Issue with an X88R build

JazzGuitarGimp · Post by **JazzGuitarGimp** » Fri May 03, 2013 4:49 pm

I might suggest disconnecting the rear input jack (at the front input jack) to determine if that run of coax is the culprit. You can always reconnect it after you get the issue sorted out.

MGW · Post by **MGW** » Fri May 03, 2013 5:59 pm

JazzGuitarGimp wrote:I might suggest disconnecting the rear input jack (at the front input jack) to determine if that run of coax is the culprit. You can always reconnect it after you get the issue sorted out.

I'll give that a shot as that is something I haven't tried.

Working on the pics. Just picked up a Galaxy S4...so when I get them uploaded....they'll be nice and clear.

MGW · Post by **MGW** » Mon May 06, 2013 11:08 am

Tillydog · Post by **Tillydog** » Mon May 06, 2013 3:20 pm

Just had a quick look - couple of things:

The solder joint where (what I think is) the ground wire joins the preamp board is bad (photo 6 top centre / photo 4 bottom left);

I can't see any ground wire from the power supply board to the chassis - the connection that is near the -ve ends of C32 & C33 on that board / top left centre on photo 8. I don't think there is a photo of the other side of the board here, I may have mised it, but I can't trace any of the other wires back in this direction. You also need a ground here, as there is no other ground connection between the PCBs);

[^^ Put those two right and I think you'll make progress.]

Are you sure that none of your ground busbars between the pots are touching the chassis? (Difficult to tell from the photos) They need to be clear of the chassis;

Are you using an AC filament supply? If so, I think you have created a heap of trouble by running the heater supply through as busbars. I don't think it's your current problem (I still think it's the grounds to the two PCBs), but you might want to re-work this when you overcome the current hurdle.

0.02p

MGW · Post by **MGW** » Mon May 06, 2013 4:03 pm

Thanks for the input.

Take a look at pic 13. The wire at the far left on the power supply board by the standoff is the ground wire going to the pre board. It is grounded to the ground trace just below the leftmost LDR.

I'll revisit the input jack ground this afternoon.

I think my ground buss between the pots is ok but I'll take another look again at that as well.

Your comment about the filament supply going through the buss bars...I'm not sure I follow. I tried with twisted pairs and there was no change so I went back to the solid core a la Mike Soldano. See the pic below...actual soldano preamp.

Also, what is an acceptable reading on the meter when measuring the ground continuity? Typically it is less than one ohm, but at what point is it an issue that needs to be addressed. I ask because I'm concerned about the continuity between the different parts of the chassis.

Thanks.

Tillydog · Post by **Tillydog** » Mon May 06, 2013 5:47 pm

MGW wrote:..Take a look at pic 13. The wire at the far left on the power supply board by the standoff is the ground wire going to the pre board. It is grounded to the ground trace just below the leftmost LDR.

Ahhh! Sorry, I'd completely missed that on the layout - Ignore my ranting about needing 2 grounds, then.

Your comment about the filament supply going through the buss bars...I'm not sure I follow. I tried with twisted pairs and there was no change so I went back to the solid core a la Mike Soldano. See the pic below...actual soldano preamp.

I was only bothered about it not being twisted - does the original have DC heaters?

Also, what is an acceptable reading on the meter when measuring the ground continuity?

Generally very slightly more than you get shorting the leads together (0.1 or 0.2 ohms, maybe).

MGW · Post by **MGW** » Mon May 06, 2013 6:40 pm

The original does not have DC heaters. Also, if one did add DC heaters to this set up, the current connection of the heater pins wouldn't be appropriate would it? Pin 9 would be unconnected and you'd wire pins 4 and 5 to + and - correct?

Depending on where I hit the chassis I'll get any where from .1 to 4 or 5 ohms. wondering if that may not be a clue as to what is going on.

Tillydog · Post by **Tillydog** » Mon May 06, 2013 9:21 pm

Heater pins just depend on whether you're running them at 6.3 or 12.6 V - AC or DC immaterial. If the original works, then I guess it's OK.

4 - 5 ohms is definitely not right. 0.1 is the ballpark.

Did you always have the grounded input jack, or is that something that's changed since your first post?

MGW · Post by **MGW** » Mon May 06, 2013 11:57 pm

I have tried just about every grounding point you can. I've disconnected the ground between the two boards and grounded the pre board on the front panel and the power supply on the back panel... no change. Ive had the both grounded at the back...both at the front...neither made any difference.

I checked the chassis and the chassis itself is fine. Reworking the inout jacks again. I tried isolating and grounding to the rear chassis lug, but that didn't help. I've got new bell ends coming tomorrow...so we'll see what happens there.

About the DC, I'd leave it connected as is and hit 4&5 with 6.3 and connect 9 to ground?

Tillydog · Post by **Tillydog** » Tue May 07, 2013 4:36 pm

MGW wrote:About the DC, I'd leave it connected as is and hit 4&5 with 6.3 and connect 9 to ground?

You can disconnect the transformer heater winding and connect 6V to the busbars that you have. No need to connect anything to ground.

MGW · Post by **MGW** » Tue May 07, 2013 11:36 pm

Well, the endbells make no difference. It looks cleaner though.

MGW · Post by **MGW** » Wed May 08, 2013 12:46 am

should the transformer make noise when it is moved and makes or breaks contact with the chassis? The buzz doesn't change, but if I move it around and touch the chassis with it, it pops...kinda like touching the end of a guitar cable. That just doesn't seem right to me.

Tillydog · Post by **Tillydog** » Wed May 08, 2013 10:30 pm

I don't think the 'pop' when the transformer touches the chassis is significant...maybe adding to the evidence for problems with the grounding arrangements, maybe not.

