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 Forum index » DIY Hardware and Software » Ken Stone designs - CGS
CGS65 Tube VCA Troubleshooting
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echoer



Joined: Dec 10, 2009
Posts: 40
Location: lawrence, ks

PostPosted: Thu Dec 10, 2009 9:05 am    Post subject: CGS65 Tube VCA Troubleshooting Reply with quote  Mark this post and the followings unread

Hello Everyone,

Just finished building a couple Tube VCA's. One works great. The other not so much. I am not getting any sound output from the VCA. I am using a 1J24B Submini Tube I got from Bridechamber.

I've done a bit of troubleshooting already and have compared voltages between the working unit and the non-working unit. I think the problem is a faulty tube but would appreciate your opinions.

Tube Pin Voltages:
Pin # Non-Working Unit
1) 13.68
2) -14.96
3) -.4
4) -13.68
5) -17.47
6) -17.28

Pin # Working Unit
1) 13.68
2) -14.96
3) -.8
4) -14.96
5) 11.81
6) -15.02

Pin 5 seems to be the problem. Followed the schematic and voltage is the same at Pin 1 of the LM358 on both units so I don't believe it is a problem there. Quadruple checked the pins of the Tube are going to the proper holes on the board. Triple checked wiring and polar components.

I also traced the audio signal from the Audio In on the board all the way to the tube and have a strong audio signal the whole path. No output from the TC of the Tube though.

I would be happy to include pics if that would be of any help but I think the Tube Pin Voltages tell most if not all the story.

Thanks in advance for your help.
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andrewF



Joined: Dec 29, 2006
Posts: 1171
Location: australia
Audio files: 4

PostPosted: Thu Dec 10, 2009 4:34 pm    Post subject: Re: CGS65 Tube VCA Troubleshooting Reply with quote  Mark this post and the followings unread

echoer wrote:

5) -17.47
6) -17.28



Hi
welcome welcome to e-m

What power supply are you using to get -17V?

Yes a dead tube is possible, worth exhausting other possibilities before replacing it. i haven't used the 1J24B, can you see the heater glow?

I would have expected the voltage on pin 5 to be the same as that on pin 1 of the LM358, will check on one of mine later (after brekky).
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andrewF



Joined: Dec 29, 2006
Posts: 1171
Location: australia
Audio files: 4

PostPosted: Thu Dec 10, 2009 6:06 pm    Post subject: Reply with quote  Mark this post and the followings unread

LM358/1 : -11.8V
tube pin 5 : -10.8V

.....that probably confuses things just perfectly Confused
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echoer



Joined: Dec 10, 2009
Posts: 40
Location: lawrence, ks

PostPosted: Fri Dec 11, 2009 7:26 am    Post subject: Reply with quote  Mark this post and the followings unread

Thanks for you quick help Andrew.

I'm running off 15 V Power Supply (MOTM 900)

There is a 100k resister between Pin 5 of the tube and Pin 1 of the LM358 so that should make the voltages between the two a little off.

I can not see the heater glow on either working or non-working unit. The LED's light up underneath though.
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echoer



Joined: Dec 10, 2009
Posts: 40
Location: lawrence, ks

PostPosted: Sat Dec 19, 2009 8:55 am    Post subject: Reply with quote  Mark this post and the followings unread

Update: Scott from Bridechamber sent me another tube for no charge. I installed it last night and the VCA works great now. Thanks to Scott and Andrew for their help.
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