# Bassmaster shock/no sound



## analogteletube (Jan 26, 2011)

I have a 70's bassmaster thats been my go to amp for a couple years now, sounded great at a show on friday. Last night it started making a high pitch screeching at high presence settings, I reflowed any suspicious solder joints and played around with the presence control lead dress. Turned it back on and now no sound at all, replaced all tubes, still no sound, and now its shocking me when I have one hand on my guitar strings and one hand on the chassis, also shocked me with nothing with nothing plugged in and I touched the ext. speaker jack.


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## dtsaudio (Apr 15, 2009)

You need to retrace your steps. If it wasn't shocking you before you opened it, you've done something wrong and need to back track.
Also does this amp have a 3 prong plug or 2 prong with death cap?


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## analogteletube (Jan 26, 2011)

3 prong cord yes. Thats exactly what I thought, I must have done something wrong, but I only re-flowed a few solder joints in the preamp, and pushed a couple of wires around, nothing I could think of that would cause current to be flowing to the chassis. 

I took some readings and im only getting 228VDC right after the rectifier, which was formerly a solid 450 or so, could a dying PT account for the screeching and shocks? I'm getting 1.6 ohms across the primary 12 ohms across the secondary, looks like the meter wants to go to megohms from primary to circuit ground but to high to measure, but only around 1 ohm at the transformers ground, everything is still soldered in the circuit.


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## dtsaudio (Apr 15, 2009)

A couple of things come to mind. Check around where the output transformer connects to the high voltage (probably a red wire). Make sure there are no nicks in it and it's a partial short to chassis. Filter cap may be bad, or a bad rectifier.
One way to check your power transformer, is to leave the amp in standby and measure the AC voltage at the rectifiers. With it in stand by, you won't be powering the amp. You should measure 320 - 340 VAC. Be careful this is quite lethal. 
Check also around where you re-soldered. If there are any burns on the insulator layer, these will conduct. You may have contacted something there.


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## jb welder (Sep 14, 2010)

It's best to tackle the safety issues first, then move on to sound problems.
You said you got a shock touching the chassis and the strings at the same time. They should be connected. Your guitar cord ground (sleeve) should measure 0 ohms to the chassis. The ac cord ground pin should measure 0 ohms to the chassis.
With guitar plugged into the amp, there should be 0 ohms from the strings to the ground pin on the amps AC cord.
The exception to this would be a guitar that does not have the strings grounded, but this is a rare case.
Do those checks and see what you get.


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## loudtubeamps (Feb 2, 2012)

Check for secure connection on any transformer center tap leads that would be chassis grounded as well check filter cap ground paths.


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## soldierscry (Jan 20, 2008)

Has the ground switch been removed from the amp with the 3 prong plug mod? if not flick it in the other direction.


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## analogteletube (Jan 26, 2011)

Im getting 330vac at the rectifier with the amp in standby, all ground connections checked, swapped the filter caps with ones i know are good, still no luck....


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## nonreverb (Sep 19, 2006)

Did you try a continuity test with a meter between the ground lug of the plug and the chassis?


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## dtsaudio (Apr 15, 2009)

> Did you try a continuity test with a meter between the ground lug of the plug and the chassis?


For that matter, did you check ground continuity from the chassis to the input jacks. Make sure all ground points are good.



> Im getting 330vac at the rectifier with the amp in standby


Power transformer's probably good. We now need to narrow down the search. Once you have verified that *all* ground points are connected we need to find where the voltage is being lost. This will most likely tell us where the chassis is being livened up. 
*Note that this can get tricky and dangerous. Safety First.*
With amp off check the diodes of the high voltage. Once they are verified as being good, disconnect the HV feed to the small signal tubes. We want only the voltage going to output transformer and output tubes. There should be a 4700 ohm resistor off the main cap tied to another cap. Lift one end. 
Here's where it gets dangerous. *Don't leave it running any longer than you have to, as voltages will rise possibly above what the caps are rated for.
*Remove the output tubes. Check that there is no voltage on the filter caps. Check continuity between the output transformer and chassis at both output tube sockets. There should be none. If there is, there's your problem. If not, turn on the amp and measure the voltage at the center tap of the output transformer. Then shut it off. The caps will stay charged and you should be able to make more voltage measurements at the tube sockets. If these are all correct *drain the filter caps through a resistor *and reconnect the power supply. Double check the supply is drained before re-connecting.
You can then moved down the line. 
I would normally bring this up on a Variac so as not to damage anything. This is also assuming you have the skill level to do this, as it's not the safest thing to do. If you don't feel comfortable powering it don't. You might find the problem just checking for continuity.

I apologize if I'm being a bit over cautious and condescending. I don't know your skill level or what equipment you have. I don't take safety lightly.


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## analogteletube (Jan 26, 2011)

So after taking more readings in the amp I noticed some strange stuff around the cathode follower that reminded me of when I first got the amp (preamp had be heavily molested by previous owner) a bunch of solder had bubbled out under/ between the boards and was making connections where they shouldn't be, I pulled the board up and sure enough the cathode, anode and grid of the cathode follower were tied together under there. I pulled the wires out heated up the turrets/rings(?) and blew the excess solder out, rewired and voila! I guess re-flowing those few joints in the preamp really did make a difference, though I'm guessing if the job had be done properly this would not have happened! 

So the amp is up and running, sounding great except for the squealing I mentioned in my initial post, it only happens when the channels are jumped the amp is cranked and the presence above 7 or so.


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