Alembic Guitars Club
Alembic products => Owning an Alembic => Troubleshooting => Topic started by: Thomasio on August 19, 2022, 09:42:33 AM
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Hello -
I have an Alembic F-1X preamplifier, and the direct out (XLR) on the front of the panel is not outputting any signal.
The preamp is in new condition, and I took the top off to have a look. All of the plus are in place, and the vacuum tube appears to be soldered in place (as opposed to being pressed into a connector). Is that how these came from the factory?
Any thoughts on what may be wrong, or how I could try to troubleshoot this? Would Alembic do a repair if I send it to the factory?
THANKS for any advice.
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I think it should be in a tube socket. You can plug it in with the top off and see if it glows.
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The tube is in a tube socket, but appears to be soldered in to the tube socket as opposed to just pressed in. I'm wondering if this is something the factory did. (Or, it is possible I am misunderstanding what I am looking at... but normally it is easy to remove a tube, this one does not budge.)
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If it is the same as mine i had problems getting the tube out until i found that the sleeve is in two parts with the top half needing to be slid down to expose the top of the tube. Once I did that the tube pulled out easy. phew.
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That may be what I am seeing. I'll take another look, thank you.
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Thank you for the suggestion. The metal cylinder slides back which releases the vacuum tube.
I verified that the vacuum tube is fully seated. No change in the direct output (still nothing).
Everything else functions great and sounds fine, just no output at the XLR.
Any ideas?
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Hey Thomas,
The first thing to do is exercise that "Pre" and "Post" switch near the XLR about 50 times. It's self-cleaning and might need to be - woken up. The next thing to try is exercising the Effects Loop "Return" jack on the rear panel = plug and un-plug a few times. Sending a signal into that "Return" jack and having the front panel switch set to "Pre" should work like a simple passive DI box. You could even try connecting a 1/4" cable from the Effects Loop "out" to the "return" and see if that does anything. Just some ideas.
Jimmy J
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Might be some dirty contacts on the jacks (inside the box). Some have sets of 'points' like an old time car. I will also use a 9mm gun cleaning brass brush on the jacks from the outside just to make sure where they contact the guitar cables is clean.
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Great suggestions, I'll give those a try tonight.
ThAnK yOu!
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Update - I exercised the switches, no change. Also said the effects loop cables in and out several times, no change. Still zero output from the 3 pin direct out.
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Does the tube look normal? Do the heaters glow evenly? Do you have another tube to try out?
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I'll try swapping the tube next.
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Not sure if this will help you but here is the schematic for the F1-X. The forum I found it on said it was drawn by Ron W but no way for me to confirm.
I know nothing about electronics but if everything else works as expected I doubt that the tube has anything directly to do with the output from the D.I. Maybe that can be determined from the schematic?
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... Everything else functions great and sounds fine, just no output at the XLR.
So there's no sense in changing the tubes or pursuing anything else deeper into the circuit if all is working EXCEPT the XLR out. This almost has to be something you can see.
The diagram (which does look like could be Ron's handwriting) shows "point A" on the return jack of the Effects Loop. The other side of "point A" is the Pre/Post switch in the "pre" position. So with the switch in that position the signal comes straight from the "return" jack, into the transformer and out of the transformer to the XLR. There is very little that can go wrong with that simple circuit path. If you've got the lid off, visually inspect the wire from the "return" jack to the Pre/Post switch, the wire from that switch to the transformer, the other transformer leads which connect to ground and the XLR, and that's all that is involved.
Tell us, what are you connecting that XLR to which is not showing any input? The input of a mixing board or an external mic pre? Have you tried another mic cable?
We'll figure this out!
Jimmy J
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Thanks Jimmy. Everything looks perfect under inside the chassis.
I’m using the XLR out to go into my RCF mixer. I’m getting zero signal from the XLR.
I made sure each of the connectors was fully seated while I was inside the the chassis.
I’ve tried other XLR cables.
I’m out of ideas.
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Well that is a strange failure indeed. I found this pic of the internal works showing the expected great build and wiring quality. Is this what your unit looks like? Anything not match this photo?
Jimmy J
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Maybe try unplugging and plugging back in the XLR connector where it connects to the board inside the F-1X. Also check all solder points for the wires where they connect to the XLR plug and switches inside the F-1X.
Do you have another mixer or device you could test it on to see if that is the source of the issue rather than the F-1X?
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This has got to be something simple, perhaps to the point of silliness. I think it's really unlikely there's anything seriously wrong.
Here are a few troubleshooting suggestions:
- Are you connecting to a mic input (XLR) or a line input (phone jack) on the mixer?
- If you're connecting to a phone jack, are you using a balanced cable with female XLR at the F-1X end and TRS phone plug at the mixer end? Do you know that the cable is actually wired correctly?
- If you're connecting to a mic (XLR) input, is phantom power on (it doesn't need to be, leave it off)?
- Do you have an ohmmeter? If so, what is the resistance between pins 2 and 3 of the direct out XLR on the F-1X?
