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.
Last post by zayance - January 17, 2021, 01:14:33 PM
So, i'm only able to work on this one during week-ends, so i slowly just keep at it.
Plugged your SSR03 PSU, after this one working good without any load, meaning H1 / H2 lighted up, output voltage within specs no prob (+/-15V). But as soon as this one is hooked up to the QSXM2 (Both channels), H1 / H2 lights up for a sec and then lights go out, since there is around 1V voltage drop, from 15V to 13.5/14V on both rails. Didn't want to post on the PSU section, since this one seems to work as is. I tried to the PSU to only one channel, and the PSU behaves well, no voltage drop.
Anyway, hope you can find some time to shed some light on this. Even if the T51-54 section seem to be having different behavious then yours since your tests were with 550/560's and not 850/860... (might try to match BC550/560 in this section and see if it gets better values), the circuit seem to be doing fine, and so i wanted to hear if it was passing Audio etc.. but stumbled on this with the SSR03 + QSXM2 so...
Last post by zayance - January 07, 2021, 10:36:58 PM
Allright, fired the thing up, and tested all points, for now with Lab PSU. The build is a mix between SMD and TH., and using SSM for now for ease of troubleshooting, i'l try the BC847BS and 857BS tough for comparing. Here are the values, if you have some time? I have some values low and some higher because of BC's beeing CG, but not sure of the lower ones, and i could maybe better match some points, any suggestion is welcome.
Thanks for your help, and sorry to bring this back after all this time :-)