DC-3 Rhythm Channel help

[Jimmy74]

After a long and time consuming repair I finally got rid of the hum problems in the amp and the short that was happening in the LV supply on my DC-3. The shielded rhythm channel gain pot wire was creating some sort of interference with the input jack wire and I changed out some of the resistors that go to pins 1 & 6 of the pre amp tubes, and I changed out the LDR's, the NE5532 chip and also one of the 4N33 chips... but I was still getting hiss mainly on the rhythm channel.

So I decided to go through the whole board checking all the resistors and voltages and all the resistors seem fine except the 680 ohm voltage dropper resistors between the two 30uf 500v filter caps. There are 8 of these resistors, 6 of which measure around 630 ohms and the other 2 measure 645 ohms.... do you think these could be a possible cause for the hiss? I was also thinking of the MPSA transistors in the equalizer/output volume section or maybe even the 2N6426 transistors in the footswitch section. All the voltages off the preamp tubes are fine and I've drawn up a schematic of the LV supply including the section that is not shown on the original mesa schematic:

http://farm4.static.flickr.com/3611/3580861903_7cbf239d59_o.jpg

 

[Monsta-Tone]

If it's only hissing in channel 1, I would replace the Plate Load resistors on V1 & V4.

100k ohm resistor tied to pin 1 of V1
100k ohm resistor tied to pin 1 of V4
100k ohm resistor tied to pin 6 of V4

I wouldn't worry too much about power supply resistors just yet.
On just about every vintage Fender and Marshall amp I've seen, the plate resistors develop some type of hiss.
They will measure correctly, but simply make a hissing noise.

Try this 1st and see if it helps.

 

[Jimmy74]

Well I've changed just about all the 100k plate load resistors and that didn't change anything, on pin 6 of V1 there was a 62k resistor which I replaced with a 100k & 150K in parallel. The hiss is mainly present and louder in the rhythm channel and comes out when I turn the gain up, it is also present on the lead channel but much quieter.... could it be the MPSA transistors on the EQ or maybe the plate load resistors on V3b?

I was also thinking of changing out the other 4N33 IC's, could they be a cause of hiss? I also noticed that the gain pot on the rhythm channel gets hum as I turn it up but when I hit full volume it disappears.

any ideas?

thanks

 

[Monsta-Tone]

If it is more prevalent in the clean channel, then I would not worry about the channel switching at all.
I would definitely replace all of the channel switching circuit if there were a problem with channel switching, it doesn't sound like you have that type of problem.


It's more likely a bad wire placement or a failed component in the clean channel.
Or, maybe a bad tube, I know you changed them, but I have seen many bad tubes right out of the box. This has driven me crazy troubleshooting and brainstorming, only to find out the problem was where I started.


If you can't find it, I would take it to a tech.

 

[Jimmy74]

Well that's why I thought of the MPSA transistors in the EQ/ output section.... it could also be a bad wire or just a dirty connection, I dowbt it's a problem with the tubes because the old ones were giving me the same problems. I've also replaced these plate load resistors:

220k off pin 6 V3
220k off pin 6 V5
10k off pin 1 V5

That would leave me with 68k & 120K plate resistors off V6. Anyways I just finished putting the amp together and as I always do, before putting any tubes in, I test the voltages and that negative voltage is showing up again all over the circuit.... last time I fixed that problem replacing the two 1N4744 zener diodes in the LV supply with general 15v 1.3w zener diodes and that did the job until today. Looking at the schematic posted here, what exactly do those zener diodes do?

http://www.flickr.com/photos/73835524@N00/3592697774/sizes/o/

Also is this resistor supposed to be 2.2ohms?

http://www.flickr.com/photos/73835524@N00/3356870910/sizes/l/

thanks

 

[Jimmy74]

Can anyone tell me whether the 1N4007 diode in parallel between the 2 relays on a DC-3, is supposed to be giving low dc resistance readings both ways? I changed it out but that didn't change anything, I'm getting around 330ohms both ways.

thanks