HELP! MXL Genesis Power Supply Repair

Fenderguy81

New member
Rather than try to get on the leads of the regulator, the leads of the caps you replaced are long enough to maybe get clips on. Measuring there will show the voltage going into the regulator and hopefully coming out. I believe the new cap nearest the regulator should read about the 14.33vdc you got before. The other cap should be quite close to 12vdc. If you don't get 12vdc the regulator itself is a likely suspect and the reason the D12 LED doesn't light.

The schematic shows a diode D11 from pin 3 of the regulator to GND(ground). I don't think that is there in your power supply and pin 3 goes to ground. The 4th point at the regulator I think may be a solder mount point for the heatsink.

The leads on the new caps..... are they long enough that the tips could have touched the grounded case if the circuit board was mounted back in place?

I'm going to look at your pics again and see if there is an easy place to check the high voltage if you haven't already.


Hey there,

the new cap near the regulator measured 13.52vdc and the other cap is reading 0vdc so it sounds like the regulator must be the problem.

Also, the lead caps are a bit long and it is possible that they may have been grounded with the case when I put it back together. I will cut the ends a bit back and it sounds like I should probably order a new regulator from Mouser.
 

Fenderguy81

New member
High voltage check... measured across the two zener diodes D5 and D6. Hopefully the points below are clear enough to show where to put the probes at the ends of the two diodes. Schematic states 200 vdc. I don't if that is what you'll see or not, but measure it and make note of what you get.

If you get your head around all of this, the next step will be building yourself a nice little mic preamp :eek: :D

The high voltage measured 212vdc so I think that should be within tolerance of the 200vdc. Can't wait to build the preamp though ;) haha
 

ecc83

Well-known member
Hey there,

the new cap near the regulator measured 13.52vdc and the other cap is reading 0vdc so it sounds like the regulator must be the problem.

Also, the lead caps are a bit long and it is possible that they may have been grounded with the case when I put it back together. I will cut the ends a bit back and it sounds like I should probably order a new regulator from Mouser.

Ooo! That is a bit low going into the regulator. You generally need at least a 2 volt differential. Can you measure the AC voltage going into the bridge? Probably best to take the regulator out first as it might be loading the supply. Do you have any odd resistors about the place of about 100 Ohms? You might have buggered a diode in the bridge but you need to load the circuit by 100mA or so to prove it. Easiest to replace it with a 4 diode encapsulated bridge rated at 400V and 2 amps, cheap as chips, will never fail.

Gets tricky don't it!?

Dave.
 

arcaxis

Active member
Ooo! That is a bit low going into the regulator. You generally need at least a 2 volt differential. Can you measure the AC voltage going into the bridge? Probably best to take the regulator out first as it might be loading the supply. Do you have any odd resistors about the place of about 100 Ohms? You might have buggered a diode in the bridge but you need to load the circuit by 100mA or so to prove it. Easiest to replace it with a 4 diode encapsulated bridge rated at 400V and 2 amps, cheap as chips, will never fail.

Gets tricky don't it!?

Dave.

Something may have changed as the read across the ends of the diodes that feed that cap was 14.33 versus the recent read of 13.52, which may have been marginally OK to see something on the 12 volt side of the regulator unloaded even if less than 12 volts. Specs for the 7812 seem to show 14.5 as a minimum to attain a regulated 12vdc.
I don't believe a read of the AC going into the bridge was ever noted.

If the regulator is pulled, take an ohmmeter reading across the cap that read '0' volts. It should settle to be 2kohms or higher depending on the ohmmeter lead polarity after the cap charges. Unplug the power supply and let it set a while before doing the check with the ohmmeter.

Fenderguy..... can you get a look at the side of the regulator chip and confirm it does indeed say 7812? I found some information the someone 'thinks' it may be a 7809. The tube in the mic can be run at 12v or 6v depending on how the filament is wired. A 7809 seems odd unless they run the 12 volt filament at a lower voltage for some reason.
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ecc83

Well-known member
Yes Mark, it could be a 709. The 'jackup' diode would give about 9.7V and it is quite common to under run valve filaments in critically low noise situations to reduce noise, heat and prolong life.

We shall see!

Dave.
 

Fenderguy81

New member
Something may have changed as the read across the ends of the diodes that feed that cap was 14.33 versus the recent read of 13.52, which may have been marginally OK to see something on the 12 volt side of the regulator unloaded even if less than 12 volts. Specs for the 7812 seem to show 14.5 as a minimum to attain a regulated 12vdc.
I don't believe a read of the AC going into the bridge was ever noted.

If the regulator is pulled, take an ohmmeter reading across the cap that read '0' volts. It should settle to be 2kohms or higher depending on the ohmmeter lead polarity after the cap charges. Unplug the power supply and let it set a while before doing the check with the ohmmeter.

