Belton-copy tube sockets are that mojo uses in their kits now.I'm going to punt this socket for a real Belton and see where that gets me.The pins fall out before you even use the damn things! Definiyely NOT Belton!!
  This could be my issue cause I got the amp to at least motorboat which it didn't do before.I tightened the pins on the socket and moved the cap and then it started motorboating.Didn't do that when I moved the cap before.
 If it turns out to be the problem Mojo is going to get an earful from me! I buy thousands of dollars a year from them,but not kits.The rest of the parts in the kit are decent;wonder why they cheaped out on the sockets???

[TubeGeek]

Voltages:
1)198v
3&8) 1.97v
4&5) 3.3vac
6) 290v
9) 3.3vac

 New developement: I moved the .047 cap again and the amp now motorboats,no difference when turning trem controls.Moved the cap back to the tube socket and back to where I was before.
  I didn't mention how crappy these Chinese Belton-copy tube sockets are that mojo uses in their kits now.I'm going to punt this socket for a real Belton and see where that gets me.The pins fall out before you even use the damn things! Definiyely NOT Belton!!
  This could be my issue cause I got the amp to at least motorboat which it didn't do before.I tightened the pins on the socket and moved the cap and then it started motorboating.Didn't do that when I moved the cap before.
 If it turns out to be the problem Mojo is going to get an earful from me! I buy thousands of dollars a year from them,but not kits.The rest of the parts in the kit are decent;wonder why they cheaped out on the sockets???

I had the pins fall out of my princeton reverb mojo kit sockets just a week or two ago.  At first I was like WTF and then realized I could push them back in the socket and after soldering the wires to the socket pins they stayed put.  I still don't like the fact that pins can be pushed out so easily.  I am not impressed with mojo's choice to use the belton copies.  The whole reason I have been choosing mojo kits this past year or two is because I wanted to get away from buying the "walmart quality amp kits" (Weber) kits.  Even one of the Weber tech's told me they themselves are the walmart of amp kits.

[phsyconoodler]

Well changed the tube socket to a real Belton.No difference at all.The tube fits way better though.
  This one is giving me a headache to say the least.
I'm almost ready to chuck it out the window.Hours and hours spent and STILL no tremolo.What in the world can be causing this?
  I have built lots of amps and 18 watters with trem and they just don't give me fits like this one is.The layout if funky but the Mojo tech dude says they don't have issues with them at all.
  Well off to redo all the grounds.......gotta do something before I lose my mind.......too late I'm a veg.....

[sluckey]

We need to see some hi rez pics.

[phsyconoodler]

Here you go. Keep in mind I've removed and re-attached wires all over trying to get this functioning.Pics show the .047uf cap on the board and off.Not shown re-attached to pin 6 of v3

[Willabe]

Shot in the dark, maybe a bad ceramic disk cap?


           Brad     

[phsyconoodler]

second set already.

[phsyconoodler]

Ok.I'm definitely missing something here but I'm at a loss to figure out what exactly.

What about me using wires under the board and the lead to the preamp running underneath the wires?
  Maybe that causes an oscillation that makes it go into cutoff immediately when running the intensity control?

[phsyconoodler]

Nope. That is it either. Changed it and still nothing.
Very frustrating indeed.
   So I guess its go completely to the original 18 watt trem layout and ditch this silly non functioning crap.

 

[sluckey]

Where's the jumper? See pic.

There are only 7 underboard jumpers in the pics of Mojo's board, but there are several more on top of the board. I assume you put all of them on the bottom? Only you can tell. I would use an ohm meter to verify zero ohms resistance between all turrets that should be jumpered together.

Mojo's layout is not very logical IMO but it appears to match the schematic. I know the schematic works.

[phsyconoodler]

That jumper is und er the board. I have verfied all connections with my ohmmeter.

[sluckey]

That jumper is und er the board. I have verfied all connections with my ohmmeter.

Are you absolutely sure? If so, check the ground connection on the other end of that cap. If it's OK, replace that 50µF cap.

