AMP only plays when I strum really hard.

[dan92y]

I have a fender harvard reverb ii.  The pots were scratchy so I replaced them all.  Now, the amp puts out really quiet tones. If I strum really hard, it gets loud.  It seems to sound better on single notes rather than chords.  Does anyone have any idea what could be bad?  Thanks

[joecool85]

I have a fender harvard reverb ii.  The pots were scratchy so I replaced them all.  Now, the amp puts out really quiet tones. If I strum really hard, it gets loud.  It seems to sound better on single notes rather than chords.  Does anyone have any idea what could be bad?  Thanks

Are you sure you used the right pots when you replaced the old ones? 

[DJPhil]

I'm not sure, but I'll take a wild guess.
Do the replacement pots have the same taper as the old ones? It sounds like what would happen if you swapped a linear pot for a log taper pot.

I'm not sure what your level of expertise is, so I'll toss in a link on pot tapers. My apologies if it's something you've mastered.

Hope that helps.

[J M Fahey]

Do you have the schematic?
If so, post it.
I have a hunch but would like to see it first.
Google it and post a working link or post a scan of yours if you have one.

[DJPhil]

I think I found the schematic.

Poking through it now.

And the parts list.
And the manual.

Looks like it uses:
Reverse audio (anti-log) - gain, treble, and bass
Audio (log) - volume
Linear - master, mid, reverb

I think I should take Teemuk's advice and poke through more schematics. I was looking at the output stage and thought, "Oh wow, one of those Sik. . Sizl . . not Darlington pairs!" (sorry Mr. Sziklai)

[lots of edits as I added stuff]

[J M Fahey]

That "gating" effect you have makes me think that you have a problem worse than wrong taper.
Probably you miswired something when replacing them, shorted some pad to another when soldering or cracked some track.
1) Plug your guitar or a CD/MP3 player straight into the "Power amp input / J103" Do you have good sound or it's still sputtering?
2) Please measure the DC voltage on IC1 Pins 1 and 7, IC3 Pins 3 and 7 .
Thanks.

[dan92y]

I have a fender harvard reverb ii.  The pots were scratchy so I replaced them all.  Now, the amp puts out really quiet tones. If I strum really hard, it gets loud.  It seems to sound better on single notes rather than chords.  Does anyone have any idea what could be bad?  Thanks

Are you sure you used the right pots when you replaced the old ones?  

Pretty sure. I am not an electrician or a TECH but as well, not a complete newbie either.  I spoke with an amp tech from the place I ordered them.  Some were marked 15A-50kohm, 15-C 50kohm and one was just B-50kohm. The guy said the pots I got would satisfy all different markings and that the A could replace the B and C but not the other way around.  Did I get shammed? Not sure.  Other than the letters, they are the same as the old ones.  I checked each one with a meter before installation, no short circuit soldering job, all are tight...getting really frustrated.

**administrator edit - fixed quote code in post**

[J M Fahey]

Well, pots are coded A, B, or C for a reason, they are *not* the same.
Any way, today many suppliers carry only "B" (linear) ones, being the cheapest. :trouble
Yes, if you have *nothing* else, they will replace A and C, sort of.
Better than nothing at all.
Their effect on sound will be crammed towards one end or the other.
Anyway, your problem today is worse than that; any news on those voltage measurements?
Good luck.

[dan92y]

That "gating" effect you have makes me think that you have a problem worse than wrong taper.
Probably you miswired something when replacing them, shorted some pad to another when soldering or cracked some track.
1) Plug your guitar or a CD/MP3 player straight into the "Power amp input / J103" Do you have good sound or it's still sputtering?
2) Please measure the DC voltage on IC1 Pins 1 and 7, IC3 Pins 3 and 7 .
Thanks.

It sounds much better plugged into the power amp input.  Does this mean I am hosed?  I am guessing that this means the problem is in the pre-amp circuit.  This is the part where I may be over my head.   Voltages at both spots you suggested are 0.00  Thanks for the help.

[dan92y]

Well, pots are coded A, B, or C for a reason, they are *not* the same.
Any way, today many suppliers carry only "B" (linear) ones, being the cheapest. :trouble
Yes, if you have *nothing* else, they will replace A and C, sort of.
Better than nothing at all.
Their effect on sound will be crammed towards one end or the other.
Anyway, your problem today is worse than that; any news on those voltage measurements?
Good luck.

The voltages at both spots are 0.00 I am assuming that is bad.

[J M Fahey]

Sorry, I wasn't clear enough.
What I want is DC voltage from each of those pins to ground; that would be 4 values in total.
Anyway, if they are close to zero (say, less than 10 or 20 mV), that's fine, then my suspicion was wrong.
I forgot, did you test the old standardof connecting from pream out to power amp in with a known good guitar cable?
If nothing of this works, you can next  try signal tracing, the Enzo way.
But let's go one step at a time.

[joecool85]

Sounds like it's totally fixable, you just need to go through the motions and diagnose it.

Like Juan just said, you should really try plugging the preamp out into another amp and see if that sounds ok.  Sounds to me like a short somewhere in the preamp though.

[dan92y]

Sorry, I wasn't clear enough.
What I want is DC voltage from each of those pins to ground; that would be 4 values in total.
Anyway, if they are close to zero (say, less than 10 or 20 mV), that's fine, then my suspicion was wrong.
I forgot, did you test the old standardof connecting from pream out to power amp in with a known good guitar cable?
If nothing of this works, you can next  try signal tracing, the Enzo way.
But let's go one step at a time.

All voltages are zero.  I grounded to the 'bus' that seemed to be common.  Pre-amp out to power amp gets a really pretty hum.  Nothing else. 

[joecool85]

All voltages are zero.  I grounded to the 'bus' that seemed to be common.  Pre-amp out to power amp gets a really pretty hum.  Nothing else. 

Is that the preamp out on the broken amp to the power amp in on *another* amp?

[dan92y]

All voltages are zero.  I grounded to the 'bus' that seemed to be common.  Pre-amp out to power amp gets a really pretty hum.  Nothing else. 

Is that the preamp out on the broken amp to the power amp in on *another* amp?

Right.  Broken/pre-amp out to good amp input, good cord.  Good amp hums and no more.