Marshall 8100 head hum query

[g1]

Measure again at ZD1 & ZD2 but set meter to AC volts. This should show you if you have any excess ripple on the supplies.

[Chuck H]

Measure again at ZD1 & ZD2 but set meter to AC volts. This should show you if you have any excess ripple on the supplies.

This^^^^^^^^^^^^^^^^^^^^^^

And if your "meter is junk" it probably doesn't have auto ranging. Start with a range that accommodates the supply voltage and then range down from there if you can to get the most accurate number.

[Roob65]

Thanks for the help guys, really appreciated! It's why I chose this forum after doing my initial research! Ordered a true RMS DMM and will redo my measurements. Just not trusting my my current meter for this stuff.

[Enzo]

meanwhile, on your junk meter, try this test. Set the meter to AC volts. Now take a 9v battery and measure the voltage. If it says something for a moment but settles to about zero, the meter is useful. If it measures a 9vDC battery as about 12v AC, then the meter cannot be used for our test in posts #16,17.

[Roob65]

Thanks Enzo. Yeah I ran that test as a result of your earlier post asking about clean power - it failed the test hence 'junk'. I'd had concerns for a couple of month but just used it for continuity stuff so didn't bother upgrading but this application has now given me the excuse for a new toy!

[Enzo]

I have some cheap meters (free from Harbor Freight), I leave around the shop and warehouse for just such things. Check for open speakers, check for dead batteries, whatever.

[g1]

If you don't want to wait for your new meter, you can put a cap in series with your probe to block the DC. Something like .047 or .1uF should work.

[Roob65]

Tried the old meter with a cap but was getting weird readings. Got my new meter today and is true RMS and passed with flying colours. Busy with other things but will get testing done tomorrow evening and report back with the results.

[Roob65]

ok got back to testing. +/- 15v clean as a whistle. Interestingly I was rechecking all my previous measurements and was just checking again for DC on the speaker output - rock solid at 15mV on both clean/crynch channels which was different from previous when I was getting higher/climbing numbers. Tapped about the board with a wooden kebab skewer and was around IC7, TR3,C38 when it suddenly started to climb then settle around 70 mV. Gonna read up and test those (IC7, TR3,C38), but if anyone has any tips it would be most welcome.

edit:
Scratch that, I think it was coincidence as wasn't able to repeat the result. Seems more like a 'timed' event so letting it cool down again and will se how long it takes without any interference by me.

[g1]

Sorry, wrong valvestate, wrong thread.

[Roob65]

Ok picked this up again today and decided to check some more voltages. I have been looking at the pre-amp and decided to check the heater voltage (read somewhere this can be a source of hum).
I apologise in advance as I think I'm probably missing somthing and showing my rookie colours!

I read the voltages applying the probes to the two heater pins (tube removed) - the heater centre isn't attached - i get 60VAC. I knew there was something wrong here so started to read up. I assume this high apparent reading has something to do with heater elevation and an artificial centre tap but I'm damned if I can figure out how to properly test the voltages! Does the tube need to be in place to properly measure against the elevated ground? Any pointers appreciated!

[Jon Snell]

The heaters must be measured with the valve in place as incorrect voltage will be measured otherwise.
Measure the heaters between pins 4 & 5.
There should be no need to measure the voltage as the heaters glow enough to be seen. The heaters are not elevated.
If this stage is producing hum, it will be silent when VR10 is turned down.

[Roob65]

Excellent, thanks Jon!

[Roob65]

Ok so I got the second head up and running and.....

...almost identical level of hum/buzz. Maybe it's inherent to these amps? Seems a bit too loud to be from factory level though.
Another possibility is the caps I replaced as it's the only consistent thing I have done between the two. I replaced C40, C41 and C61 & C62 on both. Maybe underperforming fakes? (I got them via eBay doh!)
The capacitance values check out when measured though. Hmmm...

[Jon Snell]

At this point I would lift the vol ground and connect it to the point at which R26 meets ground, on that exact spot. Turn VR13 down and observe what happens. It should go quiet.
If it doesn't check for HF oscillation in the main power amp, if it does then the hum comes from the pre amp side.
If ... it is quiet ... at reasonable levels ,,, then you've fixed the ground issue.