Fenderguy..... can you get a look at the side of the regulator chip and confirm it does indeed say 7812? I found some information the someone 'thinks' it may be a 7809. The tube in the mic can be run at 12v or 6v depending on how the filament is wired. A 7809 seems odd unless they run the 12 volt filament at a lower voltage for some reason.
Gearslutz Pro Audio Community - View Single Post - MXL V69 Power Supply Help Needed

Sorry for the delay I got tied up and haven't had a chance to get back to troubleshooting this power supply. Anyway, I pulled the regulator and it does say 7809 (KA7809). I checked the ohms on the cap that was reading 0 volts and it would not produce a resistance measurement. Would a bad capacitor here cause this problem as well? I replaced both caps with new caps, but perhaps one of them was faulty?

Thanks for all of your help guys!
 

Fenderguy81

New member
sorry for the delay i got tied up and haven't had a chance to get back to troubleshooting this power supply. Anyway, i pulled the regulator and it does say 7809 (ka7809). I checked the ohms on the cap that was reading 0 volts and it would not produce a resistance measurement. Would a bad capacitor here cause this problem as well? I replaced both caps with new caps, but perhaps one of them was faulty?

Thanks for all of your help guys!

bump
 

arcaxis

Active member
Sorry for the delay I got tied up and haven't had a chance to get back to troubleshooting this power supply. Anyway, I pulled the regulator and it does say 7809 (KA7809). I checked the ohms on the cap that was reading 0 volts and it would not produce a resistance measurement. Would a bad capacitor here cause this problem as well? I replaced both caps with new caps, but perhaps one of them was faulty?

Thanks for all of your help guys!

My inclination is to think the 7809 regulator is toast. The ohms check on the cap was to ensure it wasn't shorted and pulling the voltage down to zero. It seems you got a high resistance or infinity reading, so that should be OK. If your PS is like the one in the schematic possibly reversing the ohmmeter leads on that cap may show a reading from the LED power light.

Mouser didn't seem to have the 7809 with the insulated type tab. Digikey appears to stock it... L7809CP STMicroelectronics | Integrated Circuits (ICs) | DigiKey They're cheap and you'll get hit with shipping, so buy a few.

You'll also need a small amount of heatsink compound to apply to the tab and heatsink to ensure good thermal conduction.
 

Fenderguy81

New member
My inclination is to think the 7809 regulator is toast. The ohms check on the cap was to ensure it wasn't shorted and pulling the voltage down to zero. It seems you got a high resistance or infinity reading, so that should be OK. If your PS is like the one in the schematic possibly reversing the ohmmeter leads on that cap may show a reading from the LED power light.

Mouser didn't seem to have the 7809 with the insulated type tab. Digikey appears to stock it... L7809CP STMicroelectronics | Integrated Circuits (ICs) | DigiKey They're cheap and you'll get hit with shipping, so buy a few.

You'll also need a small amount of heatsink compound to apply to the tab and heatsink to ensure good thermal conduction.

Thanks! I'll order a few of them.
 
If you already have the regulator pulled it would be easy enough to test it, as far as I know.
Check before doing it but I'm pretty sure you can just have a small cap between 1+2 and 3+2, where two is ground.

Then 1 is >9VDC input and 3 should be 9VDC regulated output.

Wear goggles. ;)
 

marsel

New member
hello everyone,
i just want to ask. how can i use an MXL Genesis Mic in Europe. Is there an simple switch that change it to 220 from 110 or soldering is needed?
Thanks
 

ecc83

Well-known member
hello everyone,
i just want to ask. how can i use an MXL Genesis Mic in Europe. Is there an simple switch that change it to 220 from 110 or soldering is needed?
Thanks
No idea, ask the agents? But I would doubt if it was multi-voltage. The current drain is so low that you will be able to find a 110 to 220V auto transformer fairly easily and cheaply (will have a shufty)
NB! Keep such transformers a good mtr away from any audio wiring and especially anything that has an audio transformer in it.

Dave.
 

ecc83

Well-known member
No idea, ask the agents? But I would doubt if it was multi-voltage. The current drain is so low that you will be able to find a 230 -115V auto transformer fairly easily and cheaply (will have a shufty)
NB! Keep such transformers a good mtr away from any audio wiring and especially anything that has an audio transformer in it.

Dave.
 

marsel

New member
thanks for answer, im thinking to buy it from US and use it in EU. Couldn't find the power supply manual and trying to figure out how to use it. If there is a simple switch from 110 to 220 i will buy it. i guess i need the schema but couldnt find it on the web
 

ecc83

Well-known member
thanks for answer, im thinking to buy it from US and use it in EU. Couldn't find the power supply manual and trying to figure out how to use it. If there is a simple switch from 110 to 220 i will buy it. i guess i need the schema but couldnt find it on the web
You should be able to download a user manual for the mic? That SHOULD give you the mains power draw, if not email the buggers and ask! I would say any COMPETENT manufacturer would have that information readily available and IF they didn't I would avoid their products!

Dave.
 
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