by erichayes » Tue Aug 01, 2006 10:28 pm
Hi All,
I'll bet there are a lot of guys reading this who are jealous of the stuff you have, which covers about 90% of the problems likely to be encountered. You didn't mention if you had a variable AC source like a Variac, but at this stage, it's not that critical.
I want to say before starting that I'm not being intentionally patronizing when I "Heathkit" a procedure. If you know how to set your test gear up, do it. There are some folks out there who might have similar equipment that they've been afraid to use; the step-by-step is to assist them.
First thing is to try to locate the source of the hum. Put a dummy load on the output (8Ω resistor or 20W 12V automotive lamp), short the inputs, hook the scope to the output (make sure you've located the trace first), set the scope to 5mS/div and turn the amp on. After 10-15 seconds you should be able to find the hum on the scope screen by varying the vertical sensitivity. What you'll see could be a really dirty sine wave, a sawtooth wave, or some combination of the two. If it's predominantly sawtooth, go directly to the filter caps and replace them.
Ascertain if the heaters on the preamp/mixer amp tubes are running on AC or DC. Go to pins 4 and 5 of one of the ECC83s with your DMM set to AC volts. (If 4 and 5 are connected together go between them and pin 9.) You should see either roughly 6 or 12 volts or less than .5 volts. If it's one of the former, the tubes are AC heated; if it's the latter, switch the DMM to DC volts and check the reading for 6 or 12 VDC more or less. If it's signifcantly lower, and/or if the AC voltage is over .5, look for a high capacitance, low voltage electrolytic (usually no less than 500 µF @ 50 V or less) and replace it.
If the heaters are AC operated, remove the shields, if any, and grab each tube with your fingers while watching the scope. If the hum level doesn't change, or changes uniformly for each tube, go to the next paragraph. If one tube has more hum than the others, pull it (yes, while the amp's on) and check the scope. If the hum drops substantially, that's your problem tube, or stage. Try substituting another tube and see what happens.
If no one tube stands out, remove the preamp tubes one by one while watching the scope as outlined above. Handle the same way as above.
If the amp still hums with no preamp tubes, move on to the tone control amp stage. Fortunately, design usually follows function, even in England, so the TC amp should be close to the tone controls.
Keep proceeding from stage to stage until the hum diminishes. If it doesn't by the time you get to the output stage . . . well, tomorrow's another day.
Caveat: I've been doing this professionally since 1967, and a lot of my technique has become second nature to me. I try to be as thorough as possible in explaining my procedures, but some things fall through the cracks. If you have questions, please ask. " 'Tis better to ask stupid questions than to make stupid mistakes."
Last edited by
erichayes on Thu Oct 12, 2006 1:07 am, edited 1 time in total.
Eric in the Jefferson State