[BUILD] CAPI VP2X~500 Series~Preamp Kit~Official Support Thread

[haima]

someone with a better understanding of opamp design theory would have a clearer idea of what would be damaged by reversed ClassAB NPN PNP pairs.

my thinking was - they are cheap parts, just replace them and get on with life - but i have a large stock of these kind of parts, if you have to order new resistors it'd be worth checking them first.

when you have the transistors removed you could measure the resistance of the resistors - if they check out ok you're probably fine... no need to replace them.

personally i wouldn't worry about the brand of either the resistors or the transistors - just use what you can get easily.

 

[Whoops]

Hi,
first of all I want to thank Haima, Jeff and Silent Arts for the helping me in my build.

I replaced the transistors in the OPamps, re-checked all the solder joints and components in the boards, and I also soldered the components from the top of the Pre board and not only from the bottom.

The Opamps are not burning any more, they seem fine, although there's no sound coming out of the preamp.
I can't figure out where the problem resides.

I did some measurements,

measure the voltages present at DOA sockets - without the DOA installed

at socket V+   there's  15,57 volts
Socket V-  there's   -15,59V


I measure the resistances also, my DMM doesn't measure resistance above 2Megs , so above that there's no read.

A) what's the resistance of from the opamp output pin socket to ground/0v?

Above 2Megs, no read

B) resistance from +ve to -ve on the opamp sockets?

Above 2Megs, no read

C) resistance from +ve on the opamp socket to ground?

Above 2Megs, no read


D) resistance from -ve on the opamp socket to ground?

Above 2Megs, no read


Sometimes when I measure the resistance in this cases, the multimeter starts to read 1500 and then gradually increases the value , until it reaches 2M and them stops reading, like if it went to +infinity


I checked the resistance after the T Pad attenuator, and it seems to be performing fine, the resistance value decreses when you turn it up, reaching zero when it's turned full clockwise


there's any other tests I could perform to help the troubleshooting?

thanks a lot once again
my best regards

Tiago

 

[3nity]

try another DOA in working condition first..
If it burns then the fault its on the VP board..

 

[jsteiger]

Might be a silly thing to say but is the "Mute" switch functioning at all?

 

[3nity]

could be that other components burnt when the darlingtons were switched.

 

[Whoops]

try another DOA in working condition first..
If it burns then the fault its on the VP board..

Hi 3nity, I've tried 3 Opamps today, the 2 I already had, and another one that was working in a 312 pre.
None Burned , but there was still no sound coming out from the pre

Might be a silly thing to say but is the "Mute" switch functioning at all?

Not silly at all, I thought exactly about the same thing.
I measure continuity from the orange and red wire of the output transformer to the switch, and when in normal position  there's was continuity from those wires going to the mute switch and then to the Phase switch.
When the mute switch was pressed there was no continuity going to the phase switch at all.

Maybe there's some better way of checking this....

thanks a lot guys

 

[Whoops]

could be that other components burnt when the darlingtons were switched.

Maybe, but all the resistors in the VP26 board seem fine, they retain the blue colour as new.
The capacitors also look fine, at least i couldn't notice any change, but I don't know how do you see if a capacitor has burned.
The resistors I know that they burn and you can easily see it, the capacitors I dont know if they blow or can be damage inside without you even noticing it.

 

[haima]

whoops - do you have a way to trace signal - like a signal injection probe and a little amp? if not you should make one... you can make a probe out of a old cable and a DC blocking capacitor. have a search online.

that way you can start at the input or output and see where the signal stops (if it does) - you shouldn't need to even have the preamp powered up to do most of this.

A) signal should make it from the inputs on the card edge to the opamp input

B) signal should make it from the opamp output pin to the output on the card edge.

 

[Whoops]

whoops - do you have a way to trace signal - like a signal injection probe and a little amp? if not you should make one... you can make a probe out of a old cable and a DC blocking capacitor. have a search online.

that way you can start at the input or output and see where the signal stops (if it does) - you shouldn't need to even have the preamp powered up to do most of this.

A) signal should make it from the inputs on the card edge to the opamp input

B) signal should make it from the opamp output pin to the output on the card edge.

yes Haima I have a probe that I used in the past to trace signal in some guitar pedals I built.
What signal should I inject? what should I use?

I don't need to power the preamp to trace it?

thanks

 

[haima]

hey whoops, you need two things:

1) a signal source (signal generator, ipod etc) with a cable that ends in alligator clips or some other type of clips

2) a signal probe to listen to the signal source


with the opamp removed and the preamp NOT powered, sitting on your bench, check the input up to the opamp non-inverting input:

A) connect signal source, to "In Hi" & "In Lo" (pin 10 & 8 on the edge connector, or the side of the 750R resistors that doesn't connect to the 160R resistor)

B) connect the 0v/ground of the probe to a 0v point on the  preamp.

C) probe the non-inverting input pin on the opamp - you should hear the signal source.

D) if you don't trace the signal from the input, following the schematic - see where it stops.


i'll write similar instructions for the output section if you need it...

 

[Whoops]

Hi Haima ,
thanks a lot for taking you time to explaining.

