Kit 88 Hum Problem.

[Fred Shaw]

Hi,

I have a 16mv hum on the right channel (& 0.2mv on the left). If I press down hard on V3 the hum reduces to around 4mv. Switching V1 & V3 makes no difference but again pressing on the 'new' V3 reduces the hum. It appears I have a bad connection somewhere but is it likely to be a poor connection between a valve pin and the base, or that flexing the PCB improves a connection elsewhere? Any suggestions before I take the bottom off to investigate will be welcome.

Fred.

[John Caswell]

Hi Fred,
Have a look at the soldering around the valve bases as I have had cases of fractured solder joints on the pins due to the thermal cycling. Some times it strips the plating off the pins and the only solution is a new base.
Do take care not to overheat the tracks on the PCB as they will lift quite easily.

John aka Dr John

[Fred Shaw]

Thanks Dr John for the info. I re-soldered many of the joints a few months ago when I felt that there had been a decrease in sound quality. I found a number of the joints around the valve pins had what looked like a small circle around the pin itself. I suspect that what I could see was cracking as a result of thermal action. Whether I re-soldered all the joints at the bottom of V3 I can't remember, however, at the moment the amp is running quietly - after being on for most of the morning. I suspect that the heat/expansion has tightned-up a bad joint. I'll wait until the hum appears again (at next switch-on!!) before getting out the soldering iron.

Best Regards,

Fred.

[Fred Shaw]

I have now resoldered all joints in the V3 vicinity. (As I thought, I had already resoldered the actual V3 joints when I had the board out the last time.) However, to get at the joints more easily, I unsoldered the FB cables (Maplin Van Damme) and noticed that the conductor was possibly in contact with the carbon inner screen. (The polyethylene insulation had retreated under the heat of the iron.) Would/could this account for the 16mv hum? Now it is all back together the hum is down to 3.9mv, far better than 16mv but not as good as the left channel at 0.3mv. I have tried rotating the mains tx and moving the earth cables around but I can't seem to get lower than 3.9mv on the rt channel. Has anyone any further suggestions? On a slightly different 'hum' matter, a while ago I added a 10 ohm res in the earth wire to the inputs as I was getting an annoying hum on both channels. Is it usual to have to put an earth lift resistor in a KIT88 to stop earth loop currents? Or have I possibly a wiring problem associated with my connected equipment?

Fred

[John Caswell]

Hi Fred,
Hum lift resistor, not usual but can be very helpful in cases of stubborn hum, and most certainly will not harm.
I and others had a few hum problems with this amp and I found that routing the feedback wires from LS termls across the top of the power tx and thens to the F/b tmnls helped but not in every case. Try rerouting them to see what happens. it is possible to get both channels "silent" ie 0.2-3mV. Another thing I did was to move the power tx to the top of the chassis between the opts this also reduced hum and made for a cooler running amp.

Regards

John aka Dr John

[Fred Shaw]

Thanks Dr John,

I'll have a go with your suggestions when I next have the bottom off as I can live with the low level hum for a while.

Regards

Fred