Hi all,
So in order to entertain myself during these lockdown days I decided to put together the ez1290 mic pre kit I bought many moons ago (https://www.diyrecordingequipment.com/products/ez1290-preamp-pcb)
I made an lm317 based power supply PCB for this project a while back, which seems to work fine as far as I can tell... It provides +24V and +48V, but I can't really check any further than this - my oscilloscope is 50 miles away locked in someone else's workshop! But it's a pretty standard lm317 circuit, the kind of which we've all used a hundred times. I had plans to use it with bigger projects in the future though, so I added a "high current" section consisting of a 2955 and 3055 - again, nothing particularly interesting but for me it is at least an unknown. I've sketched up a quick schematic of the 24V section - hopefully attached - in case I've just made an arse of it!
After populating the ez1290 pcb it was getting late in the day, so stupidly I just plugged the lot in. Poof, immediate smoke! The 1290 output stage 3055 heatsink was shorting to 0V it turned out - stupid really. I burnt out a 1W resistor which provided power to that half of the circuit (the cause of the smoke). I isolated the heatsink and replaced the resistor, and on it came. There was some buzzy power supply noise and some hum (I had it all in the same case, power transformer and all - part of the fun!) but it was working. It sounded nice and full, like a mic amp should. Big plosives were dealt with properly, not crapping out the power supply, happy days. It was noisy, but it was correct. Phantom worked fine with a 414, and all sounded good with a 57. Time for bed!
The next morning I switch on expecting a day of experimenting with power supply earthing and positioning, but instead I got a thin and distorted mic amp. I had changed nothing. It's like a ringing buzzy distortion at even tiny input levels. The overall sound, if you can listen through the mess, is much brighter too. If I put a larger noise into the mic you can hear the whole thing crapping out - it goes silent for half a second, as if the power supply can't keep up. This happens with both dynamic and condenser microphones, so I'm inclined to think the 48V is not to blame.
All the voltages check out/are in spec for all 9 transistors in the circuit, no capacitors or resistors look like they've failed...
So I was really looking for any suggestions/advice before I start tearing it apart. Power supply or mic amp? It sounds like a failure on the mic amp, but looking at the schematic, I think the rogue power ran through the primary of the output transformer, onto the collector of the 3055, then through the heatsink and straight to 0V - it didn't really pass through anything... So then maybe the power supply, but it appears to be working normally! Can lm317's partially fail?
One moment of stupidity in not beeping out the simple connections before switching on has added an unknown into the diagnosis! I'd say live and learn, but I've done it before and I'll do it again!
Any help or advice would be greatly appreciated.
R
So in order to entertain myself during these lockdown days I decided to put together the ez1290 mic pre kit I bought many moons ago (https://www.diyrecordingequipment.com/products/ez1290-preamp-pcb)
I made an lm317 based power supply PCB for this project a while back, which seems to work fine as far as I can tell... It provides +24V and +48V, but I can't really check any further than this - my oscilloscope is 50 miles away locked in someone else's workshop! But it's a pretty standard lm317 circuit, the kind of which we've all used a hundred times. I had plans to use it with bigger projects in the future though, so I added a "high current" section consisting of a 2955 and 3055 - again, nothing particularly interesting but for me it is at least an unknown. I've sketched up a quick schematic of the 24V section - hopefully attached - in case I've just made an arse of it!
After populating the ez1290 pcb it was getting late in the day, so stupidly I just plugged the lot in. Poof, immediate smoke! The 1290 output stage 3055 heatsink was shorting to 0V it turned out - stupid really. I burnt out a 1W resistor which provided power to that half of the circuit (the cause of the smoke). I isolated the heatsink and replaced the resistor, and on it came. There was some buzzy power supply noise and some hum (I had it all in the same case, power transformer and all - part of the fun!) but it was working. It sounded nice and full, like a mic amp should. Big plosives were dealt with properly, not crapping out the power supply, happy days. It was noisy, but it was correct. Phantom worked fine with a 414, and all sounded good with a 57. Time for bed!
The next morning I switch on expecting a day of experimenting with power supply earthing and positioning, but instead I got a thin and distorted mic amp. I had changed nothing. It's like a ringing buzzy distortion at even tiny input levels. The overall sound, if you can listen through the mess, is much brighter too. If I put a larger noise into the mic you can hear the whole thing crapping out - it goes silent for half a second, as if the power supply can't keep up. This happens with both dynamic and condenser microphones, so I'm inclined to think the 48V is not to blame.
All the voltages check out/are in spec for all 9 transistors in the circuit, no capacitors or resistors look like they've failed...
So I was really looking for any suggestions/advice before I start tearing it apart. Power supply or mic amp? It sounds like a failure on the mic amp, but looking at the schematic, I think the rogue power ran through the primary of the output transformer, onto the collector of the 3055, then through the heatsink and straight to 0V - it didn't really pass through anything... So then maybe the power supply, but it appears to be working normally! Can lm317's partially fail?
One moment of stupidity in not beeping out the simple connections before switching on has added an unknown into the diagnosis! I'd say live and learn, but I've done it before and I'll do it again!
Any help or advice would be greatly appreciated.
R
