Kit 34 at it again

[jiten]

Hi,
My Kit 34 amp decided it would not fire up. The fuse at the mains input was blown. This has been replaced twice and still blows the moment the amp is switched on.
Have looked at everything and seems in order.
For information:
a. No sound quality problems before this event.
b. Amp had been left on for about 24 hours.
c. Amp was rebuilt a few months ago with all new components and tag boards - but not the transformers.
So how do I find the short circuit ( I guess that is what it is). A flow chart of how to trace this would be a help.

As always, grateful for any and all help!
Jiten.

[Richard]

Hi Jiten,
A good check over using the old Mk1 eyeball is the best start , hopefully just a wire off somewhere. My friend Gerry B just had similar when the ground lead became unsoldered from the smoothing cap in Phono and gave him a shock so it must have wandered over to the B+ fortunately no harm done to man nor beast

[John Caswell]

Hi Jiten,
As the fuse blows immediately it points to either a faulty power transformer, a faulty HT rectifier or a faulty input RF filter capacitor (if fitted)
So...
1) Remove the 5U4s, fit a new fuse, and power up - if the fuse blows straight away, it is probably the power transformer or filter..
2) Remove input filter, fit a fuse (if fitted), and power up again. If the fuse blows again this looks serious.
3) Disconnect all the transformer secondary wires from their respective points and isolate.
Fit a new fuse and power up. If the fuse blows again I am afraid you have a faulty power transformer
4) If not, then reconnect the secondary wires a pair at a time, fitting a new fuse and powering up after reconnecting each pair. If the fuse blows after connecting a specific pair, then the fault lies with the circuitry associated with that pair,
Report back here accordingly and /or pm me with a phone nr. and i will try to sort it over the phone.

John

[jiten]

HI Richard and John,
Tried the eyeball and hand manipulation trick.All seems firm and in place.

I then followed Johns advice and removed 5U4s. The fuse blew just as fast.

Was looking at the pack of fuses and it says fast blow when the instruction book says get slow blow fuses because they are better for a high current start. If I am using the wrong fuses, I will get some new ones tomorrow and try again.

I don't think I have an input filter as I built this as per kit with no extras. If I do have the filter, is there a component name or position? I can look for it.

Finally, John thank you for your offer. I will get the new fuses first and then contact you. if indeed the transformer is faulty, is it possible to get new ones?

Best wishes,
Jiten

[Richard]

Hi Jiten,

Yes you certainly need slow blow or "T" (time delay) fuses. The supplied ones were 1.6A T.

Maplin sell them; 20mm glass fuses.

The standard value one often goes after while (due to fatigue of being on the limit at switch on I suppose) and after mine blew I used 5A in all my WAD amps.

You may just want to use another 1.6A T to make sure all is well and it fires up ok, but if it gets tedious consider a 2A, 2.5A, or 5A.

[jiten]

Thank you. I will pop into Maplins and get the T fuses tomorrow.
Not sure if switches go after a while but a switch socket is only £3.20 so will get a new one as well.
Will let you know how I get on.
JT.

[bob orbell]

My KAT 6550 would not work with the 1.6 amp fuse, they blow immediately, 2.5 amp did the trick, as Richard has said, it is the current surge, of course, if you have a major fault, this size of fuse will also blow, do all of the test that John has suggested, this will point you to the problem. I do not think it will be the input socket and switch. BOB

[jiten]

Hi all, Plugged in the 5 amp fuse and it blew as well. It did so with and without the 5U4 valves in place.
This time, with a 5U4 in place, I could hear the normal hum of the amp start up for about 5 seconds or less before the fuse blew.
So we are not getting much further than switch on.
Had another look and feel for any loose connections and none apparent. Correct me if I am wrong but if it is not the power transformer, then it has to be a short circuit in the central tag board. Can it be tested by checking for resistance at different points?
Also when I built the amp and tag boards again, I was getting a lower voltage which was possibly caused by lower voltage mains power. Could this have been an early sign of a fault in the transformer?
best wishes, Jiten.

[Greg.]

Rather than making things more complicated at this stage, surely the thing to do is re-read the instructions John has given you and if you are confident, carry them out to the letter. There is a high probability that doing so will isolate where your fault lies.

[jiten]

Thank you. Up to point 3, the fuse blows. I dont have a filter.
Now it is a matter of taking off all the secondary wires. I was trying to see if there is any way of testing the circuits without taking off the secondary wires which probably means disturbing a lot of other connections too.
If that indeed is the only next step , then I will have to do it when I am free for a whole day..
So, in the absence of any other suggestions, will do it on Thursday.
JT.