QSXM2 problem

[zayance]

Hi There,

well back to it, after all this time.

Schematic was printed and DC checking was also before, but i'll start again by redoing that, and see what's up.
But at last check all seem to be fine.


EDIT: No actually i talk too fast, after searching my files, i see that i had measured and noted all voltages corresponding to the ones you provide on your website, and some don't match at all, and here they are, i rechecked to make sure these were the latest:

Power supply +/-15V feeding board correctly

Input side:
R1 at 56K
R2 Jumpered

LED H1 ON
LED H2 OFF

So left channel, MM mode, BC all position, OP07, No Input Bias, input not shorted, SSM2210 and 2220:

DZ1 to DZ4 OK
R1 0.16V
R6 to R17 OK
R26 ok

R27 0.8V
R28 14V
R29 0.8V
R30 14V

R31 to R35 OK

R38 2.3V
R39 2V
R42 0.45
R43 0.66
R44 0.45
R45 0.66
R46 0.45
R47 0.66


Voltage check with Schematic showed that on the RIAA section some values don't match, all other transistors seem to have equal values from positive to negative rail circuit, aside of these:

T51:   
C 9.4V
B 15V
E 15V

T53:
C 0.2V
B 9V
E 9.4V


T52:
C -1.5V
B -15V
E -15V

T54:
C -0.01V
B -9V
E -1.5V

DC Servo

IC1:
pin 1 15V
2 0V
3 0V
4 -15V
5 0V
6 2.4V
7 15V
8 15V


While all parts seem to be "ok" on DMM test, i'd like to have your tought on that.


Thanks a lot for the help

[peranders]

If you short the input, what will the output voltage of IC1 be? Within normal range? Right now it's less than +- 13.5 V = DC servo is working.

What is the voltage at TP1?

If you use "B" transistors instead of "C" you will be increased base currents and by this increases voltages across som base resistors like for instance R39.

You have obviously some problem in the RIIA amp. The pre amp seems to work. Are you having the same problem in both channels?

Voltage across R58, R59, R62 and R63?

DC voltage out at the ouput?

IC2, pin 6? Less than +- 13.5 V?

[zayance]

So with input shorted here are the readings:

IC1:
Pin 1 -15.8V
2 0V
3 0V
4 -15.8V
5 0V
6 +3V
7 +15.6V
8 +15.6V

TP1 0V

Same problem on both channels yes.

R58 5.4V
R59 5.4V
R62 5.5V
R63 5.5V

IC2 pin 6:

-5V shorted input
14.8V not shorted

DC voltage out:

0V shorted input
-12V not shorted.


Thanks again for the help

[peranders]

Since you have the same problem in both channels you must have some mounting error. H2, does it work? Mounted correctly? You can test this with an multimeter and diode test. It should light up some with the test current.

The input stage of the RIAA section seems to be OK.

Do you have the IC5, IC6 installed? I recommend that you short the input when you investigate the DC levels.

Can you measure the voltage across R74 and R75?

[zayance]

Hi there,


IC5 was not installed on previous DC check, but IC6 yes.
I'll have input shorted for testing OK.

R74-75 are at 0V, input shorted, all IC's in place.
H2 is not ON only H1, The LED are at 1.4V on diode check, and they were both working I tested before soldering in place.

After visual check on the RIAA section all transistors are the good ones and they test fine on dmm, and resistors too
I always dmm check before soldering, and I had already gone thru the board before for checking resistors,
So I know they are the right ones in the right place, will recheck again, but I only see a cap value error maybe,
Electro caps are all on right direction etc....

Thanks again!

PS: on lab PSU I read 110mA current draw with all IC's In etc.... Seems it's doing good in terms of power feed, am I wrong?
I think you said about 75mA with BC550, since I use BC550C it draws a little more...


Pretty weird really, Passes Audio tough, but with distortion and lots of crackling...

[peranders]

R74/R75 = 0 mA

What about R72/R73?

