Fender Twin Amp 6G8

[sluckey]

Does your treble pot work correctly now?

I don't think flipping coupling caps around is going to fix your hum issues. Assuming there are no more wiring errors or incorrect component values, it's time to tackle the layout issues. I'm out of ideas so I'm gonna repeat what I think is the real issue. Read this again...

I think that most of your hum issues are caused by poor layout based on your choice of chassis. The 6G8 chassis dimensions are 26.1875" x 1.586" x 8.25", think long and narrow. But you chose to build a short and wide chassis (why???), so now you have to squeeze the tube lineup and front panel controls lineup to fit your chassis. But you are still using a board designed to fit the long and narrow chassis. This causes some very long wires to go from the board to the sockets and control panel. Plenty of opportunity to pick up extra hum.

The cap board needs to be moved away from the sensitive preamp section and input jacks. The first three filter caps are used in the power amp and have lot's of BIG 120Hz AC ripple (hum) on them. This hum is easily picked up by the very sensitive input section. The wiring from all 4 of your input jacks pass right by all this noise and hum. So, move the cap board.

Moving the cap board reveals another big layout problem... OT placement. All 4 of your very sensitive input wires are passing right by the OT wires. The OT wires are carrying the biggest signals in the amp. This is a huge design flaw.  Also, your speaker jacks are mounted near the first preamp tube and your OT secondary wires (high current carriers) are routed very close to the preamp tube. So, move the OT and the speaker jacks and keep all OT wires away from sensitive preamp circuits.

Using properly shielded wire for your 4 input wires will help with the present cap board and OT placement issues, but I'm afraid it's like putting a band aid on a broken leg. I know that moving that board and OT will be more difficult to do than to say. And you won't know if things will improve until you move them. I'm really skeptical about the success of this project in the confines of the chassis you have chosen.

I know you have a lot of time and money invested in this amp. And I love the sound of the harmonic vibrator circuit. So, what to do? I'd cut my loses on the time and put all those components in a proper chassis. There are 6G8 chassis for sale on eBay right now. Then study the original Fender layout. Collect as many pics of 6G8s as you can. Then wire the amp as close as you can to Fender's proven layout. And ask questions about anything you are not sure of, especially concerning grounding.

This is the first time I've ever recommended changing the chassis, but I really think it will take less time than trying band aids on your present chassis. An I'm confident you will have better results in a proper chassis.

[shooter]

For *future* builds here's a link, for the present, check-do what Sluckey is suggesting.

http://el34world.com/Forum/index.php?topic=11427.0

[uki]

For *future* builds here's a link, for the present, check-do what Sluckey is suggesting.

http://el34world.com/Forum/index.php?topic=11427.0

Man this is great thread thanks for this link !!

Does your treble pot work correctly now?

Yes it does thank you !

I don't think flipping coupling caps around is going to fix your hum issues. Assuming there are no more wiring errors or incorrect component values, it's time to tackle the layout issues. I'm out of ideas so I'm gonna repeat what I think is the real issue. Read this again...

I'm going to do that anyway, better lil improvement than no improvement at all.
I did read that several times, and probably more, I'm in a mission Sluckey , the hum must be gone. Gonna grab some mesh to insulate the AC wires, as suggested above.



 

[PRR]

> something nobody has mention in this thread

Because it hardly matters.

Yes, you can get hum on an UN-loaded cap when you touch it. Sometimes a very dramatic difference one way or the other.

In circuit the difference is far-far less.

Also you can't touch the caps when both your hands are on the guitar.

If you must explore this: *very carefully* touch caps with the amplifier on (and dangerous). That will hum much more than normal (no finger) operation. Any cap which seems exceptionally hummy, see if its orientation matches the direction suggested in that guru video.

