Trace Elliot GP7-SM Running well, but, mute problem...or something else?

[aavatech1]

I've just noticed that this probably should have been posted in the "Maintenance, troubleshooting and repair" Sub-thread. If the mods here deem this to be so I'll understand the moving of this post. Apologies for any inconvenience.

Greetings, I've got a stable, working Trace Elliot GP7-SM (Model 3001) bass head (280W) with the infamous "Bi-polar Bear" "issue 7" output board. Not long ago I performed the updates stated in Paul Mathews 2001 service note and the amp is biased fine with no crossover distortion and has a +/- DC offset of less than 2mV variance.
My problem is that when the amp is turned off it makes this sort of "Thweert" sound through the speaker. And I'm sure the speaker does NOT appreciate it. I've included a scope-meter "trend" display of what happens at power up and power down at the output jacks. I should state that I've replaced the G5V-2 relay, the C1 22uF/25V cap and the D9 16V zener diode in the relay timer circuit. The R1/R5/R18 resistors are spot on. The ground of the issue 7 amp board is also benefiting from the recommended soldered eyelet at the ground/screw hole of the board and wired directly to the chassis ground. So what happens at power up? As you can see on the scope image included at power up there is about a 200mV positive swing which produces a barely audible sound through the speaker, as it should be, and the relay clicks in a fraction of a second later. Great. But at power down (the second DC spikes in the image) there is an almost -600mV spike proceeding an even larger +1.5V spike. The cause of the "Thweert" through the speaker I'd assume. Once the amp is on there is no hum or distortion and the amp is stable and plays loud and clear. The circuit description provided in the SM explains the muting function at power up and I'd assume that it'd be presumptuous to assume that the opposite is true at power down. So I was hoping that someone could shed light on what should be happening at power down and what might be happening here, where I your friend and comrade, am being assaulted by the evil "Thweerts at power down.
Ive included a copy of the correct SM for the "Issue 7" "Bi-polar Bear" board (starting on pg.10) that is present in this amp as well as the image previously mentioned.
I'm at a loss here and not sure where to check next. Any assistance would be most appreciated. Thanks much
Mark>

NOTE: I just realized that the "test" speaker cabinet I've been using measures 16 ohms (it's a guitar cab originally) and the Trace sports 2 - 8 ohm parallel output jacks. It may not matter in this case, but it indeed [read, probably] may matter. Either way I'll be checking this problem out again tomorrow with a proper 8 ohm cab and updating this post.

[Jazz P Bass]

I would check the high voltage dc rails.
Check the Vac ripple.
If a supply capacitor is going bad it may cause the issue.

[aavatech1]

I would check the high voltage dc rails. Check the Vac ripple. If a supply capacitor is going bad it may cause the issue.

Thanks J.P.b, considering the main filter caps are circa. 1996, I thought that replacing them as well might be a good idea. I'll do so since I've got those on hand.
Thanks for the input. M>

[aavatech1]

I would check the high voltage dc rails. Check the Vac ripple. If a supply capacitor is going bad it may cause the issue.

So I replaced the two 4700uf - 80VDC filter caps and did the remainder of the electrolytics while in there and the spike remains. The main rail voltages increased from ~ +/- 65V to +/- 70V and the spike at power down increased a slight bit as well. So it wasn't the caps. I replaced 1N4007 Diodes 1,2,5,7-10,15 and 51V Zeners D3,D4 in the Mute and Swing limiting portions of the circuit and the spike remains. I also replaced the two 2N3904 matched transistors in the mute timing circuit. The result is the same. Generally I don't like heading toward "Shotgun" mode, especially when there's the possibility of learning something from those more capable of understanding circuitry than myself. So I'm just chalking up the replacements to "scheduled maintenance" while still hoping for help. The amp is still stable and working just fine but the power down spike still remains.

[Enzo]

Just thinking here, but the output is bound by the power supply voltages, your 70v. When you turn it off that voltage drops, and at some point the amp gets unstable for a moment. However, during that period, the output is still bound by whatever is on the power rails. And to me this means your thweep is never any louder than the peak output from a slap on a bass string. SO if your speaker can stand the loudest it can play, that noise won't hurt it.

But you want it gone so... First that mute circuit is labelled as a power ON mute, it has no effect on power off. And while I don't know what the pair of 2N3904 would be matched for, they are not in a balanced circuit, they are just wired in darlington - one drives the other for more gain. In my world I think I might have used a single MPSA13 or something. In any case, in that circuit, C1 is the timing cap. V+, your +70, charges C1 through 470k R18. Once that charges up to a volt or so, it turns on the two transistors, which in turn energizes the relay. So the cap makes the relay pause a moment, allowing the amp to power up[ and stabilize. You tell me how long it takes, a couple seconds I'd guess.

The rest position of the relay is drawn on the schematic. The bases of the drivers are shunted to ground through pairs of diodes. Two diodes is about the same as two junction drops. (The driver and output makes two) Personally I think a speaker relay would be more effective, but this ought to work. But note it repeats that it is a turn on mute, not a turn off mute. Relay energizes and the contacts transfer and the shunt is opened.

