G Pultec complete except high boost isn't working

I'm not sure why. I wired ioaudio's inductor up according to the diagram in this thread: http://www.groupdiy.com/index.php?topic=26067&highlight=inductors

I've been checking solder points w/ an ohm meter and I'm not finding any conductivity issues. I'm not sure if the signal is going through the high boost filter circuit or not at this point. What I am sure of is that the high boost/high boost freq. select and high boost "Q" have no effect on the audio.

All the other filter adjustments are working and sounding great.

Does anybody have any hints as to what would cause this? Like accidentally grounding that part of the circuit or something? Not sure what I'm doing wrong.

Thanks,
Jon

look for signal at this points if all other bands are working OK.

good luck

What input transformer are you using?

Thanks guys.

I'm using the Edcor replacement instead of the Lundahl. I'm forgetting the model number, but the one Tat provided those adaptor boards for.

Why?

Tat had the same problem I believe, and Kevin had a similar problem. From memory, I think the 10K:10K input transformer was the problem... Kevin immediately solved his problem by using a 600:600 input transformer. There was a thread about this recently. I don't have it on hand, but you should be able to find it.

I am currently building a few Pultecish EQs, but I'm using old 600:600 iron, so I do not have experience with this problem.

Just passing along some (hopefully good) information. :guinness:

Ah, okay, I had no idea. Thanks.

I actually went ahead and built up a second one which I was also working on and found the same problem, so . . . we'll see. I'm off to find that thread. Thanks Greg.

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Please don't kill me for asking (I'm not trying to be insulting, but sometimes the obvious gets overlooked)...did you link the low and high filter boards together at all link points (there are 5 of them)?

I just built two of the same pultecs with the same iron (wired from a breadboard) with the same inductor (see pic). They work fine.


http://i302.photobucket.com/albums/nn94/lookn4tone/Pultec/Ebay002.jpg

Yeah, I made those 5 connections between the boards.

Okay, I think that probably for my initial tests I wasn't driving it hard enough. Regardless of whether I can get some high boost or not with these 10K:10K trannies, (XS1100 is the model number btw) I think I'm gonna try to either find a 600:600 or 10K:600 input transformer to substitute and see how it sounds.

In this regard, would I also need to change the input loading resistor to something closer to 600R?

I'm guessing simply changing that resistor WITHOUT changing the input transformer isn't going to be the best way to do things. Is this right?

After reading that thread for an hour or so, a lot of it was quite a bit over my head, but I do understand now that the filter network probably ideally needs to be driven by a lower impedance than the 10K:10K input plus 10K resistor are providing.

Luckily, all the gear I can think of in my studio that I'd be plugging into this should be happy driving a 600ohm load, so a 600:600 input should do the trick for me.

I do have on hand a couple of Yamaha PM1000 600:600 output trannies. Do you guys these these would be okay wired in reverse?

first try to hook up a 620ohm resistor across the input transformers secondary (instead of the 10k or just parallel for now, solder one leg, turn on the thing and listen if there´s a difference when you connect the other leg - dont touch HT). you can do that without replacing the input transformer, though the yamaha 600:600 is surely worth a try.

-max

[quote author="JW"]...
I've been checking solder points w/ an ohm meter and I'm not finding any conductivity issues. I'm not sure if the signal is going through the high boost filter circuit or not at this point. What I am sure of is that the high boost/high boost freq. select and high boost "Q" have no effect on the audio.

All the other filter adjustments are working and sounding great.

Does anybody have any hints as to what would cause this? Like accidentally grounding that part of the circuit or something? Not sure what I'm doing wrong.