I did signal probe tracing today, the sound was interrupted in the connection from the phase switch to the Mute Switch. The traces are on the top of the board, but you cannot reach the pins in the switches from the top, to solder them also.
I redid this soldering points as much as I could, problem seems to be solved (I have continuity now from phase switch to mute switch) the signal doesn't seem to be interrupted any more.

I will check tomorrow in the studio in the rack and with a microphone.

Thanks a lot for all your help, I'll let you know how the conclusions tommorow.

 

[haima]

let's hope that's it!

 

[surfkat]

HAIMA - thanks for writing those explicit troubleshooting directions. I would appreciate it if you could write instructions for troubleshooting the output as well as I am still struggling to get one of my VP26 pres to work properly. I took sometime off for Christmas and went back to it today. Jeff suggested that I might have a problem with the attenuator or the output trans wiring. I double checked all of that today but found no problems. I made a signal probe today and was going to check the circuit tomorrow and came here looking for help with that. Thanks

 

[haima]

no probs... here's one way to trace the output stage:

opamp removed, no power supply attached, preamp sitting on the bench.

A) connect your signal source - attach the "signal" lead to the opamp "O" (output) pin socket. attach the "0v/sheild/ground" lead to a 0v point on the preamp.

B) connect your probe "0v/sheild/ground" to the same 0v point on the preamp.

C) probe the opamp "O" (output) pin socket to confirm you have your signal source. you should hear it!

D) probe the output transformer primary "red" connection - you should hear signal. if not try actuating the phase and mute switches. still nothing? do step E. otherwise skip to step F

E) trace along the schematic to follow the signal out of the opamp output pin, from left to right (C10, phase switch, mute switch) till the signal stops - your problem might be around there.

F) now test the output all the way to the end - with your signal generator still connected as before, connect your probe "0v/sheild/ground" to the "black" lead on the output transformer or card edge pin 4.

G) probe the "blue" lead of the output transformer - got signal? if so,

H) probe the card edge pin 2 - turn the output attenuator - got signal? if not look at the attenuator wiring, everything on the output transformer secondary.

GOOD LUCK!

BTW - it's always good practice to be careful with your alligator/jumper clips - not that you can hurt anything with the preamp un-powered like we are with this test - but if you DO have the unit powered up for other tests it can be easy to cause damage if you let the clips short across power supply rails etc...

 

[Whoops]

Hi Haima,
tested the Pre today in the studio.

Everything seems to be working fine, so the late problem was really realted to the connections between phase and Mute Switch, the traces are on top of the boards but it's impossible to solder the switch pins , because they are close together.

Anyway, I'm really happy.

thanks for you help and Jeff also.

Now I need to fix one of the Gar Opamps that burned, I replaced the resistors ans BD transistors and it still doesn't work, but I keep that to the respective thread.

Thanks

 

[haima]

whoops - glad to hear 

i'm sure you'll get the opamp going - even if you have to replace ALL the components. in fact, sometimes with a little circuit like this it's the easiest way - life's too short  ;D

 

[jsteiger]

Very good to hear whoops.

Interesting situation that was your problem. All of the holes are "plated thru holes" so technically, the top and bottom solder pads are connected by the plating that runs thru the hole on the PCB. I think it is good, solid build practice to solder any part or component you can from the top and the bottom. Not 100% necessary but still good. Naturally, some of the components cannot physically be soldered top and bottom. Like on the PB switches and the 2622 for instance. The tails are thin and rectangular so a nice flow of molten solder down into the hole is definitely doable and preferred. I use a very small and conical tip so I can "get down into" the holes as much as possible. I'll tell you one thing, desoldering a 2622 that had real good flow down into the holes can be a real PIA.  ???

Anyhow, sorry you had to go thru all of this with the pre.

BTW Haima, your trouble shooting tips are very detailed and well written.   I have been working on a detailed, step by step assembly guide for the VP2x pres the last few weeks. I can't even begin to count the hours I have into it. I have maybe 30 more pics to crop and insert into the text and then I am done. I have been thinking about adding some basic test procedures that could/should be done. The posts here the last few weeks are perfect info for some testing. I am directing the guide more towards the DIY virgin but hopefully there is a little something there for others too. A lot of first timers have been buying the kits as of late.

Cheers, Jeff

 

[haima]

the step by step guide sounds good jeff!

basic signal tracing skills and concepts are essential if you want to fix just about anything audio/electronics related - even just if it's just troubleshooting everyday studio problems...

and DIY is a great way to learn these skills - anyone who has to FIX a problem on their preamp will learn a lot more than someone who manages to get it right first time 

 

[Whoops]

Thanks a lot for the words Jeff, and for the the continue support you provided.
Although I think it's not a difficult build the Step by step guide will be quite useful for everyone,
even for more experienced people as it will make the build faster.
It's great that you are doing that.

and you are right all the tips on testing and signal tracing from Haima in the last weeks were great.

my best regards

 

[surfkat]

I have followed haima's trobleshooting tips and tracked my problem down. The output section is fine. The input has a greatly reduced level on the + input rail as compared to the - input at the opamp pins. I traced the + input and the signal is good until pin 5 of the input transformer. I inspected and re-soldered this pin but the problem remains. Is it possible I have a bad transformer? I don't know what else it could be. Any ideas?