Help finding the odd fault

[pucho812]

Trying to figure this fault out. Got a unit on the bench that is 8 channels of eq. Line level in and out at +4dbu via that balancing chips.  Running audio through the unit the level is +4 on every channel in bypass. If I turn in the eq and leave it flat, level is still +4. This is all normal operation.

So here is the fault one channel out of 8 drops level by about 1.5 db when the eq is inserted as described above(with the eq flat) , I have gone over everything and all parts are the same per channel.  All the chips are the same and all parts are the same. All things seem to test out the same.

I am out of ideas. About the only thing I haven't done is swap the 10uf bp input caps the are in series with each leg of the balanced input going in right before the balancing chip as well as swap the balancing chip input.  Hmmmm...

 

[JohnRoberts]

Is the 1.5dB drop flat wrt frequency? 
Is boost cut symmetrical (same amount of boost as cut?

how complex is EQ?

When EQ in does level drop at input to eq or eq output?

It's always the last thing you check.

JR

 

[ruairioflaherty]

If you have working examples you should be able to scope/probe your way through the circuit and determine where you are losing the level in the bad unit relative to a good one.

 

[pucho812]

Is the 1.5dB drop flat wrt frequency? 
Is boost cut symmetrical (same amount of boost as cut?

how complex is EQ?

When EQ in does level drop at input to eq or eq output?

It's always the last thing you check.

JR

Yes it is... o.k. the unit is 8 channels of a tone control, single knob, 7 out of the 8 work normally.  Each channel is balanced with that chips for the I/O. the channels with out the eq inserted have same level in and out. The 7 working channels  will have same level  with or with out the  tone inserted as long as the tone is set to flat(no boost or cut) the one odd channel has the same level in and out without the eq insterted and  does a 1.5db drop when the eq is inserted and the tone controls on that channel are set to flat.  So the fault is that the unit is not behaving like it's brothers when set to the same conditions.  I know it's not the opamp and I know it's not the pot. All the resistors test the same and all the caps are the same. So I am down to either the that chips on the in or out or caps in the audio path but was thinking that of it was a cap there would be a bigger difference in level

If you have working examples you should be able to scope/probe your way through the circuit and determine where you are losing the level in the bad unit relative to a good one.

Agreed, except I can probe the working channels and the faulty channel and I get the same results.  which is even more odd.

 

[Brian Roth]

Do you have a schematic to post or link so we can take a look?

Bri

 

[joaquins]

If aren't resistor, opamp or caps there are two other (but strager) possibilities the PCB and the switch... what other components has it?

JS

 

[pucho812]

Do you have a schematic to post or link so we can take a look?

Bri

I do not, but I can sketch one out. It's a simple circuit. Normally I see things like off by 6db due to it unbalancing for some reason at the balanced output or the level being way down to a faulty opamp but the 1.5db drop is a good one

If aren't resistor, opamp or caps there are two other (but strager) possibilities the PCB and the switch... what other components has it?

JS

It's not the switch, I swapped it out  just in case even though the switch was testing perfectly.

 

[JohnRoberts]

I am repeating myself, does the EQ work properly otherwise? Is it flat when set flat, symmetrical boost/cut.

Is the 1.5dB loss occurring inside the EQ? i.e. how does input to the eq stage compare to good channels, how does output compare?

It must be something, but should be relatively easy to see where it is losing the signal compared to other good channels.

JR

 

[pucho812]

Yes John the 1.5 db loss is  happens when the eq is in the path.  I have the unit on the bench and will in full force monday. I'll keep you guys posted

 

[12afael]

I would test every stage with a scope and a signal generator,  comparing with a working channel. those 1.5db are going to somewhere.

 

[pucho812]

got it. Don't take long to probe it out.  it"s always the last place you look too. faulty that chip. Replaced the chip and all good now...

 

[12afael]

nice