Magnatone-Revibe 10A-Sluckey Mod--Low Volume and No Output Level Adjustment

[wsscott]

About 3 years ago I built the attached Reverb/Vibrato headhunit with incredible support from Sluckey and this forum.  It was like a Revibe, but using the Magnatone 10A circuitry for true Vibrato, and Sluckey modified the circuit.  Attached is the Schematic of the finished product.

Over the years I've wanted to reduce some of its hum, and yesterday I was running a Test Tone through the unit to try and trace the signal, and the output voltage of the signal was higher than I thought, probably 650mv or maybe a little more, and after awhile I lost almost all of the Output signal volume.  The signal was still coming through, but the Output Level pot was no longer responsive, and the only way I could get any output sound was to turn the Volume pot almost all the way up, where previously 10 o'clock on the pot worked fine and the Output Volume was great, with plenty of headroom left in volume.

The hum has never been affected by adjustments to the Volume pot, only the Output Level pot, and if I had it turned low enough it was almost gone.  The only problem was that I would have to turn up the Volume on the Amp to get the volume I wanted.  So I was trying to get something close to unity gain.

So I'm hoping someone can help me figure out what's going on.  I don't know if it messed up one of the pre-amp tubes, or a resistor or cap.  Nothing looks blown or dirty, and all the tubes light up.

The only "good" thing is that the Hum I was trying to eliminate is now completely gone.  It's just that I have to turn the Volume pot all the way up, and the Output Level pot no longer responds.

Thanks for your help.

[sluckey]

Surely you have a better copy of the schematic???

The hum issue is probably due to a ground loop. Disconnect the power cord green wire from the chassis and see if hum improves.

[wsscott]

I'm attaching a pdf that I think you sent me, if this helps.

Right now the hum isn't the issue.  The problem is that the Output Level pot no longer responds, and the Volume Pot has to be turned all the way up to get any sound out of the unit to the Amp.

So it seems like something in the circuit is busted.

[sluckey]

Tube substitution first. Then voltage checks.

When you fix the low output the hum will likely return. BTW, the ground loop only exists when the revibe unit is connected to the main amp.

[wsscott]

Yes, I'll see if I can find a 12AX7 to test it with.

The HUM was gone when I played it through the Main Amp after all this happened.  I just had to turn the Volume all the way up on the Mag ReVibe to get any sound to come through to the Amp.  Looks like now there's some minor responsiveness in the Output Level Pot, but only when it's turned way up.  All much different than before.

I did check the voltages on the Filter Caps and they are all reading similar to the schematic.

[wsscott]

I pulled a 12AX7 from another amp, and no difference when I swapped it with the 2 in the Revibe.

I didn't have a 12AU7 but did have a 12AY7, and swapped that for those tubes and for the 12DW7, but no difference.

[wsscott]

This morning I checked the DC voltages on all of the tube pins and they were consistent with the values shown on the As Built Schematic.  Some a little lower but not significant.

Next step?

[wsscott]

So I injected a 1kHz tone at 150mv to the Input jack and checked out the AC voltage of the signal at both ends of the 1M resistor before it connects to the 100K Output Level pot.  It read about 22VAC going in and about 1.6VAC coming out.  It maintained that reading to the Input lug on the Output Level pot.  No changes in the voltage when the Output Level pot is turned.  Then I checked the AC voltage reading on the Center Lug of that pot, and got 0 ACV readings regardless of the the position of the wiper of the pot.

So I guess the Output Level pot is bad, and that's the problem?

[sluckey]

Sounds suspicious to me. Also take a look at the output jack. If you happened to connect the switch lug to the ground lug, remove that connection as per the schematic.

[wsscott]

Sluckey--When you say the "switch lug", are you referring to the "switch" on the tip?  In other words just have the Output Jack wired for the input to tip and the sleeve to ground but not bridged to the "switch"?

As to the replacement pot, I have a A100K that reads about 92K, and a B100K that reads about 104K.  Which do you recommend I use?

Thanks.

[sluckey]

Yes and use the log taper pot.

[wsscott]

Thanks, I'll keep you posted.

[wsscott]

No luck.  Just like it was before.  I am now getting AC at the Center Lug on the Output Level Jack and the Output Jack of a maximum of about 1.7 Volts AC when the Main Volume pot is turned up all the way. That level now varies with Volume and Output adjustments.

I can't imagine what's going on.

There's no DC getting past the .047uf cap, and about 20 VAC going into the 1M resistor and about 1.7 VAC coming out that then goes through to the Right Input Lug on the Output Level pot and then out from the Center lug on the Output Level pot on to the Output jack.

If I'm getting that much AC voltage on the Output jack, why is the Volume so low?  Could it be the 1M resistor? Should there be that much of a voltage drop across it?

