Amplifier fuse occasionally burns

Status
Not open for further replies.
Hi,

I have an old Arcam Alpha 6 plus amplifier which uses 37v-0v-37v voltage configuration. The amp works fine.

However, occasionally the fuse blows when I switch on the amp. I believe this is due to insurge current. When I connect the amp thru a current limiter (330w halogen light bulb), the fuse does not blow at all even after months of usage.

How do I prevent the insurge current? I heard you can use a thermistor? But what type? Where should I install this thermistor?

[Updated: Solution found: see the last post]

Thanks,
Dino
 
Last edited:
It would be useful to know what rating, type and position of fuse are we talking about.

Its 1.25A @ 250v which give rise to around 300watts. I have tried 2A fuse, which does not blow, but the power transistors burnt instead and its full short circuit where the 2A fuse also blows. However, when I limit the current via light bulb approach amp plays beautifully thru my Kef (6 ohms) or Mission speakers (8 ohms). I dare not try it on my ELAC Unifi FS U5 floorstanders at 4 ohms in fear it might blow these brand new floorstanders

The fuse is located in the main input line inside the Amp where the power sockets connect.
 
Last edited:
… Silver also oxidizes with time...

I doubt it is the fuse... replaced it many times... but using normal fast burn fuse which is easily bought n cheap.

Back to my question, should I use a Thermistor and where should I use it? Which kind?

I've attached the power supply schematic for this amplifier. Clearly it shows using delayed spring loaded fuse.
 

Attachments

  • PSUArcam6.jpg
    PSUArcam6.jpg
    175.8 KB · Views: 172
Last edited:
Back to my question, should I use a Thermistor and where should I use it? Which kind?

I've attached the power supply schematic for this amplifier. Clearly it shows using delayed spring loaded fuse.
Yes, NTC can help. You can put NTC in Phase Line (L) or in Neutral Line (N) (where there is a free space). Nominal can be about 5 Ohms (or little more), diameter 9-11-15 mm.
Starting point for estimating resistanse of NTC-termistor is active resistance of power transformer primary winding - NTC nominal may be about the same (depends of...)
 
Last edited:
Yes, NTC can help. You can put NTC in Phase Line (L) or in Neutral Line (N) (where there is a free space). Nominal can be about 5 Ohms (or little more), diameter 9-11-15 mm.
Starting point for estimating resistanse of NTC-termistor is active resistance of power transformer primary winding - NTC nominal may be about the same (depends of...)

Would these NTCs be ok? It is rated at 6A and 4.2A, however, the amp only uses 1.25A at the primary winding of the transformer. Or should I just get the thermistor with 2A nominal current at 240v?

1. https://my.element14.com/epcos/b572...c-ntc-5-ohm-20/dp/2101626?st=5 ohm thermistor

2. https://my.element14.com/epcos/b57235s0509m000/thermistor-ntc/dp/9751866
 
Last edited:
The schematic clearly shows 1Amp for 230v and 2Amp for 120v Both are Slow Blow or Time Delay type.
Why not repair the cause; problem solved then.
Either values are common in both 1 1/4" and 20mm sizes.
Even 1A25 is available in both sizes; Cartridge Fuses | RS Components

I managed to find the slo blo fuse for 1.25A at Element14.com. However, it is expensive for just a fuse, and I still thinking of putting a Thermistor to manage the insurge current.
 
I think you should monitor the NTC temperature (very carefull, it's dangerous, switch off it from wall socket), and find 1 A fuse.
9.5 mm NTC will be hotter then 15 mm, and will have smaller resistance after several seconds-minutes.

And what is your power transformer primary winding resistance?
 
Last edited:
I managed to find the slo blo fuse for 1.25A at Element14.com. However, it is expensive for just a fuse, and I still thinking of putting a Thermistor to manage the insurge current.

https://my.element14.com/bel-fuse/5st-1-25-r/cartridge-fuse-time-delay-1-25a/dp/2844497?st=20mm fuse

These ones are only 0.44USD each and have a MoQ of 1.. so not that expensive really.

Or treat yourself and get 25 for about $6USD:

https://my.element14.com/multicomp/mc000882/fuse-cartridge-1-25a-time-delay/dp/2461213?st=20mm fuse
 
But the amp works fine... on insurge current destroys the fuse

But why is there an inrush current now? Surely the amp has not always exhibited this behaviour from new?

If your fuse keeps blowing, then there is something wrong. Always. Yes you can soften the inrush with a thermistor but really that only masks a problem which may only get worse over time.

You mention that if you use a too-high rated fuse your output transistors burn. As Kay pointed out, there must be something wrong with your amp because a properly functioning 'healthy' amp would not burn up if you were to bridge the fuses.

Have you modified the amp or replaced anything that could contribute? Installed decoupling caps on the output transistors perhaps?
 
Status
Not open for further replies.