testing capsule in condenser mic

D

drembo

Member
I have MXL V69 tube condenser. I like this mic, I use it in my home studio to record myself. I used it a couple of weeks ago, no problem. When I went to use it this past weekend, I could only get a veeery small signal out of the mic. The mic had been left on for a couple of days( I had to leave for an emergency, didn't have time to shut down). I finally got back to turn it off, but the mic stayed out on the mic stand for another week. I have an autistic son who might have gotten in there and banged it around, but if he knocked it over, he wouldn't have put it back where it was, so it appears that that was not the case.
So, while the mic is still less than 3 years old( the MXL warranty period), I am unable to find the receipt, and the place that I purchased it from online does not keep records or whatever, that far back. MXL does offer to replace this mic out of warranty for $150, which is very reasonable of them. Add sales tax and shipping to that.
If I knew it was the capsule, I would just buy an upgraded one and put it in myself. Something like the Peluso CEK-89 is $120, that would be less than having the mic replaced by MXL.(I think this capsule would work in the V69, No?)
Is there some way I can test the capsule to see if it is the problem? I do have some electronics back ground, I build Hi-Fi and musical instrument tube amps. I have a scope and a couple of nice meters.
I have ruled out cables, preamp, tube, and I have 120v at the back of the power supply for the mic.
 
Last edited:
Have you checked the PSU output? Measure voltage at each pin of the cable to the mic, and post your results. I would think if something was gonna let go, it'd be in the power supply. After that, check voltage at each tube pin. Unless the capsule was subjected to moisture, I wouldn't look there first.
 
I'll do that when I get home tonight. I'll open up the power supply and look around also, maybe something obviously cooked.
 
You said you had 120V on the PSU. Do you also have filament voltage? Probably 12V. That's probably the first thing I'd check. If that doesn't turn up a problem, make sure there's voltage at the capsule.

The capsule is about the last thing I'd expect to just suddenly fail. My first guess would be a cold solder joint, followed in short order by a bad cable, a leaky capacitor, a shorted or open power transformer, and finally a shorted or open audio transformer. The capsule would be somewhere around number one hundred on the list. :D
 
Well, I've got (looking clockwise at the end of the cable to the mic), 212vdc,8.9vdc, 212vdc, ground, and the last two have no voltage, I assume that they are the signal returns from the mic. However, at the tube, I only have 4.4vdc on the plates. The heater has about 6vdc on it, and the tube lights up. The heaters are hooked up in parallel, so that voltage should be adequate. I am thinking that something in the mic circuitry is fried, and not giving me the plate voltage I need. I don't think that is something I can fix.
 
drembo said:
Well, I've got (looking clockwise at the end of the cable to the mic), 212vdc,8.9vdc, 212vdc, ground, and the last two have no voltage, I assume that they are the signal returns from the mic. However, at the tube, I only have 4.4vdc on the plates. The heater has about 6vdc on it, and the tube lights up. The heaters are hooked up in parallel, so that voltage should be adequate. I am thinking that something in the mic circuitry is fried, and not giving me the plate voltage I need. I don't think that is something I can fix.
Click to expand...

My guess would be that it got warm enough that some already weak solder joint (under tension for some reason) popped. Look for cold solder joints on the board.
 
4.4vdc on the plates is far too low. There might be a short somewhere on the wires feeding the plate. Of course it could just be a failing tube. If you have access to a cheap but functional 12A_7 model tube (12AX7, 12AT7, 12AY7, 12AU7) you could swap the tube temporarily to see if the problem goes away or stays the same. That would at least prove or rule out the tube.

P.S. If you need a capsule to test, I have a few similar to the one in the V69M. P.M. me if you want me to send one for testing.
 
Last edited:
Philgood,
I don't think there is any use in doing anything unless and until I can get that plate voltage figured out. No tube is gong to work with 4.5v. I already tried a known good tube, to no avail. I am going to pull apart the guts of the mic a little further so I can see where the wires all go. There are two boards with the wires running between them, makes it hard to trace anything.
 
well, hopefully I have found the problem. A zener diode is no good on the board. I am going to get a new one today, put it in tonight and cross my fingers that that is the only issue.
 
If anyone cares, I finally fixed the mic. It ended up not being a bad zener(checked it again when I wasn't so tired, the meter was set in the wrong mode), but was a shorted .01 uf cap off of the power supply to the plate resistor. Shorted all the voltage to ground. MXL wouldn't give me a copy of the schematic. It was a bit tricky sorting things out with two dual-sided printed circuit boards in the mic.
 
drembo said:
If anyone cares, I finally fixed the mic. It ended up not being a bad zener(checked it again when I wasn't so tired, the meter was set in the wrong mode), but was a shorted .01 uf cap off of the power supply to the plate resistor. Shorted all the voltage to ground. MXL wouldn't give me a copy of the schematic. It was a bit tricky sorting things out with two dual-sided printed circuit boards in the mic.
Click to expand...

Just be glad it only has a single layer. :D
 
You must log in or register to reply here.
Back
Top