[Ed_Chambley]

Coupling caps with the outer foil orientation installed backwards will not cause the hum you are getting.  I promise

Now if you had a small amount of floor noise and had use all MF resistors and wanted to get dead quiet maybe then it would be something to try.  The problem is stuffing 10 pounds of manure into a 5 pound bag.  Something is bound to stink.

I have built a 4 power tube amp in a chassis the size you are using, but I also shielded the power section from the preamp.  All 4 power tubes were between the transformers.  Choke was inside and 2 cap cans, one for the power side and the other for the preamp.  Also, inside the chassis I installed a plate separating the rear of the chassis from the preamp with a hold drilled and a rubber grommet for the signal wires to pass through.

When I installed the chassis cover it essentially isolated all the PA eddy currents from the preamp.  I wish I had a photo, but the amp is long gone, but I did have to design my own board for the preamp and the bias board was affixed to the side of the chassis.

I made the same mistake on my second scratch build by wanting to stuff a high gain 2 El34 amp into a 10 x 6 x 2.5 chassis and put all the tubes in a row.

Now when someone ask me about a chassis I will say layout one to fit and then get a one larger.

The main problem is if you do get it quiet a simple tube change in the future could bring it back.  Yes, shielded will and can help, but you have already spent most of the money on your parts.  You can use the chassis you have for a smaller amp that I am sure you will want.

I have used a 19 x 8 x 2 to build a Marshall Super Lead and put it in a head box 22 wide, but I still put the power tubes towards the back of the chassis and preamp tubes to the front.

Search for a Soldano SLO 100 watter.  He uses some nice ideas to layout a shorter chassis, but they are wider.

[Willabe]

Uki, where did you buy the nice blue epoxy ceramic disk caps? 

[uki]

Now if you had a small amount of floor noise and had use all MF resistors and wanted to get dead quiet maybe then it would be something to try.

I read somewhere , carbon composition resistors do good at high voltages, but where the voltage isn't that high, they are noisy and don't do much in tone terms at those spots, so something else could be used, like the MF as you mentioned in those noise sections of the circuit, how do I find where to use the MF in the circuit?

I have built a 4 power tube amp in a chassis the size you are using, but I also shielded the power section from the preamp.  All 4 power tubes were between the transformers.  Choke was inside and 2 cap cans, one for the power side and the other for the preamp.  Also, inside the chassis I installed a plate separating the rear of the chassis from the preamp with a hold drilled and a rubber grommet for the signal wires to pass through.

There was hum? How much of it worked? 

Search for a Soldano SLO 100 watter.  He uses some nice ideas to layout a shorter chassis, but they are wider.

Will do.

I got a pic and video of an 6G8 with some MF resistors, are those at key spots?
https://www.youtube.com/watch?v=7dChBeEYIkM

Uki, where did you buy the nice blue epoxy ceramic disk caps?

Here  shipping was pricy thou, no cheaper options.
http://www.tedweber.com/amps/capacitors?limit=all

[Toxophilite]

I would suggest you use all flameproof metal film resistors everywhere.
Safer, quieter, less prone to drift and failure
I don't see the advantage of carbon comp resistors myself.
I try to never use them unless I need one while I'm building and it happens to be all I have or it's all I can get in a particular value.
In my opinion, that's on par with orientating capacitors


I think addressing layout  would be a more efficient way of tackling hum, provided you've sorted out your grounds etc

As many people have suggested.(ALL of them WAY more experienced than me)


-Your output transformer placement. It should be as far from your inputs as possible
 Right on top of the inputs is usually the worst position possible (sorry)
-And your power supply, which also should be far from your preamp section (keeps the high B+ away from your sensitive inputs and preamp)


Your inputs and preamps are the most sensitive portion of your amp regarding picking up noise.


I've never found it too hard to move transformers and or chokes if they are sitting on top of the chassis(not half inside) It's merely a matter of a couple holes and a little rerouting of a few wires.

[Willabe]

Thanks Uki. 

