Strange EQP1A issue

[stitch-o]

Hi all,
I'm building a pair of EQP1As and have hit a snag on the second one.
These are, P-T-P using a variation of the RecProAudio layout.

So the first EQP1A passes audio and does all the equalizery things a Pultec should do.

The second Pultec (and I've spent some hours A/Bing the 2 builds for any edits/miswires/shorts/etc)
has a strange thing going on:
ONLY WITH BYPASS ENGAGED: the LF Boost acts as a volume knob.
All the way down, nothing passes.
Turn it up, and the unaffected signal passes direct to the output.
When bypass switch is engaged, ALL audio is cut.

I thought I had either the bypass or some of the front panel knobs wired wrong (somehow input is going directly to output amp stage somehow) but I can't seem to find the problem anywhere.

I'll be checking voltages and pull out the scope tomorrow, but
I thought I'd toss this out for any idear from the experts...

Any idears?

CHEERS!

 

[radiance]

I thought I had either the bypass or some of the front panel knobs wired wrong (somehow input is going directly to output amp stage somehow) but I can't seem to find the problem anywhere.

either way...your bypass wiring is wrong. Sort that out first before moving on to the lf boost issue...

 

[stitch-o]

Thank you Radiance.

That was my first inclination in troubleshooting
also, I'm pretty sure the bypass and LF boost thingy are one in the same issue.
and I did double check the bypass switch wiring...

Now I will triple check it!

Cheers!

 

[stitch-o]

Hey stitch,

Check to make sure that the wiper on your LF BOOST pot is at ground potential. One of the other terminals should also be dangling in the wind (not connected).

Find the the common connection on the bypass switch that always has continuity to the common of the LOW FREQUENCY boost switch (not the LF ATTEN side). Check to verify this in both positions. Now bypass the unit, and verify that this same common connection on the bypass switch is at ground potential.

Once you've verified that, move over to the OTHER common connection on the bypass switch and verify that it has continuity with the wiper of the BANDWIDTH pot when the unit is NOT bypassed. Now turn the LF ATTEN pot to the maximum cut position, bypass the unit, and that same common point on the switch should have continuity with the wiper of the LF ATTEN pot.

You can do all of this with the power off. Maybe some pictures would help...

Dr Pat!
As always, with the great tips.
I'll buzz from the switch and take some pics.

Cheers!

 

[stitch-o]

Hey stitch,

Check to make sure that the wiper on your LF BOOST pot is at ground potential. One of the other terminals should also be dangling in the wind (not connected).

Yup

Find the the common connection on the bypass switch that always has continuity to the common of the LOW FREQUENCY boost switch (not the LF ATTEN side). Check to verify this in both positions. Now bypass the unit, and verify that this same common connection on the bypass switch is at ground potential.

yup, yup and yup

Once you've verified that, move over to the OTHER common connection on the bypass switch and verify that it has continuity with the wiper of the BANDWIDTH pot when the unit is NOT bypassed. Now turn the LF ATTEN pot to the maximum cut position, bypass the unit, and that same common point on the switch should have continuity with the wiper of the LF ATTEN pot.

yup and yup
ug...

You can do all of this with the power off. Maybe some pictures would help...

Here ya go.

Cheers and, as always, thank you for your time, Dr. brudda Pat!

 

[Kingston]

Great unit!

excellent point to point work.

extra points for using russian PIO caps, but also DIY-award-of-the-month for latching them down with garden hose ties!

That thing has absolutely no chance of sounding anything but brilliant.

 

[stitch-o]

Sorry Dr. for the short grunts.
Its more about juggling the kids and this right now...frustrated, a little.

I mean "not bypassed" as in "In". When Bypassed, all audio cuts out.


-Thanks for the kind words Kingston.
Sill in the fidgeting stage but they do sound promising!
and built like a tank.

 

[stitch-o]

ONLY WITH BYPASS ENGAGED: the LF Boost acts as a volume knob.

