Simetrix graphic EQ

[Ice cream man]

Hi, I am in need of a bit of help here to repair a Simetrix 531E graphic equalizer.

The problem is that the -25db GRN, the 0db GRN and the +10 Amber leds are on all of the time whatever I do with the unit.

I have tested the voltages with respect to the schematic and all seem to be good. I just cannot get to the fault yet.

I have included the schematic and flow chart. If you don't mind taking a look at page three on the schematic drawing where there is the LM339 voltage comparitor and the circuit showng the LEDs.

Any pointers and help is much appreciated. Thank you.

[Jazz P Bass]

List the comparator input voltages.
At idle & also with signal.

Also list U2's pin voltages.
IT may not be the LM339's fault if IC U2 is bad.

[The Dude]

Is there hum on the output? If so, check the power supply for excessive AC. A bad filter cap will cause these things to "wazz out".

[AudioMan]

In order to turn on all 3 LEDs, the outputs of U1-A,B,C must be "off", which under normal conditions means that the DC voltage output of U2-A will exceed 3.462VDC.
Hence, Jazz is correct to ask what voltages you actually measure on U1 and U2.

[Ice cream man]

Ok at last, I got some voltage measurements;

U2, 2068M

pin1, 14v
pin2, 8.1v
pin3, 6.6v
pin4, -15v
pin5, 6.6v
pin6, 6.6v
pin7, 13.8v
pin8, 14.6v



U1, LM339

pin1, 5.85v
pin2, 3.9v
pin3, 14.6v
pin4, 0.82v
pin5, 5.6v
pin6, 0.58v
pin8, 8.6v
pin9, 5.1v
pin7, 5.6v
pin10, 3.2v
pin11, 5.6v
pin12, GND
pin13, 1.9v
pin14, 0.17v


These are the voltages that I get with and without the input signal applied. So the voltages at the non-inverting inputs of U1-A, U1-B and U1-C are all higher than the non-inverting inputs that's why the LED's are on, that's correct, yes?

With the U2 2068M dual op-amp the output on pin 1 is the voltages of pins 2 & 3 added together, and the output voltage on pin 7 is also the addition of the voltages on pins 5 & 6 added together. so is that ok, I don't think that seems right but I could be very wrong.

Should I test on back in the circuit before RLY1 for more info?

I am applying a good 1v input signal,
I did check the PSU caps and they seem ok.

I have included the pin-outs of both chips for quick reference and a copy of page 3 of the schematic.

Thank you.

[AudioMan]

U1-A pin 7 has 5.6V on it and thus turns all LEDs on. That looks normal.

I think U2 is not working correctly.
For example, an opamp has infinite input impedance (not in the real world) and thus no current can flow into the + input, or the - input.
Yet U2 pin3 shows 6.6V when it should be 0V.
U2 pin 2 is higher than U2 pin 3, so the output, pin 1, should be negative, but your measurement say +14V.

[Ice cream man]

U1-A pin 7 has 5.6V on it and thus turns all LEDs on. That looks normal.

I think U2 is not working correctly.
For example, an opamp has infinite input impedance (not in the real world) and thus no current can flow into the + input, or the - input.
Yet U2 pin3 shows 6.6V when it should be 0V.
U2 pin 2 is higher than U2 pin 3, so the output, pin 1, should be negative, but your measurement say +14V.

Yes +14 on U2 A pin1, Is that a type of voltage follower configuration though?

I am figuring that the 6.6v on U2a pin3 is developed across R23. I'm not sure my self. I'll have to go back to page 1 of the schematic and work my way forward from IN+ & IN- (SH3) and follow through from there.

Thank you.

[AudioMan]

U2-A is a 1/2 wave rectifier circuit and charges C3 to the peak value of the AC signal. It uses an inverting configuration. R23 serves as balancing the input offset current to minimize voltage output offsets.
However, U2 pin 3 does not take in any current (except leakage). Therefore, there is no current flow through R23 which in turn means U2 pin 3 must be zero volts. Any higher voltage is due to a malfunction.

U2-B is a differential circuit of unity gain. you could also see U2-B as a balanced-input to single-ended output amplifier.

What voltage do you see on OUT+ and OUT- of U2-B?

[Ice cream man]

Thank you AudioMan,

At U2-B pin5 it is +6.6v and at pin6 it is also +6.6v. I'll get more readings tomorrow at OUT+ & OUT- and
I'll start from page 1 of the schematic and go right through it from J1.

Your help is much appreciated. I do have a scope and a function generator at my disposal if those would be of any use also.

Thank you.

[Ice cream man]

Hi, Things have changed. The main T100mA fuse on the PSU board blew when I switched the unit on. I need to get some new ones and test the PSU board before I can proceed. I'll order up a new 2068M, U2 also.

I have little hope left for this repair and it's draining me out quickly.

[Ice cream man]

Thank you AudioMan, you re-introduced me to electronics, !!!!

[AudioMan]

Does this mean that you got your unit working?