Carvin vt112 low output

[nosaj]

I have a carvin vt112 with low output and some voltages that don't seem right. Can anyone help with this?

The schem is for a vt2800 but in the notes they say the vt112 is same schem just drop 2 power tubes.

The PI voltages are
1 335vdc
2 76
3 102
6 326
7 77
8 102

Should be per schem
1 258
2 120
3 142
6 258
7 120
8 142


same voltages on both el-34
pin 3 480
pin4 477
pin5 -47
pin 6 478

[nickb]

I have a carvin vt112 with low output and some voltages that don't seem right. Can anyone help with this?

The schem is for a vt2800 but in the notes they say the vt112 is same schem just drop 2 power tubes.

The PI voltages are
1 335vdc
2 76
3 102
6 326
7 77
8 102

Should be per schem
1 258
2 120
3 142
6 258
7 120
8 142


same voltages on both el-34
pin 3 480
pin4 477
pin5 -47
pin 6 478
[ATTACH=CONFIG]34620[/ATTACH][ATTACH=CONFIG]34621[/ATTACH][ATTACH=CONFIG]34622[/ATTACH]

Nice info - well done +1.

The current in the tube is about half what it should be. The real pin 2 and 7 voltages are a bit tricky to measure as they are at an elevated voltage meaning your meter will load it down and change the reading. It's better to measure between the cathode and the grid. For that reason it's hard to be sure if the 22K and 220 ohm resistors are OK so go and check their resistance. There's a good chance that the tube is end of life too - try a substitute.

[nosaj]

New JJ
The PI voltages are
1 318vdc
2 82
3 112
6 319
7 83
8 111

Pic attached. resistors look ok. Tested the 22k 220r 270k all within 10%. Would the 47ks have any bearing on this or is this gonna fall back to a filter cap? They are can style mallorys 1979, all other electrolytics have been replaced.

Thanks,

nosaj

[nickb]

New JJ
The PI voltages are
1 318vdc
2 82
3 112
6 319
7 83
8 111

Fixing to pull the 22k and 220 ohm out of circuit to test output still not there yet.

Thanks and will update soon,

nosaj

There is no need to pull them to measure their resistance if the amp is off. Also, you should measure the voltages across the 22k and 220 ohm when running. They may be off the expected values but I really don't think they are the root of your problem. Check that V5 really is a 12AT7 - I suspect that is the reason for voltage problem.

Assuming that is the case, I think you need to be able to scope this out by putting a 100mVpp 1KHz signal on the input and following it through.

Anyway, let's see where we are once you've got that info.

[nosaj]

Ok put in a JAN 12at7

pin1 265
2 109
3 139
6 257
7 112
8 138

okNot sure on the measuring voltage across means a lead on each side of the resistor or one to ground.

On the 220r with a lead to ground one side is 138 the other 134vdc

On the 22k weird things One lead to ground measure one sideI get 134 measure the other 0. Swapped in a new resistor same thing.

Thanks,
nosaj

[g1]

Those voltages seem good now. One end of the 22K is only 100 ohms away from ground, so the low end will be close to zero.
So you can probably look elsewhere for the problem, as nickb suggested, inject a signal and follow it through.

[nickb]

Ok put in a JAN 12at7

pin1 265
2 109
3 139
6 257
7 112
8 138

okNot sure on the measuring voltage across means a lead on each side of the resistor or one to ground.

On the 220r with a lead to ground one side is 138 the other 134vdc

On the 22k weird things One lead to ground measure one sideI get 134 measure the other 0. Swapped in a new resistor same thing.

Thanks,
nosaj

Yes, 'across' means put the probes on either side of.

Before we trace the signal check the DC volts on each op-amp output i.e on pins 3,4, 10 and 12.

To trace the signal, you'll need some equipment:-

A signal generator and a scope is ideal.

PC or phone can be used instead of a signal generator
A meter on ACV can be used instead of the scope, but it's a poor substitute.

[Enzo]

Yes^^^

Think of it like measuring a bridge over a river. How far is it across the bridge? Measure end to end, not how high each end is.

[nosaj]

Yes, 'across' means put the probes on either side of.

Before we trace the signal check the DC volts on each op-amp output i.e on pins 3,4, 10 and 12.

To trace the signal, you'll need some equipment:-

A signal generator and a scope is ideal.

PC or phone can be used instead of a signal generator
A meter on ACV can be used instead of the scope, but it's a poor substitute.

Some reason I didn't get notification of new replies. Will get the voltages for you.

I have a tenma 20 mhz scope with function gen built in. has 1khz tone can do sine square, triangle.
Thanks,
nosaj

[nosaj]

Yes, 'across' means put the probes on either side of.

Before we trace the signal check the DC volts on each op-amp output i.e on pins 3,4, 10 and 12.

To trace the signal, you'll need some equipment:-

A signal generator and a scope is ideal.

