Legend A30 humming

[bfglp]

my A30 is humming through the speaker when i turn on the standby and i have no sound. although i still have a fully functioning preamp section when i run through the preamp out into another amp.

I'm a great parts changer but my trouble shooting skills are lacking a bit and i could use some advice.

[gtrplayr1976]

my A30 is humming through the speaker when i turn on the standby and i have no sound. although i still have a fully functioning preamp section when i run through the preamp out into another amp.

I'm a great parts changer but my trouble shooting skills are lacking a bit and i could use some advice.

Check this page. It may help. How old is the amp , tubes ?

Excessive Hum

[bfglp]

amp is from '82 it has a solid state power amp. preamp runs two 12ax7's and they are new

[gtrplayr1976]

amp is from '82 it has a solid state power amp. preamp runs two 12ax7's and they are new

Have you tried wiggling the input jack to see if it changes ? If it is a pcb mounted jack it may have broken solder joints.

[Jazz P Bass]

What you need to do is disconnect the speaker.
Get out your volt meter, set it to read Volts dc & measure across the speaker leads.
If there is a dc voltage measured, do not hook up the speaker again until you get it sorted out.
You may have a blown output section, which can damage the speaker.

[gtrplayr1976]

What you need to do is disconnect the speaker.
Get out your volt meter, set it to read Volts dc & measure across the speaker leads.
If there is a dc voltage measured, do not hook up the speaker again until you get it sorted out.
You may have a blown output section, which can damage the speaker.

I had this in a Crate bass amp. I never got that one straight. Future project

[52 Bill]

We started discussing this amp in the schematic forum. Preamp works fine, problem is in the power amp. He has 2-3 vdc on the speaker output and there was a bad diode or something.

Please recap what you have already done so we all know what is going on with the amp.

[bfglp]

OK, so far i tried jumping preamp to power amp and no sound, but if i turn on standby i get a loud transformer hum through the speaker and no sound.

so next i tried preamp out to another amp and had sound. everything functioned except when i switched on standby there was loud humming through the speaker and no sound. this seems funny that it works fine with the standby off and the power on.

next i disconnected the speaker and tested the output for DC voltage and found there was 2.7v DC

i looked inside to see that there were signs of excessive heat where Q12 and CR18 were pulled together with a zip-tie this was done @ the factory maybe some sort of heat-sink. anyway the zip-tie melted away.

could there be a bad transistor in the standby circuit?

[52 Bill]

could there be a bad transistor in the standby circuit?

The standby switch turns on and off the connection to the speaker. There are no active parts in that circuit.

In the other thread did you mention changing caps. What did you change?

You will need to test the diodes and transistors in the power amp. Does your meter have a diode test setting? If it does, start by testing the four diodes in the power amp CR15, CR18, CR19, and CR20 (I think).

Turn off and unplug the amp. Let the filter caps drain for a bit and then check the diodes. Put the red lead on the end of the diode away from the stripe and touch the black lead to the end closest to the stripe. Your meter should read some value, like 0.6 or 0.7 volts. Now reverse the two meter leads. Your meter should now show no reading, just like when the two leads are not touching anything.

See what you find out.

[g1]

schematics:

[bfglp]

In the other thread did you mention changing caps. What did you change?

You will need to test the diodes and transistors in the power amp. Does your meter have a diode test setting? If it does, start by testing the four diodes in the power amp CR15, CR18, CR19, and CR20 (I think).

Turn off and unplug the amp. Let the filter caps drain for a bit and then check the diodes. Put the red lead on the end of the diode away from the stripe and touch the black lead to the end closest to the stripe. Your meter should read some value, like 0.6 or 0.7 volts. Now reverse the two meter leads. Your meter should now show no reading, just like when the two leads are not touching anything. See what you find out.

I changed out C43 C44 C46 C47 and C36

[Mark Black]

Q12 and CR18 were pulled together with a zip-tie

Pretty common to physically attach a bias string diode to the output devices for thermal tracking, but the zip-tie is a pretty cheesy way to do it (quick & dirty - probably made sense from a manufacturing standpoint).

[bfglp]

Pretty common to physically attach a bias string diode to the output devices for thermal tracking, but the zip-tie is a pretty cheesy way to do it (quick & dirty - probably made sense from a manufacturing standpoint).

yeah there kinda boutique looking but designed for mass production

[bfglp]

You will need to test the diodes and transistors in the power amp. Does your meter have a diode test setting? If it does, start by testing the four diodes in the power amp CR15, CR18, CR19, and CR20 (I think).

Turn off and unplug the amp. Let the filter caps drain for a bit and then check the diodes. Put the red lead on the end of the diode away from the stripe and touch the black lead to the end closest to the stripe. Your meter should read some value, like 0.6 or 0.7 volts. Now reverse the two meter leads. Your meter should now show no reading, just like when the two leads are not touching anything.

See what you find out.

they all tested OK 0.5 and nothing on the reverse

[bfglp]

I also tested CR61 CR17 CR21 CR22 CR23 CR24 the same way and all read 0.5 and 1.3 on the reverse