When bypass switch is engaged, ALL audio is cut.

These two statements seem contradictory... no?

please to see my sig
In one statement I am speaking of the switch itself.
The other I am speaking of the action of the switch position.
I guess this is not clear.

to repeat: the bypass switch, when 'IN' will pass audio when only the LF boost is up.
The switch, when flipped down to 'BYPASS' passes no audio.

I read 169.3Ω from ground to first lug on the LF Boost pot with the pot is set to max and the bypass switch is set to 'IN'.
Though the HF Boost and switch and Bandwidth do seem to work when LF boost is at max.

 

[stitch-o]

I read 169.3Ω from ground to first lug on the LF Boost pot with the pot is set to max and the bypass switch is set to 'IN'.

OK that number is too low. Depending on where your LF ATTEN pot is set, you should not see below 2K there. This is where it could get complicated because I don't know your routing scheme, and we need to disconnect a few things.

As a template and starting point, I used the RecProAudio layout:
http://www.recproaudio.com/diy_pro_audio/diy_files/eqp_1a/eqp-1a_layout_final.jpg

With the LF boost pot at Min, and measuring from the open tab to ground I get 9.6K



Cheers Doc!

 

[stitch-o]

OK, that helps a lot...

Let's disconnect the whole filter to verify that the amp is doing what it should be... which I believe it is. Disconnect the red wire from the HF BOOST pot and connect it directly to the junction of your 10K and 1K resistors. Turn the unit on, and your level should jump up ~23db and be fairly flat regardless of where anything is set. Verify this and resolder the red wire back on to the HF BOOST pot.

As expected, that 9.6K reading looks good.

disconnected the red wire (bypass toggle switch common to left lug of HF boost knob) and connected it to junction of 10k and 1k.
You are correct: passes audio with filters doing nada.
still no audio passes when bypass switch is set to "bypass".

reconnecting now.

very interesting development - with the red wire hanging, and nothing attached to the LF boost knob, "bypass" works!
Attaching the LF switch pole connection on LF boost connection point with the red wire, is what kills the bypass from passing audio.
Attaching the HF Atten connection on LF boost connection point with the red wire, drops the signal considerably in "BYPASS" and also does that 'volume knob' thing back up to full volume when "IN".

With JUST the red wire going to the LF Boost left solder tab:
"BYPASS" and "IN" both seem to work as they should.
HF boost and switch works when "IN".
LF Boost works like HF Atten when "IN".

So the issue is somewhere around the LF switch maybe...

SO nice to have bypass going!
So far so good, doc!

 

[stitch-o]

Sounds like a grounding issue with the LF BOOST filter caps. Verify that you have a solid ground to the common side of the caps. I'd ground this directly to the bypass switch... where the pink wire is connected.

Which set of LF Boost caps that are tied together?
The 56n, 27n, etc. should be tied to ground?
The 330n et all set are good. I didn't see a connection for the other set...

 

[stitch-o]

No those are the HF ATTEN caps....

You need to verify that the 330n, 1µ5, 820n and 220n caps all have solid continuity to ground. Move the black wire over 6 terminals to the right (at the 330n cap). But before you modify it, check the continuity to ground as it is now.

BTW, what are you using as a ground reference when you're checking for continuity?

Ah yes, I just got this as far as which set of caps.

I am using one of the ground point chassis bolts as ground reference for measurements.
All the grounding bolts reference to earth via the IEC.

The 330n, 1.5u, 820n and 220n all reference well and strong to ground.
You mentioned 6 terminals...What other two points (besides checking continuity to ground from the 330n, 1.5u, 820n and 220n) should I check continuity with?

Thanks for the patience doc...

 

[jackies]

Way to go stitch-o!
Russian caps and hose clamps, now that's serious!
Got any pics from the front?
Sounds like you wired something wrong, hope you'll figure it out.
Thanks to drpat for his help and patience!
Which exactly filter schematic did you use?

