Repairing a guyatone tube echo

[fuzzo]

Hi,

I have since few month a guyatone AD-X which is a analog delay using a tube under 12Volts DC. It isn't working , actually it works but not very well.

The dry signal is passing but the delayed signal gives a very loud oscillation that can be "tuned" with the "time" pot .

The PCB is clean , any passive "burned" component (visually) , the double AOPs have a correct bias on pins (6V) .

The circuit uses a an electronic bypass (maybe  it's damaged and creates this problem ?  ) 

3XJRC4858
MN3101
Mn3011

here's the schematic :



I've to say I'm not familiar with analog delay  and his circuitry ,

Any clues to help me  ? things to check more than others ?

[fuzzo]

no one ?

well, I'm doing the "audio probe" technique.

I got sound until C4 (after the tube part and the second buffer) , clean signal which can be bigger when I tun up the "input" knob (increasing the signal coming into the first triode).

After that, the signal from the NE571N is satured (taken at C12 and after the filter of U6b at C16) with the repeat pot the saturation becomes bigger (with the input too but It's normal by the fact it'is placed like a "gain" control).

this part must stay clean , no ?

At C26 , I've a very low signal , lower that bypassed I think, the "input" pot does nothing on the "dry" part .   

So, the NE571N could be dead or damaged, right ?

[maarten]

Yes, the N571 should compress the signal by a 2 to 1 ratio, so the signal thereafter should stay clean. At C 26 both dry and wet signal pass (dry signal is split of before your input pot, so this pot should not do anything on the dry signal). Something is wrong, but I am not sure that it is a damaged 571. Could also be a cap which causes the 571 to malfunction - or something else...
May be you should try to bypass the 571 temporarily, to see whether that gives you a better result (though be it it with some more noise without the 571), then you would have made sure that the problem is somewhere in the 571 and its supporting circuit.
Maarten

[fuzzo]

thanks for your answer

I went seeing the price of that chip , the 571 , a little bit expensive (6 euros at banzai) So I wanna be sure it's the guilty before changing it.

I took some datasheet , I understand the role of the BBD line and its associated driver clock,  but I don't really see what the 571 does on the circuit....

How a cap could produce this malfunction ? 

[maarten]

Shortly: the 571 gets rid of noise by compressing the signal before it enters the BBD, and expanding it afterwards (to restore dynamics of your playing). You might test it by pulling the 571 (observe the orientation of the chip, or make a legible photograph so you can put it back right later on) and connect pin 5 to pin 7 (push a uninsulated copper wire in the pinholes) and check if the sound now is improved after turning the thing on. If it is not, then connect pin 5 to the positive side of cap C12, to see whether this helps. If this doesn't help either, the problem may not be with the 571.

The 571's operation will depend upon some capacitor's like C9 and C11 in the scheme.
For a replacement, you also might have a look at the NE 570 or the SA570 and SA571; essentially the same chip, pin compatible, slightly differnet voltage but probably that wont matter here. I also remember that there is a chip u 1571 (u = the mu-sign) which is the same - however not the NE 1517 IIRC. These might be a bit cheaper, though banzai's price is not that bad - the 571 is not produced any longer and is getting extinct at the moment (as is your BBD chip, might try to find a replacement for that now as well, if you want to use it in years to come and then turn it over to your grandchildren...)
Let us hear if this helped!
Maarten
edit: did not think of this before: when you take out the 571, you are also losing the expander part: to bypass this, connect pin 15 by some kind of wire sticking into it, to pin 10 when you do the experiments above.
M.

[fuzzo]

Thansk again for your answer maarten !

So, I removed the 571 and put an audio probe at "pin10" , there's some sound (with a lot of noise  ) and delayed signal . The sound isn't "clean" like it should be.

But no distortion anymore !

(the delay time seems really short)

[maarten]

So the oscillation is gone? Sound should be lees clean without the 571, but delay should be as normal. To make sure, maybe you'd better probe along the signal path at pin 5/7, before and after C12, R17, R18 etc. all the way to U6a and C22, just to see if the noise enters at a specific place - it sounds like there is too much noise, even without the 571.  I can't understand the short delays; did you flip S2 and fiddle with the delay time knob?

Maarten

[fuzzo]

hi,

I put the circuit aside for a little while and I just went back to it today . And I have clean  sound with delayed signal at output !! 

perfectely clean and the delay controls work great (I even got some beautiful  typical oscillation of analog delay when i play with the time control ) 

The only thing unusual is a tinny "bipppp" (nothing compared to the oscillation noise and chaos of the begining) behind the sound when the repeat and "time" control are at max . I assume that why we use that 571 chips .

So , the 571 was the guilty ? 

thanks maarten for your precious help !

(the pcb is really small and tight to work on )