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TZFM SAW VCO
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sduck



Joined: Dec 16, 2007
Posts: 442
Location: Nashville
Audio files: 5

PostPosted: Mon Aug 27, 2012 6:10 pm    Post subject: Reply with quote  Mark this post and the followings unread

So I replaced both of the diodes in the B206-B206 spots. They were fine when I installed them. Turned it on, checked all the voltages, everything is fine. Turned off power, installed the CA3280. For about a minute after powering up the unit it seemed fine, but then the CA3280 started getting hot about a minute into it. When powering off it seems the diode in B205 has blown - it's now a wire.

So what the hell is going on? Do I need a higher value zener? I used mouser 512-1N5223B, which is a 2.7v 500mW zener. Anyone have a recommendation for a bigass zener that won't blow out?

And thanks Dave for monitoring this and being helpful - I may just switch the thing over to your conversion method - it might be easier and faster.
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davebr



Joined: Jun 09, 2007
Posts: 181
Location: portland, or

PostPosted: Mon Aug 27, 2012 10:31 pm    Post subject: Reply with quote  Mark this post and the followings unread

sduck wrote:
So I replaced both of the diodes in the B206-B206 spots. They were fine when I installed them. Turned it on, checked all the voltages, everything is fine. Turned off power, installed the CA3280. For about a minute after powering up the unit it seemed fine, but then the CA3280 started getting hot about a minute into it. When powering off it seems the diode in B205 has blown - it's now a wire.

So what the hell is going on? Do I need a higher value zener? I used mouser 512-1N5223B, which is a 2.7v 500mW zener. Anyone have a recommendation for a bigass zener that won't blow out?

And thanks Dave for monitoring this and being helpful - I may just switch the thing over to your conversion method - it might be easier and faster.

Normal current draw is less than 100mA and you are dropping 3V so you are at 300 mW which should be fine if everything is working correctly. However the current is probably much higher to heat up the chip and that is taking out your zeners. I would just put in wire for now until you can get the current draw under control.

The easiest way to blow up an OTA is to source too much current into the Iabc pin. Section B is sourced from a resistor so that should be fine if the resistor value is correct.

I would assume your OTA is toast by now, but you might still be able to use it to debug. I would put it in with both pin 3 and 6 lifted. See if the heat issue goes away. If so, then I would use your multimeter to measure the current between the lifted pin and the socket. There are no pin numbers on the schematic so I'll assume section A is pin 3. I would start with pin 6 first. If the current is <2mA then I would bend that pin back and into the socket. My guess is your current for pin 3 will be way too high. That is controlled by the expo converter and it could be shorted. There is a 6K8 resistor in series which should prevent more than 2 mA with a voltage drop of 13.6 volts. The Iabc pin is one diode drop above the negative rail, so if the collector of the expo converter goes above ground it could damage the part. You could connect the end of the 6K8 resistor to the negative rail with pin 3 lifed and monitor the voltage on the emitter of the expo converter as you adjust the frequency over the range.

I would verify all the voltages on the OTA socket as well to make sure something else isn't haywire.

Sometimes when I'm debugging a system like this, I will bypass the expo converter with a simple trim pot and the right series resistor to ground. This should give you some frequency control. Likewise you can connect the series resistor from the expo converter to either the negative rail or a diode drop above it and monitor the voltage. It allows you to divide the circuit into two halves that are independent for debug. You also have two sections to deal with. Isolate it to a section, and then to what is causing the excessive current.

Just a thought.

Dave
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sduck



Joined: Dec 16, 2007
Posts: 442
Location: Nashville
Audio files: 5

PostPosted: Wed Aug 29, 2012 7:52 pm    Post subject: Reply with quote  Mark this post and the followings unread

Thanks Dave.

This is taking me too long, as I've had visitors and zero time to actually deal with the thing.

Today I pulled the ca3280, and replaced the B205 diode just so things would be correct so I could check voltages. While checking something, I don't know what, the other diode blew out again - the one in the B206 location. Probably just an accidental contact made where one shouldn't have been made, unrelated to the other problem. This is getting very frustrating.

I went over the thing with a magnifying glass today, no solder shorts, cold solder, missing solders, etc. Also checked resistor values - everything's correct. As I mentioned earlier, I was particularly meticulous about getting the parts right on this build after the stupid part problems I had on the last one.

Could a fake/blown CA3280 be whats causing all this excess current to be drawn? I have another CA3280 here, but don't want to sacrifice it until I know it won't overheat and/or blow out immediately.

