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PAS 2, Lost high freq w/ mod

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I have a friends PAS2 that has had the Curcio RIAA mod done. I bypassed the tone controls and lowered the heater voltage to 11.6, it was over 13VDC. This thing sounds really detailed and clear with just the linestage working but the phono stage has lost all the high freq response. it sounds muffled. I changed the caps between the 12ax7 triodes with .047 black beauties, the .1's caused motorboating. the output caps i started with .1's and the sound was dead and lifeless on the high end, .05's made it better and .031's made it even better. Is it a problem with the RIAA mod? What else can i do to bring back the highs?

Second issue, when connected to my DIYtube st-35 i have almost no hum from the preamp, i connected it to the owners counterpoint sa-100 and it hums like crazy. no grounding combination we could come up with fixed the problem. when i disconnect the interconnects the hum goes away for the most part. is there something i have to do to the loading of the preamp output to get rid of the hum when connected to the counterpoint? it's a 60hz hum that gets worse when i grab the interconnect, changing out the interconnects did not fix it.

Help!
Paul

Follow Ups:

RE: PAS 2, Lost high freq w/ mod - GP49 21:22:07 03/08/11 (21)
Are you still using the original Dynaco selector switch?

If so, did you remove the equalizer parts on it?

GP49
- RE: PAS 2, Lost high freq w/ mod - paulcblais 22:44:51 03/08/11 (20)
  no the selector switch is stock, i did wire the grounds direct though. is that where the highs are lost? how should i change that part? bypass or wire direct to the board?
  - RE: PAS 2, Lost high freq w/ mod - GP49 07:15:08 03/09/11 (18)
    There are parts on the selector switch which are in the original RIAA equalization scheme. They need to be removed, because the Curcio modification has all of the equalization parts on the circuit board.
    
    The original Dynaco design had equalization parts on the selector switch because it provided for the preamp to be used with either a moving magnet/iron phono cartridge, or a tape head without a gain stage. To do this it had to be switched between RIAA phono equalization and NAB tape head equalization. It also had a provision (the SPECIAL input) for use of the phono stage as a flat amplifier for use with microphones.
    
    Most users only use phono in their systems, so most modifiers wire the phono input directly to the input of the PAS phono stage without provision for those alternate uses of the phono stage, thus removing switch contacts from the signal path. But in any case if you do the Curcio RIAA modification...or any of the others...those equalization parts need to be removed from the selector switch. Also, the wiring that goes to/from the switch to change equalization among RIAA, NAB and flat, needs to be eliminated.
    
    Follow the instructions from Curcio...or anyone else...to the letter.
    
    GP49
    - RE: PAS 2, Lost high freq w/ mod - sbalfour 17:43:37 03/20/11 (1)
      I always wondered whether the selector switch parts belonged to the
      RIAA or NAB/special equalization. I need the NAB and microphone
      equalization because I have a 1964 Sony reel-to-reel with tape head
      output. How do I do the RIAA mod, and keep the others?
      Stuart
      - RE: PAS 2, Lost high freq w/ mod - GP49 20:12:19 03/20/11 (0)
        
        In Reply to: RE: PAS 2, Lost high freq w/ mod posted by sbalfour on March 20, 2011 at 17:43:37
        
        You cannot do that with Joe's modification.
        
        If you want to do that, you may as well leave the stock Dynaco parts in place. Or recalculate all the RC values.
        
        GP49
    - RE: PAS 2, Lost high freq w/ mod - paulcblais 07:51:02 03/09/11 (15)
      Thanks, that makes perfect sense. i'm cleaning this thing up after someone else did the mod and refused to even consider he did it wrong or incomplete. i'll wire the phono input direct and put the output coupling cap back to the .1uf it is supposed to be.
      Thank You so much
      Paul
      - RE: PAS 2, Lost high freq w/ mod - GP49 11:27:24 03/09/11 (1)
        
        In Reply to: RE: PAS 2, Lost high freq w/ mod posted by paulcblais on March 09, 2011 at 07:51:02
        
        "...I'm cleaning this thing up after someone else did the mod...
        
        NOW we know that someone else did the modification, so you started off with his mistake. That can be difficult to trace.
        
