[BUILD] CAPI VP28~500 Series~2-Stage Preamp~Official Support Thread

[johnlukeyoung]

.....If I measure the same point multiple times, the readings vary drastically.....

Readings that vary can mean a poor connection for your probes or more likely bad solder joints.

If you are injecting a 0dB signal I assume you are in line mode? If that is the case, are you sure the preamp gain is set to the Unity position (and knob is properly located) and the output fader is at 0 (and knob is properly located)? With 0dBu going into line mode, you shook have appx 0.39V AC at test point #6 which is just before the 1:2 output transformer which is providing 6dB of "free" gain.

I am in line mode, and I've checked that both sets of gain knobs are properly oriented. I will try reflowing all accessible solder joints and see if that allows for more consistent readings. Also, should I test this with or without opamps installed? (I apologize if this has already been asked, I couldn't find it) Thank you!

 

[Zamoht]

I reflowed the solder everywhere but the same symptoms persist.  I did remove the opamps and the IC before soldering, so they were re-seated before I tried again.  But after it was reassembled before I tried it out I still get that 4.06k ohm reading when I measure the opamp sockets between "+v" and "c".    And the reading is exactly the same for both opamp sockets.

I did not reflow the solder on the hpf board though.  I will do that as well.  I saw in another post that the audio goes through there no matter what, right?

I am getting the same reading between the DOA sockets on both of my preamps. this can't just be a coincidence right?

 

[jsteiger]

I am getting the same reading between the DOA sockets on both of my preamps. this can't just be a coincidence right?

The A1 and A3 +/-V sockets are directly connected to each other so they will measure the same. I have a pair of properly working VP28's here and 4.06kΩ is correct for +V to C. The -V to C DCR is 6.4kΩ.

 

[Zamoht]

I am getting the same reading between the DOA sockets on both of my preamps. this can't just be a coincidence right?

The A1 and A3 +/-V sockets are directly connected to each other so they will measure the same. I have a pair of properly working VP28's here and 4.06kΩ is correct for +V to C. The -V to C DCR is 6.4kΩ.

Ah ok, I got confused because the values given in the VP2x manual said it should be over 200KΩ.
-V to C gives me a resistance of around 100 KΩ (although it starts low when first measured, but steadily climbs). might be a bad solder joint somewhere? (or does DCR means it should be powered?)

 

[jsteiger]

I am getting the same reading between the DOA sockets on both of my preamps. this can't just be a coincidence right?

The A1 and A3 +/-V sockets are directly connected to each other so they will measure the same. I have a pair of properly working VP28's here and 4.06kΩ is correct for +V to C. The -V to C DCR is 6.4kΩ.

Ah ok, I got confused because the values given in the VP2x manual said it should be over 200KΩ.
-V to C gives me a resistance of around 100 KΩ (although it starts low when first measured, but steadily climbs). might be a bad solder joint somewhere? (or does DCR means it should be powered?)

This is normal as its a cap charging. DCR is a resistance measurement and in this situation there should be no power applied. FYI, the results shown in the VP2x Assembly Guide are specific to a VP25 or VP26.

 

[nicholasdover]

I've been puzzling over my 4th VP28 build for hours now - the input transformer 1-3 and 2-4 are both measuring 2 ohms, then 5-8 is 1.6k and I'm getting very low output level but correct functionality across all controls. Are these readings odd?

A separate issue in same unit is that I stupidly put 4558 in reverse and it blew, replace it and now only 48v LED works. They're all in correct polarity - could IC blowing have damaged LED's?

Thanks for any advice
Nick

 

[jsteiger]

I've been puzzling over my 4th VP28 build for hours now - the input transformer 1-3 and 2-4 are both measuring 2 ohms, then 5-8 is 1.6k and I'm getting very low output level but correct functionality across all controls. Are these readings odd?

Are these measurements when the 2622 is soldered in circuit?

The LED's are in a string starting with the 48V and going towards the top of the module. They really won't have anything to do with issues regarding the chip opamp.

 

[nicholasdover]

Yeah it's in circuit still - only have a sucker so desoldering a bit of a worry...

 

[jsteiger]

Yeah it's in circuit still - only have a sucker so desoldering a bit of a worry...

Well, I have recently found shorts on a few boards fresh out of the package. My gut tells me yours is one of them. Can you send me an email please?

 

[nckptmn]

hello all!
Sorry if this has been asked before, but i'm working on my first build of my VP28 and just finished populating and soldering the resistors. While i was double-checking everything with my multimeter, i found that R20 and R21, despite being labeled and color-coded for 6.8k, were returning a measurement of 3.7k each time. Is this normal or something I should be worried about?

 

[jsteiger]

hello all!
Sorry if this has been asked before, but i'm working on my first build of my VP28 and just finished populating and soldering the resistors. While i was double-checking everything with my multimeter, i found that R20 and R21, despite being labeled and color-coded for 6.8k, were returning a measurement of 3.7k each time. Is this normal or something I should be worried about?

I assume you are trying to check after they are soldered in circuit? You cannot do this unless one lead is lifted. You will get all kinds of false readings.

 

[nckptmn]

thanks for the tip jeff! I'm trying to be as cautious as i can with this build but sounds like i was just setting myself up for confusion by checking the resistors in circuit.

another question -- i made the mistake of soldering in my EA2622 without placing a spacer underneath. Am i setting myself up for big problems when i plug this into my rack?

finished the main boards and assembly today, building the opamps tomorrow. this build's been a lot of fun to work through!