To recap (apologies for a quick cut-and-paste job)....

The buzz I'm getting is a combination of 180hz and 300hz and other harmonics of this.

I discovered ... the buzz goes away with the trafo isolated from the chassis. However, when I do this, the 60hz grows much louder.

There is also something interesting happening with the gain pots, if they are turned all the way down the buzz gets louder

If I ground the grid of the second stage the pre is dead quiet.

....the closer the input wire is to the chassis wall the louder the 60hz gets...much louder.

The preamp is now grounded at the input jack on the chassis. The power supply is grounded to the preamp board

I can probe the grid of V1 and can hear the noise but barely. Probing the plate of the input stage...same noise but louder.

Depending on where I hit the chassis I'll get any where from .1 to 4 or 5 ohms. wondering if that may not be a clue as to what is going on.

Chassis is OK

I have tried just about every grounding point ... I've disconnected the ground between the two boards and grounded the pre board on the front panel and the power supply on the back panel... no change. Ive had the both grounded at the back...both at the front...neither made any difference.

Well, the endbells make no difference. It looks cleaner though.

Going back to the beginning... Do you still have all these symptoms, or have any of them changed / improved?

DID YOU HAVE THE INPUT JACK ISOLATED FROM THE CHASSIS ORIGINALLY?

HAVE YOU TRIED DISCONNECTING THE REAR INPUT COAX? Did it make a difference (it should, as there is no short on the input without it).

180 or 300 Hz is more likely to be oscillation than PSU noise, IMHO. Does this happen when there is no jack in the input?

If the noise is louder with the gain pots down, it suggests that there is a poor ground between the gain pot and the rest of the circuit. Likewise if the 60Hz gets louder when the input cable is near the chassis, it suggests that the chassis is not at the same potential as the circuit ground. Your ground wiring looks correct as far as I can tell, which leaves poor connections / dry solder joints, etc.

Where exactly are you 'grounding the second stage - connecting what to what?

When you probed V1 grid, did you have a jack plug in the input socket? If not, the shorting jack will kill the noise.

Can you take DC voltages to the ground on the PSU board at each pin of each valve?

Other than that, double check component values & placements, and look for dry joints.....

my 0.02p....

MGW · Post by **MGW** » Thu May 09, 2013 3:50 pm

Here goes...(and many thanks for the help!)

Going back to the beginning... Do you still have all these symptoms, or have any of them changed / improved? All symptoms but one still exist. The 60hz getting louder the closer the input wire is to the chassis has been corrected. Not really sure how except that I completely replaced the input cabling all the way around.

DID YOU HAVE THE INPUT JACK ISOLATED FROM THE CHASSIS ORIGINALLY? Yes

HAVE YOU TRIED DISCONNECTING THE REAR INPUT COAX? Did it make a difference (it should, as there is no short on the input without it). I did and it buzzed like crazy for the reason you mentioned above.

180 or 300 Hz is more likely to be oscillation than PSU noise, IMHO. Does this happen when there is no jack in the input? Happens all the time IF the transformer is in contact with the chassis.

If the noise is louder with the gain pots down, it suggests that there is a poor ground between the gain pot and the rest of the circuit. Likewise if the 60Hz gets louder when the input cable is near the chassis, it suggests that the chassis is not at the same potential as the circuit ground. Your ground wiring looks correct as far as I can tell, which leaves poor connections / dry solder joints, etc.

Where exactly are you 'grounding the second stage - connecting what to what? There is a ground plane on the preamp board and everything is grounded to that as needed along the length of the board.

When you probed V1 grid, did you have a jack plug in the input socket? If not, the shorting jack will kill the noise. Can't remember, I'll have to try when I get home. However, I believe that I probed V1 plate with nothing plugged in and I got buzz...not as loud at the plate of the second stage, but that is to be expected.

Can you take DC voltages to the ground on the PSU board at each pin of each valve? Not sure I understand what you're asking on this one.

Other than that, double check component values & placements, and look for dry joints..... I've gone over the board many times, re-made the board and used new coupling caps all the way around on it and it made no difference.

I'm gonna try reworking the trafo mounting this afternoon to see if that has any impact.

Tillydog · Post by **Tillydog** » Thu May 09, 2013 5:03 pm

MGW wrote:All symptoms but one still exist. The 60hz getting louder the closer the input wire is to the chassis has been corrected.

DID YOU HAVE THE INPUT JACK ISOLATED FROM THE CHASSIS ORIGINALLY? Yes

OK, that's that one explained and put to bed.

180 or 300 Hz is more likely to be oscillation than PSU noise, IMHO. Does this happen when there is no jack in the input? Happens all the time IF the transformer is in contact with the chassis.

Odd! And it stops if the transformer isn't in contact with the chassis??

Can you take DC voltages to the ground on the PSU board at each pin of each valve? Not sure I understand what you're asking on this one.

I was just asking for voltages at each pin on the base of each *tube* (sorry!) with the meter black lead on the HV ground on the power supply board. It would ultimately be useful, but I've thought of a different tack to take (this was before the info about the transformer which I'm still pondering).

Does the schematic below represent your build? (It's from this thread.)

If so, you could pull certain tubes to disable two of the channels:

Using the numbering on the PDF below, pull U2 and U5 to disable the lead channel, and U3 to disable the clean channel. What's left is just the crunch channel. Is there still a problem? Try it with the switching set to each channel in turn. If that's OK, put U3 back in and try again, then pull U3 again and put U2 and U5 back in, etc... (You can't disable the crunch channel)

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