Dynacord VRS-23 bbd delay - mod for endless repeats

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beatnik

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Oct 18, 2009
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Hi everybody

I just purchased a vintage Dynacord VRS23 analog delay and figured out that mine is the later version, it doesn't go into endless repeat mode with the feedback control turned all the way up

I was wondering if there could be a way to modify the unit for self oscillation ?

I guess something has to be changed in the feedback path of the delay signal but I don't really know how

Maybe someone more expert can have a quick look at the schematics and suggest a modification ?

Here you can find a copy of the service manual and larger scans of the schematics which are clearer to read

http://www.florian-anwander.de/dynacord_vrs23/

It seems that R040 on the front panel board is the feedback control pot, this is connected to ne570 companders on main board. Maybe something in this path needs to be altered in order to increase the feedback amount ?
 
BBDs degrade the signal with every pass through so multiple passes will turn into noisy mush.

Using high regeneration gain to get a lot of repeats can run away if regeneration gain is more than unity.

Be careful what you wish for....

JR

PS: A companding noise reduction compresses input signal 2:1, and expands output 1:2 to reduce BBD noise. Manipulations of the compressed audio will get expanded 1:2 by the output expander, making high repeat density more difficult (expanded 1:2 so will decay faster).  Regeneration can be performed outside the companding loop but that can introduce other problems while solving the faster decay.
 
beatnik said:
Hi everybody

I just purchased a vintage Dynacord VRS23 analog delay and figured out that mine is the later version, it doesn't go into endless repeat mode with the feedback control turned all the way up

I was wondering if there could be a way to modify the unit for self oscillation ?

Seems to me that the right way to do this would be to take the BBD chain output and route it back to the BBD chain input, instead of feeding the instrument input to the delay chain.
 
I do not recognize the BBD ASRs  but I know my way around ne570 companders and they are doing some unusual tricks to that one. 

Pin1 (16) of ne570 should normally be a cap to ground to smooth the rectified signal for the 2:1 compressor... Instead they connect the 0.47UF cap to an opamp that bootstraps the cap at low level but is clamped at  2 diode drops so very fast for less than 2 diodes, normal 0.47uF slow attack above 2 diodes. As I recall that node inside the 570 is something like a 10k feeding a base junction (actually current mirror) so cute non-linear fast attack compressor. They do a similar fast attack trick to the expandor smoothing pin (16). They add some Dc current into both pins 1 and 16 which will limit how much noise reduction it will have, (less compression boost, and less expansion attenuation). Perhaps to prevent mistracking at low level.

JR

[edit-  I am pleasantly surprised to see the trick fast attack circuits around the NE570 compander IC and dynamic range manipulations. I thought I was the only person doing stuff like that.  ;D Good chance the rest of the design is good too.

That said too much recirculation gain is not wonderful sounding, and path frequency response errors get exaggerated by multiple loop throughs..  So have fun but perhaps lower your expectations.  [/edit]
 
First is this schematic the older or newer version? (Didn´t see a mention of that...)
Sorry no technical expertise from me as usual, but isn´t R024 the ´Regeneration´ pot?

How about patching up an external feedback path with pos. gain via your mixer, just for kicks?
I know that delay and it is wonderful, just the incarnation of the much cited  ´analogue warmth´!
 
I did this simple mod as posted on the Swedish 99musik.se forum to mine, and after owning three different vrs-23s  I finally have one with "lagom" feedback.

http://www.99musik.se/showthread.php?342825-Modifikation-av-Dynacord-VRS23-vill-ha-evigt-eko&p=3093051#post3093051
 
you got the SM ?
Did you do the test and adjustment instructions ?

i recently repaired one of these and had the opposit problem that there was only a small margin in which the unit wouldn´t go into endless repeat .
most of these older Dynacord units do have problems with dried up Tantalums  , a recap followed by adjustments did the trick .

here´s the SM
 

Attachments

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thanks to everybody for your replies

from the information i found online, mine should be a later version, that's why it doesn't go into endless feedback

i don't think there is any problem really  (I've had already replaced all electrolytics and tantalums and calibrated) from what i have found out online this is just the way the later versions work

it seems that altermark solution should work, and i am going to try that tomorrow. thank you altermark that's a very nice first post !

 
i have modified the unit as suggested and it now goes into endless feedback

the mod just consists of altering the resistor values of R016 and R017 on the front panel board.

original resistors are 39K, I have added 100K in parallel as suggested, worked perfectly

thank you !
 
nashkato said:
you got the SM ?
Did you do the test and adjustment instructions ?

i recently repaired one of these and had the opposit problem that there was only a small margin in which the unit wouldn´t go into endless repeat .
most of these older Dynacord units do have problems with dried up Tantalums  , a recap followed by adjustments did the trick .

here´s the SM

Just got mine and it has this problem - it starts to self oscillate way too quick just past 4. Maybe you could assist where and what has to be done to fix this problem?
 
vytenisstaugaitis said:
Just got mine and it has this problem - it starts to self oscillate way too quick just past 4. Maybe you could assist where and what has to be done to fix this problem?
If he increased the recirculating signal by lowering the 39k resistor, maybe make it larger to reduce the recirculating signal...

Is the signal clean otherwise? There may be some other set up trims that need adjustment.

JR 
 

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