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170DC offset odd behavior

ocdecio

ocdecio

The proof of the restoring is in the listening.
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I have a 170DC with a very strange quirk in one of the channels - the DC offset starts off at about +4V, goes down enough to click the relay and starts going in the other direction and when it reaches -2V the relay of course goes in protection.

I read here that it could be an open trace somewhere, I looked and tested as many locations as I could, and replaced the usual suspects (small signal and pre/drivers) and still continue to have this. The dual FET seems to be OK and the diff pair is a new matched pair.

Truly stumped.
 
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Roger_88_K said:
Is this the 170DC we were discussing yesterday?
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Believe it or not.. it isn't. It's another one I had sitting here forever that I decided to work on since I was already knee deep in one. The one we were discussing is doing very well, knock on wood.
 
Roger_88_K said:
Have you checked the diodes? I believe the zeners are ok as the system is regulating the voltage.
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I replaced the diodes in front of the driver heatsinks. I may replace all of the ones that sub to 1N4148 (don't think the flybacks would matter). I'll check the zeners just in case (I've seen odd behaviors in the 2325 amp if the zeners are not matching). Thanks!
 
@Roger_88_K checked zeners - one is 31.1V the other 30.7V that seems inside the tolerance I guess? I'll get some ordered just in case.
 
ocdecio said:
@Roger_88_K checked zeners - one is 31.1V the other 30.7V that seems inside the tolerance I guess? I'll get some ordered just in case.
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Seems the zeners are fine. What rail voltage do you have?
I recall another thread discussing the DC offset problem, that might be the 300DC of which the circuit is very similar to this one. Seemed to be a FET problem.
Do you have anything in stock to try? a 2SK117 or 2SK170 is fine.

For the zeners, if you'd like to order them, I used the 30v type. For Digikey part number please refer to my list in the other thread. There's a link below the restoration.
 
Roger_88_K said:
Seems the zeners are fine. What rail voltage do you have?
I recall another thread discussing the DC offset problem, that might be the 300DC of which the circuit is very similar to this one. Seemed to be a FET problem.
Do you have anything in stock to try? a 2SK117 or 2SK170 is fine.

For the zeners, if you'd like to order them, I used the 30v type. For Digikey part number please refer to my list in the other thread. There's a link below the restoration.
Click to expand...
I'll get me some FET - they seem to be very pricey. I found the part number for the Zener on your very detailed and helpful thread!

Meanwhile I think I will borrow the FET from the other channel just to see what happens.
 
Roger_88_K said:
Thanks, note that the 2SK117 is also a possible replacement.

Let me know if that 2SK109 from the good channel is working correctly in the faulty channel. (and vice versa)
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BTW - do they really need to be purchased matched or can I just get two and install?
 
ocdecio said:
BTW - do they really need to be purchased matched or can I just get two and install?
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Simple answer is yes. They work together as differential pair. It is the "cascoded FET differential amplifier"
You also want to have them thermally coupled as explained in my restoration. It isn't difficult at all.
 
Roger_88_K said:
Simple answer is yes. They work together as differential pair. It is the "cascoded FET differential amplifier"
You also want to have them thermally coupled as explained in my restoration. It isn't difficult at all.
Click to expand...
Excellent, thank you so much! I put the other channel chip on this one but I am now getting minus rails, it may be damaged as well. I'll set it aside until I get the parts. I appreciate your help!!
 
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