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Old 8th April 2015, 09:31 PM   #1
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Default suggestions for oscillation on neg half of output

Hi I fixed a Dynaco st-80 amp, and on one channel (one I fixed) when you put a load on the output the neg half of the waveform oscillates. the system has all new transistors, new caps for output zobel(c8), and bypassed the speaker out cap(c7). if you scope at the the transistor out it is really ringing, at the speaker out (after the cap\coil (c7\L1) it is subdued. When I added a mylar bypass on c4 it rang all over the sine wave so I took it out.
Your thoughts are appreciated. thanks
john
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Old 8th April 2015, 09:34 PM   #2
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Default schematic for ST-80

oops hit enter too soon
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Old 8th April 2015, 11:03 PM   #3
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Quote:
and bypassed the speaker out cap(c7)
That would make it not work by itself ... why? It's not a DC
coupled amp ?

OS
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Old 8th April 2015, 11:06 PM   #4
rayma is offline rayma  United States
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Quote:
Originally Posted by ostripper View Post
That would make it not work by itself ... why? It's not a DC coupled amp?
I think that he means that he added a small cap in parallel.
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Old 8th April 2015, 11:10 PM   #5
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Originally Posted by rayma View Post
I think that he means that he added a small cap in parallel.
Why ?

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Old 9th April 2015, 12:24 AM   #6
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Q4 small pf capacitor between base and collector.

I had the same problem in a quasi design.
The gain on the lower driver needs taming a bit with a capacitor.
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Old 9th April 2015, 03:21 AM   #7
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Default screaming st-80

thank you all for your help, Yes I added a bypass cap to the large elctro output cap, to improve the top end. I will try the Q4 cap add in and let you know. Thanks for your time.
John
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Old 9th April 2015, 03:29 AM   #8
Art M is offline Art M  United States
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Where did you get Exactly equivalent transistors as replacements ?
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Old 9th April 2015, 05:35 AM   #9
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Default the cap worked 12pf on q4

Hi Nigelwright7557, spot on that work , clean signal now. I would have thought the Q3 would need it, glad I got your help

Hi Art M, there is a guy from Jersey on Ebay selling kits for most Dynaco kits.
I had run out of fixes so I did a shot gun try, turned out it was a broken lead on the 5v zener . then the output oscillated on load, so I came here. the best people are here.
thanks everyone.
john
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