PC or phone can be used instead of a signal generator
A meter on ACV can be used instead of the scope, but it's a poor substitute.

opamps are 4136
opamp1
pin3 -.03 pin4 -.11 pin10 .19 pin12 .14 power pin7 -14.92 pin 11 13.85

opamp2
pin3 -13.71 pin4 -.03 pin10 -.01 pin12 0 power pin7 -14.94 pin11 13.88

eq board which is bypassed right now

opamp3
pin3 .02 pin4 13.97 pin10 .02 pin12 .02 power pin7 -14.94 pin11 14.54

opamp4
pin3 .02 pin4 .02 pin10 .02 pin12 .02 power pin7 -14.94 pin11 14.54

I am not sure about the above opamps
CMOS switching 4016

pin1 .05 pin2 .06 pin3 .13 pin4 .13 pin5 .13 pin6 .13 pin7 .13 pin8 .03 pin9 .05 pin10 .05 pin11 .05 pin12 .05 pin13 -.48 pin14 -.46


I think the possible problem maybe in the cmos switching no 7 volts. I know my 14v rails were good and they use a voltage divider down to 7 that is missing. So I will follow that down, included the cmos jpg.

[nosaj]

voltages of cmos without chip in
pin1 -4.75 pin2 4.33 pin3 6.42 pin4 6.40 pin5 6.40 pin6 4.30 pin7 6.46 pin8 0 pin9 0 pin10 0 pin11 0 pin12 4.40 pin13 4.30 pin14 -6.60

So I'd say bad cmos switching chip and since signal runs through it it's is at least in part part of issue.

nosaj

[nickb]

voltages of cmos without chip in
pin1 -4.75 pin2 4.33 pin3 6.42 pin4 6.40 pin5 6.40 pin6 4.30 pin7 6.46 pin8 0 pin9 0 pin10 0 pin11 0 pin12 4.40 pin13 4.30 pin14 -6.60

So I'd say bad cmos switching chip and since signal runs through it it's is at least in part part of issue.

nosaj

I'm not getting notifications either.

OPAMPS look Ok.

pin1 -4.75 Wrong - should be ~0V
pin2 4.33 Wrong - should be ~0V
pin3 6.42 Wrong - should be ~0V
pin4 6.40 Wrong - should be ~0V
pin5 6.40 OK
pin6 4.30 Wrong - should be ~ +7V
pin7 6.46 Wrong - should be ~ -7V
pin8 0 Wrong - should be ~ +7
pin9 0 Wrong - should be ~ +7
pin10 0 Wrong - should be ~ +7
pin11 0 Wrong - should be ~ +7
pin12 4.40 Wrong - should be ~ +7
pin13 4.30 Wrong - should be ~ +7
pin14 -6.60 Wrong - should be ~ +7

Go and re-do those measurements as they make no sense at all. For example, pin 5 is connected to pin 9 get you have different readings. Is you meter black lead always on ground?

[nosaj]

I'm not getting notifications either.

OPAMPS look Ok.

pin1 -4.75 Wrong - should be ~0V
pin2 4.33 Wrong - should be ~0V
pin3 6.42 Wrong - should be ~0V
pin4 6.40 Wrong - should be ~0V
pin5 6.40 OK
pin6 4.30 Wrong - should be ~ +7V
pin7 6.46 Wrong - should be ~ -7V
pin8 0 Wrong - should be ~ +7
pin9 0 Wrong - should be ~ +7
pin10 0 Wrong - should be ~ +7
pin11 0 Wrong - should be ~ +7
pin12 4.40 Wrong - should be ~ +7
pin13 4.30 Wrong - should be ~ +7
pin14 -6.60 Wrong - should be ~ +7

Go and re-do those measurements as they make no sense at all. For example, pin 5 is connected to pin 9 get you have different readings. Is you meter black lead always on ground?

Ok redid them common black on meter clipped to ground. As a side note this amp was given to me with all the IC's pulled, by using voltages present at the pins in conjunction with a datasheet a tried to use the data I got to choose correct positions. These are measured without cmos chip in. I made a jig to test the cmos switching and the 4 switches all switch with a 9 volt battery. I also included a pic of the ic slot and what I am calling pin1.

Thanks,
Jason


Pin1 .04 Pin14 -6.45
pin2 4.30 Pin 13 4.25
pin3 6.37 pin12 4.30
pin4 6.38 pin11 0
pin5 6.38 pin10 0
pin6 4.25 pin9 0
pin7 6.37 pin8 0

[nickb]

Well, if you are really sure about those voltages then I would say you have pin 1 in the wrong place and therefore the chip was in backwards.

[nosaj]

Well, if you are really sure about those voltages then I would say you have pin 1 in the wrong place and therefore the chip was in backwards.

On this chip positive input is pin7 and negative input is pin 14 right? Those measurements were chip out. With the chip in should I have the FS plugged in? I can measure chip in in one direction and flip it around and measure another way.

nosaj