The places to maybe use CC R's is where there's a high DCV, plate R's, CF R's. Grid return/grid leak R's and K R's don't have high DCV on them so no advantage to using CC's in those places.

You'll have to try them for yourself to see if you think they make a difference in the tone/sound. 

[Toxophilite]

Uki
That's just my opinion on Carbon comp resistors, as Willabe suggested do some testing and decide for yourself
It really is a debatable point and like many things there's a lot of opinions on all sides of the argument, like capacitors, tubes, wire, speaker magnets, etc etc etc.
I tend to fall into the , 'if it can't really be proven one way or the other then it might not be a factor', camp

I think the layout stuff is really important though

Willabe - My local electronics store sells those very capacitors, I bought a couple today for a Baxandall preamp I build into the Akai/Roberts preamps I convert
I'm betting digikey and similar companies have them as well.
They certainly do the job and are relatively inexpensive.

[Willabe]

Thanks Toxo. 

[Ed_Chambley]

Now if you had a small amount of floor noise and had use all MF resistors and wanted to get dead quiet maybe then it would be something to try.

I read somewhere , carbon composition resistors do good at high voltages, but where the voltage isn't that high, they are noisy and don't do much in tone terms at those spots, so something else could be used, like the MF as you mentioned in those noise sections of the circuit, how do I find where to use the MF in the circuit?

I have built a 4 power tube amp in a chassis the size you are using, but I also shielded the power section from the preamp.  All 4 power tubes were between the transformers.  Choke was inside and 2 cap cans, one for the power side and the other for the preamp.  Also, inside the chassis I installed a plate separating the rear of the chassis from the preamp with a hold drilled and a rubber grommet for the signal wires to pass through.

There was hum? How much of it worked? 

Search for a Soldano SLO 100 watter.  He uses some nice ideas to layout a shorter chassis, but they are wider.

Will do.

I got a pic and video of an 6G8 with some MF resistors, are those at key spots?
http://www.youtube.com/watch?v=7dChBeEYIkM

Uki, where did you buy the nice blue epoxy ceramic disk caps?

Here  shipping was pricy thou, no cheaper options.
http://www.tedweber.com/amps/capacitors?limit=all

A Picture is worth 1,000 words.

Here is a Hiwatt.  Look at the layout.  It would be possible to reduce the chassis size more, but with your build you have a Tremolo to consider.  If you eliminate the tremolo you lose everything special about a Blond Twin.  Might as well build a Showman.  While there is nothing wrong with a Showman as Dick Dale seemed to get by with one, the Twin is very special IMO.

This is what I was talking about.  If you want a amp that is not as wide, you have to put your tubes in separate rows.

[uki]

Thanks Uki. 

The places to maybe use CC R's is where there's a high DCV, plate R's, CF R's. Grid return/grid leak R's and K R's don't have high DCV on them so no advantage to using CC's in those places.

You'll have to try them for yourself to see if you think they make a difference in the tone/sound. 

- What is CF ?
- K is kathode yes?
Thanks Willabe

__________________________________________________________________________

... I think addressing layout  would be a more efficient way of tackling hum, provided you've sorted out your grounds etc
...
-Your output transformer placement. It should be as far from your inputs as possible
 Right on top of the inputs is usually the worst position possible (sorry)
-And your power supply, which also should be far from your preamp section (keeps the high B+ away from your sensitive inputs and preamp)

Don't be sorry, you are right, I did put all of it together without some very important key information about how to do the layout, I appreciate your honest and that is a BIG help to get it working the way it should, I been thinking about it since you 1st mentioned to move things around, I've been gathering all this information and still processing it. The amp does work for the most part, have a marvelous sound, its a big sound amp, just that darn hum .....   

Your inputs and preamps are the most sensitive portion of your amp regarding picking up noise.

I've never found it too hard to move transformers and or chokes if they are sitting on top of the chassis(not half inside) It's merely a matter of a couple holes and a little rerouting of a few wires.

