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 Post subject: OSD amp not working
PostPosted: Sat Aug 29, 2020 4:47 pm 
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Joined: Tue Oct 16, 2018 9:34 pm
Posts: 5
I built the OSD amp several months ago and it was working great. I just recently bought an attenuator/DI Box to connect my amp to my Audio interface to my computer for recording. The attenuator is 8ohm, my speaker output is set to 8ohm and my speaker is 8ohm. I connected my amp to my attenuator input then my output to my speaker, it was working no problem. Today I turned on my amp, it worked for about 1min then nothing, my amp stopped working. I didn't see any smoke, no noise, nothing just stop working. I took voltage readings from my amp and got a negative voltage readings:
B1 -192
B2 -192
B3 -192
B4 -197
B5 -195

Has anyone seen this before? Does anyone have an idea what could be wrong?
Thanks


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 Post subject: Re: OSD amp not working
PostPosted: Thu Sep 03, 2020 5:21 pm 
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Joined: Wed Sep 12, 2018 5:17 pm
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Location: Shawnee, OK
Are you using the multimeter on the vDC setting?

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 Post subject: Re: OSD amp not working
PostPosted: Thu Sep 03, 2020 5:24 pm 
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Joined: Wed Sep 12, 2018 5:17 pm
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Location: Shawnee, OK
Or maybe PM Coco?

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 Post subject: Re: OSD amp not working
PostPosted: Fri Sep 04, 2020 1:21 am 
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Joined: Sat Dec 04, 2010 3:42 pm
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Location: Qualicum Beach, BC
I was thinking it was a meter problem, too. Either the meter doesn't work right or it's not being used right. The power supply can't make negative high voltage. Set the meter for DC volts and make sure the negative probe is firmly connected to chassis ground for the measurements. Also make sure everything in the amp that's supposed to be connected to ground still is.

Does anything work on the amp - like the pilot light or the tube heaters? Is the fuse OK? Are the patch cords good?


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 Post subject: Re: OSD amp not working
PostPosted: Sat Sep 12, 2020 11:35 pm 
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Joined: Tue Oct 16, 2018 9:34 pm
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When I took my initial readings that came out negative I did a little troubleshooting and found the fuse was bad. It didn't look blown at first but when I checked continuity there was none. So I replaced the fuse and got proper readings. I played my amp for about an hour and the fuse blew again. So I knew something was wrong. I replaced the fuse again and thats when I noticed the power tube was glowing super bright and very hot. I tested the power tube and sure enough it went bad. I ordered new tubes which came in today, mullard 6v6, put them in, biased the amp to 22Mv and it sounded fine. After about an hour or so the amp started having problems again. This time the sound gets really low and muffled sounding like its breaking up in a bad way. Tomorrow I will check all my voltages and trouble shoot some more. Does anyone have an idea what may cause the problems im having or where to start.
Thanks


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 Post subject: Re: OSD amp not working
PostPosted: Sun Sep 13, 2020 12:57 am 
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Joined: Sat Dec 04, 2010 3:42 pm
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Location: Qualicum Beach, BC
Voltage readings at the tube sockets are always a good place to start. They are the amp's vital signs, especially at the plates, screen grids, control grids, and cathodes of the power tubes. Also at the positive terminals of all the filter capacitors for the power supply.

Are the fuses still good?

If all that checks out OK, then look for bad solder connections in the signal path.


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 Post subject: Re: OSD amp not working
PostPosted: Sun Sep 13, 2020 8:54 am 
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I took my readings today with new mullard 6v6 tubes and new mullard 12ax7 tubes, here's what I got:

B1: 460, B2: 453, B3: 442, B4: 300, B5: 291

V1 Pin1: 170, Pin3: 1.78, Pin6: 170, Pin8: 1.80
V2 Pin1: 194, Pin3: 1.58, Pin6: 196, Pin8: 1.55
V3 Pin1: 311, Pin2: 38.7, Pin3: 61.5, Pin6: 307, Pin7: 38.5, Pin8: 61.4
V4 Pin3: 455, Pin4: 451
V5 Pin3: 456, Pin4: 451

Now when I bias my power tubes I get two different readings on the bias points. So I'm looking from the back of the amp, the bias point closest to the V5 power tube I'm calling that the left bias point and the one closest to the V4 power tube the right bias point. When I check the readings on the left bias point if I set that to 22mV, the bias point on the right reads 26.0mV. If I set the right bias point to 22mV, the left bias point reads 18.4mV, is that normal? For now I left it so the right reads 22mV and the left reads 18.4mV so I don't blow my tubes again.

Im going to continue to check my wires for continuity and my solder points to see if I can find anything else.

As far as the fuses go, there are still good. I replaced the HT fuse with another .5A SLO BLO fuse.

Thanks for all your help.


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 Post subject: Re: OSD amp not working
PostPosted: Mon Sep 14, 2020 11:59 am 
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Joined: Wed Sep 12, 2018 5:17 pm
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Location: Shawnee, OK
Maldo, are you sure about the fuse? OSD Bill Of Materials indicates 250V, 5 X 20mm, 2 Amp Slo Blo fuse! EDIT: Apologies, I haven't built an OSD and didn't realize there are 2 fuses! I'll keep quiet now :oops:

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