Thermionic Deluxe - voltages but no sound...

T

terribleamps

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I've built up a few of the 'one-channel' Brown Betty boards before without incident - this is the first time trying the twin. I admit (and it'll be obvious from the pics) that I'm working on a mod to one channel, presently no clipping diodes at all across the 'top' channel - and at present no "on/off" indicator LED's populated. Otherwise - best as I can tell, everything's fitted and in its' proper place. Bypass signal is fine - zero signal (other than a little click/pop when switching the R/H 'channel' switch), just hum that doesn't appear to change relative to gain/volume controls or anything else. I've confirmed that I'm seeing ~9.1V on pin 8 of all seven TL072's on the board (socketed) - but so far I haven't gone digging too much further. I'm kicking myself for not having tried to fire it up with just the top row of pots fitted before soldering in the lower row, although I do have an electric vacuum desoldering pump that'll hopefully make it not too awful to remove pots if need be.

I'd be thrilled if someone spotted a smoking gun that didn't involve having to dissect the whole thing... if anyone can suggest smart first-things-to-sniff-out on this board, that'd be extremely helpful. If a closer/better photo will help - by all means, let me know. All wiring is the prebond stuff from lovemyswitches, using Alpha pots from Tayda on the lower row (yes, with the dust covers), I don't recall who makes the ones on the top row but I get them locally from pedalpartsaustralia.

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Update: my VRef voltage at IC1 is about 1.43V with respect to ground. Vdd is the same. These reflect out onto the other six TL072's. I don't have any reference to match against - does that look like we're in the right ballpark?

Edit: Also successfully traced paths from input/output jacks through to R1A/R1B and R24A/R24B. I've got some more TL072's out in the workshop (that I think have come from a different source) - I might try them just in case. Got me stumped that both channels are completely 'dead silent' with respect to passing any kind of signal though - no variation/volume-increase-of-hum relative to gain/volume.
 
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zgrav said:
Are you testing with an audio probe? That seems like the best way to find out where your audio signal is getting stopped.
Click to expand...

I hadn't gotten to that stage last night (very cold in my workshop, wasn't inspired to go and fire up the sig-gen/CRO). I braved it this morning, and traced a sine wave through. On both channels, the results were similar - top channel not having clipping diodes was a little stronger most of the way through the path, but both channels simply seem to keep "losing" signal until they get to about R13 (between the two stages of IC3), where it peters out almost entirely. For a circuit that I expect to be at unity or greater at each consecutive stage - what I'm seeing is progressive loss until it withers entirely. Gets me wondering more about the Vref/Vdd values and whether those are correct?

I swapped out all seven of the Tayda TL072's for the others I had - different markings/code - but had the same result with the other set.
 
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seems like your v ref should be closer to 4 volts than the 1.43 you are getting. vdd should be about 8.8 volts or so.

what voltage readings are you getting at the power input pad on your board, and on pins 1 and 6 on IC1?


do you have a different power supply you could try? or a battery?
 
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Aha... It looks as though I've managed to apply 200K resistors to R32/R35, in lieu of the intended 20K. I suspect that's the cause of my downfall (by a factor of ten)... I'll get the desoldering station going in the morning and swap those two resistors over. On a quick glance of the two Brown Betty boards that I was about to fire up - it looks as though I may have replicated the same mistake, so I'd better get on and fix those two as well! I got a huge bulk-lot of resistors a number of years ago, and I've clearly slipped a wrong-value slice off a reel into the wrong drawer... never mind!
 
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