Need help fixing Russian delay unit - is this how a broken BBD chip sounds?

[living sounds]

I bought an obscure Russian analog 2-channel delay. Pretty nice sounding unit, but one channel drops out at lower input amplitutes, distorts and has greater input amplification than the working channel. Since it's build up modular I could nail the problem down to a card, but wasn't able to find the culprit there.

The higher amplification might just be due to a former owner turning trimpots on the card...

Unfortunately, the schematics are in Russian and all the active parts are eastern ones, as well.

Anyway, here's an mp3 of a recording run through the defective part, where the drop outs and distortion can be heard, me varying settings (and dialing input volume up and down at a certain point in the beginning as well):

https://dl.dropboxusercontent.com/u/15815132/VenectTest.mp3

Here are schematics of the card - is there an obvious culprit? 

https://dl.dropboxusercontent.com/u/15815132/SchematicsVenec.jpg

The drop outs occur with the wet signal, longer delay lines may be cut off abruptly if they get too weak in amplitude.

Or could this simply be misaligned trimpots (e.g. too much input amplification causing distortion and maybe an expander/noisegate on the output acting at too high an amplitude) - does this make sense from the schematics? If so, which pot would do what?

It appears to me that the line labled "5" is the input and "6" is the outpout (connectors to the right as well as the corresponding numbers within the schematics). Voltages measure as they should.

I'd love to get this thing up and working, it's a pretty unique unit. Thanks!

 

[JohnRoberts]

Well if you find a BBD in there, the symptom you describe sounds like the input DC bias is set way too low or too high for the BBD. So for low levels it stays saturated high or saturated low, and only passes audio when higher signal level gets up/down into the input voltage range to pass changing audio.  However, looking at that schematic I see some digital memory notation (RAS<CAS< WE), so that suggests digital delay, perhaps an old delta-mod (i bit encoding ).  Perhaps some DC error in a delta-mod input encoder could cause the result to stay saturated one direction of the other for low levels.

Good luck I can't follow that schematic with any clarity.

JR

 

[ungifted]

I can help with translation and searching russian datasheets.
Yes the line labled "5" is the input and "6" is the outpout, here is the full legend:

конт. (contact) / цепь (circuit)
1A - Вход (input)
1Б - -15v
2Б -15v
5А - control Ft
6А - common (ground(?)
7А - common
7Б - indication mode 4
8Б -  indication mode 2
9Б -  indication mode 1
10Б -  indication mode 3
11Б - channel 2 on
12А - +5v
12Б - channel 4 on
13А - +15v
13Б - channel 3 on
14А - +15v
15А - output
15Б - channel 1 on
16Б - operation Ft

 

[jackies]

It is indeed a digital delay - the most common problem I encountered with those were with КР565РУ5Г ram chips - try reseating those if they are socketed.

 

[living sounds]

Thanks everyone! Unfortunately, now the unit doesn't pass audio once it's switched on. The individual delay cards don't get power from the PSU, protective circuitry seems to be either broken or getting a signal not to power the cards.

Here's the entire schematics:

https://dl.dropboxusercontent.com/u/15815132/vensxem.pdf

I don't have much hope however, without circuit descriptions and readily availible parts.

There are no socketed parts, I tried reseating the cards to no avail.

jackies, have you seen one of these before? Is there a western unit this was more or less based on? Poor man's AMS DMX?  8)

 

[Ilya]

The individual delay cards don't get power from the PSU, protective circuitry seems to be either broken or getting a signal not to power the cards.

The PSU seems to be a very simple transistor type with no protection. Check the PSU first, you may have s burnt transistor there.

 

[s2udio]

And if you get the PSU back.......check for clock signals on a scope

 

[living sounds]

The PSU is working, it's the delay cards that don't get powered. There's signal getting through while the unit is turned off, which gets cut off with power on.

 

[StephenGiles]

It would help if the Russians learnt how to draw schematics properly : :

 

[living sounds]

It would help if the Russians learnt how to draw schematics properly : :

I agree.

 

[synthiaks]

Im going to throw in my standard reply... Caps?

Most of my "broken" gear worked fine after a Recap. If this unit is from late 70s early 80s and have as good quality caps as the units in East Germany had, you are going to have at least 10 leaking Electrolyts in there. The recap is something you are going to have to do at some point anyway if you want a unit that is working as it should.

But im not in anyway an expert

/seb