Sjöström Audio DIY forum

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

Hi,

Under no load attached  Pin 2/3 are exact:
Positive Voltage
pin2: +7.41V
pin3: +7.41V

Negative Voltage
pin2: -7.36V
pin3: -7.36V

Under load (both channels of QSXM2) they are off:
Positive Voltage
pin2: +7.31V
pin3: +6.79V

Negative Voltage
pin2: -6.62V
pin3: -7.21V

Those this means both IC's dead?  :-o

You seem to have the same trouble in both channels so concentrate to one channel to start with. Disconnect the power for the second channel.

First you must cure the very high offset.

Short the input to ground. Measure the voltages for the preamp. Start with this.

Greetings!

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.

Thank you!
Dan
W1DAN

Check that the SSR03 really regulates. Measure the voltage at pin 2 and 3 on the opamp. Those pins should have the same voltage with a millivolt or so.

QuoteMake sure that your incoming voltage is at least 20 V. Your sense inputs are both connected, tin blob on the solder side.

I'm using 15VAC Transformer, this one exhibits 17VAC wich give around 24VDC after Bridge, so.... edit: 22.5VDC To be precise at X2
Blobs are there yes.

Thanks

The SSR03 should be rock stable if you aren't in the current limitation mode. Make sure that your incoming voltage is at least 20 V. Your sense inputs are both connected, tin blob on the solder side.

Hi Again,

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...

Thanks for the help and clarifications,

T.

Hi Again,

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  :-)

Quote from: zayance on December 31, 2020, 10:56:08 AM
C52?  typo prob on the Schematic?. for MC

Yes, type but I can't change that, I'm afraid due to an old design.