Fuse Blowing Problem

[ESK]

This follows on from my K&K phono stage build.

http://www.hifiwigwam.com/forum19/14937.html

I'm doing some testing and getting nowhere. Kevin's on holiday for afew of weeks so I'm asking for some Wampower.

Having completed my build I was ready to start my testing. Here's a snapshot of where I got to;

With all valves in place andthe time delay switchdisconnected, I turned on the amplifier.

The valves glowed gently and the blue LED lit into life. I then measured the voltage across the tubes and they were inthe upper end of the expectedscale.

I turned off and fitted the time delay switch.I switched on again and waited for the click to tell me the time delay switchhad kicked in.Nothing. Except I noticed that the fuse had blown.

So,I removedthe switchagain and replaced the fuse witha spare. I then switched on again (remember this was fine before) and the blue LED lit for a fraction of a second before the fuse blew again.

I then tried disconnecting some wires from the 10VAc/Fil connections on the main board and the first resitor in the circuit as well as the rectifier tube on Kevin's instructions. Some times the fuse blows and some times, it doesn't...

The result of all this testing is that I have an intermittent catastrophic failure somewhere that appeared to start when I first installed the time delay switch and continued after its removal.

Kevin's instructions before he left on vacation are to check that I don't have any solder joints where I shouldn't on the main board. Of course I'll check that. (there's none on the top of the board.)

Can anyone offer any other suggestions?

One question, When I solder compenents from the bottom of the board, is it vital that the solder bleeds through to the top of the board to make a proper connection? Some of mine (a lot if I'm honest,) do not.

I know this is a bit vague but any pointers gratefully received?

Any experts near Edinburgh who would like to assist, I'm more than happy to make dinner/buy beer. I'm a pretty good cook!

Thanks

 

[Dik Dolan]

I'm far far from being able to offer much help, but with regard to soldering, If the pcb tracks are on the underside, then as long as the component and the solder pad of the pcb track are soldered ok then that should be fine. It's only when there are pcb tracks on the topside too that you need to ensure the solder has flowed all the way up- or solder both sides.

With pcb mounted valve bases it might be a good idea to ensure they are soldered top and bottom (if there are solder pads on the topside, of course) just to help with mechanical rigidity and thermal expansion etc.

As for your fuse blowing problems, sorry to hear.

Wasn't your kit designed with solid state rectifiers originally, then offered with valve option? It might just be something as simple as requiring a different rated fuse to match...but that IS just a guess, please don't just put a larger fuse in and see. It might be worth asking on the k&k forum too.

http://www.audioasylum.com/forums/KandK/bbs.html

 

[felix]

Are you replacing the fuse with the right time rating? Often mains input fuses are required to be time-delay time, to cope with the turn-on inrush current. These fuses are usually stamped with a 'T' on the endcap along with the voltage/current rating.

 

[i_should_coco]

When you say "sometimes the fuse blows and sometimes it doesn't", that could be because the fuse is marginal for the inrush current at startup.

Although the steady state current may be reasonably low, it's not unknown for the inrush on cold tube heaters to be 10x the steady draw.

You could try increasing the fuse rating by 50% to see if that helps. That will still offer nearly the same protection, but will allow a little more headroom. Also make sure the fuse is "T" rated, i.e. slow blow, fast blow fuses have little or no headroom for surges.

If the PCB is through plated, it shouldn't matter if it bleeds through.

Edit: Oops, sorry Felix, just saw your comment on T-rated fuses above.

 

[ESK]

Yes I'm using the correct fuses. T315ma. I got two with thekit and bought another pack from Maplins. I suspect I'll be getting another pack soon!

I may ask about a larger fuse on the K&K Asylum page to see if any other UK builders have had similar issues. The kit is designed for US voltages, I wonder if its possible that UK (especially mine) is causing problems?

I think that the board is through plate

d. Here's a picture of it.