That isn't something to easy to accomplish in my case things are really tight there! I did this image edition to see what it would looks like moving things around. Tough thou and this is just an image edition. I have no idea where to put the choke.

__________________________________________________________________________

A Picture is worth 1,000 words.

It does !!!

Here is a Hiwatt.  Look at the layout.  It would be possible to reduce the chassis size more, but with your build you have a Tremolo to consider.  If you eliminate the tremolo you lose everything special about a Blond Twin.  Might as well build a Showman.  While there is nothing wrong with a Showman as Dick Dale seemed to get by with one, the Twin is very special IMO.

This is what I was talking about.  If you want a amp that is not as wide, you have to put your tubes in separate rows.

This is good point. Although I'll barely be using the tremolo/vibrato, when I saw the schematics/layouts online all what I was concerned about was an amp with 100watts, I did search a lot on about Fender amps, this is the one I found that did match, latter I found some other twin with 100w but no tremolo/vibrato the 5F8-a (that is what I should have build), but I was already building the 6G8. I see now that I did reduce the chassis way too much, but man it looks so cool. 

You see I'm green about all this electronic stuff, I didn't know any of it when I started, details... details...

Now about the tremolo/vibrato, it isn't working , do I need a foot switch ?

[Willabe]

K = cathode, CF = cathode follower.

[sluckey]

Now about the tremolo/vibrato, it isn't working , do I need a foot switch ?

The 6G8 vibrato will work just fine without a footswitch.

You keep finding more stuff that isn't working. It may still be possible that a wiring error is contributing to your hum. I know your layout is not good and is likely the real reason for the hum, but disregarding the layout for a moment, I just don't have any confidence that the amp is wired correctly.

I'd like to revisit something for a moment. Would you recheck the AC voltage on the red wires coming from the power transformer? One probe connected to chassis, the other probe on a red wire where it connects to a diode on the small board. Do this for each red wire. What do you get?

Now, with your meter still set to measure AC volts, move the probe to the junction of two diodes and the red wire that goes to the first filter cap on the cap board. I know there's big DC volts at that point but I'm not interested in that. I want to know how much AC voltage (ripple) is at that point. What's the number?

[uki]

Now about the tremolo/vibrato, it isn't working , do I need a foot switch ?

The 6G8 vibrato will work just fine without a footswitch.

You keep finding more stuff that isn't working. It may still be possible that a wiring error is contributing to your hum. I know your layout is not good and is likely the real reason for the hum, but disregarding the layout for a moment, I just don't have any confidence that the amp is wired correctly.

I'd like to revisit something for a moment. Would you recheck the AC voltage on the red wires coming from the power transformer? One probe connected to chassis, the other probe on a red wire where it connects to a diode on the small board. Do this for each red wire. What do you get?

Now, with your meter still set to measure AC volts, move the probe to the junction of two diodes and the red wire that goes to the first filter cap on the cap board. I know there's big DC volts at that point but I'm not interested in that. I want to know how much AC voltage (ripple) is at that point. What's the number?

I did read the red wires and the reading on each wire is 320vac   both do jump back and forth between 319-320.

The junction of the diodes, with the meter in AC  it does 992 ,, what?? , with the meter in DC it does 450vdc.

[sluckey]

I did read the red wires and the reading on each wire is 320vac

Did you measure that 320VAC since you read my last post?

Your meter may not be able to read AC ripple. What name brand and model is your meter? How much did it cost?

[uki]

I did read the red wires and the reading on each wire is 320vac

Did you measure that 320VAC since you read my last post?

Your meter may not be able to read AC ripple. What name brand and model is your meter? How much did it cost?

Yes , I just read it just few minutes ago, the amp is on right now.

Here is the meters I got, cheap stuff 50 bucks both:

[Willabe]

I had to clip a .001/630v film cap in my red meter lead to get it to read ripple voltage on the B+.