- If you have an ohmmeter, set the pre/post switch to pre, plug a guitar cord into the Send jack of the effects loop and measure the resistance from tip to sleeve on the guitar cord, what is it?
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Thank everyone, I will try these things tonight and report back.
The photograph looks identical (from memory), I will compare side by side with my F-1X.
To answer some questions: the RCF mixer works fine, I can use the exact cable and input with another device and all works fine.
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We don't know which RCF mixer you have and which input you are using. It should be a balanced XLR mic input - as in - you should see signal on that channel from a microphone plugged into that cable. Unplug the mic and plug that cable into your F-1X output and you should have signal. The transformer in the F-1X cancels out any phantom power so that's not a big concern.
Try some of these ideas tonight if you have the chance and let us know if there are any breakthroughs.
Jimmy J
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Testing the input with a known source like a mic is a great idea. Mind the signal levels when moving the cable to the F-1X!
It's absolutely correct that phantom power should be no problem at the F-1X end. I am wondering whether it could have an effect on the mixer input, though.
I'm going by the schematic posted here. It shows that the transformer in the F-1X does not have a center tap that can be grounded. Without a ground reference at this end, phantom power could cause the inputs of the mixer to float up to 48V DC.
This should not harm the mixer inputs (otherwise, they would be damaged if phantom power was on with nothing connected to the input), but since usually the use of phantom power means drawing current from these lines and thus pulling down the DC voltage, the inputs might not function correctly if they are being held near 48V.
We don't know what the input circuitry of the mixer is. If the mic inputs are transformer isolated, the DC shouldn't be a problem. But that's a lot of high-quality audio transformers (one per channel). If the inputs are directly connected to active circuitry (like op amps or transistors) without capacitors to block DC, they might not function correctly.
I'm really guessing here and trying to come up with testable hypotheses about where the problem could be. I don't think phantom power is a likely explanation, but it's not impossible and it should be easy to verify that it's off and eliminate the possibility of this particular problem.
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These are the only two pins that show continuity:
(Unit is turned OFF.)
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If I'm interpreting this correctly, the meter is set to the 20 kohm range, and is reading 0.13 kohms, or 130 ohms. That seems like a reasonable DC resistance for the transformer, so I think you've now verified that the transformer's secondary winding and its connection to the XLR output are good.
Did you measure DC resistance using a guitar cord plugged into the effects loop send jack?
Can you tell us a little more about how you're connecting the direct out of the F-1X to the mixer? Are you connecting to a mic input on the mixer (XLR) or a line input (phone jack)? That could also give some clues.
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I'm fairly certain I had my effects loop send/return plugged in to my pedal board when I did that measurement.
It is an XLR cable from the direct out of the F-1X and into the RCF mixer. I can put a different preamp (Mesa WD-800) into the exact same cable and that works fine.
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Okay - it looks like the XLR output of the F-1X has a good connection to the secondary of the transformer; and you've tested the cable and mixer input and verified that they work.
The next thing to try is to measure the DC resistance at the effects loop Send jack. With the F-1X turned off, unplug the cables that are connecting Send and Return to your pedalboard. Plug just a cable into the Send jack and measure the DC resistance between the tip and sleeve of the cable. This should probably be "a few kohms" (I don't know the actual specs for the transformer, so this is just a reasonable guess). If it's up around 500 kohms, that would indicate a problem with the connection to the transformer primary. If it's around 1 Mohm, there's a problem with the switch in the Return jack.
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There is a small IC chip (it is an op-amp), that has caused the same issue from research that OJ Dorson (thanks!) found.
I'm going to try swapping with a new one... I'll report back.
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Thank you JimmyJ for the photo (top photo here), this is a photo of my F-1X (bottom photo here). Check out the wiring at the XLR jack, could this have something to do with my problem?
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Hey Thomas,
Sorry you're still chasing this problem. So strange! The only thing that looks odd to me is the solder joint at the top of your XLR. As if the jack may have been replaced at some point? I would suggest reheating at least that top "chassis ground" lug to get the solder to flow better. While you're at it, you should confirm that the other connections are solid: the 2nd black wire soldered to pin-1, white to pin-2 (bottom pin) and yellow to pin-3.
If that top solder joint is indeed "cold" enough to not be making the connection, that would mean the ground pin of the XLR would be lifted regardless of the "ISO" and "GND" switch position. It's unclear to me if THAT would stop the audio signal from passing to your mixer, but it's possible.
Let us know what you find.
Jimmy J
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Oops, forgot to attach my pic...
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Oops, forgot to attach my pic...
That does look suspect. Reflowing it won't take but a moment... Good luck!
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* * * S O L V E D * * *
Thanks to the excellent investigative work of Mr. OJ Dorson, I was able to fix the XLR output.
It was as simple as replacing the chip (which is an OP-AMP), see the photos for part number (the top line of the P/N is what matters).
* * * S O L V E D * * *
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Love a happy ending. Love a community that helps each other.
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If you are getting output from the rear sockets the XLR out is the issue and can be either the transformer to the DI or a connection around it.