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Topics - SteveBAnalog

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Tape Tech / Echoplex EP-2 Erase Bias
« on: September 03, 2015, 07:44:44 AM »
Hello All. Thus far in my troubleshooting my echoplex, this thread: on this forum had the most valuable information.. so I thought I ask a related question to the techs here. Thanks so much for your time. Any advice or links will be gratefully received.

I own an echoplex EP-2. When I purchased my EP-2, it had very weak repeats. I replaced most of the caps. The filter caps were replaced by the previous owner with a higher value multican 40/20/80/80. The voltages appear within spec . I have tried different tapes and physical adjustment to the heads. Demagged and cleaned numerous times.

I traced the problem to the oscillator circuit. Replacing the .047 c15 with a fresh .01 and setting the R26 trimmer to very low resistance gives me strong full echoes, repeats and self oscillation when driven. The unit doesn't fully erase. When the black lead from the erase circuit is scoped I find a distorted sine wave which looks to be 25 khz. (full disclosure, I am a new to my oscilloscope but I think I got the math right) This seems low for ac bias. The Orange lead to the record head shows the same frequency but smooth and a much higher amplitude with the r26 trim lowered. I don't totally understand how the bias choke and capacitor work. I tried adding a variable capacitor, c17 but I don't register a change at V2B per the schematic instructions. Do you have any recommendations for further tests? How might I raise the ac bias for the erase head?

I realize I am a little out of my league on this circuit but, I feel like I am so close and not ready to give up just yet. Especially when i got the echoes to sound amazing... I just want deeper erasure!

thanks so much for your time.

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