        Some people can't decipher the physical layout of a circuit board as it relates to the schematic. That could have happened here.
        
        GP49
        
        RE: PAS 2, Lost high freq w/ mod - paulcblais 12:09:03 03/09/11 (0)
        
        In Reply to: RE: PAS 2, Lost high freq w/ mod posted by GP49 on March 09, 2011 at 11:27:24
        
        I agree completely, i hate fixing the mistakes and laziness of others. at this point i have verified not only the curcio mods but the stock circuitry. the only changes i made was cleaning up the grounds to a common point at the new Cap board instead of the old can cap. the highs are still muffled but the bass seems strong and clear. i'm considering adding the tone controls back in to see if that makes a difference. Suggestions?
      - RE: PAS 2, Lost high freq w/ mod - paulcblais 10:22:35 03/09/11 (12)
        
        In Reply to: RE: PAS 2, Lost high freq w/ mod posted by paulcblais on March 09, 2011 at 07:51:02
        
        Ok so i have everything done as per the curcio mod, the phono input is wired directly to pc-6. all the components are exactly as they are supposed to be. i have been changing the output coupling cap to everything from .015uf to .22uf with minimal improvement on the highs with smaller value caps. the bottom end is great the midrange ok but the highs sound like there is a heavy wool blanket over the speakers. the linestage sounds great with the tone controls bypassed as per Curcio. Where do i go next to get the highs back?
        
        RE: PAS 2, Lost high freq w/ mod - xaudiomanx 15:07:17 03/09/11 (11)
        
        In Reply to: RE: PAS 2, Lost high freq w/ mod posted by paulcblais on March 09, 2011 at 10:22:35
        
        I would go back and retrace all the steps and make sure the correct components are in their proper postions as well as the proper values. This all has nothing whatsoever to do whether the tone controls are in the circuit or not. The tone controls are within the linestage part of the preamp.
        
        That phono is a as simple a circuit as can be. What is wrong has to be something simple and just being overlooked. But make sure you deal with the caps on the selector switch as well.
        
        If need be, unsolder all the RIAA components and redo everything. As I said before, "it can't be much!"
        
        Paul
        
        RE: PAS 2, Lost high freq w/ mod - paulcblais 18:07:22 03/09/11 (10)
        
        In Reply to: RE: PAS 2, Lost high freq w/ mod posted by xaudiomanx on March 09, 2011 at 15:07:17
        
        sounds good to me, i'll remove and verify all the RIAA components then reinstall as per the directions.
        
        RE: PAS 2, Lost high freq w/ mod - sbalfour 17:57:23 03/20/11 (3)
        
        In Reply to: RE: PAS 2, Lost high freq w/ mod posted by paulcblais on March 09, 2011 at 18:07:22
        
        while the components are off the board, don't just read off the values;
        check the components with a DMM or LCR meter. You might have a defective cap. Is the treble bad in both channels, or only one?
        Stuart
        
        RE: PAS 2, Lost high freq w/ mod - paulcblais 20:35:00 03/20/11 (2)
        
        In Reply to: RE: PAS 2, Lost high freq w/ mod posted by sbalfour on March 20, 2011 at 17:57:23
        
        it's equally bad in both channels. the only caps i haven't checked directly are the RIAA caps, the 820pf and the 2200pf paralleled with the 470pf. they are labeled with the proper values though. These came from Curcio audio so i assumed they are correct. i'll check those out next.
        
        RE: PAS 2, Lost high freq w/ mod - paulcblais 10:40:43 03/21/11 (1)
        
        In Reply to: RE: PAS 2, Lost high freq w/ mod posted by paulcblais on March 20, 2011 at 20:35:00
        
        i think i found it. I assumed these parts came from Curcio Audio since i am fixing what someone else has done but they couldn't have. measuring capacitance with my meter i come up with 3.18nf on one channel and 3.21nf on the other. This is my whole problem isn't it? i need to get the right values of 2200pf and 470pf to get 2670pf.
        
