fried output opamp on inovonics 235

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kc8gpd
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fried output opamp on inovonics 235

Post by kc8gpd » Wed Jul 20, 2016 4:03 pm

well i think i have a burnt out op amp in my 235. i accidentally shorted the positive audio rail to ground when disconnecting a piece of equipment and now i have low tinny audio on the output.

problem is, even with the schematic the output IC(s) are not readily apparent. it appears to be 3 IC's connected to the output but i am not sure.

i have attached the schematic in pdf form to this message.

if someone with more experience could give it a look see and tell me which IC's to replace it would be appreciated.

it is an LF353N that i am sure of. just not sure which if the almost 30 of them in there it is. and i am sure it is the result of shorting the positive audio rail to ground. i am assuming the positive half of the circuit is the only part blown since there is still audio it's just not loud and full sounding.

thanks

Rob in denver
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grich
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Re: fried output opamp on inovonics 235

Post by grich » Wed Jul 20, 2016 5:01 pm

I suppose IC23 and IC25 might be the problem, but I'd be worried about Q9 thru Q12...looks like they're doing the heavy lifting on the output. IC23 and 25 appear to be driving the transistor pairs for each output leg.

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Re: fried output opamp on inovonics 235

Post by kc8gpd » Wed Jul 20, 2016 5:36 pm

I'll have to look in the parts list to find out what the p/n's are for q9-q12. i will likely just replace that whole circuit then IC23/25, q9-12.
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Re: fried output opamp on inovonics 235

Post by Deep Thought » Wed Jul 20, 2016 11:03 pm

It is highly unlikely that shorting the output to ground did anything to the circuit. There is a 100 ohm current limiting resistor on the output line as well as 10 ohm emitter resistors that would pretty much prevent it. Are you sure you didn't feed a voltage back into that instead?

Op amp failures are easy to diagnose...they usually go to the supply rail. In this circuit it would also result in one of the output transistors being at the supply rail since the whole thing is DC coupled. Is there a voltage on the + output?

You should also be able to check this by connecting headphones between ground and either the + or - output. They should be the same just inverted.
Mark Mueller • Mueller Broadcast Design • La Grange, IL • http://www.muellerbroadcastdesign.com

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Re: fried output opamp on inovonics 235

Post by kcbooboo » Thu Jul 21, 2016 6:25 am

The main output is the negative (-) output from IC25A, Q11, Q12. This signal is then inverted to drive the positive (+) output with a unity gain stage IC23A, Q9, Q10. The output should be short-circuit proof however when pulled to a voltage greater than +/- 9V, odd things could happen.

A voltmeter on each of the output terminals should be sitting at 0VDC with or without audio passing through it. Obviously with audio there will also be some amount of AC voltage present. If you find DC on one output terminal, the other should be the exact opposite polarity. Then work your way back through the circuit looking for non-zero DC.

An oscilloscope would probably reveal exactly what's going on. Feed a sine wave into the unit and terminate each of the output lines to ground with a 600 ohm resistor temporarily, so you can verify that the output signals are sinusoidal and symmetrical around ground. If you turn the output level up all the way, it may or may not go into clipping. Remember that this is an AM processor and it could have an asymmetrical output with some settings, so putting the unit into "proof" mode would be useful.

By the way, the schematic sheets insert quite nicely into the on-line manual.

Bob M.

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Re: fried output opamp on inovonics 235

Post by kc8gpd » Thu Jul 21, 2016 9:38 am

just got an email from Jim Wood, he says he can probably take care of it for $100.00 plus return shipping. sounds like a good deal to me and i will likely go that route rather than risk hosing a 2100.00 processor trying to unsolder and replace q9-12.
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Re: fried output opamp on inovonics 235

Post by Deep Thought » Thu Jul 21, 2016 10:43 pm

Did you bother to do any of the basic troubleshooting suggested here?
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kc8gpd
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Re: fried output opamp on inovonics 235

Post by kc8gpd » Fri Jul 22, 2016 9:19 am

yes and jim agrees with my assessment, i am just not comfortable trying to desolder the q9-12 double sided pcb. i tried to do those before (luckily was cheap behringer crap) and messed it up. not gonna take a chance with a 2100.00 retail processor.

Jim as well as me thinks the outputs may have gone.

i do own a scope and vom (as well as other test gear) and did test it.
kc8gpd

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Re: fried output opamp on inovonics 235

Post by kcbooboo » Fri Jul 22, 2016 10:55 am

It will be interesting to hear:
1. how long it takes from the day you pack it up and send it out until the day you get it back and put it in service.
2. exactly what parts were replaced. You should ask for a full report.

Bob M.

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Re: fried output opamp on inovonics 235

Post by COMMENG » Tue Jul 26, 2016 6:18 pm

kc8gpd wrote:well i think i have a burnt out op amp in my 235. i accidentally shorted the positive audio rail to ground when disconnecting a piece of equipment and now i have low tinny audio on the output.

problem is, even with the schematic the output IC(s) are not readily apparent. it appears to be 3 IC's connected to the output but i am not sure.

i have attached the schematic in pdf form to this message.

if someone with more experience could give it a look see and tell me which IC's to replace it would be appreciated.

it is an LF353N that i am sure of. just not sure which if the almost 30 of them in there it is. and i am sure it is the result of shorting the positive audio rail to ground. i am assuming the positive half of the circuit is the only part blown since there is still audio it's just not loud and full sounding.

thanks

Rob in denver
How does anyone damage the output drive circuit with the covers on the unit?

Shorting the pos supply rail to ground might only damage the +9V positive regulator.

The LF353's are driving a complementary-symmetry set of bi-polar transistors, 2N3904 and 2N3905 with current limit resistors in each output.

The 2N3904 has a 40 volt Vceo while the 2N3905 has an 80 volt Vceo. The internal supply rails are +9V and -9V.

The LF353 can take up to 30V input differential voltages and PS voltages up to 36V. The supply voltage is only 18Volts.

Sounds like a screwdriver got loose while the cover was off.

COMMENG

kc8gpd
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Re: fried output opamp on inovonics 235

Post by kc8gpd » Tue Jul 26, 2016 6:55 pm

i had it feeding a carrier current transmitter with an unbalanced input and i had a barrel connector with neg to ground and pos to tip. i forgot to turn the 235 off before disconnecting the audio lead from the carrier current Tx. the connector dropped down and landed on top of the metal casing to the PSU running my ham radio with the positive lead touching case ground for an extended period. the 235 worked perfectly up to that point after that it had low hollow sounding audio.
kc8gpd

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Re: fried output opamp on inovonics 235

Post by kc8gpd » Sun Jul 31, 2016 10:16 am

turns out it was the negative rail on the balanced output. the entire negative half of the balanced output was blown. Jim fixed it and did some factory mods for me as well. turns out it was a newer unit as well with a late serial number and this series debuted in 1999, so it is a relatively new processor and not as old as i though it was.

anyway it is on it's way back to me all repaired with some small factory mods to improve sound quality and a lesson learned.

he had to replace the output IC and two transistors. also did some mods to the release and attack timing on each band and added a mod to stop the popping after long periods of silence.
kc8gpd

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Re: fried output opamp on inovonics 235

Post by kc8gpd » Fri Aug 05, 2016 5:30 am

got the 235 back yesterday and installed it, listening to it this AM. it sounds much better than before it broke. Jim did a good job on it. my little part 15 AM sounds like other AM's on the dial within it's small primary service contour on my Carver TX11B.
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