Phono section of this SP-11 preamplifier never worked -- traced it back to a wonky MC7812K which was depriving the phono section filaments. A few weeks ago the entire thing started popping fuses.
Well, digging in I've replaced the 1N4006's with UF4007's and upgraded the filter caps. Just wondering if the entire MOSFET/MC34071P regulator isn't worth an upgrade while the top is still off!
I just posted the link to the rest of the schematic on the AR db -- ARCDB - SP-11
Well, digging in I've replaced the 1N4006's with UF4007's and upgraded the filter caps. Just wondering if the entire MOSFET/MC34071P regulator isn't worth an upgrade while the top is still off!
An externally hosted image should be here but it was not working when we last tested it.
I just posted the link to the rest of the schematic on the AR db -- ARCDB - SP-11
Expensive enough ? Despite the complexity and the number of regulators !!, the 6.3V supply is rather hit and miss with no output decoupling to ground. What happens on start up is the regulators go close into current dropout limit/shutdown...limited by the large series resistors R140/141 ....(tungsten near s/c cold resistance).....long term reliability a question. I never like these types of circuits. Maybe the L series 1.5A versions are a better regulator.
A 25V rated electroytic cap working at 22V (0.9) unadjusted for AC variations questions reliability. A 40V rated component space permitting would do the trick.
richy
A 25V rated electroytic cap working at 22V (0.9) unadjusted for AC variations questions reliability. A 40V rated component space permitting would do the trick.
richy
Get it working correctly first. Then modify.
I doubt you'd get much out of messing with the FET/op-amp regulators. Granted the MC34071 (Motorola TL071 equivalent?) doesn't have the highest AVOL or lowest noise in the world, but I doubt you can get significantly better ripple rejection without running into issues with HF stability. You might be able to get better 1/f noise in the regulator by using a bipolar rather than a FET for the pass device. But honestly, I'd leave it alone.
~Tom
I doubt you'd get much out of messing with the FET/op-amp regulators. Granted the MC34071 (Motorola TL071 equivalent?) doesn't have the highest AVOL or lowest noise in the world, but I doubt you can get significantly better ripple rejection without running into issues with HF stability. You might be able to get better 1/f noise in the regulator by using a bipolar rather than a FET for the pass device. But honestly, I'd leave it alone.
~Tom
Turns out that one of the windings on the transformer was shot -- (the one for V2) what I discovered was that the secondary windings are twisted pairs of enameled wire. When I un-soldered the transformer it turned out that only one of the pair were soldered to their respective diodes. The other wire wasn't connected on one end! Thus the remaining wire had to handle 2x the current and failed.
The regulator for V2 should have had a diode across it --
The regulator for V2 should have had a diode across it --
This will probably cause heart attacks in Minnesota -- slow-start filament regulator for V2:
An externally hosted image should be here but it was not working when we last tested it.
Was thinking that they were using twisted pair for the transformer wiring. This is the "offending" 12V secondary which had become detached (thus the phono pre had no filament voltage). Seemed strange that only 1 tube would cause a problem. The wires are twisted for some structural integrity, not lower resistance. There is, however, a similar problem for the transformer primary which has to be tackled.
An externally hosted image should be here but it was not working when we last tested it.
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