 

[stitch-o]

Clamp on to the upper side of the 330n cap and check for continuity to the center tap of the secondary of the power supply transformer? Now check for continuity from the same point on the 330n cap to the gray wire on T1 that connects to the 620R resistor?

Yessir, checks positive for continuity on both points.

I'm poking around the switch right now...

 

[stitch-o]

I'd check all of your ground connections in reference to the secondary center tap of the power transformer. You should have continuity between here and all of these points:

-The bottom of the 620R resistor that's located directly above T1
-Pin1 of T2 interstage
-Center tap of T2 interstage
-The bottom of the 1K resistor located directly above T2
-The top of the 62K resistor that's located just below the 12AU7
-Pin1 on both XLR connectors
-LF BOOST pot wiper

Hey Pat,

Back on it now...coffee and a digital multimeter

Just went through this list.
Everything has continuity back to the PT secondary center tap (Hammond 270CAX - red/yellow lead).
I'm using the Sowter 9330 and cant find docs on the pins but did follow the color codes from the
layout diagram.

I'm pulling the LF switch to examine and trace all those connections now.

-Jackies - Thanks brudda!
Did you ever wind up getting that russian PIO source?
I'd love some more of these, they sound like golden ear butter.
I think its the original filter section: http://www.recproaudio.com/diy_pro_audio/diy_files/eqp_1a/eqp1a_filter_schematic.jpg

Dr. Pat has the Rx FO SHO!

 

[jackies]

Did you ever wind up getting that russian PIO source?
I'd love some more of these, they sound like golden ear butter.

Yeah well, it didn't quite work out. But I will be going around the world some more later and will definitely look into it. Totally loving the PIO sound though. I was experimenting alot with speaker crossovers and all polyprop caps seem to have some sort of harsher coloration on high frequences, even expensive "audiophile" ones, while PIO seem to sound more natural.

I can also suggest you try russian teflon caps, FT-2, FT-3. They are rather large and only come in smaller values, like 0.22 uF max. In russian teflon is called "ftoroplast", hence the FT designation. This specimen on the picture is like, 2" long!

 

[stitch-o]

Did you ever wind up getting that russian PIO source?
I'd love some more of these, they sound like golden ear butter.

Yeah well, it didn't quite work out. But I will be going around the world some more later and will definitely look into it. Totally loving the PIO sound though. I was experimenting alot with speaker crossovers and all polyprop caps seem to have some sort of harsher coloration on high frequences, even expensive "audiophile" ones, while PIO seem to sound more natural.

I can also suggest you try russian teflon caps, FT-2, FT-3. They are rather large and only come in smaller values, like 0.22 uF max. In russian teflon is called "ftoroplast", hence the FT designation. This specimen on the picture is like, about 3" long!

There is one K72 Teflon I used on the HF side. They are hefty to say the least!
Jackie, do you (or anyone) have experience with burn-in time and PIO caps?
Do they need some 'exercising' before reaching stable sonic performance?

Sorry to hear the hook up didn't work out.
These cost me a bit extra, but, so far, seem worth every penny.

Cheers!

 

[jackies]

Nah, can't say anything about burning in.

 

[stitch-o]

ah...heh heh...uh...

I had the 10K to pole of the LF rotary switch wired to the 12n - pole side of the switch.
Switched the LF Boost connect to the other Pole of the LF select switch and it would seem I have Pultec.

Dr. Pat yooz a lifesaver. MANY humble thank you's for your patience and help!

Heres a pic for ya Jackies:

 

[Kingston]

experience with burn-in time and PIO caps?
Do they need some 'exercising' before reaching stable sonic performance?

No they don't. God only knows how long the russian caps I generally use have been sitting on the shelf. Manufactured from sixties to late eighties. They sound the same the first time you install them and after active duty. One of the great things about PIO caps really. They last "forever".