I just came from my shop. I opened my 18W and unsoldered the positive end of that cap. This simulates an open cap or a missing/bad jumper. Guess what??? I had no tremolo and the Intensity pot would cause a volume drop as it was turned up. Everything else worked fine. Sound familiar?

[phsyconoodler]

Im on it as we speak. I will let you knome Sluckey!w. Thanks for taking the ti

[phsyconoodler]

Replaced the cap,veirfied connections and grounds. Still the same. Time to tear it out and start over.

[Platefire]

Wow! Can't believe all you've done on this amp and still no trem. Got to be frustrating but you will get it because now it's become a showdown between you and the amp---but I have every confidence you will win!!!  No suggestions, just keep searching till you find the missing link 

[jim]

"tear it out" but build a TMB.  Your friend will love it just as much.    Jim

[silverfox]

How hard is it to replace the existing Trem circuit with one that works differently and reliably?

Silverfox.

[phsyconoodler]

That is what I have to do. This plain doesnt work. Its all there,volyagrs,connections,tubes,sockets,pots and it still wont work. Frustrating beyond words!

[SILVERGUN]

Hey just for the record....
I recently did a build with one of those BLACK fibre boards,,,and after hours of meticulous wiring......

IT TURNED OUT TO BE CONDUCTIVE.....turns out that there might be carbon in the coloring 

Just a heads up and something else to look for.......that could be where your losing it

[phsyconoodler]

Thanks bro,but this board is garolyte not fibreboard.
  Nothing is out of place here. Frustration building...................

[terminalgs]

Voltages:
1)198v
6) 290v

The schematic shows triode V3B (1,2,3) as the oscillator and V3A (6,7,8) as the gain stage. The voltages you have above look like you've switched triodes from one to the other.  as long as you switch grids as well, it doesn't matter, but if you switched plates accidentally, it won't oscillate (but I think some guitar signal will still make it to the vol/tone controls via the shared cathode.. or maybe not with that 50uf cap... I'm not sure).

[Ed_Chambley]

I cannot see it very well in the photo, but it looks like the footswitch jack is wired differently than the layout.  It is probably correct as it took me a long time to see the shielded wire to pin 7 of v3b.  Worth another look.

I have built this exact amp before and other than a couple of oscillation problems, the trem worked well.

[phsyconoodler]

You have to use the mojo schematic. They switch the triodes. I ripped it apart again,removed 90% of the underboard wires,changed a couple of groinds and still nothing.

[Ed_Chambley]

Attached are the Mojo Schematic and Layout.

[terminalgs]

so,  you've wired this correctly as the diagram and schematic that mojo provided.  Either you continue to rip out and re-wire, or you can suspect the mojo oscillator (maybe it is too finicky?).  I'd turn to a known good oscillator, for example a Bandmaster AB763.

Compared to the Fender: from plate to grid, caps in series .02, .01, .01. after each cap, resistance to ground is 1M (pretend the speed knob is set to "1M")

The mojo one has the same cap and resistor values except they've got 2.2M at the last resistance to ground instead of 1M.  also in  "1M to ground at each phase shift" camp are several old Marshall trem circuits, Vox circuits, and the WEM Dominator 18 watt circuit (for which this 18W, and the Marshall 18W are based upon).

Unlike this mojo circuit none of those old ones (that I looked at) have that .047 plate to ground cap (I don't see how that'd work, its seems too much signal would be zapped to ever oscillate).

You might try this (make the oscillator look a little bit more like a fender):

Make that 2.2M be 1M. and lift that .047 again.

now you've got 1M to ground for each.  It should oscillate.   you can put an LED across one of those 1M's to see it oscillate if you don't have a scope.

About the only other issue would be that the oscillator cannot bootstrap or kick-start itself itself. 

[sluckey]

Unlike this mojo circuit none of those old ones (that I looked at) have that .047 plate to ground cap (I don't see how that'd work, its seems too much signal would be zapped to ever oscillate).

