Problem in VCO / Voltages [SOLVED]

by osku on Thu May 28, 2009 4:20 am

Hello,

I'm building my fourth self-sourced x0xb0x and now after finishing VCO, I met a bit confusing problem. :oops:

Voltage 5.333V somehow drops down to 2.21V even if it is measured from ioboard (J4). I swapped ioboard from another device (fully working one) and the problem still persists, so I would assume the bug is in VCO circuit somewhere (without VCO, PS outputs correctly 5.333V).

There is no voltage dropping in 12V output.

I currently have no idea what causes this and unfortunately I don't have too many spares, so all ideas are very welcome...

--Oskari
Last edited by osku on Sat May 30, 2009 4:41 pm, edited 1 time in total.
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Re: Problem in VCO / Voltages

by guest on Thu May 28, 2009 2:06 pm

it is strange that the 12v is not drooping as well
as the 12v is a strict multiple of the 5.333v

there is probably a short someplace in the vco
check if any parts are getting warm
compare your voltages to the ones given the troubleshooting faq
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Re: Problem in VCO / Voltages

by osku on Thu May 28, 2009 3:07 pm

Thank you for your reply!

I couldn't find warming or short and double checked items once more. I changed C31 with no success and then replaced AN6562 in VCO.

There is some progress now, I'm getting 2.21V instead of 5.333V and 4.92V instead of ~12V, so it looks a bit brighter today, but I'm still stuck.

--Oskari
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Re: Problem in VCO / Voltages

by guest on Thu May 28, 2009 4:50 pm

do you only have the vco section built now

what is the resistance between 5.333v and ground
on the mainboard

its possible a transistor is in backwards
so it only becomes a short when its turned on
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Re: Problem in VCO / Voltages

by osku on Thu May 28, 2009 5:27 pm

Hello,

Thank you again!

Yes, only VCO is completed (and PS in ioboard as in manual). Resistance is 20.2k from mb j4 pin 1 to ground.

- I checked all resistors once again
- Measured following points:
Q26 (from mb down to top):
2.21V
2.21V
1.63V
3.74V
2.17V

Q8 (from mb left to right):
4.51V
2.26V
4.06V

These two look quite ok to me, but the voltage is low?

- Checked transistor alignments (visually) and types
- Checked trimpots and tuning knob types once again
- Warmed every soldering point once to ensure there is no "cold solderings"
- I tested every 2sa733p's hfe's before soldering and sorted them to find hi gains, so I believe those were ok at least before soldering.
- I didn't test 536f's and sk30ao :?

--Oskari
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Re: Problem in VCO / Voltages

by guest on Thu May 28, 2009 5:43 pm

double check that r1 is correct
what are all the voltages on ic23
is ic23 getting hot
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Re: Problem in VCO / Voltages

by osku on Fri May 29, 2009 2:20 am

Hello,

Thank you again for your reply!

R1 is 100Ohm (1%) and IC23 is not getting hot or even warm (it was powered about 30 minutes).

I measured following voltages from IC23:

J4 connected
4.97 |\_/| 21
2.08 | | 2.15
2.22 | | 2.21
0.00 | | 5.33

J4 disconnected
11.8 |\_/| 28.6
5.33 | | 5.33
5.33 | | 5.33
0.00 | | 5.33


Edit: I was able to do some minor analyzing (with a cheap handheld scope) to waveform from wave selector pin. It outputs square and saw (switch works), but I think pulse width of square is not regular (top part of the pulse is too short).

Tuning knob seems to work ok.

--Oskari
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Re: Problem in VCO / Voltages

by guest on Fri May 29, 2009 3:42 pm

does your multimeter have a dc current measuring function
can you measure the current going on the 5.33v and 12v lines
going in to the mainboard

it is strange that ic23 is not getting hot
it looks like 76mA is passing through it
which would give at least 1W of power dissipation

can you try connecting a dummy load to the ioboard
with the mainboard disconnected
like a 1k resistor from 5.333v to ground
and a 1k resistor from 12v to ground
and see what the voltages do

do you have ic9 ic11 and ic12 populated on the mainboard
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Re: Problem in VCO / Voltages

by osku on Fri May 29, 2009 5:30 pm

Hello,

Thank you again!

