Trident Series 65 troubleshooting puzzler...

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leigh

Well-known member
Joined
Jun 4, 2004
Messages
394
Location
Portland, OR
I'm stumped! I've done a fair amount of troubleshooting on my S65 in the past, but this is a weird one...

The setup: A Trident Series 65 (32x8) that all channels but one are working fine on.

On the problem channel: it's not passing signal to the Right remix buss, or to any of the even-numbered groups (groups 2, 4, 6, 8).

So that pretty much rules out a pin connection issue, since it would have to coincidentally be an issue with "every other" pin in the bottom strip of pins. I cleaned the pins anyways, but it didn't fix this.

My next thought was naturally the panpot. Checking the schematic (attached, see bottom right area), we see that is where the mono signal through the channel gets split into Left and Right sides. From there the signal passes through the solo switch, then to the group assign switches, then to the summing resistors, and then out the card. Nothing else going on there!

I cleaned the panpot and the switches. No go.

With an ohm meter, I then checked the resistances from the "top" of the panpot to each pin output. They are nearly identical. So it seems I have a continuous path for the signal out each side (L & R). If so, then where is the signal being lost??

One other bit of info I forgot to mention: as I turn the panpot to the right, at about the 3 o'clock point the signal through the Left output starts sounding like a square wave. My test tone is sine wave. (So that would suggest that some stage is getting fuzzed out?) When the panpot is all the way to the right, I can still hear the fuzzed out tone, but since I'm only hearing the left side of things, it is very faint (probably just leakage).

Leigh
 

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That's a great question, and I should have mentioned that: it has the same issue when the solo button is pressed.

As you may know (or can tell from that schematic), this board has a "solo in place" feature, so when soloed, the panning should work. But, just like when sending to the Remix buss or the Groups, there's no signal on the Right side of the Solo buss. The "fuzzing" that I described, once the panpot is at about the 3 o'clock point, also happens when soloed.

(Though I should clarify, there's nothing magic about the 3 o'clock point of the panpot - that's level dependent. My test tone is computer generated, at -12 dB, sent out a +4 dBu converter output, to the board with line trim and fader at both 0. If I increase the gain, it happens sooner.)

Leigh
 
OK, this is odd: a spontaneous fix just happened.

I was double-checking my last statement, about the fuzz happening sooner when the channel trim was turned up. I maxed the channel trim, and then noticed that the board was passing signal on the right side again.

I've seen loud signals "wake up" a problem transistor before, but in this circuit, once a channel's signal is split to left and right, the signal path is all passive until it leaves the card. So I'm trying to figure how a loud signal might have "woken up" anything in the right side, and I'm stumped.

Leigh

 
pucho812 said:
possible cold joint, failing lytic cap?

Cold joint would be my best guess. That part of the circuit is all resistors and switches - there's not even a cap to fail!

Thing is, I checked continuity with an ohmmeter, and was careful to check from the pan pot's lug, not from the solder blob on the circuit trace side. So if there was a cold joint there, that should have caught it - but continuity measured fine.

I suppose if it's a real borderline case, that the voltage put out by the ohmmeter was great enough to jump a cold joint, but the signal voltage (until I cranked it up) was not?

It's still working now. We'll see tomorrow morning, when the board has cooled down overnight again.

Thanks for the suggestions.

Leigh
 
I have a 16 channel 4 buss version  trident 65 sidecar. fwiw you can swap all the tl071's out for opa134's and the sound really improves. It gives a more open top end with more clear mids and bass. You should also up the power supply amperage meaning put in an acopian or something. that  Trident PSU was often a big weak point on tridents. I also  swapped out capacitor c52 to change the high frequency shelf point. Lowering the value will increase the frequency. I tried a few different values before I settled on a value that I happen to really like.



also have you tried moving the module to another slot to see if it follows the module?

are you certain that the switches do not need a cleaning or replacement.  maybe you need a new 2 mix switch on that module.
 
hey Pucho,

Thanks for the upgrade tips. I am fortunate in that this board was already worked over when I picked it up a couple years back. Jim Williams had done 4 channels, swapping out the TL071's for LT1357's, bypassing many of the signal path electrolytic caps, and upgrading the EQ caps. Jim had also done similar work to the master strip. The previous owner had then done the rest of the channels with LME4970 swaps, and the same cap bypasses Jim had done. PLUS he had assembled a mighty pile of Acopian power, 15 amps per side!

