kel84 channel imbalance

[chambers67]

This is my first post and concerns my KEL84. There is a discernable channel imbalance, with the right being noticeably quieter than the left. The valves have about 100 hours on them. Swapping the valves from left to right has no effect. The problem remains when I use my Revolver speakers or Heybrooks. My knowledge of electronic circuitry is sparse to say the least, but any suggestions, in abc form, would be most welcome. I can use a soldering iron, although my technique is not perfect.
I purchased the amp from Ebay, to be used in another room. I also have a Pure Sound A10, which performs faultlessly, making the KEL fault even more frustrating.

[Richard]

Hello and welcome
Faults like this are usually a dry/poor solder joint, a broken pcb track, or a failed electrolytic capacitor.
How big is the channel difference? Is the R channel just a bit quieter or is it almost not working?
Do you have the circuit and build sheets?
Do you have a digital multimeter?
If so check the voltages around the circuit and compare each channel as they will often pinpoint the area of fault.
Check also ohms reading from pin 7 of the ecf80 to chassis looking for 750R. This is made via the feedback wire and is sometimes broken.
If you're not sure about how to do any of the voltage or ohms checks or need the circ etc post again. There are high voltages involved and it's far better to ask than wish you had...

[chambers67]

Cheers for that. The quieter channel is such that, to achieve anything like a stereo image, I have to be substantially off centre. I have trawled through many of the posts on the forum. I have upended the amp with the bottom panel removed and tapped the pcb with a pencil -- no crackling. I have copies of the circuit/schematic, but not the build sheets. I own a basic meter, but am clueless as to how I would check voltages. I believe I can test the ohm reading on pin 7 and will do this tomorrow. Many thanks for your advice.
Kind regards

Brian

[Richard]

Hi Brian,

If you send me your email address via personal message I'll send the build instructions inc test voltages etc.

Some faults will show in the voltages and some not.

For example, does R channel distort at high vol? If so it could be an el84 not working which would show in voltages.

Otoh, if it doesn't distort and is simply quieter it may be the cathode capacitor and resistor of the ecf80 is out of circuit for reason of failure or bad solder joint. That would give lower gain but would not show in the voltages, so worth going over the joints with a hot iron and fresh solder.

[chambers67]

Many thanks, Richard, for the pdfs.
Now, this will indicate my ignorance when it comes to valve amplifier testing ---
when testing, for example, the resistance between pin 7 of the ecf80 and ground, should the amp be switched on, with either the speakers connected or some form of dummy load? Same applies when testing the valve voltages? Should the valves be in situ?
In the meantime, I have found another multimeter, a Clarkes, that I was using when working on some old Roberts and Hacker radios. Of course, transistor radios do not pose a risk of potentially lethal shocks and I could prod around pcbs willy nilly.
I am hoping for your forbearance.

[Richard]

Hi Brian,

Yes no power and unplugged for ohms testing, continuity checks, soldering etc. Be aware the main electrolytic capacitors may hold charge for a few minutes so check across them with a volt meter before starting work.

Power on for voltage testing. Attach speakers or dummy loads to the outputs. Valves in place (as they will load the supply and affect all voltages) then select an input and attach shorting plugs or a CD player to it (to block possible rf input). The most dangerous shock would be across the heart so get into the habit of just using one hand when probing. Attach a clip probe to the chassis for example then you just need one hand to probe the voltage source.

Mark the pcb for L and R as it can get confusing upside down. Check it's been wired correctly by the builder. Mark your test points first, especially valve pin numbers, and follow through the voltage test list methodically