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Old 24th December 2005, 09:44 AM   #1
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Default chinese 845 amp blowing fuses

ok here are the symptoms.

the power supply is a C-L-C affair

All connections dissconnected after the first C.

ie no chokes, output transformer, input stages.

Removing just the ht connection, the fuse doesn't blow and everything is ok, however, resoldering it in, leads to the fuse blowing.

I have replaced the first Capacitors, and still it blows, and its taking the 2x 5408 diodes with it, too.

any ideas, please, do I need a new mains tx?

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Old 24th December 2005, 02:17 PM   #2
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Default Have you a schematic?

Working a bit in the dark here, but could be the choke has gone to deck or even one of the O/P valves--Have you tried taking out the valves and trying it?

You could dis the choke, and use a bridgeing link in place of it, Do you have any resistance readings at all, With no valves in, the HT rail should read very high resistance after the caps have charged to the meter.

It would be very difficult to diagnose a fault without a test-meter (AVO/DVM etc) and a basic idea of the schematic of the amp.......

The 845 runs at a quite high HT voltage, so things like chokes and O/P Transformers are running at say, around 1.2kV (1200 Volts) , so insulation can easily fail at these high potentials, unless they are well made!

Please take Extreme Care testing out the PSU stages, Even a small value cap of 1uF can KILL at these voltages, and easily can remain charged for some hours/days with nothing to drain them--Just ask guys that repair Microwaves!

When testing out this sort of fault, the best way is by resistance readings to deck, after having ensured that the PSU and other HV caps have been discharged first
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Old 24th December 2005, 02:40 PM   #3
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Default Bad tubes?


Have you tried to pull the tubes, and then chek if it blows the fuse? The mains xformer is not the problem since its okay when you have disconnected the ht load.

Best regards, and a merry christmas from,

magnus kofoed
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Old 24th December 2005, 02:42 PM   #4
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hi all, yes this is with all tubes pulled and just the connection to the first filter stage, chokes are not in circuit.
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Old 24th December 2005, 04:25 PM   #5
hacknet is offline hacknet  Singapore
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is it a jolida?
the jolidas have a design defect of having the delayed relay before the cap bank, when the relay closes it causes the fuse to blow very often.
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Old 24th December 2005, 04:51 PM   #6
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hi its not a jolida but there is a delayed relay, however, it blows pretty much straight on switch on.
with luck it could be this and hopefully no problems with it, what do I do, uprate the fuse?
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Old 24th December 2005, 04:54 PM   #7
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Default Helpful service/diagnostic technique

If the tubes are all pulled out you can save component failures like your diodes and the fuse during diagnostics by temporarily wiring a 40-60 watt incandescent 120 volt light bulb in series with the mains input (the power cord). The lamp should dull to a very dim glow once all is well (with no tubes). If it glows brightly and does not diminish as the caps charge you have a short in the HT supply.

I would be looking at the choke or perhaps an output transformer as the culprit. I would not want that to be the case though, the output xfmer will be hard to source and expensive.

What brand/model amp are you having trouble with?
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Old 24th December 2005, 05:30 PM   #8
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hi Bob, the only things that are connected with I re-connect the HT is this circuit, and it blows the fuse, I have undone connections to chokes or output transformers, which come after this first cap bank, or valves in the output or further to the input stages.

All other connections like heaters are ok and no blowing of fuses happens..

the caps were replaced just in case it was those, but its still doing it.

as it happens, I can get replacement output and mains tx's, from a good maker, but not tried the manufacturer, as you say, perhaps hard to get in touch with for parts.

The audio circuit is very nearly identical to this one, the power supply is slightly different, there is no voltage doubling as far as I can tell, and I think the delay circuit is near enough identical.
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Old 24th December 2005, 05:38 PM   #9
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Try disconnecting the transformer outputs (take care to insulate the bare ends)

If fuse continues to blow it would point towards transformer insulation breakdown.
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Old 24th December 2005, 05:40 PM   #10
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Hi pete, you mean the HT secondary?

(indeed. there is over 700 volts there.)

yes that has been disconnected in fault finding, and its ok when that's open circuit, it only blows the fuse when that same secondary HT is reconnected, which suggests caps but I think they are ok, I changed them to be safe.
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