Aside from some cleaning, all I've had to do so far is put in some LEDs to light up the master meters and the solo indicator. I am planning to pull those auto mute CMOS switches from each channel, and drop in a dummy replacement (by inserting a DIP "component header" that has been properly jumpered). Also the groups and aux sends haven't had their chips swapped out, so of anything, that's next. Other speculative upgrades are planned as well, I just haven't had the time.

Anyhoo, getting back to this particular issue...

pucho812 said:
also have you tried moving the module to another slot to see if it follows the module?

are you certain that the switches do not need a cleaning or replacement.  maybe you need a new 2 mix switch on that module.

Once I got the right side to pass signal yesterday (by briefly cranking up the channel trim), it stayed working. However, this morning it's cold again, and if the problem returns, I will indeed swap the module to another slot, to see if the problem follows it.

It's not the switches, far as I can tell. It would have to have been all the assignment switches (the 2-mix, the 4 group switches, and the solo switch) that were faulty. And then all of them started working again at the same moment yesterday.

Thanks again for the ideas, and I'll report any further insight and fixes here.

Leigh
 
The Right side dropout is back in the problem channel today - but I've figured it out more. If I tap on the channel strip near the panpot, it starts passing signal. If I tap on the channel strip all the way towards the bottom, it cuts out (and stays cut out). I can't visually see any bad solder joints or broken circuit traces, but I'll reflow the solder at the panpot lugs and see if that solves it.

I'll be happy to write up the changes done to the master channel, later today maybe. I should mention too that on the channels modded by Jim Williams, the old faders (Alps) were swapped in with new ones (Sellmark). I don't have documentation written up by Jim about what he did, but since the Sellmark faders appear on only the 4 channels he modded, it's reasonable to assume that they were his work as well.

Leigh
 
Reflowed the solder around the panpot, and a few other "right side" connections, but no dice yet. Also tried putting the trouble channel in a different slot, and it has the same behavior. So the problem is definitely onboard the channel strip.

Might try jumpering the long circuit trace between right panpot and the switches. It takes so little pressure on the face of the channel to make the signal cut out, I'm thinking it might be a hairline break in the circuit trace.

Arg...

Leigh
 
Could be a loose connection where the pan pot track is riveted to one of its legs. Hence why testing the circuit for continuity from the leg to the switch showed as OK.
I've had this before......
 
Walrus said:
Could be a loose connection where the pan pot track is riveted to one of its legs. Hence why testing the circuit for continuity from the leg to the switch showed as OK.
I've had this before......

Good idea - but that doesn't seem to be it either. I jumpered the "top" panpot lug to the wiper lug, thereby effectively taking the panpot out of the circuit, and it still is a flaky connection.

Still trying...
 
Ah, I found it, or at least, most of it... see the attached photo. Back of the fader's shield would get pressed into those lugs coming off the back of the buss assign switches, grounding out the right hand side (only because a lug connecting to the right hand side happened to be sticking up the most). This isn't an issue stock faders would have, the stock Alps don't have a shield there. So I put two layers of electrical tape in there, to prevent those lugs from grounding out.

The reason I couldn't measure this when I had the module pulled is that the shield only gets pressed into those lugs when the channel is inserted into the board (I guess the card is a bit off tolerance in its positioning).