First, I have to took my word back about not warm ic23. I have probably soldered too much recently because I could not verify this until I touched ic23 with my pinkie finger and it was hot.

I measured current from ioboard to mb:
- pin1 (5.333V): starts from ~45mA and decreases to ~36mA within couple of minutes. It may go lower, but not fast.
- pin3 (12V): 1.5mA (hmm)

With dummy load I measured following values (both cases):
11.8|\_/|27.8
5.33| |5.33
5.33| |5.33
0.00| |5.33
-> no effect

IC9, IC11 and IC12 are soldered to mainboard. I measured following voltages from these (powered with only ac adapter connected to ioboard, no power to R90):

IC9 (V):
2.14|\_/|2.06
1.03| |0.01
0.01| |0.00
0.30| |0.00
0.80| |0.01
0.00| |0.00
0.01| |2.06
GND| |0.00

IC11 (V) (I replaced this one earlier):
1.63|\_/|4.98
2.24| |0.58
2.24| |0.58
GND| |0.19

IC12 (V):
0.77|\_/|4.78
0.77| |0.00
0.77| |4.37
0.77| |0.57
0.77| |0.57
0.01| |0.57
GND| |0.09


--Oskari
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Re: Problem in VCO / Voltages

by guest on Fri May 29, 2009 8:10 pm

the problem is definitely on the 5.333v line on the mainboard
your ic9 might be faulty
check that none of its pins are shorted to one another or ground
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Re: Problem in VCO / Voltages

by osku on Sat May 30, 2009 4:40 pm

Hello,

Thank you so much!

I tried to find a short near IC9 (tested all pins to all pins), but I found only one direct connection:
from this ->|\_/|<-to this

There was no shorts to ground.

I was lucky and I found a spare for IC9 from my previously built x0xb0x and then replaced it. Tada!! Voltage is ok and now everything works!

When building next one, I will use sockets for IC's and transistors. It is cheap and desoldering IC is quite violent operation and I wish not to do it again.

I couldn't have solved this problem without your help and knowledge and I hope this thread will also help other people, with same kind of problem.

Thank you once more and I wish you have nice weekend there!

--Oskari
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Re: Problem in VCO / Voltages [SOLVED]

by guest on Sat May 30, 2009 5:06 pm

im glad you got it up and running
ive seen a lot of ic9 problems
where they just draw too much power

i am beginning to think that it is not a good idea
to test the vco with ic9 in place

looking at pin2 and pin5 voltages on your ic9
they were both sitting at midrail
which should never happen for a digital output
they should be either high or low

this makes me think that the floating inputs on ic9
are causing the problem

since there is nothing connected to the inputs of ic9
at this stage of the build
they can float to an arbitrary voltage
and if they float to midrail at the input
the output will get confused as to wether it should be high or low
so it sometimes tries to do both
this turns on both the pullup and pulldown transistor in the output stage
basically shorting power to ground
once this happens the chip can fry itself
such that it wont work when returned to normal operation

this doesnt happen very often
but could be the cause of the few ic9 problems that have occured
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Re: Problem in VCO / Voltages [SOLVED]

by zeropoint on Fri Aug 07, 2009 8:36 am

guest wrote:i am beginning to think that it is not a good idea
to test the vco with ic9 in place


Guest, do you think it might be best to leave IC9 out until testing of the entire analog portion is complete?
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Re: Problem in VCO / Voltages [SOLVED]

by guest on Fri Aug 07, 2009 10:33 am

it wouldnt hurt
it doesnt do anything until the digital is connected
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Re: Problem in VCO / Voltages [SOLVED]

by johey on Thu Mar 21, 2013 9:35 am

Sorry to wake a thread up from sleeping. I'm having a similar or maybe exactly the same issue. Power drops from 5.33 to 2.xx and from 11.81 to 5.xx when inserting the IC9. I have built the PSU and VCO strictly by the book. I've socketed all ICs so far, but not the transistors (should I?)

The IC23 is running warm when IC9 is connected. IC9 itself stays cool though.

I have not been running for a long time in a row in this configuration. Less than one minute I guess. I quickly realised something bad is going on and started my googling adventure.

Should I continue building the next part now and hope for the IC9 to be ok when everything else eventually are connected, or should I order a new 74AC174 now? Can I continue anyway without IC9 (I suppose, as guest suggest so)?
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