[zayance]

R72 0.2V
R73 0.5V

I can't find the time to sit down
and test it all for hours these days unfortunatly, so your help
is much appreciated.

[peranders]

Here you have a voltage less than 0.65 V + some voltage drop over R74/75 meaning shut off V51/52.

You have to go backwards in the chain.

Voltage across R68+R70 and R69+R71?

[zayance]

Back again, took some time.

R68: 0.2V
R70: 0.7V

R69: 0.2V
R71: 0.8V

Going backwards in the chain we have:

R62: 5.5V
R64: 4.5V

R63: 5.5V
R65: 4.4V

R59: 5.5V
R61: 9.5V

R60: 9.5V
R58: 5.5V

Thanks again for your help.


T.


PS: I've tried the preamp using another turntable/Amp etc... a Revox B790 mine is a Rega RP3, and funnily only one channel exhibits
the dirt in the signal using the Revox turntable and amp  :-o, so i'll compare readings again from one channel to another and see what's up.
I'll keep posted if i finally find the fault in this one

[peranders]

Voltage across R72 is 0.2V (should be 1.3 V)

What is the voltage between base V50 and negative rail? Should be 1.3 + 0.65V

Is the transistor right type and correctly mounted?

[zayance]

V50 and T50 is a BC550c un the right direction.
I read 1.062 V between base and negative rail.

Here is a small Audio capture of each channel recorded with Portable Mic recorder right on front of one speaker,
just for you to hear what i'm talking about, well it seems it's more dominent at startup then after some time,
but the distorted sound is there on one channel.

Right Channel (Seem to be a good one)
https://soundcloud.com/zayance/right-channel

Left Channel (you har the distorted sound, it's obvious on the first fade out and the middle section of the sample)
https://soundcloud.com/zayance/left-channel


I'll need to check overall sound when this channel is hopefully up and running, and tweak things, the working channel seem to exhibit a little too much bass for my taste, but not there yet.

[peranders]

I'm going to vacation so I can't help you for a while but we have this:

One channel is working, that's good.

The other channel seems to have problems at the VAS stage, V52 etc. Go backwards and see why the base voltage is so low at V50. Trouble is in the input stage or the in the current generators at the emitters of the input stage.

Compare my values with your working channel, all voltages. Calculate all currents (U/R).

Check all resistor values!

[zayance]

Hi Peranders,


No problem, have a nice one!

And yes since one channel is working then i can compare etc...
I supsect it could be some transistors that are bad, i remember encountering
same kind of problem with some other build, and it was because of the transistors.
I had checked all resistors twice, i'll do on the problematic spots, but i'll change transistors that
seem to be having wrong values in the circuit as well, might solve the problem.

Anyway, thanks for the support and i'll keep in touch.


Best,


T.

[zayance]

Hi Per,

As you know i left this one aside for some weird probs concerning the build for years;
I'm taking my time and try to give it another chance.
I've desoldered everything on one channel, and kept preamp stuffed on LEFT Channel, stuffed with BC550/560 transistors (the circuit that wasn't quit working right).
The Right Channel is stuffed with SMD BS850/860 and all BC847/857BS.
This for giving me a comparison, because i suspected that my batch of BC550/560 might be causing some problems, so tought of comparing with smd ones, might be stupid, might be a good idea, will see.....
Anyway, still didn't powered and started the test according to your build, and didn't stuff the RIAA, going step by step for now.

I just wanted to clarify some infos on the build, because circuit description on your website and the infos provided on the Schematic confuses me a little, if you can just confirm these:

From Circuit description on your website:
R1 + R97 are the values that should match the MM pickup
R3 + R97 are the values for matching  MC Pickup

Info On Schematic:
R2 Resistor for MM pickup ?  (circuit description gives this one in addition to C2? as EMI filtering)
C1 for MM
C52?  typo prob on the Schematic?. for MC

Thanks for your help and clarifications...

Will ask questions or update on the old forum thread if this comes to an end haha.

Best,


T.

[peranders]

C52?  typo prob on the Schematic?. for MC

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