The circuit works. The original has that cap. My homebrew has that cap. Every schematic I've seen has that cap. You don't need a ton of trem signal when you are only gonna wiggle the low level first preamp stage. Look at the attached original schematic.

The Mojo layout is poorly thought out, but it does agree with the schematic. It should work. I still think there is a wiring error or faulty/wrong value component. Hoffman's law.

[phsyconoodler]

I agree about a faulty component. Im dead positive there is no wiring error. Off to get caps and resistors I guess.

[sluckey]

How much resistance do you measure from V3-7 to chassis ground?

[phsyconoodler]

I measure 2.25megs of resistance there.

[terminalgs]

I measure 2.25megs of resistance there.

doublecheck the same resistance to ground for each phase-shift cap  in that .02-.01-.01 series of caps (i.e. from the 2.25M side of the .01, what is the resistance to ground on the other side of that .01, and then the  same thing between the the last .01 and the .02).

phsyconoodler, do you have a scope?

[phsyconoodler]

2.25meg,1meg,2.5meg respectively. Yes I do have a scope but I'm not sure what I would be looking at on it for this circuit.It's a dual trace Heathkit,so accuracy is not going to be paramount!

[phsyconoodler]

Changed the 2.2meg resistor to 1meg.No change. Lifted the cap,now it makes the amp motorboat even with no instrument plugged in.
  Put the cap back on V#,pin 7 to ground.Stops the motorboating.Incidentally,plugging in the footswitch stops the motorboating. Either way,STILL no trem!!!!
  The amp motorboats with the .047uf cap in the stock location,but stops with the footswitch plugged in.
There is definitely a war of capacitance going on in this circuit somewhere.......

[terminalgs]

Changed the 2.2meg resistor to 1meg.No change. Lifted the cap,now it makes the amp motorboat even with no instrument plugged in.
  Put the cap back on V#,pin 7 to ground.Stops the motorboating.Incidentally,plugging in the footswitch stops the motorboating. Either way,STILL no trem!!!!
  The amp motorboats with the .047uf cap in the stock location,but stops with the footswitch plugged in.
There is definitely a war of capacitance going on in this circuit somewhere.......

does motorboat change speed with your speed-knob pot changes?

with a dual trace scope, put one trace on the grid of the oscillator triode, and one on the speaker out,  see if the motorboat sine wave matches the oscillator sine wave.

When you say  footswitch plugged in,  do you mean plugged in with switch closed?  (If so, then that's how the switch is designed to work. turns oscillator on/off, ground to grid -- and it sounds like the oscillator is, in fact oscillating).

[phsyconoodler]

Yes the pitch and speed changes when you turn the intensity pot when it's motorboating.
The footswitch does not need to be closed to stop the motorboating.
  Ok so it's oscillating.Now to get it to oscillate the other half of the damn tube!

 Ok scope patterns do not match on grid and output.Output trace is a smooth wave and the trace disappears on the grid.

[terminalgs]

... it has a two button footswitch.One button shorts the trem to ground and the other switches in a 1meg resistor on the intensity control.

The schematic shows the 2nd footswitch to switch a 1M across the speed control.  If this is typo, then forget it, but if you wired the switch on the intensity control, it'd be something to fix (maybe not the culprit).

So with the trem F/S switch closed (i.e. the oscillator is disabled), and the intensity turned all the way off, you should have a 820ohm to ground for the cathode.   Now, the trem channel should function just like the normal channel.  Is this the amps behavior?   Is does it function correctly like this?

[phsyconoodler]

Actually the ground is attached to the center lug on the intensity control and the 1 meg resistor goes to the speed control.
  The amp works just fine on both channels without the trem engaged.
 It just loses volume as the intensity control is turned up and goes totally silent when it's all the way up.It just shuts off the tube altogether or is going into a high frequency oscillation or whatever the hell it's doing.
  This circuit may indeed work but it sure isn't working for me.