1 end of cap in red gator clip the other end as prob to B+, meter set for ACV.

[uki]

I had to clip a .001/630v film cap in my red meter lead to get it to read ripple voltage on the B+.

1 end of cap in red gator clip the other end as prob to B+, meter set for ACV.

I only have .0033/630v at the moment would that do ?  What kind of reading it will be? And will the cap charge up ? 

Thanks for the hint !!

[Willabe]

Yes, I think that value should be fine. You set your meter for ACV.

I'm not sure about it charging up, be careful and check it with your meter for dcv after you use it. 

[uki]

I did the readings and I got the following

With the digital meter at 200vac i got readings at 0.03 , 0.04 , 0.05 and some peaks at 01.2
Also I've measured with the analog meter at 10vac, so it does about +- 1  , see pics.

I this good or bad, what does it mean?

[uki]

Found a possible bad cap  at the rectifier board. When detached from the circuit the hum didn't get worse, stayed the same, so gonna replace that cap, probably both 8µ-150v and see if hum goes away. If not then back to the tests.

Got some pics of the shielding done as suggest in previous posts. Thanks for the tips !!

[tubenit]

I am concerned about your layout with filter caps being where they are causing your hum?

with respect, Tubenit

[uki]

I am concerned about your layout with filter caps being where they are causing your hum?

with respect, Tubenit

Hey Tubenit !

I've been looking at every possibility, if gets to the point where that board need to be moved from there, it will move.
For now looking after what is easier to deal with, that bad cap isn't doing what it should be doing. No effect in smoothing the signal.

[Willabe]

The 8 x 150uf cap is part of your -dc bias supply it's not a filter cap for the B+ power supply.

[sluckey]

I've been looking at every possibility, if gets to the point where that board need to be moved from there, it will move.
For now looking after what is easier to deal with, that bad cap isn't doing what it should be doing. No effect in smoothing the signal.

It's been at that point from the beginning. Just waiting for you to actually move it. Same goes for the OT.

I don't think a new bias cap will do any better than the one that's in there now. There is such a light load on the bias supply that a single cap is often used by Fender in some of it's most popular amps.

[Willabe]

Uki, have you got the vibrato to work yet?

I agree with Sluckey, that if the vibrato is miswired that could be a source for your hum. 

[uki]

Uki, have you got the vibrato to work yet?

I agree with Sluckey, that if the vibrato is miswired that could be a source for your hum.

I haven't got the vibrato working yet, gonna check that part of the circuit all over.

Any hints ?

Thanks

[Ed_Chambley]

Uki, have you got the vibrato to work yet?

I agree with Sluckey, that if the vibrato is miswired that could be a source for your hum.

I haven't got the vibrato working yet, gonna check that part of the circuit all over.

Any hints ?

Thanks

I got a hint for you.  Order a doghouse cover and move your filter caps to the outside of the chassis.  I did not notice your cap board was right at your input jack(s)

You can try another experiment if you need more convincing.  Un-bolt your input jacks and swing them over to the back of the amp and ground them to the chassis and see if your hum drops.  Think of it as if you were installing your input jacks on the back of the amp.

I am not sure if the tremolo is causing hum or not, but you can bypass the tremolo for the moment.

[uki]

I got a hint for you.  Order a doghouse cover and move your filter caps to the outside of the chassis.  I did not notice your cap board was right at your input jack(s)

You can try another experiment if you need more convincing.  Un-bolt your input jacks and swing them over to the back of the amp and ground them to the chassis and see if your hum drops.  Think of it as if you were installing your input jacks on the back of the amp.

I am not sure if the tremolo is causing hum or not, but you can bypass the tremolo for the moment.

Hey this sounds great !!  Also bypassing the tremolo is a good hint !

Thanks !!
 

[uki]

Hey guys

I did some tests, and by removing every preamp tube one at a time, like: remove V1, listen, put back; then remove V2 listen, put back, then V3 and so on; I have found something very interesting !!

