No percussion decay

[johnny b3]

I seem to have made a mistake re-wiring my AO-28 after a re-cap job.
First, turning on the percussion effect causes a thump in the speaker and terminal K falls to 0 vdc.
Percussion tones are triggered but do not fade.

The cutoff pot simply acts as a sort of volume adjustment, the voltage on the pot ranges from 33 to 39V. When bringing the voltage back up on the pot swiftly the voltage is delayed on its climb as if its waiting for a capacitor to charge.

In this version of this amp terminal K passes through R51/4.7K to both pins 5 and 6 (screen and plate?) of V6, but the signal path ends there. The schematic says that terminal M should connect to this point via R55/82K, but instead, R55 connects M to pin 1 of V6. This is the only discrepancy I can find between the schematic and the amp in front of me. A different schematic shows K > R51 > pin 1,6 > R55 > M, R56... not sure whats up here. I don't recall changing up the wiring in the amp like this...

Thanks!

[Tubejack]

This is the first time I've studied the schematic for the B3, so bear with me. The circuit is a little different than the M100 series, but worse than that, the JPG I'm looking at doesn't show exactly where L, M, and N (and two other wires with no letters) go, only that they connect to the percussion switch. That aside, one thing you said caught my attention, namely that the percussion note never fades away. The first thing I would do in that case is to check for the presence of +37V at the cathodes of V7 (percussion amplifier). If that voltage is missing or low, V7 will never cut off.

If this isn't the problem, let me know. I'll look for a better schematic and try to walk through the circuit with you.

[johnny b3]

The voltage on the V7 cathodes is 37V to 41V depending on the extreme end-position of the cutoff trim pot. Like I said, it climbs back up with a slight delay.

http://www.bentonelectronics.com/ima...percussion.gif

http://captain-foldback.com/Hammond_...ao28_under.gif

thanks to geo and lars for keeping these things hosted!

Notice how the wiring diagram shows K terminating through R51 to V6 pin 1/6, with no connection to M or the bias voltage!

[Tubejack]

Your readings at V7 seem normal to me. Remove V4 (the 6C4 that has its grid and plate connected together) and see if the percussion sounds are still audible when you press a key. If they are, try turning the cutoff pot again to see if you can silence them. Let me know the results.

Also, it would help if you could clarify two things. First, when you said that "percussion tones are triggered but do not fade," did you you mean that they are triggered only when you press a key? I know that seems obvious, but I have to ask. Also, when you said "The cutoff pot simply acts as a sort of volume adjustment, the voltage on the pot ranges from 33 to 39V," what specific point in the circuit where you measuring? I'm confused about this because your last post implies the measurement was at the cathode of V7, not the pot, even though the voltages you noted on the cathodes aren't the same as those in your first post. In any event, I may need to know what's happening at the wiper of the pot (center terminal) when you turn it from one extreme to the other.

[johnny b3]

By V4 I think you mean V6. Percussion tones are triggered properly when playing a key. I swear every time I take a reading I get different values.

Pulling V6 still yields percussion tones. The thump from turning on the percussion goes away. The cutoff pot has started behaving differently - yielding a full perc volume at either end of its swing, but giving a drop in volume (not completely gone) at about 1/4 through its CW swing.

Re-seating V6 yielded no difference in cutoff pot effect - the volume is the same at both extremes of the pot position with a slight dip 1/4 into the CW swing. The thump in the perc on/off switch comes back.

Voltages are as follows: at the wiper I have a low of 0V at the ground side and a high of 35vdc at the top side. At the top terminal I have 35vdc or 32vdc depending on the pot's extreme position. The V7 cathodes are 40.5vdc to 37.5vdc depending on the pot's extreme positions. The lower values are of course when the pot is full CW to ground.

[indianajo]

Are you using a DVM (averages over 4 seconds) or a VOM or VTVM (can follow pulses down to 0.1 second) or a scope (can show pulses as short as .1 microsecond)? For a signal that has about a 1/2 second life, I recommend one of the last three.

[Tubejack]

By V4 I think you mean V6. Percussion tones are triggered properly when playing a key.

Yes, sorry, I meant V6.

Pulling V6 still yields percussion tones. The thump from turning on the percussion goes away. The cutoff pot has started behaving differently - yielding a full perc volume at either end of its swing, but giving a drop in volume (not completely gone) at about 1/4 through its CW swing.

That's not what it should do. With control tube V6 removed, the grids of V7 should become more negative. Somewhere in the rotation of the cutoff pot, as you move its wiper toward ground, V7 should stop conducting and stop amplifying the percussion tones.

