Bass guitar amplifier blows fuse when powered on

Thread Starter

stuartm

Joined Aug 17, 2019
11
I live in a neighbourhood where people throw out good things. A few days ago, I found a Hartke A70 bass amp in a pile of rubbish waiting for collection outside a nearby house.

I carried the 21kg amplifier home, plugged it in and switched it on, but it did not power on. I unscrewed and removed the electronics from the speaker cabinet and looked inside. There was no obvious sign of anything wrong – no bulged capacitors, or scorched power resistors, etc.

I discovered that the fuse inside the power plug was missing. Writing on the back of the amplifier said 115V 2A / 230V 1A. I live in Australia, which has 230V 50Hz mains power, so I bought a replacement 1A fuse. I bought a few F1AL250V fuses (F = fast acting, L = low breaking capacity).

When I inserted the new fuse and switched it on again, the fuse blew, and I suddenly had an insight as to why the original owner was throwing out their amplifier.

I am comfortable with simple low-voltage DC electronics, but mains-powered AC is outside of my experience. I have tried a few things already, but I am feeling a bit stuck, and I would really appreciate any suggestions.

Here is what I have tried so far…

The removable fuse holder in the chassis mount AC plug can be inserted two ways, which selects 110-120V input or 220-240V input, with the label for the selected voltage upright (and the other voltage label upside-down).

I thought that I might have inserted it the wrong way but, just to be sure, I followed the wiring from the plug to the switch to the transformer, and unplugged the orange wire between the switch and the 110V input on the transformer. I inserted a new fuse and powered it on, but the fuse blew again.

Next, I thought that something on the circuit board might be shorting out, so I unplugged connector 7, which connects the output of the transformer to the bridge rectifier and voltage regulators, etc. This means that the only components being powered are: the input plug (with in-jack fuse), a power switch (with an LED indicator), a thermistor and fuse, and the toroidal transformer. I powered it on, but the in-jack fuse blew again. I guess this means that the problem is isolated to the first part of the input power circuit (see attached circuit diagram).

I have run out of ideas, and my knowledge of AC electronics is weak. Could the 1A fuse be blowing because of the inrush current as the transformer is initially receiving power? Would this be fixed with a slow-acting 1A fuse? Could there be a fault with the transformer, even though there are no signs of physical damage or scorching? How would I test this before buying a new transformer? Have I missed something that is obvious to everyone except me?
 

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AlbertHall

Joined Jun 4, 2014
8,363
Yes, it should be a slow blow fuse and it should probably be 2A.
Is the switch marked 'K' a manual on/off switch? If so try a new fuse with this switch in the 'off' position first.
 

bertus

Joined Apr 5, 2008
20,051
Hello,

Mains fuses are usualy slow blow.
They must resist the inrush current of the transformer.
There seem to be two positions for the fuse.
One for 110 V (5,6) and one for 220 V (4,6).
Be sure in wich position the fused is placed.

Bertus
 

MrChips

Joined Oct 2, 2009
19,275
With connector 7 unplugged, that leaves you with few components to check in order to find the fault.

My guess is:

1) A bad power transformer.

2) A short in the wiring to the primaries or secondary windings.

3) A short in the power receptacle.
 

Thread Starter

stuartm

Joined Aug 17, 2019
11
Thank you everyone! You are right that I am using the wrong fuse (and I'm an idiot).

I found the manual online and it says:

Fuse Type: 3AG slo-blo (US & Japan), 50T time delay (UK & European)
Fuse Rating US & Japan: 1.0A (A70), 1.6A (A100)
Fuse Rating UK & European: T500M (A70), T1ALM (A100)

I have done a bit of research to try to decypher the fuse codes. 3AG is a form factor, rather than a rating. I am having trouble finding 50T, but I am guessing the important part of that is the T (slow acting).
I am assuming that T500M for the A70 amplifier is really meant to mean T 500mA, and T1ALM for the A100 amplifier is meant to mean T 1A L (L = low breaking capacity, and the M makes no sense as a fuse can't be simultaneously T and M - Slow acting and Medium acting).

