Dynacord VRS-23 bbd delay - mod for endless repeats

[beatnik]

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 ?

 

[JohnRoberts]

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.

 

[Andy Peters]

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.

 

[JohnRoberts]

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]

 

[L´Andratté]

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´!

 

[altemark]

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

 

[nashkato]

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

 

[beatnik]

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 !

 

[beatnik]

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 !

 

[vytenisstaugaitis]

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?

 

[JohnRoberts]

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 

 

[sonashi]

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

Would you be willing to share the adjustments you made? I’m dealing with the same issue and would be incredibly grateful. I love this unit but it start to self osc way too fast.

 

[sonashi]

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?

Did you figure this out?

 

[JohnRoberts]

Did you figure this out?

vytenisstaugaitis was last online here in 2018 so don't expect a quick answer....

JR

 

[nashkato]

@sonashi
how about you introduce yourself to the community first before jumping right in . seems to me you only joined to find a quick and easy fix to this specific problem.
and i cannot tell you anymore than what i wrote years ago .
the manual is still online , read it , use it . if problems arise during that i'm sure there are people that can help you .
but please dont expect an easy fix from me alà : turn pot 5 to 3 o clock , ready .

 

[sonashi]

The introduction part sounds fair, I’ll give you that, I missed the welcome convo. That said, there’s not much to say about me—I’m not exactly a DIY person, but I’m curious and willing to learn, especially since it seems I have a love for vintage hardware. I’ve already reviewed the manual and was seeking clarification. I’m here to learn, not to look for shortcuts. Yes, I joined because of this issue, and I don’t see the problem with that—everybody needs to start somewhere. However, I’m not feeling very welcomed so far.

 

[sonashi]

For those in need of genuine help, there’s a thread about this on the Sequencer forum too, albeit in German. I’ll give this a try (with the support of my service guy), as it seemed to work for someone.

Translation:

"The delay is essentially made up of five BBD chips. Each of these chips has a trimmer (P20, P23, P25, P27, P28) that determines the initial volume of the corresponding delay position (Dynacord refers to this as the 'Kopfpotis' in the adjustment instructions). So, they are still thinking in terms of tape echo with multiple playback heads.
Additionally, there is a trimmer P1 or P3 for the output level behind the expander output: on the schematic of the motherboard on the left, you’ll find outputs A1 and A2. In front of these, there is a buffer stage with I 001 and I 003 (each 1/4 RC4136). The trimmer is located in their feedback loop.
I believe that their setting also affects the ability to self-oscillate. I would start by adjusting P1 and P3."

So, it seems worth starting with the trimmers (P1 and P3), adjusting them to reduce feedback gain incrementally, and then tweaking R016 and R017 if necessary. But take this with a pinch of salt, as I really don’t know what I’m talking about. I'll report back in a few weeks

 

[nashkato]

i didn't want you to feel not welcome .
sorry if my reply induced such feelings , that wasn't my inention .
however , it's been too long since i had my vrs23 for repair and don't know a thing about it anymore .
i got 2 kids now and my diy time is very very restricted , the little time i've left i try to make more music again and if necessary service my gear , not to speak of the cupboards of parts and unfinished projects and repairs i will ever have to time to attend to .

you however should be aware that there's a specific subforum for introductions and that i'm the wrong tree to bark at .

i wish you all the best in translating the manual and your repair .
cheers

 

[sonashi]

I appreciate your follow-up, and I’m confident I can figure things out from here. I took care of the introduction right after, so that’s all set now. Yeah life can get pretty busy, especially with so many things to juggle. I’m sure everything will work out smoothly with the fix. Thanks again, and I wish you all the best with everything on your end too!