Urei 545 EQ Band Bypassed

[Siegfried Meier]

Hey guys,

I just acquired a pair of these eqs in mint shape from the original owner - somewhere around 1977-1979 these were built.  One of them works perfectly, and silent as anything.  The other, I noticed when the mains were cranked there was a bit of mid range hiss - not crazy loud, but definitely much louder than the first unit.  Upon further inspection, I noticed that the mid-band wasn't working at all - no boost, no cut.  I swapped the IC's from one band to the next as they were the same, but it made no different.  What exactly would cause this bypassing of the mid band? 

Thanks!
Sig

 

[abbey road d enfer]

It looks like the biquad filter is not working. You have to check the outputs of the IC at pins 1, 7, 8 & 14. With a sinewave of suitable frrequency applied to the unit and some boost dialled, there should be signal at all the outputs and no offset.
It's also possible a pot is broken.

 

[Siegfried Meier]

Weird, I opened up the bottom and observed that the pot had been resoldered at some point.  It looked like there was a cold solder joint, so I reapplied it to all the pads on that pot.  Now the entire channel won't work when I engage it...?

 

[Siegfried Meier]

I'm checking the IC marked R16 directly to the right of the pot (not sure why ICs are labelled R?) - it has no resistance across pins 1/2, but all the others do.  It appears the IC is dead?

They're 16 pins - what are these?  Not sure if I can get these...

 

[Siegfried Meier]

Super pain in the arse getting this thing out too.  I've put in a socket now - why would they socket the others and not these?

IC reads 1736 RA08-5.1KN

 

[Siegfried Meier]

This appears to be it?

http://www.halted.com/ccp19088-dip-5-1k-16-pin-8-isol--resistors-ra08-5-1kn-1478.htm

What are these things?

 

[Siegfried Meier]

Hmmmm now that I've resocketed that IC, it appears the measurements are fine compared to the others in the circuit...perhaps I wasn't getting a proper reading before.

Why is there absolutely no signal now though?  All I did initially was resolder that pot...

Edit - swapped out IC3 from the working unit, still no go - so I guess the IC is fine.  I went ahead and swapped out all the ICs from the working unit, and they're all fine.  Any ideas on what else to check?  Most bizarre thing ever...

 

[mjrippe]

Hi Sig,

They are not socketed because they are not ICs.  That is a resistor pack - eight 5.1k resistors in a 16 pin DIP package.

Did you remove the old solder from the pot before adding new solder?  It could be contaminated and still not making good contact.  Check the rest of the board for cold solder joints as well and check that pot for proper resistances while you've got it open.

Mike

 

[Siegfried Meier]

Ya, I figured out those were resistor packs...don't see those too often.

Yup, I even took out the entire pot and checked to make sure nothing was shorting out underneath.  Quite a mess now - even with a desoldering tool, the pads are so ****** and thin that they lift.  Put it all back together, tested all the points and still no sound.

I don't get it...

 

[Siegfried Meier]

Got it...the previous band resistor pack has to connect to one of the pins of the next pot.  Trace seemed to be dead, even though it was being fed on to the diode.  I jumpered it and it came back to life.  It's unfortunate I damaged so many other traces and wasted so much time on this though...it wasn't until the pot was fully removed that I even noticed those were supposed to be connected.

Thanks for your help!
Sig

 

[mjrippe]

Hi Sig,

Glad you got it sorted!  Nothing like a cheap PCB to make life difficult.

Mike

 

[Siegfried Meier]

Haha I know eh?  Of course, if I had just jumpered right away I would have saved myself a lot of trouble...but alas, that's the way it goes with troubleshooting.

And my silly brain, I'll forever think that the 2nd EQ won't ever possibly sound as good now because of what I've done to it.  Why does that happen to me?  Just get over it dude...