But the reason I say that was "most of it" is that, while the right side no longer dies completely, when I put pressure on it, the right side is dipping by about 2 dB, and that fuzzy tone I described before is heard out the board's output (the sine wave test tone sounds like it's getting squared). And this varies depending on the position of the panpot... so I might wind up replacing that anyways.

Leigh

 

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Turns out I do have a spare panpot around. Have you guys swapped out a panpot on one of these board before? Do I have to detach the circuit board from the faceplate, or can I just squeeze stuff in there without going to those lengths?
 
I think your pan pot is fine. you should be able to measure the pot with an ohm meter. Compare it to another pot and you will find it to be close. I would not be changing out the pot unless it was doing the above symptoms without applying pressure, leaning on the module. you've got the answer in front of you in the fact when you apply pressure to the module it drops in volume and sounds fuzzy. The band aid is to not lean on the module. The fix is to find where exactly your grounding out when you lean on it. Obviously the fader was a big part to it all, now just look at the rest./ I must admit the 65 strip leave a lot to be desired structurally...
 
pucho812 said:
The fix is to find where exactly your grounding out when you lean on it. Obviously the fader was a big part to it all, now just look at the rest./ I must admit the 65 strip leave a lot to be desired structurally...

Well, to see if this fuzzy sound was being caused by the panpot, I jumpered the top lug of the panpot again to the wiper, for the right side (same as what I did a few troubleshooting steps ago). And this got rid of the fuzzy test tone sound on the right side - so odd as it seems, I seem to have a messed-up panpot that makes test tones fuzzy.

I know pots go bad, but this panpot is fairly new, replaced a few years back when Jim Williams did the mods to it.

I'll take another look at it tomorrow morning, and verify that this jumper also prevents the ~2dB drop in level.

Aside from the panpot, there are a pair of resistors shown (R95 and R96) dropping off each panpot wiper, connecting to ground. If those (well, specifically R95, for the right side) had cold solder joints, that could affect level as well. But since a cold solder joint there would lift a connection to ground, it would raise the signal level, rather than lower it.

Still, it made me curious what the value of those resistors are, and in the Trident S65 manual, the component schedule lists them as "A.O.T.". Adjust-on-test, to even out the level of the left and right sides. And looking at a few different modules, they are indeed of different values. On this module, for example, they are 3k3 and 4k7.

OK, I'm rambling at this point, will look at this issue again tomorrow with a clear head.

Leigh
 
OK, I checked again today, and with the panpot jumpered on the right side, that eliminates both the ~2dB drop in level, and the fuzzy tones.

So, out with the panpot, in with a new one (after one more attempt to just clean the connections - I already re-flowed all the solder, but I'll pull it all off this time, scrub the lugs with contact cleaner, and try a re-solder).

I can't tell yet if I can squeeze a new panpot in there without dismantling circuit board from faceplate. If I can't, there's 14 nuts that need to come off the faceplate for the two to be separated...

And, I found an idea of what was causing the fuzzy test tone tones in the first place. With this jumper in there (again, from the top of the right pot to the wiper) if I turn the panpot left, fuzz starts creeping in again around the halfway point (9 o'clock, this time). If I go all the way left, the signal gets very quiet and thin. Why? Take a look at the schematic. I'm jumpering the top of the right pot to its wiper - but when the panpot is turned all the way left, the right wiper is sitting at ground.

So, in short, with jumper in place and panpot all the way left, I'm asking the preceeding gain stage (IC 11, the fader buffer amp) to drive a 0 ohm load. And it's not happy about that!

In my first post, I wrote "as I turn the panpot to the right, at about the 3 o'clock point the signal through the Left output starts sounding like a square wave" - which now makes sense, since the right side was being shorted out by the fader shield. So I was likewise then asking that IC to drive a 0 ohm load.

Leigh
 
i think your going to find the easiest way to get the pot in and out is to remove the 14 nuts to get the faceplate off. it may take longer but  squeezing it in may take longer. Sometimes the long way around is the only way.
 
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