Serious imbalance with SSM Version

[Cel]

heaveno P-A,

I'm building a QSXM2 SSM Version for MM pickup, power supply is JSR04 +/-15V. I have two problems right now:

1.I have actually finished the board itself, and as I have gone through the voltages I have measured some really disturbing values around the servos IC1/IC3. I think I have a huge imbalance on both channel:
R27 0.755V
R29 0.762V
R28 13.44V
R30 13.44V
DC-ADJUST_L to ground measures 14.27V
Preamp out is on 9mV
The exact same happens on the right side as well. What could cause this?

2. Another problem are the voltages of R42-R47/E42-47, they all measure over 500mV. The temperature of transistors T31-T36/V31-V36 goes up to 85-90 celsius. I understand in this case one should shunt the LED with a resistor. I measure 2V on my LED, in the VAS stage runs 11mA(?) , so my LED represents around 182Ohm resistance. Normal voltage would be 1.6V so I need to bring down this resistance to around 146Ohm. That would mean a shunt resistor of 1Kohm (with the LED it would make 154Ohm). Is that correct?

Otherwise everything seems to work, other voltages seem to be fine.

Thanks in advance!

Cheers,

Marcell

[peranders]

The pre amp seems to work and you have the same error in the correction amps. It's a build error I think. Do you use BC transistors for the input stage of the corrections amp?

I have measured in almost all points. Please measure yourself and see where you start to have a big difference.

Short the input when you do the measurements.

[Cel]

heaveno P-A,

thank you for your reply. I use BC550C and BC560C everywhere except for the couple of SSM-s. I have measured all the points, most of them are around 5%-10% of your values, except for R27-R30 and maybe R38,R39 and R14. I have remeasured some of the resistors around of them, but I could not find anything suspicious.

Should I try to disconnect R93 (RIAA amp) and measure everything without the IC1 and IC3 in place? Or this would damage something?

I post all my values, both L and R side (on R side I measured the equivalent parts of course), input shorted:

Measured part	Reference	MyAmpChnLeft	MyAmpChnRight	DeltaL  (%)	DeltaR  (%)
DZ1	6,96V	6,75V	6,75V	-3	-3
DZ2	6,96V	6,76V	6,75V	-3	-3
DZ3	7,08V	6,76V	6,72V	-5	-5
DZ4	7,09V	6,82V	6,71V	-4	-5
R1	-9,5mV	0V	0V	-100	-100
R6	491mV	442mV	462mV	-10	-6
R7	438mV	518mV	508mV	18	16
R8	5,85V	5,71V	5,68V	-2	-3
R9	5,88V	5,63V	5,64V	-4	-4
R10	5,86V	5,73V	5,70V	-2	-3
R11	5,90V	5,64V	5,66V	-4	-4
R12	4,80V	4,71V	4,65V	-2	-3
R13	4,83V	4,63V	4,62V	-4	-4
R14	200mV	266mV	272mV	33	36
R15	299mV	274mV	281mV	-8	-6
R16	5,64V	5,39V	5,41V	-4	-4
R17	5,61V	5,47V	5,44V	-2	-3
R18	5,64V	5,40V	5,41V	-4	-4
R19	5,61V	5,47V	5,44V	-2	-3
R20	5,64V	5,40V	5,42V	-4	-4
R21	5,61V	5,48V	5,45V	-2	-3
R22	5,64V	5,41V	5,42V	-4	-4
R23	5,61V	5,48V	5,46V	-2	-3
R24	508mV	529mV	529mV	4	4
R25	518mV	526mV	526mV	2	2
R26	1,65V	1,712V	1,714V	4	4
R27	9,15V	823mV	829mV	8895	8960
R28	3,65V	13,51V	13,53V	270	271
R29	9,12V	812mV	825mV	8804	8946
R30	3,62V	13,51V	13,50V	273	273
R31	1,68V	1,701V	1,702V	1	1
R32	1,56V	1,646V	1,645V	6	5
R33	1,58V	1,638V	1,641V	4	4
R34	926mV	1,008V	1,003V	9	8
R35	926mV	1,001V	1,009V	8	9
R38	834mV	1,071V	1,151V	28	38
R39	855mV	1,308V	1,324V	53	55
R42	325mV	362mV	371mV	11	14
R43	319mV	389mV	369mV	22	16
R44	321mV	381mV	372mV	19	16
R45	319mV	365mV	374mV	14	17
R46	316mV	368mV	381mV	16	21
R47	323mV	366mV	368mV	13	14
C3	6,46V	6,21V	6,23V	-4	-4
C4	6,40V	6,29V	6,27V	-2	-2
C19	5,28V	4,63V	4,49V	-12	-15
C20	5,30V	4,79V	4,62V	-10	-13
H1	1,82V	1,84V	1,84V	1	1

