Need Some Insight Into Troubleshooting an AB763 Tremolo (?) Problem

[cjmusial]

I have recently completed a Hoffman AB763 retrofit and have run into an issue that has me scratching my head.

I have installed the 47K resistor to ground that feeds the 220K bias splitter on the vibrato channel.    Per the layout, the second 220K bias splitter connects to the 0.47 coupling cap on the normal channel.   With the 47k resistor in place, the volume of the amp is substantially reduced on both channels.   A volume setting at 10 sounds like what I would normally expect with the amp set on 1.   In this configuration, the tremolo circuit sounds great: the modulation is smooth and works across the entire range of speed settings and levels.  Reverb working as well. 

When the 47K attachment to ground is lifted, the amp volume becomes much closer to "normal".   The difference is dramatic, but likewise troubling.   The trem circuit still "works", but there is a lot more harmonic distortion at lower speed settings.  It doesn't sound nearly as good as when the 47K is in place.

All the voltages look good.  So it would seem I'm looking at a component or wiring related issue.

Given the above, I would really appreciate any insight that might narrow where to focus the troubleshooting.  I have access to a signal generator and scope.

[Tone Junkie]

Hopefully Sluckey will see this he helped me with mine and has alot of knowledge in this area.Bill

[sluckey]

220K bias splitter

Those resistors are called isolation or mixing resistors. They are used to join the normal and vib channels together to feed a single input to the PI.

It's normal for that 47K resistor to decrease the gain (signal level) when installed. But if the amp is wired correctly, it has nothing to do with the tremolo circuit.

A volume setting at 10 sounds like what I would normally expect with the amp set on 1.

That's not normal. Are you sure it's a 47K and not a 4.7K or 470Ω?

You have stated two symptoms that should not be related. However, that 47K resistor sits next to the tremolo circuit components and a wiring error that affects both is possible. I'd troubleshoot the low volume issue first. I would look for a wiring error or incorrect component value.

If you can post some hi-rez pics, some of the sharp eyes here may be able to spot something you are overlooking.

And in case you don't have one, here's a schematic for the Hoffman AB763...
http://home.comcast.net/~seluckey/amps/misc/Hoffman_AB763.pdf

[cjmusial]

Ok .. I poked around a little more.   Sorry for the confusion regarding two elements.   Lets table the trem issues for now and get the basics of the amp functional.     

For background info.   This chassis setup has a DRRI PT and Mercury OT with a 5u4 and 5881s @ ~380V.   The circuit board is pretty much stock minus a few changes on the trem oscillator for gain.

On the signal loss issue.   The 47K value is spot on.  I have measured the signal levels from the normal and the reverb channels going into their respective 220K mixers.  The input was a 100 Hz tone @ ~100mv amplitude at the input jacks.    Both channels were very similar.   Saw about 2V at the mixers with the volumes set just past 2.

When I disconnect the reverb channel from its 220K mixer, the normal channel starts working just fine.  The voltages look reasonable at the phase inverter plates: ~210 and 200.   Signals into each grid seem Ok.

When the reverb channel input to its 220K mixer is connected, I see some bad stuff at the PI.   The test signal feeding the mixer still looks Ok (the ~2V with volume on 2 or so).   However, the voltage drop across the 82K plate resistor of the phase inverter goes to almost nothing ... so that plate sees > 350V DC.  The 100K side shows about 150VDC at its plate.   Little volume output.

So this is what I know so far: the signals feeding the mixers from either channel look Ok.   But the act of connecting the reverb channel seems to send the PI into a tail spin.   I can get rid of that bad stuff when I take the 47K attachment to ground on the reverb channel.

So I'm still fooled here.   I have checked components across the board and they seem Ok.   The hookup looks good as well.   I'll try to find time to post pics.   Could this be a lead dress issue ?  Any other thoughts or suggestions ?  Thanks for the help so far.  I really appreciate it.

[sluckey]

Well, that's certainly an odd set of symptoms. Let's be sure that the NFB loop is not a factor. Temporarily disable the NFB by disconnecting the wire from the OT secondary to the 820Ω resistor. Does that help? PI voltages get happy?

[cjmusial]

Sluckey:

I wondered about the NFB as well, so with a second vote, I lifted the connection to the 820 ohm resistor.   PI voltages still out of wack unless I lift that 47K connection to ground.    Very very strange ... I think its time for a beer break.

[sluckey]

There almost has to be a wiring error.

[Raybob]

And in case you don't have one, here's a schematic for the Hoffman AB763...
http://home.comcast.net/~seluckey/amps/misc/Hoffman_AB763.pdf

Thank you for posting the schematic.  I notice the notes saying to change a resistor or a cap for 'stronger' tremolo.  I'm wondering which is the better option or should both be done for a deeper sounding tremolo.

Thanks,
Raybob

[cjmusial]

I've taken a second look at the wiring for just about every connection on the board and the under-board connections as well ... it all seems to match the schematic.   Obviously I'm clearly missing something or else she'd work to spec.  

I was hoping the behaviors might narrow the troubleshooting to a specific area of the circuit or that there might be a possible explanation for how one might observe those sort of asymmetric PI voltages in the PI.    Its frustrating to see the right signal voltages at the PI inputs from the preamp, yet the PI goes into a tail spin when I ground that 47k.   I find no fault in the PI wiring or power amp section, and, as mentioned, the normal channel works fine when I disconnect the reverb input to the 220k mixer.

The first beer break didn't help.  Time for the second