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Welcome to my DIY audio forum.

Cheers
Per-Anders Sjöström

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#21
Phono RIAA amplifiers / Re: QSXM2 troubleshooting!!
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?

Cheers,
Dan
#22
Phono RIAA amplifiers / Re: QSXM2 troubleshooting!!
Last post by peranders - January 30, 2021, 11:20:35 AM
Try to figure out if the DC servo range is too small.

If the IC5 is not populated: Apply +15V at pin 5, check output and see if it moves
Apply -15V at pin 5, check output and see if it moves

I recommend that you use a variable power supply for this test so you can adjust the voltage in pin 5 from 0 to 15 V and 0 to -15 V.
#23
Phono RIAA amplifiers / Re: QSXM2 problem
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.
#24
Phono RIAA amplifiers / Re: QSXM2 problem
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....


Thanks
#25
Phono RIAA amplifiers / Re: QSXM2 troubleshooting!!
Last post by W1DAN - January 24, 2021, 07:59:44 PM
Greetings Per Anders!

Here is my QSXM2 preamp section voltage check report.

Test environment:
Input shorted
Power applied to left channel only
IC1 (OP07C) inserted. IC5 not inserted.
B&K model 391 DVM
V+ at X3: +15.002V
V- at X3: -14.993V

Actual voltage readings in (). Questionable results have "!" added
DZ1, 6,96V (6.870v)
DZ2, 6,96V(6.698V)
DZ3, 7,08V (6.606V)
DZ4, 7,09V (6.617V)
R1, -9,5mV (00.00mv, input is shorted to ground)
R6, 491mV (492mv)
R7, 438mV (800mv)!
R8, 5,85V (5.633v)
R9, 5,88V (5.590v)
R10, 5,86V (5.551V)
R11, 5,90V (5.643V)
R12, 4,80V (4.575V)
R13, 4,83V (4.619V)
R14, 200mV (320mv)!
R15, 299mV (241mv)
R16, 5,64V (5.384V)
R17, 5,61V (5.391V)
R18, 5,64V (5.327V)
R19, 5,61V (5.360V)
R20, 5,64V (5.409V)
R21, 5,61V (5.441V)
R22, 5,64V (5.330V)
R23, 5,61V (5.274V)
R24, 508mV (254mv)!
R25, 518mV (262mv)!
R26, 1,65V (811mv)!
R27, 9,15V (14.058V)!
R28, 3,65V (203mv)!
R29, 9,12V (14.030V)!
R30, 3,62V (185mv)!
R31, 1,68V (823mv)!
R32, 1,56V (550mv)!
R33, 1,58V (540mv)!
R34, 926mV (44mv)!
R35, 926mV (53mv)!
R38, 834mV (117mv)!
R39, 855mV (71mv)!
R42, 325mV (430mv)
R43, 319mV (351mv)
R44, 321mV (447mv)
R45, 319mV (514mv)
R46, 316mV (561mv)!
R47, 323mV (500mv)
C3, 6,46V (6.100V)
Vbe, T1 580mV (632mv)
C4, 6,40V (5.970V)
Vbe, T2 550mV (553mv)
C19, 5,28V (6.453V)!
C20, 5,30V (6.440V)!
H1, 1,82V (what and where is H1?)
PREAMP_OUT_L, <+-0,2mV, changing with temperature fluctuations and air movements
(TP1 -240mv)
(J1 -13.5mv)
(preamp side of R93 lifted -182mv)
DC-ADJUST_L, -5,66V, can go between +12-13V to -(12-13)V depending of balance. (fixed and steady -14.311V)

Extra DC Voltage readings to ground:
T1 BC560 collector 5.839v
T2 BC550 collector -5.803v
T13 BC550 emitter 3.822v
T14 BC560 emitter -3.674
T15 BC550 emitter 3.720v
T16 BC560 emitter -3.650v
C17 412mv
C18 -700mv
Junction R50 and R51 -17mv

Thank you!
Dan
#26
Phono RIAA amplifiers / Re: QSXM2 problem
Last post by zayance - January 24, 2021, 03:25:33 PM
Yes Vreg is at 18VDC at pin 3
Voltage over R47/49 is at about 550mV.

hmm so i guess i should get a 2x18VAC Tranny here.

Thanks for the support Peranders, i'll keep in touch on this one, and hopefully
will be working as it should and ready for fitting in case etc....
#27
Phono RIAA amplifiers / Re: QSXM2 problem
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
#28
Phono RIAA amplifiers / Re: QSXM2 problem
Last post by zayance - January 23, 2021, 05:28:09 PM
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
#29
Phono RIAA amplifiers / Re: QSXM2 troubleshooting!!
Last post by peranders - January 18, 2021, 07:10:33 PM
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.
#30
Phono RIAA amplifiers / QSXM2 troubleshooting!!
Last post by W1DAN - January 18, 2021, 06:13:30 PM
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