been cleaning up an old 1975 bass head, unfortunately someone else worked on it first and used whatever was handy for parts but its looking pretty good now, replaced all the electrolytics and the normal channel preamp ic's, its all working now but the bass channel has a bad AC hum, I can turn down the input volume and leave the master up and use the normal channel and there is no hum in that channel, just the bass channel, and I can adjust the hum tone with the tone controls, im stumped at what part in the bass preamp could be causing this. any thoughts?
Wed, 01/31/2018 - 21:56
#1
Claytond1
You sure the input jack is
You sure the input jack is shorting properly? That's where to start-at the beginning. Does it hum with nothing plugged in? Mike.
cleaned the jacks
plenty of tension, had to pry the switch portion open to slide in 800 grit sandpaper strip
hums with nothing or sig generator
hums with nothing or sig generator plugged in, driving tone or not, but normal channel has much less hum and works ok, problem is mostly evident when input volume in bass channel is turned up, hum can also be equlaized and is effected by tone controls so my guess is that its early in the circuit, thinking about replacing some caps, already resoldered a couple of grounds.
You can shorten your search
You can shorten your search by pulling out IC's at places in the chain to see where it stops and starts or get out the audio probe and start at the front or back and chase in the opposite direction. Mike.
last one cuts it
last one cuts it from 12ax7 and quiets output but scope shows same sig from 3rd IC, when I first started on this the normal channel was dead so put 2 new ones in the normal, for diagnosis after normal channel came back I swapped them over to the 3rd and 4th position but no change in hum.
It seems to be in last section of BB-1 board
I pulled the last IC and no hum, dead quiet with power amp and 12ax7 driver all hot, I replace it and pull the two upstream IC's and the hum is back.Must be somewhere between 3 and 4, this is the output from IC 3, a bit strange but looks like a hum
poke and hope
So here we have the ac hum on the scope from the speaker jack and next image coming from the last 307H as well, which are almost identical, I checked the diodes on the power board, replaced all electrolytics and after seeing that pulling the last IC which drives the 12AX7 completely silenced the hum, I've replaced a lot of parts on the B-B-1 Bass pre amp board, the ceramics, rebuilt the bias (had no choice there it had been smoked and rebuilt with incorrect values) I ordered the 1uf filters but the ones that came (from ebay) were gigantic and wouldn't fit on the board if I wanted to.Replaced the glass zeners even though they tested good, I have read about this problem several times on various forums and have yet to see one resolved so I'm going to keep going till I replace every component or find the problem, any advice appreciated, I will post my findings for the next guy
With IC-4 in and IC-3 out do you still get hum?
Hi Clayton,
do you have hum at the wiper of the bass channel treble control if IC-3 is pulled by IC-4 is in place?
Cheers,
Lars Verholt
no hum at bass wiper
no hum at bass wiper with IC3 pulled, last night I replaced the low voltage power supply diodes even though they tested good due to the saw tooth on the supply wires coming into the preamp board, baffles me, the voltage is DC with a bad sawtooth, even with new sprague caps and new diodes.But per your request, with three pulled, no hum on bass pot wiper. I'm going to look at some photo's of a different HD100 board, I had a lot of cleanup on this from some home repairs where a guy used whatever was in his junk box in a couple spots but I thought I got it all.Thanks for your comments, I'm open to any suggestions.
sorry that was the bass, treble was the same
sorry that was the bass, treble was the same
Ok now we're getting somewhere
Ok, so the hum is not at the treble control wiper when IC3 is out. Next thing to try is to put IC3 back in and short pins 2 and 3 together. The pair of small diodes (D3 and D4) acting as voltage clamps on the input is a good place to hook your clip lead to.
(All this may of course be done with the standby switch in the off position - the output section is not relevant).
If you get the hum to go away with a short between pins 2 and 3 on IC3 the hum is more than likely a faulty connection or component between the input jacks and the IC. If the hum is still there, it may be a power supply connection problem or there is something going on with the connection to the socket (bad solder joint?).
One good thing to try, if not already done, is to measure resistance between the IC3 socket (with the IC itself out) and where it's supposed to connect to - e.g. Pin 2 is going to the junction of R13, R15, D3&D4 and C11
Hope this helps.
Cheers,
Lars Verholt
getting somewhere
Thanks Lars, I'll try this and post results