Since this modified circuit is based on the MA-10A, do you know what the voltage should be?

Thanks.

[wsscott]

I lifted one end of the 1M resistor and it reads fine, ie. 0.995M.

[sluckey]

If I'm getting that much AC voltage on the Output jack, why is the Volume so low?

Maybe the issue is not inside your revibe unit. Did you have your revibe plugged into a main amp when you made these AC voltage measurements?

Since this modified circuit is based on the MA-10A, do you know what the voltage should be?

I know roughly what the voltage should be. The output fed into a power amp in the original M10A. That power amp was expecting a line level signal, ie, approx 1V. So, I guesstimated needing about a 10:1 voltage divider to knock that level down to a reasonable instrument level (about 0.1V). And the 1M with 100K pot make a nice 10:1 voltage divider. So, you have 1.7V into the pot and you should be able to send a healthy signal to an amp input.

[wsscott]

Just one more piece of information.

Its Output jack has a 1/4" plug inserted and output voltages are read off of its lugs.  The input voltage is read directly at the input jack where the test signal is injected.

The Tone Generator is injecting a signal at the Input Jack of 155 mv AC.

When the Mod Revibe is not plugged into an external amp, but just using the plug to measure the output voltage, the Output Jack shows an Output signal of 1.55V, with Volume and Output Level turned all the way up.  So that's a 10:1 ratio.

HOWEVER, if I plug it into an external amp, the Output Voltage read at the Tip on the Jack is 0.025V!

Why would that be?  What's reducing the Output voltage?

[sluckey]

What's reducing the Output voltage?

Obviously the external amp or the connecting cable. Need to see a schematic of the amp to be more specific.

[wsscott]

Before I saw your post I changed the cable.  That was it!!

SOLVED.  THE CABLE!

Who knows why or what happened to it, but done.

It also seems like the hum is gone.  Maybe that is a result of my removing the jumper wire on the Output Jack.

Thanks for your help.

[Williamblake]

I am guessing the way the signal travels is galvanically interrupted at some point and is only transmitted over the air. That is why it is loaded down so easily. You can use an alligator clip cable to connect what should be connected and looks connected but is actually not connected to check if it makes any difference.

[wsscott]

I replaced the plugs on the cables and they are working fine now and getting full volume.  I think the hum is improved, but I haven't had time to test it out, so will try to do that over the weekend and let you know if it's improved or not.  The only thing I did to the circuit was to remove the jumper between the Output Jack "switch" lug and its GND lug.

I know the schematic that Sluckey did doesn't show it to be jumped between the 2 lugs, and I guess I did it because the foot switch was jumped.  I was wondering why use a switching jack for the Output Jack if the switch portion isn't being used?  Am I correct that it serves no purpose unless it is jumped to the GND lug, and just acts like an normal jack?

[sluckey]

Am I correct that it serves no purpose unless it is jumped to the GND lug, and just acts like an normal jack?

That's not correct. I can think of three common uses of that jack using the switch connected to something other than ground... Fender HI/LO inputs. Passive FX loop, and combo internal speaker connection such that when plugging in an external speaker cab, the internal speaker is disconnected. I have used all three of those circuits in my builds. I'm sure there are more uses but I don't want to think about this too hard.

[wsscott]

I'm just trying to understand what purpose it serves in this particular case if its not wired to something.

[sluckey]

   How can it serve a purpose if it's not connected to anything?

[wsscott]

I finally figured out a way to deal with the Reverb hum.  In the past, I set the Volume Level at about 10 o'clock, and the Output Level at about 2 o'clock.  This produced unity between the Input and the Output jacks.  The hum increases when the Reverb circuit is turned on, but the hum level is not affected by the Volume Level, just the Output Level.  So I decided to lower the Output Level until the hum is gone and that occurs at about 8 o'clock, and increase the Volume Level until I get the volume I need, and that occurs about 4 o'clock.  This cleaned everything up great.

I don't know if this setting is pushing the output of the Mod Revibe too hard or not, but it seems pretty good to me so far.

[wsscott]

Is that because the Output Level pot is just controlling the gain or output after the signal has gone through the Reverb circuit, but doesn’t control the Volume gain coming out of the preamp?

[wsscott]

SOLVED the problem.  I continued to go back and forth with this issue.  Just bugged me.  It basically was a ground loop, I think. When I wired the circuit I questioned how to wire the connection from the backside lug of the 1M Revibe Pot to the Reverb Foot Switch to ground.  I had wired the lug to the Ground BUSS, and also wired the Foot Switch jack Ground lug to that same point.  I finally realized that created a ground loop.  So I removed the connection from the pot to the BUSS, and then connected the Ground Lug on the Footswitch to a new connection to chassis ground.  So when I hit the switch it makes the connection to ground at only one point.  Seems to have solved the problem.