Cant understand...racking Mccurdy EQ (((((FIXED))))

Racking 2 x Mccurdys 8992 eqs here.
Im having some weird problems.

A balanced signal will result in kinda unbalanced signal with poor audio and low gain... When eq bypassed all is ok.
an unbalanced signal will result in a balanced signal out..

Another weird thing its that before racking them i tested them in same connectors and everything worked just fine!

Schematics is here: http://www.groupdiy.com/index.php?action=dlattach;topic=45018.0;attach=8316

Hi 3nity,
(i guess its normal but it's weird that the first input series resistors are not matched R3 and R4) i am not an expert here but have you tried to verify the CCMR of the input section,feeding an unbalanced tone to input connector + and - simultaniously and check the output of the first IC with a scope out of curiosity? or compare some measurements against a working module if you have one that is known working? i know this sounds obvious but also check the usual stuff, like visual inspection of your wiring, continuity test, your cables...
good luck

Its weird too i saw that they are not matched but i thought they were ok
i cant test against something else both of them dont work as it should.

Im not sure if they will need a recap since they are 30 years old...

i will try the tone as you said..

(((What i try last night just for sake was touching IC13 at pin # 2 and i had a balanced loud pop!))

no one else to chime in??
Thanks.

wwhat kind of connectors? are they grounding against the case possibly?

That input stage will have very poor CMRR. Bad design. They have at least tried to impedance balance the two input legs, but in so doing have made the stage not balanced for gain. It would be fine if you whacked a transformer on the front of it. Re caps, if they are thirty years old they are very suspect, with bipolar stages like this the electro's end up with no polarising voltage on them, and this causes them not to form well, and shortens their effective life. I would be trying to remove them completely.
Electro's in the signal path BAD!

Radardoug.
Are you saying what i understand?? bypass them completely..

Abe.
Connector is an EDAC 15 position just like in the 500 series.


In my opinion theres a bad electro somewhere or one IC gone bad..
Thanks.

3nity said:

Abe.
In my opinion theres a bad electro somewhere or one IC gone bad..
Thanks.

Click to expand...

Yup that would be my first attack
New electros and reflow all solder pads,sometimes I spot faults during a rework.

Check rail voltages and ripple at all ics to various ground rail points.

heres one thing while testing i checked voltages and proceed to adjust them and i heard some changes in the audio but couldnt get a good result in audio..however i got the volts adjusted well!

> not balanced for gain.

??

14,300/5360 = 2.668
10,500/3920 = 2.678

0.4% unbalance.

Gain is 0.375; appropriate for taking +26dbu into a +/-15V system.

It worked fine when McCurdy made it. Whatever the problem is now, is probably age or mis-connection.

heres the back connector.
http://www.groupdiy.com/index.php?action=dlattach;topic=45018.0;attach=8354

Im about to recap one channel and put new IC's.
We'll see what happens.
Thanks.

Well you said they worked fine before you racked them.

So what has changed?

My first reply I was referring to your input/output connectors. I know sometimes when I use trs they ground out against the case when I'm not necessarily wanting them to.

Anyway I'm sure you will get it!

before racking them i tried them on a bench and use the same connectors nothing was grounded per to say..just Sleeve to shield on the connector..

PRR said:

> not balanced for gain.

??

14,300/5360 = 2.668
10,500/3920 = 2.678

0.4% unbalance.

Gain is 0.375; appropriate for taking +26dbu into a +/-15V system.

It worked fine when McCurdy made it. Whatever the problem is now, is probably age or mis-connection.

Click to expand...

1% balance gives 40 dB CMRR. .1% balance gives 60dB CMRR. So the CMRR will be modest at best.
I get very annoyed seeing this same bad design balanced input stage used. No wonder people like transformers!
That said, I agree, it must have sort of worked when McCurdy designed it, so it could well be a component failure, but it was never a great design, and when you look at the trouble they went to elsewhere (output stage filtering), it is surprising. But it is the cheapest balancing stage you can do.

Im confused as hell here  ???
Recap done on 1 channel as well as all ICs changed.

Now PRR suggest mis-connections???
Im using TRS connectors for audio here...T= +, R= -, S= Ground.
If i do this the signal gots better but out of phase: T= -, R= Ground, S= +.

What used to be the standard 30 years ago anyone??

Thanks.

In think im done..Its working a lot better i just got to verify phase.
What i did: Tip to - and Ground to + Shield goes unconnected..

Anyone care to comment??

Well, could these be pin 3 hot?

3nity said:

In think im done..Its working a lot better i just got to verify phase.
What i did: Tip to - and Ground to + Shield goes unconnected..

Anyone care to comment??

Click to expand...

You have to be careful with the output stage; it is electronically balanced NON-FLOATING.
If you short it to gnd at pin 11, you get a very small distorted signal, if you short it at Pin 10, the level will be 6dB down and there will be distortion from the shorted opamp.