And when you power off, it takes a moment for V+ to discharge, also C1 in the timing circuit takes a moment to discharge. Those things mean the power on mute relay does not drop out when the power switch turns off. So none of those things are involved in your noise. Sorry you replaced them all.

Note the swing limit is there to guard the drivers again voltage from the opposite polarity. See D10 is reverse biased for positive voltage. It only conducts when the signal drops below zero. Once we go to negative on the base of TR13, only then will D10 conduct, and the 51v Z3 only conducts after it sees 51v. SO in other words, that pair prevents the signal from going further than 52v negative at that point. Same deal below at the opposite polarity.

You do not have a turn off mute. You could engineer one in, but I don't see your noise as a defect. MAybe not all get unstable in the exact same way, so maybe not all of this model make the noise. If I had to know I might slowly dial down the amp on a variac and see when it goes nuts. I think I would also verify that the power amp is the source of the noise. it comes out the speaker, but do we know it isn't a preamp noise being amplified? Looks like a shorting plug into the Line Out jack would short across the preamp output. DO that, does it still noise at power down that way?

Since power supplies drain over time, a power off mute needs to work instantly. Using a speaker relay, we many times find a separate power supply for it. Instead of using one of the main supplies, there will be an AC line from the power transformer - tap right as it goes to the bridge. That can be rectified and slightly filtered, or use an AC relay. SO when power switch goes off, that relay drops out instantly, opening the lead to the speaker.

[Jazz P Bass]

Try pulling the power cord directly from the wall with the unit on.

If that mitigates the 'thweep' then you may have a power switch type issue.
I have seen capacitors across the contacts to help with switch arc.

[aavatech1]

Enzo: Thanks very much for taking the time to offer this explanation. I appreciate it greatly. Read, yes. Digestion will take a little while longer but I'll be implementing your suggestions shortly. I definitely need to disqualify the preamp as well. I think my focus on the relay stemmed from the SM statement " Transistors TR1 and TR8 in conjunction with R18 and C1 set the timing constant for the power on / off muting. I also appreciate your both specific and overall take on this amp design. There's a lot I can get from this and I do appreciate your input. More to follow a little later...
Thanks M->

[aavatech1]

Try pulling the power cord directly from the wall with the unit on. If that mitigates the 'thweep' then you may have a power switch type issue. I have seen capacitors across the contacts to help with switch arc.

Thanks JPB, I'll do just that.

[aavatech1]

OK, so I made a shorting plug for the pre-amp line out jack and with the line out jack shorted i get no "Thwerrp"! I also noticed, much to my chagrin, that with the Output Mute switch enabled there is also no spike (heading) to the amp section. (.."Hey kiddies, can you say Blinders? I knew that you could") So now the only question is why. I noticed that there are 8 Tantalum caps on the pre-amp board and those I'd consider a visual "red flag". I replaced the 4 electrolytic caps on the pre board while i had it out. The Tants are still in there. There is only one active device on the pre-amp board other than the IC chips, that being the BD677 Darlington NPN. At the BD677 I've got: (E)+33.7V (C)+50.7V (B)+34.9V [Note: on my "GP7sm issue 2" pre-amp board there are no C27 and C29 installed in the "VE+ & Gnd/OV supply from power board" portion of the circuit [Factory]. After the L1 (LED) I've got +31.9V at the first contact of Mute switch and R8 at the start of the input circuit.
Also, with the Mute switch disengaged, the position of the other switches on the pre-amp board have no effect on the spike at power down.
I'll offer this now since I need to think a bit right now and I have trouble with loosing my posts when I stop for a while. But still looking forward to further input from those interested in helping. Is it safe to say this seems to be an issue stemming from the pre-amp board?
Thanks again Enzo and JPB!

[aavatech1]

Ola! So,...I tried the suggestion from JPB of pulling the plug from the wall, but no go. Spike still there. Today's observations include: Correcting a mistake I made in the earlier post of March 4, 4:22 PM, the are actually two active devices other than the three quad op-amp IC's on the pre-amp board. I had stated that there is only one (..the BD677) but there is also a J112 JFET in the input circuit. It's voltages are: (G) +3.4V (S) +20.00V (D) +23.5V. I disconnected the pre-amp board from the power amp board and the voltage spike at power down doesn't happen. The connection of the pre-amp board to the power amp board is at the PL1 connector, where there is: V+, Signal and 0 VDC. Powered up the connections read: (V+) +68.5V (0V) .1mV (Sig) +5.1mV. Monitoring these junctions at power down I find that the V+ drops steadily 0V. The 0 VDC connection remains at 0 VDC and the Signal connection "sharply" dips approx -250 mV before returning to toward the +5.1mV point. Id guess that the spike at the main amp's output is an artifact of this voltage dip "from the pre-amp"? I've been checking all the resistors to ground, jack solder joints and connecting cable solder joints as well but all are fine so far. I've ordered up some new tantalum caps for the pre-amp and those should be here Tuesday. As always, any and all advice is much appreciated.

[aavatech1]

Update: Tantalum caps came in and all originals on the pre-amp replaced. Spike at output at power down still exists. I disconnected the 7 band EQ board from the main pre-amp board and the power down spike still exists. As long as the pre-amp board is connected to the power amp board I'm getting the spike. I'm not sure where to go from here. Any input is appreciated.
M>