 

[Dik Dolan]

It does look like it's through-plated, with some tracks on the topside. It wouln't hurt to ensure that the solder has flowed all the way through, or even re-solder from the top too, but a bad solder joint wouldn't cause fuse blowing, unless you have accidently bridged a couple of the tracks...and if that were the cause I'd expect the fuse to blow everytime.

 

[Kev]

Sorry if this is a bit simplistic, but you have wired the transformer for 230V not 110V haven't you?

Kev

 

[Kev]

I've just had a look at the cct. It looks to me that the B+ is supplied through a valve rectifier so no sudden current surge. The only direct current outputs on the secondaries are the supplies to the heaters, so unlikely any issues there, but to be sure take the valves out to see if it still does it. Double check that D1-6 have been inserted onto the board the correct way round. Check that C301, 302 and L301 are rated for 230V.

Kev

 

[ESK]

D1 to 6 are installed in the correct way and yes I've wired for 230v. Got Kevin to check my work on that front to make sure.

C301 is rated at 0.01 micro farads 600V. C302 is rated at 0.27 micro farads 250V according the parts inventory. I'll need to do a bit of dissassembly to make sure that is what the actual components say.

Have removed all three valves including the rectifier and the fuse held and LED came on. Switched off and waited for the LED to dim, this took about 30 seconds. Then switched on again and the fuse still held. Repeated for a third time to be sure.

Point of interest, whilst the amp was OFF (no LED) and the valves in the L/R channels removed but rectifier still in, I went to remove the rectifier and got a very small belt whilst my hand was (I suspect) near the main transformer. Presumably, this was still holding on to some power (otherwise I would have got a slightly larger shock?).

Looks like you may be on to something.

Any suggestions for what I should try next?Should I try with channel valves out and rectifier in place?

Thanks for the suggestions!

 

[i_should_coco]

When you say you got a belt, you touched some bare wire, right? If it was anything else, e.g touching the chassis or transformer housing, etc. then something is not right and could be VERY dangerous. In which case, BE BLOODY CAREFUL! There should be NO measurable voltages on anywhere except the circuit itself.

If you're not sure, then err on the side of caution. Sorry to labour the point, but better safe than sorry.

If that all checks out, next step is to leave the rectifer out, insert the signal valves and try again. That will just be powering the heaters then, which will tell you if it's the heater surge that is the problem.

Cheers,

Pete

 

[Dik Dolan]

Capacitors can hold a charge and give you a belt even when everything is turned off.....be careful.

 

[ESK]

i_should_coco wrote:

When you say you got a belt, you touched some bare wire, right? If it was anything else, e.g touching the chassis or transformer housing, etc. then something is not right and could be VERY dangerous. In which case, BE BLOODY CAREFUL! There should be NO measurable voltages on anywhere except the circuit itself.If you're not sure, then err on the side of caution. Sorry to labour the point, but better safe than sorry.

If that all checks out, next step is to leave the rectifer out, insert the signal valves and try again. That will just be powering the heaters then, which will tell you if it's the heater surge that is the problem.

Cheers,

Pete

It was switched off and I'm not aware of touching anything, just hoverring...

Anyway, signal valves back in and rectifier out and the fuse blew...

 

[i_should_coco]

ESK wrote:

It was switched off and I'm not aware of touching anything, just hoverring...

Please be VERY careful. At this point, I'm not sure if there is a safety problem here, but if you genuinely think you got a shock from or near the casing, then I would strongly suggest getting it checked out unles you are very confident you know what you are doing. Mistakes can be lethal, and no-one wants that.

Anyway, signal valves back in and rectifier out and the fuse blew...

 

[JPG]

Remember that Valves use high voltage DC- I am not sure what voltages this kit will use, probably around 300v. This is not really enough to jump, but be careful.

You probably brushed against something- perhaps the outside of a capacitor- the cans can sometimes be live.

Please be careful. Work with one hand behind your back. If measuring with a multimeter, fix one probe to earth, and use one hand with the other probe.