Voltages are as follows: at the wiper I have a low of 0V at the ground side and a high of 35vdc at the top side.

OK, the pot is working properly. Based on these results, I think there are three possibilities. Not in any particular order, the first would be that V7 is defective. Second, C31 may be leaking, applying a positive voltage to V7's grids through transformer T5 and keeping V7 turned on. Finally, if pin "L" has been inadvertantly wired to the wrong place, that could keep V7 from cutting off when it's supposed to.

I'm suspicious of that last possibility - pin "L" - because of something you said earlier. Pin "K" (the outgoing side of R51 labeled "to percussion switch") should not go to zero volts when you turn on percussion. It should only go to zero when a) you turn on percussion and b) you press a key. If it were me, I would check the wiring of those two pins before continuing.

Jack

[johnny b3]

Thanks for the assistance thus far. I've swapped the tubes and c31 is new, I changed it up anyway and the problem persists. I've traced the signal path several times, terminal L looks good, through R57 to V6 pin 7. With the amp off I still read terminal K as being shorted to ground...maybe its a dendrite problem in the percussion switch housing? Even so, isn't the shorting of terminal K the entire basis of stopping the percussion circuit? That should be stopping the percussion signal all together.


Would it be helpful to disconnect any of the percussion switch wires? Removing K and re-testing could help locate the location of the ground short (switch or amp?).

[indianajo]

Would it be helpful to disconnect any of the percussion switch wires? Removing K and re-testing could help locate the location of the ground short (switch or amp?).

Uh, yes. Is the problem inside or outside the amp chassis? removing the wire would answer that question. Switches do short internally, though not usually from dendrites from zinc in non-hammond quality oil. Also, the wiring harness can get shorted.

[Tubejack]

Even so, isn't the shorting of terminal K the entire basis of stopping the percussion circuit? That should be stopping the percussion signal all together.

That's correct, "K" being grounded should not cause this problem. The percussion notes should also be silenced when V6 is removed. You may have a second problem regarding "K", but that will have to be checked after the percussion cutoff is fixed.

The circuit is acting as though something is holding the grids of V7 positive. Here's the next thing I would do: Remove V6 again, then turn the cutoff pot so its wiper is at ground. Be sure the wiper is actually reading zero volts DC (within a volt or so). Next measure the voltage on pin 7 of the empty V6 socket. If it's positive more than a few volts, disconnect "L" and measure again. I'm hoping this will point to the source of the problem.

[johnny b3]

Removing the blue wire at terminal K shows that the wire (or switch) is shorting to the percussion input terminal (H) ground shield. With the blue wire removed, terminal K reads 30vdc. When I reconnect the wire and remove the ground terminal from the preamp chassis the voltage at K reads 3vdc (as opposed to zero when the chassis ground is connected).

Removing V6, with cutoff pot wiper to ground (0 vdc), V6 pin 7 reads 0v.

[geoelectro]

Did you try flashing the wire from K terminal?

Geo

[Tubejack]

Removing the blue wire at terminal K shows that the wire (or switch) is shorting to the percussion input terminal (H) ground shield. With the blue wire removed, terminal K reads 30vdc. When I reconnect the wire and remove the ground terminal from the preamp chassis the voltage at K reads 3vdc (as opposed to zero when the chassis ground is connected).

I may not be able to help you with that. The "K" wire goes many places in the organ, so it's not like analysing the circuit around a tube and pointing out likely culprits. Are you absolutely sure you have the correct wire attached to "K"?

Removing V6, with cutoff pot wiper to ground (0 vdc), V6 pin 7 reads 0v.

V6 pin 7 goes to the grids of V7 through T5. If the grids of V7 are 0V and the cathodes are ~ +40V (the value you measured earlier), V7 is cut off. I don't see how it's possible to amplify percussion tones thorugh V7 like this. I guess you could pull V7, just to be sure that what you're hearing is actually coming through the percussion amplifier. Not sure what else to recommend at this point...

[johnny b3]

Flashed the K wire (not terminal) with no luck - still shorting to ground.
Pulled V7, still hearing percussion tones. The harmonic selector and percussion volume switches work fine.

[Tubejack]

Pulled V7, still hearing percussion tones.

You have a wiring error. The input to the percussion amplfier is at "H". That wire carries the percussion tones from the percussion switch. I suspect it's been connected somewhere else and is bypassing the percussion amplfier somehow.