Without V5 the amp is quiet no hum.     

The tubes are good I did swap them to make sure they are good.

Right now I'm checking if components are connected correctly. But I'm no technician, all I can do is to look for wrong connections, I have no clue how to find a bad component if that is the case.

[sluckey]

Without V5 the amp is quiet no hum.

That kills only the vibrato channel. Does the normal channel sound good with no hum?

[uki]

Without V5 the amp is quiet no hum.

That kills only the vibrato channel. Does the normal channel sound good with no hum?

I was playing and it does sound so awesome !!!

Yes the NOR channel does sound really clean quiet and no hum, the VIB channel have a very small volume, but also no hum. What is the source of the problem?

[sluckey]

Sounds like the problem lies in the vibrato channel.

[uki]

Sounds like the problem lies in the vibrato channel.

What part V4 and V5 have in this amp, do they only act with the vibrato circuit or does it have any other effect in the sound signal ?

[uki]

Sounds like the problem lies in the vibrato channel.

What part V4 and V5 have in this amp, do they only act with the vibrato circuit or does it have any other effect in the sound signal ?

I would like to understand what is the role of those two tubes.

The 5E3, 5E5 and other Fender tweed amps have interactive volume controls. When you plug into 1 channel the volume control on the other channel will also effect the amps sound and tone. Comes in very handy.

Turn the volume control on the channel your plugged into to 12:00 and the volume control on the channel your not plugged into full up and it scoops the mids and you get a nice clean tone. Keep the controls the same but plug into the other channel and you get full drive/distortion with the most amount of mids the amp can give.

Use an A/B box to act as a channel switcher. 

Does the Twin Amp 6G8 also have some of this tricks?

I did play with the amp today for about 2 hours with the NOR channel, I though it as already good with hum, but now without hum it does sound SO AWESOME !!! I was concerned about having to move stuff around the chassis but it turned out not to be the problem, It is dead quiet and crystal clean sound , the shielding did help big time with the squeal in the VIB channel that was a great hint. Now the bug is about to be found, but again I'm no technician and I do not know what to do, I'll replace the wirings from V5 to the main board, for shielded ones, that is the only thing I can think about, what can be done besides that ? 

Again I want to thank everyone that steped by and put 2 or more cents in this thread, that is very much appreciated.

Thank you !!

[Willabe]

Does the Twin Amp 6G8 also have some of this tricks?

No.

[Willabe]

I would like to understand what is the role of those two tubes.

They are this part of the circuit and are for the vibrato.

(Sorry about the poor quality.)

[sluckey]

What part V4 and V5 have in this amp, do they only act with the vibrato circuit or does it have any other effect in the sound signal ?

I would like to understand what is the role of those two tubes.

Those tubes affect only the vibrato channel. Read this pdf...

[uki]

Those tubes affect only the vibrato channel. Read this pdf...

Thanks Sluckey !! 

I'm still working to fix the problem with the vibrato circuit. Rechecked everything, all looks to be at the right places. Gonna recheck again to make sure.
I've shielded the wires for V5 and it did reduced the hum a bit.
I think the position of the diode board is interacting with the vibrato wires from the pots. Gonna redesign it and flip it back like in the original layout, to fit better with my layout, moving those AC wires alway as possible from pots and main board, then see what happen.

[sluckey]

Sounds like you are zeroing in on the source of the hum. Beginning to look like it's the V5 circuit. I've seen that circuit cause hum in another guy's amp. The tube itself was humming. So, try several different tubes in the V5 socket to see it that helps.

I've got a good way to totally bypass the V5 tube. Look at the attached pic and move the cap as indicated. You'll probably have to solder some pigtails onto the leads of that cap. Don't worry too much about how this looks, just don't let the cap leads touch anything they should not touch. The vibrato effect will not work but the vib preamp should sound just like the normal preamp and hum should be gone.

