Lost power in Vibrato Channel Bandmaster

jdwnyc · 03-19-2008, 03:42 AM

Hi Everyone,

I have a '68 BF Bandmaster, and while playing it I suddenly have very low volume on the Vibrato Channel, maybe 1/20th of what it should be. The Normal Channel works fine. The tone controls affect the volume somewhat, but not the tone. Vibrato controls work fine, but still weak. I've swapped the tubes, they're fine.

I've checked the Voltages around V2, and here's what it reads:

Pin 1 - 287 V (265 is spec)
Pin 3 - 2.3V
Pin 6 - 278 V (275 is spec)
Pin 8 - 2.2V

Is this pretty normal? Seems like it.
Also was able to measure some of the nearby resistors, and they seemed accurate. I really don't know what step to take next. Capacitors? Any other suggestions? Thanks in advance!

Dan

Twist · 03-19-2008, 05:56 AM

Which Bandmaster ?....AC568, AA763, AB763, 6GC7-A....etc.

Enzo · 03-19-2008, 06:21 AM

If the DC voltages are OK, then it is time to trace the signal through the amp and see where it quits. If there is signal on one end of a cou0pling cap and not the other, then that cap is open. Look for caps with broken lead wires too. And of course interstage resistors can open, and so can pots.

jdwnyc · 03-19-2008, 03:11 PM

Sorry, it's a '66, AB763 circuit.
And yeah, it looks like it's time to trace the circuit. I'm relatively new to this, although I'm fairly good with electronics (was able to replace filter caps and cord), so any resources, links, that would help me understand signal flow in this circuit would be really helpful (although I have a crude understanding). one question is this - if testing the pots, i'm testing for voltage, not ohms, correct? (i've already tested the pots for resistance). Thanks so much!

MWJB · 03-19-2008, 03:25 PM

It's worth checking dc voltages at V4 too.

jdwnyc · 03-19-2008, 03:29 PM

V4 is the phase inverter? Doesn't that share with the Normal channel?

MWJB · 03-19-2008, 03:33 PM

Aha! Er yes, sorry ...I was thinking Bandmaster Reverb, which came out later than '66.

Twist · 03-19-2008, 06:19 PM

I would check the vibrato LDR. Unsolder and remove the center wire from the intensity pot. This will remove any effect the vibrato has on that channel.

jdwnyc · 03-19-2008, 08:40 PM

Thanks, I'll try that. The vibrato does work, only weak like the rest of the channel. I tested the LDR with my DMM and it did respond to my flashlight, but I'm not sure what the full range should be (light vs. dark). But yeah, I'll unsolder the vibrato intensity and see what happens.

03-19-2008, 03:42 AM	#1
jdwnyc Junior Member Join Date: Mar 2008 Posts: 4	Lost power in Vibrato Channel Bandmaster Hi Everyone, I have a '68 BF Bandmaster, and while playing it I suddenly have very low volume on the Vibrato Channel, maybe 1/20th of what it should be. The Normal Channel works fine. The tone controls affect the volume somewhat, but not the tone. Vibrato controls work fine, but still weak. I've swapped the tubes, they're fine. I've checked the Voltages around V2, and here's what it reads: Pin 1 - 287 V (265 is spec) Pin 3 - 2.3V Pin 6 - 278 V (275 is spec) Pin 8 - 2.2V Is this pretty normal? Seems like it. Also was able to measure some of the nearby resistors, and they seemed accurate. I really don't know what step to take next. Capacitors? Any other suggestions? Thanks in advance! Dan

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03-19-2008, 05:56 AM	#2
Twist Senior Member Join Date: Nov 2007 Location: Springdale, Arkansas Posts: 273	Which Bandmaster ?....AC568, AA763, AB763, 6GC7-A....etc.

03-19-2008, 06:21 AM	#3
Enzo Senior Member Join Date: May 2006 Location: Lansing, Michigan, USA Posts: 10,352	If the DC voltages are OK, then it is time to trace the signal through the amp and see where it quits. If there is signal on one end of a cou0pling cap and not the other, then that cap is open. Look for caps with broken lead wires too. And of course interstage resistors can open, and so can pots.

03-19-2008, 03:11 PM	#4
jdwnyc Junior Member Join Date: Mar 2008 Posts: 4	Sorry, it's a '66, AB763 circuit. And yeah, it looks like it's time to trace the circuit. I'm relatively new to this, although I'm fairly good with electronics (was able to replace filter caps and cord), so any resources, links, that would help me understand signal flow in this circuit would be really helpful (although I have a crude understanding). one question is this - if testing the pots, i'm testing for voltage, not ohms, correct? (i've already tested the pots for resistance). Thanks so much!

03-19-2008, 03:25 PM	#5
MWJB Old Timer Join Date: Jun 2006 Posts: 1,924	It's worth checking dc voltages at V4 too.

03-19-2008, 03:29 PM	#6
jdwnyc Junior Member Join Date: Mar 2008 Posts: 4	V4 is the phase inverter? Doesn't that share with the Normal channel?

03-19-2008, 03:33 PM	#7
MWJB Old Timer Join Date: Jun 2006 Posts: 1,924	Aha! Er yes, sorry ...I was thinking Bandmaster Reverb, which came out later than '66.

03-19-2008, 06:19 PM	#8
Twist Senior Member Join Date: Nov 2007 Location: Springdale, Arkansas Posts: 273	I would check the vibrato LDR. Unsolder and remove the center wire from the intensity pot. This will remove any effect the vibrato has on that channel.

03-19-2008, 08:40 PM	#9
jdwnyc Junior Member Join Date: Mar 2008 Posts: 4	Thanks, I'll try that. The vibrato does work, only weak like the rest of the channel. I tested the LDR with my DMM and it did respond to my flashlight, but I'm not sure what the full range should be (light vs. dark). But yeah, I'll unsolder the vibrato intensity and see what happens.

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