KiT88 alarming noises

[J4mes]

Hi there,

I've been very pleased with my (2000 built) KIT88 since my tweaking efforts reached a hiatus in 2002.

To be honest, it's had quite an easy life over the last couple of years.

A few months ago I did notice that the left channel had become a little noisy - the occasional, quiet rustle and pop, but only really noticeable by me.

However, the problem is now more serious. At switch-on as the amp warms, the hustling/crackling and pop/plopping reaches a relatively high volume, enough for me to turn off in a hurry.

When the problem was less pronounced, I did try swapping around all the valves, but this had no effect at all.

Looked inside at the weekend and all seemed well.

I'd be very grateful for any suggestions - anything to point my soldering iron in the right direction.

thanks

james

[Ali Tait]

Difficult to say at a distance,but sounds like power supply or coupling caps possibly.If the amp is that old,it would be a good idea to change all the electrolytic caps anyway.Just my pennerth worth.

[J4mes]

Hi Ali,

Thanks for the help.

I'll be a bit miffed it's one of my big Black Gates that's gone west, though I've still got all the original capacitors so I can certainly try swapping out the various PSU and coupling caps.

Then again, with the PSU caps being Black Gates and supposedly bomb-proof (famous last words) that might suggest the fault lies elsewhere - just a hunch.

The coupling caps are polyprop ICWs.

cheers again

james.

[Richard]

Hi James,

Sounds like the ECC82 anode resistors which go noisy after a while, sounds just like dirty valve pins at first but cleaning them doesn't clear it and it steadily gets worse, had it myself and wrote it up at 3) here,

http://wduk.worldomain.net/forum/showthread.php?t=135

Rich

[J4mes]

Thanks again Richard,

I certainly wouldn't have thought of that. Hope you are well.

james.

[chrisbowen]

HI - I had a very similar problem with my kt88 amp. Russeling and poping sounds. Switching the valves arround made no difference. I fixed it by re soldering the joints on ( i think ) the decoupling caps ( from stock these are the orange ones - ). I just melted the solder and added a bit more silver solder, wile I was there I did this for several solder points. It fixed the problem 100%. Hope this helps

[J4mes]

Thanks Chris,

While I'm under the covers I'll try that too.

Come to think of it, I'll perform each fix separately to try to identify the problem precisely and post the result - away on hols last week, so no progress yet.

james.

[J4mes]

Hi there,

Certainly not rocket science but quite frustrating nonetheless.

A quick update:

Phase splitter anode resistors replaced (2W combinations) as per Richard's advice circa 2002 - I had forgotten

Re-soldered all valve base and coupling capacitor connections and anything else that looked suspect (LHS only) - no difference.

Replaced phase splitter capacitor - no difference.

Replaced coupling capacitors - fixed the problem.

Having thought the problem was solved, since switch-on was now quiet, I wondered whether it was still, in fact, only a solder problem. So, to satisfy
my curiosity, I returned the original coupling capacitors to the circuit.

Voila! Left hand channel now quiet as mouse and still using my beloved ICW capacitors, excellent.

However, after a couple of hours listening, the rude noises returned, particularly noticeable as I powered down, with no music playing
- loud hissing and popping on the LHS.

Conclusion:

I need to try harder. I have not removed the board yet, to get at the valve side. I originally soldered panel pins through the board, at the CC connection points so that I could more easily swap the caps, so perhaps there is a problem there.

Still a bit surprised by the nature of the problem though - maybe it's a hairline break in a track, or the coupling caps are duff after all.

First step I suppose, is to replace the caps again (with the original orange jobs) and give the amp another good run.

james

[J4mes]

Hi there,

A quick update on the state of my amp.

The bad weather has given me a chance to knuckle under.

I originally wanted to be systematic about identifying the fault, but the effort required to get at the valve side of the circuit board dictated a more practical approach.

On the LH channel, I replaced the phase-splitter anode resistors, these were the higher wattage types suggested by Richard, so were probably okay; originally positioned on the valve side to make space for bigger de-coupling caps. Also, carefully re-soldered all (LHS) joints, paying particular attention to the valve power supply connections.

This seems to have done the trick. No more left channel spitting and hissing.

Anybody got any ideas regarding the causes of such problems ?

I assume it's thermal stress on the solder joints, perhaps exacerbated by the use of a circuit-board rather than hard-wiring. Having said that, I can't say that any of the joints looked dodgy in the first place.

The 6AU6 and ECC82 valve holders were replaced with the gold plated variety - years ago. I wonder if, ironically, this makes for reduced reliability due to galvanic effects between the gold and solder - I remember reading this at sometime.

cheers

james.

[Richard]

Hi James,

Hopefully it'll stay fixed now

I've also fixed "dry" joints on various gear without ever finding the exact one causing the problem. Once I narrowed it to an area as you have but could not easily find the exact one. They all looked good but resoldering the 6 or so possible culprits fixed it so one was bad! Still remember the first, an 8 track car player, found that with a magnifying glass! Re-soldering though can be an art. It's best to remove the old solder with a pump or braid first. This then lets the new solder and flux get at the lead to wet it before closing the joint on the pad. Otherwise you just end up making a bigger hit-or-miss blob and the joint may still be poor inside.

I have my own observations from soldering different joints. Possibly it's easier to end up with a poor joint using a pcb especially if the component leads are thick. If solder is applied to both lead and pcb solder pad at the same time it takes very quickly to the thin (easily heated) solder pad but the component lead may still be cold. Trimming the lead then makes a "smeared" joint which works for a while. (With tag strips both lead and tags are similar thickness and heat is probably applied to both more evenly but I still watch carefully.)

The way round this is to apply the iron and solder to the component lead first then let it flow down to the pcb solder pad and hold for 1 second whilst you see it "wet" and flow on both. If you've not worked this way before it may be surprising how long it takes to bring thicker and longer component leads up to temp. Tinning the ends of wires before making a joint is also good practice.