I will buy a 1A slow blow (T) fuse and give it a go. Thank you again!
 

LesJones

Joined Jan 8, 2017
2,319
Another possible cause is the NTC thermistor going low resistance or short circuit. (Or someone in the past shorting it out. It's purpose is to reduce the switch on surge. When it is cold it will have a higher resistance than when it warms up.

Les.
 

Thread Starter

stuartm

Joined Aug 17, 2019
11
I bought some slow-blow fuses today and tried them in the amp. I left connector 7 unplugged, so the only components being powered are: the input plug (with in-jack fuse), a power switch (with an LED indicator), a thermistor and fuse, and the input side of the toroidal transformer.

Attempt 1: Replaced in-jack fuse with slow blow S1A250V fuse. Inserted power cable. Switched on. Blew in-jack fuse.

Attempt 2: Replaced in-jack fuse with slow blow S2A250V fuse. Inserted power cable. Switched on. Blew F1 on circuit board (F3,15AL250V). In-jack fuse was fine.

I had a look at the NTC thermistor (at LesJones’ suggestion). Measuring resistance from leg to leg gives 8.2 ohms on my cheap multimeter. The circuit diagram says 6 ohms, so this appears to be in the correct range.

Am I correct in saying that suspicion now seems to be shifting to the transformer? (as per the reply from MrChips). How would I test the transformer to verify?
 

Thread Starter

stuartm

Joined Aug 17, 2019
11
I’m not sure if this is a valid test, but I tried measuring resistance between the different wires on the transformer. The wires are:
* Primary: Fuse (black), 115V (orange), 230V (red)
* Secondary: 31.7V (blue), 0V (black), 31.7V (blue)

Primary -> Primary
Fuse/black -> 115V/orange = 1.3 ohms
Fuse/black -> 230V/red = 17.5 k ohms (!!!)
115V/orange -> 230V/red = 17.4 k ohms (!!!)

Secondary -> Secondary
31.7V/blue -> 0V/black = 1.2 ohms (for both blue wires)

Primary -> Secondary
Fuse/black -> 0V/black = open circuit
Fuse/black -> 31.7V/blue = open circuit
115V/orange -> 0V/black = open circuit
115V/orange -> 31.7V/blue = open circuit
230V/black -> 0V/black = open circuit
230V/black -> 31.7V/blue = open circuit

The reading for 230V/red input seems very high. Does this mean that the transformer is broken, even though it looks physically fine?
 

MrChips

Joined Oct 2, 2009
19,275
Here is the schematic of the power transformer:

Hartke A70 Power Input.jpg

So far, all tests suggest a bad transformer. This would be my last suspicion.

I would first look for a short in the wiring.

Can you post photos of the wiring at the power receptacle, power switch, and transformer, so that we can see the connections and if there are any disconnects to remove the transformer from the circuit?
 

AlbertHall

Joined Jun 4, 2014
8,363
Does it blow the fuse when switch K is open?
(Neither of the transformer resistances seems reasonable to me!)
 

Thread Starter

stuartm

Joined Aug 17, 2019
11
@AlbertHall No, the fuse does not blow when the K (front panel) switch is open/off. I should have mentioned this as one of the tests I ran previously.

What kind of resistances would you expect between wires on a transformer? I am a bit out of my depth on this. I vaguely remember that DC resistance can be different to the resistance/impedance that is seen by AC voltage. I would have naively thought that DC voltage (which is presumably how a multimeter measures resistance) would see something very close to 0 ohms for a transformer, as it seems like it is just a realllllly long wire.

@MrChips I have attached some photos of the power-related wiring. There is nothing obviously shorted. You will see that I have removed the blown fuse (F1) from earlier today, unplugged the orange cable between switch K and the 110V input of the transformer (not used because I am on 230V), and unplugged connector 7 (31.5V/0V/31.5V) from the transformer to the circuit board.
 

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MrChips

Joined Oct 2, 2009
19,275
Primary -> Primary
Fuse/black -> 115V/orange = 1.3 ohms
Fuse/black -> 230V/red = 17.5 k ohms (!!!)
115V/orange -> 230V/red = 17.4 k ohms (!!!)
I don't get this.

