MEQ mid-range EQ pultec style (was meq500)

[benlindell]

Hey guys, trying to get these off my bench. I replaced the 10k I used for the bass pot and disconnected my bypass switch and I'm still getting super low (like -50db) output. I can hear the EQ working but just at a way low volume. I tried swapping the opamp for a 2520 I have and it's exactly the same. Any ideas where to look/what this could be?

I checked around for bridges and reflowed a few solder points and still nothing, probing around and the biggest loss of gain is after the input transformer and it never gets much higher.

Thanks

 

[shabtek]

ben,
troubleshooting:
you can
bypass the input trans. feed a signal (cap coupled to be safe?) if that works than the xfmr may be duff. backwards?

remove xfmr-- roach clip signal in and out does it work? dont know about this but I assume it is not more than 1:4 or 4:1 ratio.
it could be terminated wrong...if it does not pas signal correctly out of circuit  I would ohm out the pins to get a better sense of what is inside

if eq is questionable
remove the doa.  take the output of the filter network into something that gives you gain...this will confirm what you said that the filter works

break the connection/ trace to the make-up gain opamp. feed it a cap coupled signal does it give signal with gain? 20dB? or whatever.

break it down into functional blocks and verify each one

 

[benlindell]

Thanks for the quick help. I bypassed the input trans and it works just fine, just a tiny bit of extra gain, but the EQ works perfectly. I tried removing the terminating resistor and it didn't affect anything when I unbypassed the tranny. Does this mean my input transformer is bunk?

 

[shabtek]

yes, send it to CJ.






or test it with an ohm meter/and try to pass signal thru it out of circuit

 

[emrr]

Ohm out the transformer windings.  Maybe they're not connected correctly.  I ohmed out all of mine (from which this one came), and they all looked the same with a meter, but I never put audio through them.    I still haven't put audio through my remaining pieces either.  Should be 600:600 input trans, I recall measurements suggesting such, but I don't remember actual DC numbers.  Totally fine to operate unbal without the input trans, with the terminating resistor still there as input load for all other troubleshooting. 

 

[benlindell]

I took some quick measurements.

in+ to in- : 70ohm
out to gnd : 90 ohm
In +/- to gnd : 13Mohm
in +/- to out : no reading, no continutity.

I'm not very familiar with transformer measurements etc, hope this shows something.

 

[shabtek]

remove xfmr-- roach clip signal in and out does it work? dont know about this but I assume it is not more than 1:4 or 4:1 ratio.
it could be terminated wrong...if it does not pas signal correctly out of circuit  I would ohm out the pins to get a better sense of what is inside

i meant terminated incorrectly inside the can/ wired wrong by the manufacturer. try inserting it (trans) in a signal path/using ohmeter to find the 2 coils with 70-90ohms resistance to determine where to make connections.

 

[ioaudio]

hi ben,

readings look ok, transformer should be working, follow the signal from the card/edge connector to the 2.2k pot.
possibly the trace from the transformer in to the front is shorted out? (it´s a bit tight there with the screw)
can you post a pic of the pcb/wiring?

-max

 

[benlindell]

When poking around with the continuity tester, the opamp beeped and showed 50ohms between the out pin and ground.

Here's some pics:

thanks for all the help guys.

 

[benlindell]

bump, anyone see what may be causing my problems?

 

[benlindell]

Hey guys, I finally got my MEQ500s working, ended up being the XLR going into my test rig (dumb). I was able to measure one of my units and the only thing now is that I'm getting a crazy HPF that won't go away. Any ideas guys?

 

[benlindell]

Bump for some ideas on what's causing the HPF.

Thanks guys.

 

[RedNoise]

Did you check if it's the same right after the input tx ?

 

[Rob Flinn]

It maybe the quality of you pictures, but some of the solder connections look like they might be a bit suspect.    Please don't be offended if you are an old hand at soldering ...... I did say might  !!

 

[pacemaker]

Hi Max !
will you have others boards or kits available ?
I would be really interested in 2 or 4 boards/kits,
best regards,
Pacemaker

 

[kilmister]

Plz plz plz  ;D
After GDIY500 there is lot of people who are intrested this eq including me.

-Paavo

 

[janczmok]

I also want two kit / boards

 

[mateus]

Hello guys,

Any news about those?
Kits or PCBs available?

Thx

 

[emrr]

I don't have all the eq caps loaded, but I have two working now.  The high boost bands are sounding cool. 

People indeed using LEDs for biasing ? What measured current-draw do you obtain ?

Those  BD139 / BD140 on the op amp adapter board get pretty hot.  I should probably take a current measurement. 

 

[emrr]

Ran some tests.  Some problems show up.   Pink noise with long settling time at about -10 dB.  

Both units flat response, showing loss of gain relative to unity.  Will need to tweak the 1.5K feedback shunt for unity.  


Both units flat response, 1 kHz normalized to unity.


Response, with (white) and without (red) input transformer.   Mine both look worse than Ben's.
Appears these specific input transformers are not capable of sufficient low frequency response.  The source has roughly 150 ohm output impedance.  Might give better response with a 0 ohm source; not many in the real world.   Maybe Ben's source was lower Z than mine.  
1 dB down @ 160 Hz
2 dB down @ 103 Hz
3 dB down @ 78 Hz
4 dB down @ 64 Hz
5 dB down @ 53 Hz
6 dB down @ 44 Hz
8 dB down @ 31 Hz
11 dB down @ 20 Hz
To be sure, I measured a complete bypass of the units and got a flat line, DC on up.  


Close detail of response, with and without input transformer.

EQ curves for both units, as stuffed so far, input transformer in circuit.  
-11.2 dB cuts
+9 dB boosts

The 4k & 5k cut freq are offset in value exactly to 5k & 7k,  7k is too high, 4k is missing.  Looks like a transcription error that dates back to the Gyraf schematic.  Othewise freq so far are reasonably close.  May or may not experiment with getting them closer.  The pairings between units are close enough to one another.  Will definitely move the upper cuts back into correct position.