Wear rubber-soled shoes, ideally work at a wooden desk with a wooden chair, and perhaps even wear rubber gloves (lates 'surgeon' type or even Marigolds).

 

[Kev]

Chances are you got a belt as the B+ leakage would be very small with no valves in. Be a tad more carefull around these voltages, they can realy sting.

Next thing I'd check would be the voltages on the heaters. They are supplied from what looks like voltage regulators, so they shouldn't be a preoblem, but just check them. Try putting in one valve at a time and measuring volts across the heater, not heater to earth.

You really need to be able to start to measure some volts around the cct. There is a possibility that you have ac domestic voltage at the high end of the scale and the transformer windings set for a nominal 230V giving a high secondary voltage. What I would do next would be to lower the supply volts using a variac. I guess you don't have one so the next best thing will be to lose some of the volts with an inline resistor in the supply after the fuse.

I'll be castigated by some for suggesting this but it's up to you if you feel competent enough. if you don't undertsand what I'm describing then simply don't do it. You will have bare supply voltages.

My guess is that the amp will take somewhere around 30 to 60 mA ish. Lets take a mid point value and look for about a 15V drop. The resistance needed will be 300R at 0.5W. I would use a pair of 680R 0.5W resistors in parallel, connected in series with the fuse after the fuse. If you do this the transformer will see a lower primary voltage and hopefully will work enabling you to take some readings.

Kev

 

[Kev]

Just reread the first post. If the cct worked without the time delay switch in how did you manage to measure valve volts? If I read the diagram correctly the delay is a relay fired by ic1. If this was out of circuit then you would have no B+ on the valves.

Removing 10Vac fil and the rectifier tube just isolates the rectifier valve completely. The fact that it's still intermittent is a bit odd. It would be nice to have a fixed refernce to go from.

In light of your inexperience with soldering double check around the dil ic sockets. Make sure you haven't got an inadvertant solder bridge. It's easy to do.

Kev

 

[ESK]

After build and re-build I have managed to get the valves to glow. IC1 is an 8 pin socket with an 8 pin 'plug' that fits in.

The first test is to switch on the amp with the 8 pin socket soldered in place but with the plug removed.

This allowed me to measure voltages at each valve by clipping metter leads to some ferrite beads that are fitted to the board bythe valves on each channel.

That allowed me to measure around 6.5 v which was within the acceptable range of 5.6 to 6.6v.

After switching off and installing the 8 pin socket I was meant to watch the voltage rise on another point in the circuit once I heard a 'click' as a time delay switch swung into action. That was when the first fuse blew.

The rest of the story is as above. Last night I disassembled the board to check for dry joints, missed solder points (found two) and I also checked that there were no solder bridges. I couldn't see any. I re-assembled and one first switch on with no IC1 plug in place the valves/LED glowed. After switching off, pausing and switching on again, the fuse blew again. Appears to hold with all valves removed but blows with channel valves in place but valve rectifier removed (still no IC1 iplug in place).

I had considerred swapping the valve rectifier for the solid state. This involves moving a couple of wires from the transformer and replacing a capicitor. IS this worth a try?

I'm not keen on trying your above suggestion, mainly as I'm not experienced enough to try it. Scared poo-less!

I suspect that if I can't see any faults in my build I'm going to need 'professional ' help before I ruin the preamp or myself!

Really appreciate the effort your all showing.

 

[Kev]

Ok it's a bit clearer, the voltage you measured was the heater volts and it's fine. Putting in IC1 allows the HT volts (B+) onto the valves. Obviously the problem is with the HT cct. It seems pretty clear that the heater cct is OK.

Does it hold with the signal valves out and the rectifier in?

The fact that it blows with the signal valves in and rectifier out suggests it's to do with the heater cct, but see my first paragragph in this post

Are both the valves the same and what are they?

Kev

 

[i_should_coco]

Do you have a circuit diagram you can post? Especially the bit showing the plug socket (IC1) thing.