        RE: PAS 2, Lost high freq w/ mod - paulcblais 11:35:26 03/21/11 (0)
        
        In Reply to: RE: PAS 2, Lost high freq w/ mod posted by paulcblais on March 21, 2011 at 10:40:43
        
        My questions, is a standard meter capable of measuring capacitors accurately? it does have that option but is it good enough?
        will measuring these caps on the board cause the numbers to change? should i remove them from the board to measure accurately?
        
        RE: PAS 2, Lost high freq w/ mod - xaudiomanx 08:55:45 03/10/11 (5)
        
        In Reply to: RE: PAS 2, Lost high freq w/ mod posted by paulcblais on March 09, 2011 at 18:07:22
        
        Very important! Just remember to do the entire mod as it is stated. Joe knows what he is doing.
        
        RE: PAS 2, Lost high freq w/ mod - paulcblais 10:28:37 03/10/11 (4)
        
        In Reply to: RE: PAS 2, Lost high freq w/ mod posted by xaudiomanx on March 10, 2011 at 08:55:45
        
        I hear you there. I don't claim to be an expert by any means. i'll let you know when i get done and what i find. i'm leaving the linestage alone since it works well and sounds incredible with the tone controls bypassed.
        
        RE: PAS 2, Lost high freq w/ mod - paulcblais 09:49:23 03/14/11 (3)
        
        In Reply to: RE: PAS 2, Lost high freq w/ mod posted by paulcblais on March 10, 2011 at 10:28:37
        
        So i finally got back to this thing and i found an issue. on the modification paperwork the pic shows a 24ohm resistor for each channel on the input to the first triode. the PAS schematic shows a 10ohm resistor for that spot and that's whats in this one is a 10ohm. should it be changed to the 24ohm like the Curcio fig-2 pic shows on the PC-6 RIAA mod? that's the only difference i can find.
        
        RE: PAS 2, Lost high freq w/ mod - paulcblais 09:46:52 03/15/11 (2)
        
        In Reply to: RE: PAS 2, Lost high freq w/ mod posted by paulcblais on March 14, 2011 at 09:49:23
        
        So i've been through every component on the PC-6 board and verified their values. the 10k resistor has been changed to 24ohm as per Joe's pic of component values. the sound is still muffled. the highs are lacking. From that point i had to change the cap between the triodes back to the stock .05 because it started motorboating with the .1 in there. the output coupling cap sounded bad with the .1uf so i dropped it to .058uf then .03 and the sound improved but not enough. Somewhere in the circuit the highs are getting clamped down.
        
        Where do i go from here? I'm sure it's something simple but i cannot seem to figure it out.
        
        I'm doing a direct A/B comparison through a DIYTube ST-35 with a set of Zigmahornet speakers and a Dennon direct drive turntable with a Shure M91 cartridge. the other Phono preamp i'm comparing it to is a $30 MCM cheapie box that sounds really clear and clean.
        
        RE: PAS 2, Lost high freq w/ mod - Corndog71 14:07:45 03/16/11 (1)
        
        In Reply to: RE: PAS 2, Lost high freq w/ mod posted by paulcblais on March 15, 2011 at 09:46:52
        
        Have you tried different tubes on the phono board?
        Rob C
        
        The world was made for people not cursed with self-awareness
        
        RE: PAS 2, Lost high freq w/ mod - paulcblais 16:06:37 03/16/11 (0)
        
        In Reply to: RE: PAS 2, Lost high freq w/ mod posted by Corndog71 on March 16, 2011 at 14:07:45
        
        i have tried multiple tubes with the same result. RCA 7025's Telefunken 12AX7's, and 5751's. everything seems to be as it should be. something is muting the highs. i hate to recommend to this guy to replace boards if this can be fixed. unfortunatly i don't know what he started with since someone else did the mod, i'm just trying to fix it. he says the stock PAS2 sounded great until this other guy convinced him to do the RIAA mod, it hasn't worked right since. i ran my little MCM phono stage thorugh the linestage and it sounded great. I'm about out of options here.
  - RE: PAS 2, Lost high freq w/ mod - xaudiomanx 06:16:31 03/09/11 (0)
    There are a couple of caps on the selector switch that has to be dealt with. It should be in the instructions to the mod kit unless you got the schematic off the Curcio site. But I think even on the site there is instructions on what to do. You have to do the full mod, not part of it.
    
    Paul

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