A friend left me this amp to look at was imported from the US and someone connected to our mains without a step down transformer.
The primary on the power transformer is open circuit, my question is what damage if any is likely on this unit?
There appears to be a number of semiconductors, IC's etc on this amp, its not a pure valve only amp.
Dont want to open a whole can of worms if I go ahead and order a new power transformer, but the cost of this amp its probably worth repairing, just want to know what Im in for.
What do most people find from experience in these situations?
The primary on the power transformer is open circuit, my question is what damage if any is likely on this unit?
There appears to be a number of semiconductors, IC's etc on this amp, its not a pure valve only amp.
Dont want to open a whole can of worms if I go ahead and order a new power transformer, but the cost of this amp its probably worth repairing, just want to know what Im in for.
What do most people find from experience in these situations?
I have a friend that is and reckons it is a waste of time, but i thought i would ask here, he says in most situations all that goes up in smoke is the primary winding, but there is no way to be 100% certain.
If your friend pays the tech, he would not consider it a waste of time.
But cascading failures due to wildly excessive AC line voltage would be expected.
Extremely dangerous voltages inside, not for an amateur.
But cascading failures due to wildly excessive AC line voltage would be expected.
Extremely dangerous voltages inside, not for an amateur.
I think he just can't be bothered to take on a big job as henis semi retired, and it is still a risk of discovering more damage and I have no idea how easy half of the components would be to find, let alone the work involved in replacing them all.
Been studying the schematic and the hardest job is pulling this heavy chassis out of the unit, it's a 2 man job!
As far as high voltages go, we'll this place is DIY audio after all and I've built my own guitar amp that runs on the same high voltages and build tesla coils running at far more lethal current/voltages so I know what I'm dealing with in that department.
Been studying the schematic and the hardest job is pulling this heavy chassis out of the unit, it's a 2 man job!
As far as high voltages go, we'll this place is DIY audio after all and I've built my own guitar amp that runs on the same high voltages and build tesla coils running at far more lethal current/voltages so I know what I'm dealing with in that department.
You COULD be lucky if 'the right AC fuse' blew purely during the charging of the PS. capacitors >
but I really don't know the odds of that 😕
but I really don't know the odds of that 😕
if the transformer core went into saturation before voltage damage could occur, you may be lucky. the primary fuse blew due to transformer saturation. check if the power transistors are blown, then it is more a total failure.
Nothing too exotic to find.
A lot can go wrong before the fuse and transformer primary melt open even when the correct voltage is applied..
Until you open it up you won't know the extent of the damage (if any) to capacitors, resistors, diodes, connectors and circuit boards.
Best of luck!
There are no power transistors in this unit, it's a 300W bass amp powered by 6x 6550 tubes.
It appears that someone has replaced the fuse at some point, but no way to tell when this had happened, other than a mark on the fuse holder from a screwdriver.
Looking at the schematic a bit further, I think its looking a bit more promising, it's possible that the electrolytic capacitors experienced voltages outside their rated voltage but largely most of the amplifier appears to be tube, the solid state components appear to be confined to the effects loop and the bias control.
Any thoughts?
https://www.bustedgear.com/images/schematics/Ampeg_SVT-CL.pdf
This is what happens when you lend gear to friends, one expensive mistake.
It appears the transformer has a 230V tap so could have been rewired to accommodate 230V mains rather than using it with this whopping big step-down transformer!
That is 'sort of' good news that the transformer has a 230V tap > but the first half of the transformer windings are probably damaged.
WITHOUT POWER CONNECTED > I suggest you start to do some measurements with a multimeter >
looking at the transformer windings and anything else you can think of regarding parts > like diodes etc.
You MUST make these 'passive' measurements before applying any 'mains power connection'.
WITHOUT POWER CONNECTED > I suggest you start to do some measurements with a multimeter >
looking at the transformer windings and anything else you can think of regarding parts > like diodes etc.
You MUST make these 'passive' measurements before applying any 'mains power connection'.
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Was just thinking about this and would this mean there is still a possibility that I could rewire it using the second half of the primary instead and remain running on 120V supply? Would permit time to test the unit before deciding to replace the transformer i guess.
I have a variac i can use to slowly bring up the voltage in a safe manner.
Will take the chassis out to inspect it further anyway.
