Preamp fail [audio clip]

jruberto · 26th January 2012

JoeMeek VC6 took a short fall and stopped working. I looked inside for obvious broken stuff, nothing. Some of the soldering looks like it wasn't great in the first place. Checked the power transformer, has continuity on both sides. Checked the big caps too, seem to be all right, but that's about the extent of my troubleshooting abilites WRT component level stuff.

Anyway, no sound coming out the output, except when engaging the phantom power I get what you hear in this audio clip. Phantom indicator light does not come on, tho there is 48v present on the mic input when engaging the switch.

The light on the "enhancer" circuit does light up dim, and that is normal. The output meter does not light up its first LED which it should when the power is on.

Any guesses?

WARNING THIS AUDIO CLIP CONTAINS A POP THAT HITS FULL SCALE AND LOTS OF LF INFO. DO NOT BLOW UP YOUR SPEAKERS.
don't say i didn't warn ya.

0.5 sec - turn unit on
10.5 sec - engage phantom

in looking for a schematic I just found an old forum reply by TF himself, replying to someone describing the exact symptoms as me:

Quote:

I'm afraid that your VC6 has a serious power supply problem. I'm afraid I don't have the circuit diagram for the VC6, but it is very similar to the VC3 and if you would like to drop me an email at ted@tfpro.com I will email you a copy of this.... But I would suggest that the fix is not simple. Once the power supply fails on these units, it's often the case that it takes out most of the integrated circuits, so the fix could be expensive.
I have an arrangement where I service all old equipment of my design, for a fixed fee of £25 plus the cost of returning it.
I can't guarantee to fix this one, but if you like I will try.

agree? any hints on how i could check if (for instance) the SSM2017 is fried?

thanks!!

henryf · 27th January 2012

Well, one way to check if the suspect SSM2017 is fried would be to swap it into a working circuit, similar to a VC6, to see if it works OK there. But, that may not be practical in your situation, and in any event it isn't the way I'd go about troubleshooting your unit.

My general strategy would be to first check the power supply voltages, fix them if they are out of spec, and then after that try to determine if anything else doesn't work. Even without a schematic, I could still measure the voltages on each pin of the SSM2017, and tell a few things from that. So, that's what I'd suggest you do: download a datasheet for the SSM2017, and measure the voltages on each pin.

Although you could use a DMM for this, an oscilloscope would be better if available, since you could see the waveforms.

If you post your measurements, there's a reasonable chance someone here could help you interpret them.

In the worst case, fixing this might require reverse engineering the VC6. Let's hope you get luckier than you have been.

jruberto · 27th January 2012

Thanks Henry,
Good plan. I will check voltages at a few points where I can get specs of what's expected, but I doubt I'm going to fix this, not without a lot of help. I do have some friends who would perhaps take a look at it. I can also pick up another one used for a couple hundred, and I just might do that to get me going again -- I actually have really grown to like this ugly little unit, and my bass rig won't be the same without it, that's for sure. Thanks again. ~j

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27th January 2012	#2
henryf Gear nut Joined: Jun 2010 Posts: 123	Well, one way to check if the suspect SSM2017 is fried would be to swap it into a working circuit, similar to a VC6, to see if it works OK there. But, that may not be practical in your situation, and in any event it isn't the way I'd go about troubleshooting your unit. My general strategy would be to first check the power supply voltages, fix them if they are out of spec, and then after that try to determine if anything else doesn't work. Even without a schematic, I could still measure the voltages on each pin of the SSM2017, and tell a few things from that. So, that's what I'd suggest you do: download a datasheet for the SSM2017, and measure the voltages on each pin. Although you could use a DMM for this, an oscilloscope would be better if available, since you could see the waveforms. If you post your measurements, there's a reasonable chance someone here could help you interpret them. In the worst case, fixing this might require reverse engineering the VC6. Let's hope you get luckier than you have been.

27th January 2012	#3
jruberto Lives for gear Joined: Sep 2008 Location: Denver, CO Posts: 564 Thread Starter	Thanks Henry, Good plan. I will check voltages at a few points where I can get specs of what's expected, but I doubt I'm going to fix this, not without a lot of help. I do have some friends who would perhaps take a look at it. I can also pick up another one used for a couple hundred, and I just might do that to get me going again -- I actually have really grown to like this ugly little unit, and my bass rig won't be the same without it, that's for sure. Thanks again. ~j