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Dyna Pas-3 restored but no bass at all

hrudu

hrudu

Guest
I have a Dyna Pas 3 pre-amp, restored, hooked up to a VTA St70 or a Dyna ST70 and it sounds fantastic with CD or tuner.
Now with a turntable it sounds good but no bass coming out at all. All tubes are still original.

I tried with a different Pre-amp hooked to the same system and the bass is 'normal'
So I am thinking something is wrong with the phono section in the Pas 3.

Maybe replace the tubes in the Pas 3?

BTW, I asked the same question over there @ Bob Latino's forum.

Please help.
Thanks
Gin
 
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By chance, do you have a tape deck hooked up to the tape monitor outputs? If so, that would explain things. The tape monitor output is not buffered- so loading it with a tape deck (commonly 47K input impedance) can cause a rolloff in the bass (the coupling caps aren't big enough to have a low enough corner frequency, with that low of a load resistance across them).

Regards,
Gordon.
 
When it was restored, what did it turn into? Is it stock boards, or are you using something different? Were any changes made to the phono section or to the tone controls?

Also, is it only on phono that it seems weak, or does this happen for all sources?
 
No, I don't have a tape deck hooked up to the Pas-3.
It has been recapped and I don't think changes were made to the phono section.
I hope you guys can see more from the pics:
Before:

And after:


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The two caps on the right edge of the phono board are the output caps. If those were replaced with a smaller value, it will kill bass output. Uncalibrated eyeball makes me think those might be 0.01uf, and I think the original were 0.1uf.
 
The cap values look right, as best as I can read them in the original image. You still have all the carbon composition resistors in there, and they tend to drift a lot. Check the value of those. I don't know how reliable the potted circuits are, but those are the eq and if they were off it could cause the bass shortage. The best thing to do with any phono preamp is plug in an inverse RIAA network and check the response. The phono stage uses metal film resistors for a couple critical locations (plates), but not the output stage. If you change the resistors in the output stage to metal films, the noise level will probably drop by quite a lot. Make sure the operating voltages are correct, including filaments. BTW, the Black Cat capacitors AFAIK are film and don't usually need to be replaced unless upgrading with polypropylene.
 
batt, I will try to clean the switch and see.
Can you or anyone show me which one are the RIAA caps?

Thanks


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The big ones on the selector switch itself, right there in the middle, though I don't remember which two are for tape and which are for phono. I'd honestly give some consideration to soldering those straight onto the board and eliminate the switching entirely. That was done to switch between tape head and phono eq settings, but nobody uses tape head anymore.
 
hrudu said:
batt, I will try to clean the switch and see.
Can you or anyone show me which one are the RIAA caps?

Thanks


~~~
Click to expand...

The 68 pf , 2750 pf , it is position 8 on both channels , i believe they are mounted on the selector switches on the left hand of the photos above .
 
Looks like an error in the RIAA Eqalization circuit. Was the problem (lack of bass with phono) already there before the mods ? If yes, it could be a wiring mistake around the input selector switch, broken wire or poor connection in the phono preamp eq circuit.
 
gadget73 said:
The big ones on the selector switch itself, right there in the middle, though I don't remember which two are for tape and which are for phono. I'd honestly give some consideration to soldering those straight onto the board and eliminate the switching entirely. That was done to switch between tape head and phono eq settings, but nobody uses tape head anymore.
Click to expand...

I concur with this. Not only does it give a better mechanical connection- but electrically- getting rid of the extra wiring length can also make the phono preamp subject to less noise and interference. By hard-wiring the RIAA EQ directly onto the board, it also would allow for all three low-level inputs (phono, tape head and whatever the other one is labeled- I don't recall right now) to act as phono inputs- allowing up to three turntables to be connected to the preamp, at one time.

I also concur with the suggestion to check the coupling cap values. If one of the caps is off by a factor of 10, like suggested, then it could definitely make for a problem- especially if it's one of the two output caps on each channel.

Regards,
Gordon.
 
The other one is "special". Non-equalized microphone input essentially.

I was actually looking at re-working that part of mine. I did the Curico RIAA thing a while back, but after some discussion here it seems that the original setup was pretty damn close. Looks like its a matter of moving two caps to the board, changing 2 to come out to the same effective value as when the selector is in RIAA mode, and pulling the wires. Easy-peasy.
 
GW, BR and G73,
Thanks a lot for the comments.
I will try to 'move the caps to the board'.

BTW, batt, I did clean the switch over the weekend and can hear the improvement now.

Thanks a lot.


~~~
 
hrudu said:
GW, BR and G73,
Thanks a lot for the comments.
I will try to 'move the caps to the board'.

BTW, batt, I did clean the switch over the weekend and can hear the improvement now.

Thanks a lot.


~~~
Click to expand...
Glad that helped , moving the caps is a better solution if you're never going to use a reel to reel tape machine
 
Hi, The same user and the same owner of the same Pas-3 here.
Question; can I use the FMa-AMb input as a line input?
 
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