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DC Offset with Cap Coupled Amp
DC Offset with Cap Coupled Amp
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Old 23rd September 2021, 11:22 PM   #21
dbxdx5 is offline dbxdx5
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Well, to Osvaldo's point, I want to test/confirm that the bass pot (either the caps or the mechanical pot itself) is the cause of the issue. I did try disconnecting the two wires going from the tone controls to C3 and R15 for just one channel. This caused some other artifacts to appear on the waveform, but it did not cause the spikes shown in the video I posted to disappear. Does this rule out the pot? Or is there a better way to test whether it's the culprit?
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Old 24th September 2021, 11:10 AM   #22
Osvaldo de Banfield is offline Osvaldo de Banfield  Argentina
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Again, try to cut the audio chain between pre an final stage to isolate both and determinate the point where the oscillation loop starts.
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Old 24th September 2021, 03:24 PM   #23
dbxdx5 is offline dbxdx5
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I disconnected the preamp board from the amp, and no change.

Looking at the power supplies with my scope AC coupled 20mV/div .2ms/div, I'm seeing the following where the 25V B1+ enters the amp/tone board:

B1+ Amp Board - YouTube

I've already replaced C410A, but nothing else in this particular part of the power supply:
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File Type: jpg Power Supply 4.jpg (88.5 KB, 65 views)
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Old 24th September 2021, 08:18 PM   #24
Osvaldo de Banfield is offline Osvaldo de Banfield  Argentina
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This important. So if disconnecting the pre the problem continues, at first instance pre is discarded. So let me know where you diconnected the amp and let me see if there is another link to break. Also try a new C3 in the 401 board (50uF), and if possible, put there 220uF or more with proper vw.
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Last edited by Osvaldo de Banfield; 24th September 2021 at 08:23 PM. Reason: Another suggestion added.
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Old 25th September 2021, 12:12 AM   #25
dbxdx5 is offline dbxdx5
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I disconnected the preamp from the amp at C1. (Circled in orange.)

For the power supply cap you suggested replacing, are you sure you want me to swap out C3 and not C2? It's the B1+ 25V, not the B5+ 25V that I saw that AC on.
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File Type: jpg Preamp Amp 2.jpg (262.5 KB, 53 views)
File Type: jpg Power Supply 4.jpg (88.5 KB, 58 views)
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Old 25th September 2021, 01:51 AM   #26
indianajo is offline indianajo  United States
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The power amp also connects to the preamp (or tone control section) at R3.C5 , C4, R15. Unusual is why Mr Banfield is not elaborating. Sort of a 4 input mixer built into the power amp board.
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Last edited by indianajo; 25th September 2021 at 01:55 AM.
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Old 25th September 2021, 12:14 PM   #27
Osvaldo de Banfield is offline Osvaldo de Banfield  Argentina
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I'm not viewing the entire schematic. The phone's screen is small. I mean the cap filter that gives supply for the preamp section. Also try to cut at C3 on the amp to completelly disconnect amp from pre and leave power stage alone. In case of doubt replace both C2 and C3 on the 401 board in parallel to both zeners.

Also it's confusing for me that there are repeated identificators for parts nubers/names and the fact that positive rail is under the scrematic and not above as usual.
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Argentine Ham Radio LW1DSE since 1987.

Last edited by Osvaldo de Banfield; 25th September 2021 at 12:17 PM. Reason: Typos. ******* small keyboard of phones.
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Old 25th September 2021, 02:12 PM   #28
indianajo is offline indianajo  United States
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Note www.www.justradios.com has more axial lead electrolytic cap values than www.newark.com or DigiKey Electronics - Electronic Components Distributor. The vishay axial lead caps from the latter may lost longer however. If you have multigang can caps, they are available from Antique Electronic Supply but I've found in organs it is a lot quicker to screw a 5 eyelet solder terminal strip across the hole and install 3 or 4 radial lead electrolytic caps. Radial electrolytic caps from panasonic, nichicon, or rubicon can be bought in service life grades up to 12000 hours, which should solve the problem I've had of replacing the same electrolytic cap over & over, up to 4 times. Antique Electronic Supply has more panel mount potentiometers than newark or digikey or mouser.
The solder terminal strips with the mount on the end are most useful for replacing can caps. https://www.tubesandmore.com/product...mon-horizontal Use elastic stop nuts on #6 screws so they don't come loose.
Try to load your film cap in the same box. The cap is <$1, the rate to ship a box is $8 up. Mouser gets me $14 shipping every time, then ships air freight for some reason.
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Last edited by indianajo; 25th September 2021 at 02:17 PM.
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Old 25th September 2021, 03:01 PM   #29
Osvaldo de Banfield is offline Osvaldo de Banfield  Argentina
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Wait a moment, dear. Let the guy find the part(s) causing his headache and after it we see how he want/can to put new parts.

His amp is motorboating. In my experience, never a tone control stage had caused such a symptom. Nor a small film cap. Usually it is as a consequence of improper bypassed power supply, be by bad design, be by degraded caps.

Salu2 para to2.
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Old 25th September 2021, 03:02 PM   #30
Fast Eddie D is offline Fast Eddie D  United States
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I was going to give an old preamp (cap coupled) to one of my friends. I hooked it up to one of my systems and everything worked. Then I hooked it up to my shop system and the power amp wouldn't come out of protect. A little sleuthing showed 0.5 volts DC between audio ground and the electrical service ground. I haven't figured out why (probably the safety circuit) but obviously something's up.

The difference between the power amps is that the shop power amp used a grounded plug.
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