If the hum is indeed gone, then the V5 circuit is the culprit and we can concentrate on fixing that. So, put the .05 cap back where it's supposed to be.

Once the hum is fixed we can move on to make the vibrato effect work. (NOTE... The hum and the broken vibrato effect may be related).

Do this simple test and report back.

[uki]

I did the test rerouting that cap/circuit with and without V5, with V5 same hum,

without V5 dead quiet and the VIB channel works like the NOR channel no hum at all. 

I had another though on the problem and might be related to the hum, remember the other thread about “missing link” http://el34world.com/Forum/index.php?topic=19225.msg198141#msg198141 there is a wire rerouted that goes to filter cap it is now passing by the V5 part of the circuit and it carries 350v, that could be part of the problem as well yes ?

[sluckey]

there is a wire rerouted that goes to filter cap it is now passing by the V5 part of the circuit and it carries 350v, that could be part of the problem as well yes ?

I doubt that wire is a problem. The B+ on that wire is very well filtered and would be no more noisy than a ground wire.

You have just proved that the hum is associated with V5. Put the .05 back in its original position and plug V5 back in. Now remove V3 and V4 and put aside until the V5 hum is fixed. Now start subbing some other 12AX7s for V5. (You can use the V3 and V4 that you just put aside). Do you find one tube that kills the hum? If not, measure the DC voltages on V5 pins 1, 2, 3 and 6, 7, 8. Measure the AC filament voltage between pin 9 and pin 4/5, not from each pin to ground (Should be about 6.3VAC). What are the measured numbers?

[uki]

The .05 is back in its place.

Tried 4 different tubes at V5 , without V3 and V4, the hum still there. I've used good tubes from other amp.

No room to put the meter probe right in some of the tube pins, so I read at the other end of the wires, does it effect the reading?
Reading from V5 pins are:

P1       -  315vdc
P2       -  0-1vdc
P3 P8 -  5vdc
P6       -  330vdc
P7        -  0vdc

P4,5+9 - 15vac

I do have the feeling that something is wrong, don't know what it is.

Note: while touching P1 and P6 with the probe it does a loud noise.

[sluckey]

P4,5+9 - 15vac

Check this again. It should be about 6.3VAC. Put one probe on pin 9. Put the other probe on pin 4. Pins 4 and 5 should be connected together. Put the standby switch in the STANDBY position so there wont be any B+ on the socket while measuring the filament voltage.

While you're set up to measure filament voltage, do so for V3 and V4 also. And make sure pins 4 and 5 are connected together on these tubes as well.

What are the new filament readings?

[uki]

Ok I did read it again, this time with standby off, the reading was 5.8vac.

[sluckey]

OK. Next, turn power off and measure resistance from V5 pins 2, 7, 3, 8 to chassis. Pins 2 and 7 should each read 1 megohm to chassis. Pins 3 and 8 (tied together) should each read 4700 ohms to chassis. What are your actual measurements?

[uki]

Ok the readings are:

P2 1.105M ohms
P7 1.077M ohms
P3/8 4.76K ohms

The readings are about what you said above.

[sluckey]

Those resistance readings are all good. Now I have two things for you to do...

1. Look at the attached pic. I've circled two .05 caps. I also circled the turret at the top of the board where a lead of each cap connects. Unsolder the cap lead at the turret that is circled in red. Do this for both caps. Just stand the cap up a bit so those loose leads will not touch anything. We're gonna leave the caps like this until the V5 hum issue is fixed. This disconnects the vibrato oscillator (V3/V4) from the modulator (V5). We don't need that in the way while we troubleshoot the V5 hum issue.

2. Look at the last pic you posted. There is a black 5 conductor cable connected between the board and V5 socket. Remove that cable and replace it with five individual wires between the board and V5 socket. And don't bundle the 5 wires together. Leave them long enough to keep them separated from each other.

When you have done both of these things test for hum and report back.