valley dynamite problems

GroupDIY Audio Forum

Help Support GroupDIY Audio Forum:

This site may earn a commission from merchant affiliate links, including eBay, Amazon, and others.

Rob Flinn

Well-known member
Joined
Jun 3, 2004
Messages
5,207
Location
Between Sussex, UK & Aude, France.
Hello

My aging valley dynamite (in the plastic box) has died on one side.

1. Does anyone have a clear readable circuit diagram.  I have a copy but the res is so bad I can't properly read the component values or designations.

2. The problem seem sto be that somewhere in the sidechain is locking the VCA control port down to -10v & the meter is permanently showing full GR.  U4 TL074 is getting pretty hot & I'm getting -10v on pin 1, & -13.5 on pin 7.  I swapped out the TL074, but no joy there.
I pulled the resistor that feeds the control port & it then passes a signal. 

Question, I'm guessing that the above proves that the TA101 is still ok ? 

Is it possible for the solid state switches to lock in this way when they go bad ?
 
Like I said I don't think it's the vca, because if you lift R88 you should find it passes a signal. So it should be fixable, since that is the only non obtainable part !

I thought it might be the make up pot gone one legged, because that provides a fixed v summed with the sidechain signal. It seems to work ok.  I'm beginning to suspect U6 which is an analog switch multiplexer thing  ( http://uk.rs-online.com/web/p/analogue-switch-multiplexer/6622659/ )


I think I'll get one from RS & a new TL074 on monday, probably Tuesday before they get it in & have a play.
 
I'm on tour till Friday, so I'd be interested to hear your findings Rob...love my Dynamite for crushing the hell out if drums and really want to get it functioning again!
 
Rob Flinn said:
Like I said I don't think it's the vca, because if you lift R88 you should find it passes a signal. So it should be fixable, since that is the only non obtainable part !

I thought it might be the make up pot gone one legged, because that provides a fixed v summed with the sidechain signal. It seems to work ok.  I'm beginning to suspect U6 which is an analog switch multiplexer thing  ( http://uk.rs-online.com/web/p/analogue-switch-multiplexer/6622659/ )


I think I'll get one from RS & a new TL074 on monday, probably Tuesday before they get it in & have a play.

Yup, some of that old 4000 series cmos can give up the ghost for no reason.

If you get schematics you should be able to troubleshoot pretty easily.

Even if a VCA were to die, you could probably roll a passible replacement from something like those new THAT transistor arrays, or just select some transistors and pot them (that's what Paiul did. ).  ;D

JR





 
Hi John

Thanks for the response. 

I'm looking at the chips available to me at RS & the only one available in single quantities is an HEX4053BP rather than the MC14053bcp.  It's some time since I did anything with CMOS chips, I'm sure I used to know what the differences in series of CMOS chips were but it totally escapes me now.  I'm thinking that there will be no real problems in subbing the chip for this application, since there's no fast switching happening etc.



 
Without looking up those specific numbers, they are probably TG (transfer gates), back to back connected mosfets to serve as analog switches. I suspect they would drop in and work, but perhaps look at the data sheets compared to old cd40xx series for any differences.

JR
 
Hi Rob
Evil bay has these ,bought some the other day for an a&h mixer
Analogue swiches ,200 ohms on resistance
http://www.ebay.co.uk/itm/CD4053BE-IC-4000-CMOS-4053-DIP16-5V-X-2-PCS-/250816288640?pt=UK_BOI_Electrical_Components_Supplies_ET&hash=item3a65d0d780
+ put a socket in  ;)
 
s2udio said:
Hi Rob
Evil bay has these ,bought some the other day for an a&h mixer
Analogue swiches ,200 ohms on resistance
http://www.ebay.co.uk/itm/CD4053BE-IC-4000-CMOS-4053-DIP16-5V-X-2-PCS-/250816288640?pt=UK_BOI_Electrical_Components_Supplies_ET&hash=item3a65d0d780
+ put a socket in  ;)

Thanks for the tip.  I can get the HEX version much cheaper at RS, so I'm going to try them first.  On the DYnamite boards every IC is socketed anyway so changing them is easy.
 
Ok I swapped out U4 & U6 and it's doing the same thing.  In fact I swapped out all the 4053's & TL072's & it does the same thing & the 5534.  So it's back to the drawing board at the moment !!

The only other chip in there is the transistor array CA3346.  Not sure how likely it is to blow or whether that could be the cause even if it was.

Anyway keep you posted if I make any progress.
 
If you have a simple VOM start working backwards from the VCA looking at the control voltage for where it is getting trashed.

While the circuit may look intimidating, the individual blocks are mostly simple opamp adders and simple log math on the side chain, so troubleshoot the individual blocks and don't be scared by the whole thing.. of course an error upstream will reveal downstream, so don't get hung up by the actual voltage at a block, only if it is working correctly for the inputs it is actually getting.

You should see the side chain voltage change as you vary threshold pot, etc.

If you have a scope you can inject a sine wave and follow the signal forward.

For any who were following the precision rectifier thread look at U3, while a little more complex than most this one is using a technique to reduce the dead band at zero crossings, Diode D3 is holding both diode connected transistors biased at roughly 1/2 a diode drop,,, this way the opamp has much less distance to slew to turn one off and the other on.

The log conversion to convert dB to mV is done in U5-U7.

The problem could be caused by something as simple as a bad solder connect or bad trim pot anywhere in the whole side chain...

I have always been a fan of Paul Buff's design work and this is one of his typical clever circuits.

A good read for folks looking to learn log transistor and opamp tricks...

JR
 
The problem could be caused by something as simple as a bad solder connect or bad trim pot anywhere in the whole side chain...

Exactly, I remember I had a bad solder joint at the threshold pot in my unit that had a similar problem...

/ nille
 
John thanks for the  tips.  The problem is definitely around U4 pin 7 from my V measurements.  Also U4 is getting very hot so it must be dumping some current somewhere.  I can't see any shorts & have reflowed the soldering in that region

Checking the bad channel against the good one some of the resistor readings are low, but if I measure the resistors lifted then they measure ok ........................ Hmmmm  :-\
 
Resistor values don't usually change and incircuit readings can be inaccurate.

U4 looks like it is not doing any heavy lifting,,,

look for valid input pin voltages wrt output state, etc.

U4 section diving VCA should have output voltage near 0V dc at equilibrium, for well behaved VCA,

If CV to VCA is pegged moderately hard off that could be hundreds of mW, not hot but warm.

If the opamp output state is valid for input voltages keep looking, elsewhere.

JR
 
Back
Top