Oh, and which is the expo converter you speak of? I don't have such a thing on my layout. Probably a dumb question, please bear with me on this.
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sduck



Joined: Dec 16, 2007
Posts: 442
Location: Nashville
Audio files: 5

PostPosted: Sat Sep 01, 2012 8:55 am    Post subject: Reply with quote  Mark this post and the followings unread

Ok, a follow up report. Dave and I switched to email - he very generously offered to help me sort through this, and I was all ears. And scope and DMM probes. Anyway, after testing everything to determine if there was a short, wrong parts, or whatever else that could be wrong in my build, Dave was able to deduce that most likely it was a flaky CA3280. So I threw in another one I had here, and it works! Everything is hunky dory - I've got reversing waveforms, everything. Now to calibrate...

I've thanked him profusely via email, but I want to make sure that Dave gets a public thanking also - it's marvelous that he's able to spend time helping others debug stuff like this! Thank you!
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neandrewthal



Joined: May 11, 2007
Posts: 672
Location: Canada

PostPosted: Tue Feb 25, 2014 12:54 am    Post subject: Reply with quote  Mark this post and the followings unread

I've been having a problem with one of my two Teezers. When oscillating at negative frequencies the sine doesn't work, the triangle is a positively offset saw and the saw is negatively offset.

The output of A2 was swinging as it should. A4 seemed to be working when I measured it so I kept looking. I noticed that when turning the initial frequency into the negative I couldn't get D12 and D13 to forward/reverse bias correctly.

When measuring I accidentally touched D11 with my multimeter probe and it started working! I sweep the initial frequency up into the positive again: all good. Then I turn it back to negative: not working. Touch anywhere on the A4b-6 junction with a metal object and it kicks in again. Turn it to positive then back to negative and it stops once again.

I reheated all the solder joints in that area and made sure I had good continuity. I swapped A4 with two other OPA2134s, one of which was happily working as A4 in my other Teezer. Still the same deal. I try a TL072 and it works!

Any idea why my Teezer is rejecting OPA2134s?

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frijitz



Joined: May 04, 2007
Posts: 1710
Location: NM USA
Audio files: 54

PostPosted: Tue Feb 25, 2014 12:23 pm    Post subject: Reply with quote  Mark this post and the followings unread

Hi Andrew --

Sorry about your trouble. A4b-6 is grounded, so touching there shouldn't have any effect. scratch

If A2's output is correct, then the trouble must be around A4. Please carefully check all the component values and connections.

Ian


neandrewthal wrote:
I've been having a problem with one of my two Teezers. When oscillating at negative frequencies the sine doesn't work, the triangle is a positively offset saw and the saw is negatively offset.

The output of A2 was swinging as it should. A4 seemed to be working when I measured it so I kept looking. I noticed that when turning the initial frequency into the negative I couldn't get D12 and D13 to forward/reverse bias correctly.

When measuring I accidentally touched D11 with my multimeter probe and it started working! I sweep the initial frequency up into the positive again: all good. Then I turn it back to negative: not working. Touch anywhere on the A4b-6 junction with a metal object and it kicks in again. Turn it to positive then back to negative and it stops once again.

I reheated all the solder joints in that area and made sure I had good continuity. I swapped A4 with two other OPA2134s, one of which was happily working as A4 in my other Teezer. Still the same deal. I try a TL072 and it works!

Any idea why my Teezer is rejecting OPA2134s?
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neandrewthal



Joined: May 11, 2007
Posts: 672
Location: Canada

PostPosted: Tue Feb 25, 2014 5:57 pm    Post subject: Reply with quote  Mark this post and the followings unread

I thought I had verified all the voltages on a previous occasion, but the B206 zener was shorted causing the power for A4 to be asymmetrical. Can't believe I didn't check that sooner. I guess the TL072 was just not as picky with it's supply voltages because it's not as fancy. Now it works with the OPA!

frijitz wrote:
A4b-6 is grounded, so touching there shouldn't have any effect. scratch


Oops, I meant pin 5 doh

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frijitz



Joined: May 04, 2007
Posts: 1710
Location: NM USA
Audio files: 54

PostPosted: Tue Feb 25, 2014 6:21 pm    Post subject: Reply with quote  Mark this post and the followings unread

neandrewthal wrote:
I thought I had verified all the voltages on a previous occasion, but the B206 zener was shorted causing the power for A4 to be asymmetrical. Can't believe I didn't check that sooner. I guess the TL072 was just not as picky with it's supply voltages because it's not as fancy. Now it works with the OPA!

D206? That'd do it. Don't know why one opamp worked and not the other, though. I hope you are enjoying the two modules. I may not have mentioned it here, but there are now videos at my utube page illustrating some of the possibilities of using a pair together.

Ian
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neandrewthal



Joined: May 11, 2007
Posts: 672
Location: Canada

PostPosted: Wed Feb 26, 2014 3:18 pm    Post subject: Reply with quote  Mark this post and the followings unread

I'm definitely enjoying them and have been for years even in crippled form. I also liked your videos a lot when I ran across them. Now that you mention it I'm gonna go watch them again.
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