Thanks,
Jon[/quote]

JW

if i'm getin' it right, HF section is not working at all, so i doubt the problem is transformer or loading related only(I'm not saying loading is not important).
The diagram that I've posted above is showing where to look for a signal.
If you don't mind, i would suggest you to:
1.Switch your pultec OFF.Disconnecting the power cord from it would be the best.
2.Disconnect your input transformer
3.Connect sound source (~1VAC) at "boost hi" pot "top".Plain old music
of your choice would be the best to test with, i would not expect much filter action if feeding 1K test tone and boost 16k on a pultec...
4.Use small "PC speakers" to monitor those points that i marked above
by touching them with the tip of the "PC speakers" input jack (tie grounds together)
5.Point 1 will show if the pot "boost hi" is OK, point2: is there any filtering at all, and so on. Use HF Q control to check inductor action...

Now, this is far from scientific method, but you'll know in no time, where
the trouble is and then you can play with the loading resistor, input transformer and so on...

Hope this helps...

Good luck

Perhaps I misread the original post. If you have no signal at all in the high frequency section, then changing the input transformer is not the solution.

Hey Syn,

Thanks a bunch. Okay, I tested like you said (just used a normal amp/speaker setup on the back end though because I don't have a computer speaker)

The 10K high boost pot is definitely working. As for the other points that I tested, I have to say the filtering is there but it is extremely subtle. I had the music cranked to about +4 on the VU meter of my board when feeding it into the top of the 10K pot, and even then I'm not really getting much boost at all. It's so subtle, that retrospectively I wonder if I was just tricking myself into hearing something. The same goes for the Q. Am I really hearing a bandwidth change? Seems like I am but not obvious enough to be absolutely sure.

Anyway, I think I must have the inductor hooked up wrong. It's weird that that thread that I linked to above no longer exists, but it had the only diagram I had seen that ioaudio posted that shows how to wire the inductor to the Gyraf board.

However, according to the pic in the first picture of THIS thread, http://www.groupdiy.com/index.php?topic=23964&highlight=inductors, it would make sense that I have it hooked up wrong. If pin 1 on the inductor is the input, shouldn't it be going to the first 110mH spot on the board? While pin five should be acting as the last leg of the 22mH inductor on the way out? Can someone confirm this?

the graph is still there, but the black market post dissapeared (they do)


the start, pin 1 of the inductor, goes to the 2,2k "high boost Q" wiper and clockwise position. first tab, pin 2, goes to 22mH, second tab, pin 3 goes to 69mH (22mH + 47mH = 69) etc.
also check with the schematics.
http://www.gyraf.dk/gy_pd/pultec/gy_pd_sch.gif

hope that helps.

-max

Thanks Max,

Well, that's how I have it hooked up, so I'm still at a loss here. Hmmm.

JW

you are welcome, glad if i could help. PC speakers (cheapest possible)
are very useful as their input will tolerate all kinds of crap, levels and mistakes, that is why i like them (paid 2.5 euro for mine) and they are self powered, I even monitored PSU (yes power sup.) and fed large amounts of DC to them (by mistake), and they are still alive. When they die I'll just pull another pair. Be careful with levels as accidentally bypassing the filter section will send around 20dB of level more to your monitoring system and speakers might not like it.... back to your problem.
If you turned the inductor "the other way around" you would still have
all functions and filter action but it would be a little strange like to sharp or too wide but very obvious, so i think something else might be an issue. Now I'd try to remove the inductor
and monitor the output at point 2 of the above diagram, as signal through the caps only would give me shelving filter, and filter action would be heard .If it still does not work, check all the soldering of surrounding circuit you might have no contact or short contact which is bypassing the
HB section all together. If it works then you have to play with the inductor and the Q pot to make them fall in place...

good luck

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Hey guys.

Just wanted to say thanks again. I finally figured out what I was doing wrong. Sort of embarrassing really. I had the in and out flipped on the wires from the filter pcb's to the amplifier.

I wouldn't have imagined that everything working except the high boost would be a symptom of this, but I guess that essentially what was happening was the signal was going through the filter network backwards and the High boost is bypassed? I'll have to study the schematic a little more to understand exactly how that was happening.

Anyway, thanks to this problem, I DO understand the circuit about 10 times better than I did when I started this project. Thanks everybody!

Great that is working and that we learnd one more of the pultec "specials"...Thanks... :thumb:

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