 

[jsteiger]

-- i made the mistake of soldering in my EA2622 without placing a spacer underneath. Am i setting myself up for big problems when i plug this into my rack?

The idea behind the spacer is to keep the bottom of the can from shorting out to the pads. Most typically, it will be OK but I like the spacer to be certain. There is no DC there so its not like anything will blow up 

 

[nckptmn]

quick update and one last question:
so after working for a while on my gar2520 kits, i got them to where i thought they needed to be, plugged everything into my lunchbox...and got the distinct smell of burning components, which i was able to confirm pretty quickly were the opamps. I don't mind being bested by them, i hadn't worked on a project that required that much precision before and i knew the risk going in, and i've got a pair of assembled units waiting in my shopping cart right now....but i wanted to ask, assuming that the issue is solely an op-amp issue, is there any risk of having done serious damage to the vp28 by powering it on for a minute as the op-amp began to burn out? with just a quick hand-check it didn't seem as though the signal IC or any of the transformers had gotten particularly hot, but of course, that can only tell so much.

Not sure if this question can even be answered conclusively without an experienced veteran taking a look at the unit itself, but figured i'd ask!

 

[jsteiger]

so after working for a while on my gar2520 kits, i got them to where i thought they needed to be, plugged everything into my lunchbox...and got the distinct smell of burning components, which i was able to confirm pretty quickly were the opamps. I don't mind being bested by them, i hadn't worked on a project that required that much precision before and i knew the risk going in, and i've got a pair of assembled units waiting in my shopping cart right now....but i wanted to ask, assuming that the issue is solely an op-amp issue, is there any risk of having done serious damage to the vp28 by powering it on for a minute as the op-amp began to burn out?

The rest of the pre is most likely OK. Common mistakes on the opamp builds are swapping and/or turning the larger BD transistors. They ARE NOT interchangeable. Backwards diodes is the #2 error.

 

[Zamoht]

I'm kinda stuck with troubleshooting here.
I've got 1 working vp28 and 1 not (yet)

The signal I get in both mic and line mode is very very low (-50 db into the DAW with preamp and fader full open)


I already put it in the rackslot that worked with the other one.
I switched all combinations of op amps and confirm they are all ok.
The signal led works (and behaves exactly the same as the other working vp28)
so, the preamp section is also ok i think?

 

[jsteiger]

I'm kinda stuck with troubleshooting here.
I've got 1 working vp28 and 1 not (yet)

The signal I get in both mic and line mode is very very low (-50 db into the DAW with preamp and fader full open)


I already put it in the rackslot that worked with the other one.
I switched all combinations of op amps and confirm they are all ok.
The signal led works (and behaves exactly the same as the other working vp28)
so, the preamp section is also ok i think?

Go back to around here http://groupdiy.com/index.php?topic=48610.msg767382#msg767382 and use the test points guide to trace signal thru both pre's and see where things change. I would start with the good one and use those results as a benchmark. Once we figure out exactly where the problem begins, we can isolate and solve it.

 

[Zamoht]

I'm kinda stuck with troubleshooting here.
I've got 1 working vp28 and 1 not (yet)

The signal I get in both mic and line mode is very very low (-50 db into the DAW with preamp and fader full open)


I already put it in the rackslot that worked with the other one.
I switched all combinations of op amps and confirm they are all ok.
The signal led works (and behaves exactly the same as the other working vp28)
so, the preamp section is also ok i think?

Go back to around here http://groupdiy.com/index.php?topic=48610.msg767382#msg767382 and use the test points guide to trace signal thru both pre's and see where things change. I would start with the good one and use those results as a benchmark. Once we figure out exactly where the problem begins, we can isolate and solve it.

Ok, I noticed that (any) opamp in the A3 position was getting really hot. Also this cap (C20) doesn't look to good either... I think this is the problem. any ideas on what the cause might be?

 

[jsteiger]

Ok, I noticed that (any) opamp in the A3 position was getting really hot. Also this cap (C20) doesn't look to good either... I think this is the problem. any ideas on what the cause might be?

Yes that doesn't look good. 1st I would measure DCR from "O" socket of A3 to ground. Do this with no power applied to module. If there is no direct short or very low resistance, I would measure DC offset just after the A3 opamp. For this A3 needs to be in place and the module needs to be under power. Set DMM to read DCV. Red probe to the + end of that leaking cap and black probe to a solid ground reference. The pad adjacent to gold finger #5 is usually the easiest. Typically there should be no more than 100mV of DC present. Looks like the DC exceeded the voltage the cap is rated for indicating a problem with the opamp.

 

[Zamoht]

Yes that doesn't look good. 1st I would measure DCR from "O" socket of A3 to ground. Do this with no power applied to module. If there is no direct short or very low resistance, I would measure DC offset just after the A3 opamp. For this A3 needs to be in place and the module needs to be under power. Set DMM to read DCV. Red probe to the + end of that leaking cap and black probe to a solid ground reference. The pad adjacent to gold finger #5 is usually the easiest. Typically there should be no more than 100mV of DC present. Looks like the DC exceeded the voltage the cap is rated for indicating a problem with the opamp.

ok, I measured DCR 0 to ground. no connection,
Next I measured 101mV  between C20+ and ground, so that seems ok. (btw, I have removed the leaking cap before measuring)