[Cel]

heaveno again,

in the meantime I have measured all the resistors in the preamp section and compared the values with my excel BOM and with the schematic as well, but all checks out well. I have checked all the transistors also, they are all soldered where they belong. As far as I can tell, everything seems to be fine.

Could it be, that the difference between the SSM2220 and the SSM2210 alone cause this much of imbalance? Could I just reduce the imbalance with changing some resistors so the servos have some more headroom, and leave it at that? Probably not a cold solder joint or damaged transistor or something like that, because both channels have the same problem.

Or should I try to measure without the servos, as I've written in my previous post? I'm a little bit worried, because I'm running out of time as well (this would be a Christmas present).

I have tested it with small signals (not with a scope yet), and it works without any audible distortion.

[peranders]

Try to reduce R27 and R28, connect 100 k in parallel over those resistors and see what happens. I think the reason is that SSM2220 and SSM2210 has so different current gain.

[Cel]

I have tried the 100KOhms parallel.The Voltage of R28 has gone down to 9V R27 gone down to 553mV, however DC-ADJUST_L to ground hasn't changed  :-(

[peranders]

You are still outside the range of the DC-servo. Can you try to measure the base current to see how much it could be?

[Cel]

Sorry but I'm not sure, where can I measure the base current? You mean the current from the JSR04? I can measure around 170-180mA on the  +15V output of the JSR04. Or you mean the current of the base of one of the transistors?

[peranders]

Connect 1 kohm across the input and measure the voltage between R96 and R97. There you have 270k + 270k.

[Cel]

I have removed the short from the input and put 1Kohm between input and ground. I can measure between ground and R97/R96 (so practically the voltage on C63) -1,132V on the left channel and -1,143V on the right.

[peranders]

1.14V/(270k+270k) = 2.1 uA rather normal since the SSM2220 has a rather low Hfe.

Do you have one SSM2220 and one SSM2210 per channel?

If you short the input, do you have zero volts across R2? R2 = 1kohm?

[Cel]

I have tried it with one SSM2220/2210 per channel (with 3.9Kohm-s) as well, but had the same problem with the servos. Right now I have put in the full 4 pairs (with 15KOhm-s) per channel. Rather unnecessary I now, I think I will go with 2x pairs per channel and 7.5Kohm at the end.

Yes R2 is 1Kohm right now and I measure 4mV on it with input shorted.

[peranders]

Let roll back a bit. You have 9 mV offset voltage on the preamp output? The only bad thing is that you could have a few millivolt less offset if the servo not was saturated.

Notice that the SSM's need 1 mA or more in collector current in order to give low noise. The BC's need 200 uA. Without changing too much one pair SSM's will be enough.

You could experiment with using lower values on the 270k resistors. Which servo opamps do you use?

Do you use red LED's?

[Cel]

Yes,I have 9mV on preamp out.

So one SSM2220 and one SSSM2210 with 3,9Kohm-s should be  the way to go?

I use the OP07 for all six opamp.

[Cel]

Yes I use red LED-s, although I had to solder 220Ohms parallel, because on the LED there was over 2V, and I had over 500mV  on the resistors R42-R47.