Would be good to check the rectifier diodes etc as you say and go from there.
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1. Disconnect all connections going to the transformers primairy.
Measure for continuity, each primary winding.
2. Disconnect all connections coming from the transformers secundairies.
Measure for continuity, each secundairy winding.
3. Rewire the transformer for 230V use.
You have then only the transformer connected to the mains.
4.Use a bulb in series with the amp and switch it on.
No bright light? Measure every secundary voltage and compare it with the schematic.
5. Success? Then come back for the next stage...
Measure for continuity, each primary winding.
2. Disconnect all connections coming from the transformers secundairies.
Measure for continuity, each secundairy winding.
3. Rewire the transformer for 230V use.
You have then only the transformer connected to the mains.
4.Use a bulb in series with the amp and switch it on.
No bright light? Measure every secundary voltage and compare it with the schematic.
5. Success? Then come back for the next stage...
Yes, that's a good place to start.
I expect I should find that all the secondaries test OK.
I won't be able to rewire the transformer gor 230V if half the primary has gone, but I should be able to use the other good half on 120V
I expect I should find that all the secondaries test OK.
I won't be able to rewire the transformer gor 230V if half the primary has gone, but I should be able to use the other good half on 120V
Short answers with good intentions:
You do not expect anything.
You have to measure and that is rule number One!
If, and only if, the primary is damaged, using the other half of the 120V will probably not work.
Usually the damaged section has some shorts in it and that will give serious problems form the magnetic foelds view.
But; you haven't measured anything so it's no option to assume that the transformer is gone.
Measure first and then take the next step.
You do not expect anything.
You have to measure and that is rule number One!
If, and only if, the primary is damaged, using the other half of the 120V will probably not work.
Usually the damaged section has some shorts in it and that will give serious problems form the magnetic foelds view.
But; you haven't measured anything so it's no option to assume that the transformer is gone.
Measure first and then take the next step.
Well so far it appears the transformer(s) are OK!
There was a "filament" fuse inside that had blown which is a bit unclear to me as it appears to be a power fuse in line with the rest of the supply according to the schematic. The transformer was reading open circuit as a result, but the windings appear all OK.
The 2 primary windings are connected in parallel from what I can gather and there is a second transformer for the filament supply.
The power transformer reads about 6 ohms on the primary and the filament transformer is 16 ohms on the primary.
I connected my variac to it and fired up.
No fireworks reported or fuse blowing which is a good sign, the heater filaments started to glow too.
The good news is that it should be possible to reconfigure this unit for 230V.
All I need to do is rewire the primaries on both transformers in series as per the service manual.
There was a "filament" fuse inside that had blown which is a bit unclear to me as it appears to be a power fuse in line with the rest of the supply according to the schematic. The transformer was reading open circuit as a result, but the windings appear all OK.
The 2 primary windings are connected in parallel from what I can gather and there is a second transformer for the filament supply.
The power transformer reads about 6 ohms on the primary and the filament transformer is 16 ohms on the primary.
I connected my variac to it and fired up.
No fireworks reported or fuse blowing which is a good sign, the heater filaments started to glow too.
The good news is that it should be possible to reconfigure this unit for 230V.
All I need to do is rewire the primaries on both transformers in series as per the service manual.
I hope you understand that when wiring two separate 120volt windings in series, you have to get the 'phase orientation' correct.
More good news, it's all working!
Was no more than a blown fuse thankfully, had a few issues while testing on the variac was causing the output tube protection circuit to kick in which initially I thought may have been damage to that part of the circuitry with the ICs involved, but was reading how bad power supplies can cause this to trip, so I went ahead and connected to the step-down transformer with no issues.
Have rewired for 230V and working sweet, so is a good success story that all hope is not lost in these situations.
Kind of strange that this fuse was on the board with no external fuse holder, huge inconvenience should it blow.
Was no more than a blown fuse thankfully, had a few issues while testing on the variac was causing the output tube protection circuit to kick in which initially I thought may have been damage to that part of the circuitry with the ICs involved, but was reading how bad power supplies can cause this to trip, so I went ahead and connected to the step-down transformer with no issues.
Have rewired for 230V and working sweet, so is a good success story that all hope is not lost in these situations.
Kind of strange that this fuse was on the board with no external fuse holder, huge inconvenience should it blow.