Last post by W1DAN - January 30, 2021, 05:12:33 PM
Hi Per Anders!
Thank you for your suggestion and guidance.
With input shorted to ground: Added variable 0 to +15v power supply to pin 5 of IC5 socket. No change at output at TP1 (which is at -184mv). IC5 socket (no chip inserted) pin 2 = 00.00mv. Pin 3 = 00.00mv. IC5 socket (MAX420 inserted) pin 2 = 00.00mv. Pin 3 = 00.00mv. Pin 5 = -371mv. TP1 is at -183mv.
Where does input to IC5 come from? Where does IC5 output enter the preamp for DC correction?
Last post by peranders - January 30, 2021, 11:15:40 AM
You should try to locate where you have the oscillation and try to cure it. Start with only one channel active and shorted input. If you can measure the frequency or even better having a FFT measurement you may come to the conclusion that it's a radio signal you picking up.
Last post by zayance - January 26, 2021, 07:27:31 PM
Allright, have some days off, yippy.... Used a 2x18VAC SSR03 is doing fine now...
Audio is passing and sounds nice, all seem to be doing well, at least to me ears... Very dynamic, had an hour listening to various stuff, mainy listening on headphones (neighbors thank me) consisting of an Audeze LCD-X, was a nice hour of listening.... EDIT: I have the same oscillation on Preamp out, but it doesn't seem to affect the overall sound as some have mentionned, so not sure what to think about that....
Still needs an enclosure, it was just dummy metal plate for now....
Last post by peranders - January 24, 2021, 09:08:57 AM
The regulators are out of the regulating range. Make sure that you have 20 V in and "in" means on pin 3 of the LM317 (IC1). Check also that your current is under the limit. Check the voltage over the current shunts (R47/R49). It should be under 600 mV
Last post by W1DAN - January 18, 2021, 06:13:30 PM
I have built and am troubleshooting a QSXM2 preamp, using BC550 and 560 transistors in the front end for a MM cartridge. This is powered by a commercial +/-15v linear power supply. Both in an aluminum chassis with proper grounding.
I have two issues:
1. A large DC offset appears at output (both channels) of about +14V DC. This exists regardless of whether the DC servo op-amps are in or not. Adding a 10uf electrolytic cap in series with R93 eliminates this issue for now. Lifting R93 to separate the preamp and RIAA amp results in low DC offset on the main output. Without DC servo op-amps and the input shorted or not shorted, TP1 rests at about -20mv DC.
2. There is a 284MHz RF oscillation in the preamp section, both channels. Adding 0.1uf ceramic bypass caps as well as 470uf electrolytic caps at the V+ and V- input terminals does not quench the oscillation. Adding a 0.1uf ceramic bypass cap across the supply rails does not quench the oscillation. The only place on the PC board where adding an 0.1uf cap to ground to quench the oscillation is at TP1, but this will affect the frequency response in the audio band. There is minimal leakage of noise at the power supply terminals. C7, C17, C19, C21 are OK.
With headphones the preamp sounds good with the exception of added noise due to the oscillation. A sine wave oscillator at the input is reproduced without obvious distortion at the output, with the exception of added RF (a free transmitter!).
My main interest is the RF oscillation right now, then DC offset. Any suggestions are appreciated.