Why is there 1.3 ohms between Fuse and Orange?
Are you certain that the receptacle is set for 230V operation?
 

Thread Starter

stuartm

Joined Aug 17, 2019
11
@MrChips Ahhh, I think we have a miscommunication. Sorry.
When I say 1.3 ohms between Fuse and Orange, I am just measuring between the black and orange wires on the transformer. The measurement is not actually passing through either of the actual fuses.

I am certain that the removable fuse holder (receptacle) in the AC plug is placed correctly. And removing the orange wire between switch K and the transformer means that there would be an open circuit if it was plugged in upside down.
 

MrChips

Joined Oct 2, 2009
19,275
I understand that if you disconnect the orange wire then the transformer is set for 220V, assuming that the fuse in the receptacle is installed correctly.

With the power switch K in the off position, measure the resistance at the transformer primaries again:

black - orange
black - red
orange - red
 

bertus

Joined Apr 5, 2008
20,051
Hello,

Do I see a neon bulb after the switch between the red and orange?
The neon bulbs do not fail often.

Bertus
 

Ylli

Joined Nov 13, 2015
776
You have measured 1.3 ohms across the 120V primary, and 17.5K across the 240V primary. Configure it for 240V input. Put in a good fuses but do not plug the unit into the power line. Instead, turn on the power switch and measure the resistance between the prongs on the power plug.
 
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Thread Starter

stuartm

Joined Aug 17, 2019
11
Thank you everyone for your further suggestions. I really appreciate all the thought that you have been giving to this problem.
I agree that it is definitely a good idea to rule out measurement errors on my part.

@MrChips I went one-better and completely disconnected the transformer from everything. This should remove any uncertainty about fuse position and other components affecting the outcome.

black - orange: 1.4 ohms
black - red: 1.712 M ohms (maybe I misread the unit scale when I said it was 17.5 k ohms previously)
orange - red: 1.710 Mega ohms (photo attached)

A vague Google search about resistance testing transformers says the the DC resistance of a transformer should be in the 1-10 ohm range, but I don't really understand a failure condition that would make resistance higher but not an open circuit.

@bertus The manual says that the switch lights up when the amplifier is on, so I guess that must be a glow lamp on the circuit diagram.
Are you saying that the globe might fail closed and cause a shorter path via the 115V tap? I can see how this might happen if I hadn't unplugged the orange 115V wire from switch K.

@Ylli After doing the measurements for @MrChips, I reconnected wires to the switch and put a fuse in the receptacle/fuse holder (in 240V position) and at F1. With the switch in the on position, I measure 1.713 Mega ohms between the L and N pins on the power plug.

I get an open circuit if the switch is off, and an open circuit if the receptacle is in the 115V position (as I still have the orange wire between the transformer and switch K unplugged).
 

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MrChips

Joined Oct 2, 2009
19,275
@MrChips I went one-better and completely disconnected the transformer from everything. This should remove any uncertainty about fuse position and other components affecting the outcome.

black - orange: 1.4 ohms
black - red: 1.712 M ohms (maybe I misread the unit scale when I said it was 17.5 k ohms previously)
orange - red: 1.710 Mega ohms (photo attached)
Two things don't make sense.

1) 1.7MΩ is very unexpected. As you surmised, a blown wiring would exhibit as open circuit. A high resistance would indicate leakage of residue across the blown debris. Your photo shows that your fingers are not in contact with the meter probe ruling out any false reading through body resistance.

2) Such a high resistance would not cause the fuse to blow, unless the high resistance is a more recent consequence after the fuse was replaced.

Regardless, it would appear that the transformer is no good. It is difficult to imagine what would have caused the primary winding to blow considering that there is no load on the secondaries.
 

AlbertHall

Joined Jun 4, 2014
8,363
And further, an open circuit transformer will not blow the fuse. It is the 230V tap that is open, but the 115V tap is still working. Connecting power to that tap will blow the fuse so I do suspect that it is being connected that way, possibly by a fault in the voltage selector or simply misunderstanding how it works.
 
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