DIY 1073/1290 mic pre - stumped!

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Redtns

Member
Joined
Apr 11, 2017
Messages
19
Hi all,

Well it seems I don't have much of a talent for this! Earlier this year during the first UK lockdown I eventually successfully built the "ez1290" diy mic pre with a little power supply I designed and built. I was a happy boy! Can't have that..! I thought I'd continue along similar tracks and try redesign the 1290 pcb as a smaller, more compact PCB - I had more time on my hands and figured it'd be a fun little challenge. Story time over.

So I laid out new PCBs, ordered them, and everything visually looks to be right.. I populated everything, turned it on (no audio connected) and hallelujah! The same current draw as the EZ kit I'd been working on (well, within a couple of mA, which I put down to my probably crap layout and smaller traces). Well it must be working! But then it suddenly "shut down" after a few minutes, dropping its current draw to 14mA. Coincidentally (or perhaps not) this is the amount of current I would measure if I disconnected the third gain stage (the one with the 3055 in it) from the circuit altogether.. Sometimes it will power up fully if it's been off for a couple of days, only to die again after a few minutes.

The circuit seems to behave normally if I combine the "first two gain stages" from the ez1290 with the third stage on my new pcb. If I combine the two PCBs the "other way around" (so the final gain stage from the ez1290, but the two previous gain stages from my new pcb), it starts dying again. (I've attached the schematic for the ez1290 if it's of use! Pins P and L from BA183/AV are a wire link on the pcb, so that's where I'm "combining" the two pcbs while trying to find a clue..). This is a horrible explanation, I can't seem to word it much better!

So it makes me think there's something wrong in the early gain stages/gain switch, but the way the current draw drops to 14mA makes me think it's the final gain stage failing! Or maybe it's all just crap! 


I know this is pretty much a ramble and it could be pretty much anything, from a layout mistake to wrong components, but how would you go about solving something like this? I'm a bit lost and I don't have the experience the think my way out! Where should I start? What would cause a circuit like this to behave like this?


Anyone kind enough to respond, thankyou! I've learned so much already reading and posting on this forum!

R
 

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Bo Deadly

Well-known member
Joined
Dec 22, 2015
Messages
2,610
Location
New Jersey, USA
Look at the DC voltages at various points. If it quits after a while it sounds like a temperature thing. Check the bias of the 3055. You should be able to troubleshoot something like this with just a meter.
 

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