Mesa/Boogie 400 keeps blowing mains fuse

[tubby]

The amp blows the mains fuse when you switch on the power (standby is in off position).
If I cycle it 10 times, it will be blowing the mains fuse 9 times.
The fuse(s) never blow once it's up and running, even under full load.

So, at first I was thinking filter caps, PT or OT.
However, if PT or OT were failing, then the fuse(s) should also be blowing during operation, right?
There is no bulging of any sorts or visible leakage on the filter caps.

Before I take the caps out of the circuit and measure them, my latest idea was that the mains switch might be starting to go south and that is triggering the fuse. Is that a possibility?
How would I go about and the check the switch? Measuring contact resistance for starters?

Very open to other ideas or similar experiences.
Schematic is attached.

[Helmholtz]

Large mains transformers (especially toroidals) can have huge inrush currents due to core saturation effects.
I need to use an inrush current limiter with my 600W isolating variac.

[Jon Snell]

No idea where you are in the world but UK/EU 240/230 volts will have a 3.15A T mains fuse and USA 120/110 volts 6.3A T fuse in series with the power in.
What size fuse are you using in the plug/socket supply to allow it to blow?

[dchang0]

Just checking: did you use slow-blow or fast-blow fuses?

[Helmholtz]

Maybe I misunderstood, but I thought that the main fuse/circuit breaker blows?

[The Dude]

No idea where you are in the world but UK/EU 240/230 volts will have a 3.15A T mains fuse and USA 120/110 volts 6.3A T fuse in series with the power in.
What size fuse are you using in the plug/socket supply to allow it to blow?

^^^^^ This. Is the amp fused correctly? Also note that "T" indicates slow blow.

[g1]

If you are powering up into 'standby' mode, OT can not be a suspect, it is not in circuit.

[tubby]

To clarify: I'm talking about the MAINS FUSE on the front panel of the amp, not the MAIN fuse on the socket or plug.
Schematic calls for a 3A SLO-BLO / T with 230V.
Thatīs what Iīve been using and they keep blowing.

[glebert]

Is the jumper configured as shown on the schematic for 230V operation? Has this ever been set up for 117V? Does it make any difference if you have the fan and/or ground filter switches in the off position when you power on (in standby)?

[Jon Snell]

What is the maker/supplier of the fuses you are using?
The correct fuse is 3.15A Time Delay; https://uk.rs-online.com/web/p/cartridge-fuses/0414049 or https://uk.rs-online.com/web/p/cartridge-fuses/0563699 dependant on fuse holder fitted.

[tubby]

Is the jumper configured as shown on the schematic for 230V operation? Has this ever been set up for 117V? Does it make any difference if you have the fan and/or ground filter switches in the off position when you power on (in standby)?

It is in 230V configuration and has been for the last 15 years.
Thereīs no ground filter switch on this unit and never has been, I would guess the schematic is from an earlier iteration.
The position of the fan switch does not make a difference concerning blowing the fuse. Pls also see my next post with measurements.

[tubby]

What is the maker/supplier of the fuses you are using?
The correct fuse is 3.15A Time Delay; https://uk.rs-online.com/web/p/cartridge-fuses/0414049 or https://uk.rs-online.com/web/p/cartridge-fuses/0563699 dependant on fuse holder fitted.

I use ESKA cartridge fuses rated for 250V and 3,15AT (=timelag/sloblo).
https://www.eska-fuses.de/en/fuses-a...fuses/6,3x32mm
The amps also blows a 6,3A of the same manufacturer, although that was a MT (=medium timelag).

[Helmholtz]

Once the amp is running, does it work as normal, i.e. no increased hum?
Are voltages good?

You might try disconnecting VR1 and VR2, which I think are varistors.

What is your measured mains voltage?

[tubby]

Once the amp is running, does it work as normal, i.e. no increased hum?
Are voltages good?

You might try disconnecting VR1 and VR2, which I think are varistors.

What is your measured mains voltage?

Measured mains is 235V. No abnormal hum. Supply, bias and heater voltages check out.
Good hint on those varistors. I remember that I have replaced those recently and I THINK I made sure I used the same value.
It says "SanKen 130K20 82" on them. Would these be the right ones? The schematic doesn't say which ones to use.
I'll take those out of the circuit and report back.

Additionally, I did some current measurements with my DMM in series with the power switch:

Max measured inrush current when switching on power (standby is off): 5,56A (out of 25 samples)
Fan switch position does not make a discernible difference.

A series of ten consecutive measurements of max inrush current gives wildly fluctuating results each time: 1,8 / 4,8 / 1,7 / 1,2 / 5,5 / 3,4 / 5,5 / 4,2 / 5 / 4 A
Any idea about those fluctuations? It would explain why the fuse only blows sometimes. Might also be to do with the sample rate of the DMMīs max function measurement (5/sec according to the data sheet)?

Once it is running (standby still off) current settles down to steady 0,82 A fan off / 0,88A fan low / 0,89A fan high
Power on and standby on while idling (with no signal applied) current is 1,9A

Peak current with power on, switching from standby "off" to standby "on" is 2,5A

I would think that rules out the fan circuit as the culprit.
OT is also ruled out as per g1īs post (#7).
That would leave what...PT, filter caps, varistors, episodic heater short on one of the tubes (is that a thing at all?)

[Helmholtz]

I don't think a DMM is fast enough to capture the actual peak current.

Transformer inrush (magnetizing) current depends on mains voltage phase at turn-on and at last turn-off.
Just google transformer inrush current.