trying to get this old univox/LEM tape echo working
https://www.google.com/search?q=lembaby+schematic&biw=1600&bih=749&source=lnms&tbm=isch&sa=X&ved=0ahUKEwjS0_myxb7NAhWIPCYKHS61D804ChD8BQgGKAE#imgrc=df06GrRwLy3VuM%3A
looks like that (mine's in a bit worse shape....)
the mixer works, passes signal. the tape loop spins and looks okay, but there is no effect. it's impossible to find a schematic so i am poking around in the dark a bit.
is there a simple way to measure that there is bias on the tape heads? can i just measure AC across the heads?
WAY too high frequency to be measured by a multimeter (think 80 to 150 kHz ), either scope it or build an RF probe:
(http://n5ese.com/rf_probe_schem.jpg)
That said, doubt your problem is lack of RF bias, trace signal along the path, either it´s not reaching recording head or is lost after playback one.
damn not too easy to trace the signal - it kind of jumps between two boards - but i think i got at least part of the problem!
near the very end of the echo section - before the volume pot - i saw the signal wasnt making it through a tropical fish cap. replaced that cap and im getting a slight echo! it's pretty quiet, though. im wondering if other tropical fish caps in this thing might have to be replaced? its not new!
k made some progress
replaced a couple of caps near the output of the tape section - cleaned everything - aligned the heads a bit better - sounding pretty good!
two main issues now:
1) unit is a bit noisy - sounds like 120hz hum - gotta scope it check it out
2) the 'repeats' knob doesnt seem to make a difference. im wondering how it works anyway...does it gradually disengage the erase head? the erase head does work - too good in fact - it erases the loop every time so i only ever get one repeat. sounds like a tricky fix. any hints?
okay dialed the noise back a bit - there are some power supply trimmers in here that ended up being useful
last issue! repeats knob:
forget what i said in the last post - i was sleepy - so the repeats are just fed back from the output back into the input, building the signal onto itself. the repeats knob does seem to be making a bit of a difference, after some tweaking, but im guessing that cranked up to ten this thing should start to self-oscillate, right??
once again, its tough without the schematic. i can trace the signal leaving from the end of the tape section where it goes back to the input - this seems to be fine. it just doesnt seem to be quite enough of the signal to send it into oscillation. there is no gain stage after the repeats knob, it just takes the signal from the output and routes it back to the input of the tape section. maybe something around the repeats knob is biting off a big chunk of the signal so it can't get quite hefty enough to self-oscillate.
any thoughts?