Can't get much help on other forums

[rockpassion]

Paul,

Thanks for the response.  I am not near any of my schematics but it only happens on two specific capacitors.  I will get which ones they are and post them in the next couple of days.

Since none of my schematics have any output voltage readings will I have to attempt to calculate them?  I would like to be able to test specific connections. 

I am learning a lot and am getting more comfortable with this process.  Since I am so new to this and I do understand how dangerous it can be so I am taking my time and trying to read as much as I can.  It definitely helps me to stop and think about what to do next as I have figured out a couple of problems. 

Thanks again,

Richard

[rockpassion]

Paul,

I rechecked the capacitor and it is the C3 (.01mfd, 600v) capacitor on the schematic.  It connects the Grid of the 6sn7 to the plate of the 6sl7.  I have hooked up my DVM with ground to chassis.  When I touch the red probe to the 6sl7 I get 91 VAC and on the 6sn7 end I get 89 VAC and in both cases the buzz ceases. 

Also, when I hook up my DVM with just the ground connected and the red probe not touching anything and the amp is turned off I get 3.5VAC when I turn it on it jumps to 5.5 VAC.  Also, I get 87.5 VAC on the rca connector to the preamp when the amp is on.  Should there be any voltage on this rca connector?

That's it for now. 

Richard

[Paul Birkeland]

I rechecked the capacitor and it is the C3 (.01mfd, 600v) capacitor on the schematic.  It connects the Grid of the 6sn7 to the plate of the 6sl7.  I have hooked up my DVM with ground to chassis.  When I touch the red probe to the 6sl7 I get 91 VAC and on the 6sn7 end I get 89 VAC and in both cases the buzz ceases. 

Hopefully those are DC volts?  There should be no voltage on the 6SN7 side of the capacitor.  If the DC voltage is the same on both sides of the cap, the cap is shorted and needs to be tossed out.

Also, when I hook up my DVM with just the ground connected and the red probe not touching anything and the amp is turned off I get 3.5VAC when I turn it on it jumps to 5.5 VAC.  Also, I get 87.5 VAC on the rca connector to the preamp when the amp is on.  Should there be any voltage on this rca connector?

There shouldn't be any voltage on the RCA jacks, at all.  How is the AC line voltage coming into the amp?  Is it a 2-wire AC cord?

[2wo]

At C3 you say you are reading ~90 VAC, are you sure you are not reading millivolts ?

Set your meter to DC, 200v scale if not auto ranging and with the black to the chaise or a known good ground recheck both sides of C3. I would expect to see something like ~100VDC one one side and <5V or so on the other, compare with C4 which should be about the same .

This amp has an input transformer at the RCA's short the inputs together and to ground, try all of the combinations and see what that does for the hum...John   

[Grainger49]

You might want to add a new power cord with a ground to chassis.  That is the first thing I do with all old equipment I work on.

[rockpassion]

Ok, I am not sure I am hooking up my DVM properly.  I hook my DVM black lead to the chassis then use my red lead to check connections.  I rechecked my C3 capacitor and got 91 VAC on one side and 89 VAC  on the other, not millivolts.  Switching to DC I got 14.5 VDC on one side and -7.5 VDC on the other.  Once again the minute I touch either side on C3 the buzz stops.  But when I test every capacitor connection I get almost the same results with the exception the buzz only stops when I test on C3. I desoldered the C3 capacitor and tested it on my DVM and got the proper reading.  So, I desoldered two other capacitors and they all tested out properly.   Also, there is a meter that tests the two 807 tubes and the VR150 tubes.  The meter registers on the first 807 but not on either the second 807 or the power tubes.  So I have something wired incorrectly I think.   

Per Grainger I rewired the input to a three wire setup and I also shorted the input wires to ground and the buzz was a little less.

My plans are to start checking my rewiring connections and resoldering all the connections to see if I have a bad solder joint.

One last question.  This amp is almost 60 years old and one of the resistors, R34, is encased  in a metal case.  The resistor is a 0.2 ohm, 2 watt wirewound type.  It appears to be grounded to the chassis.  Can I replace this with a current equivalent wire wound resistor and ground it to the chassis?

I will say this definitely test one's patience but at the same time it is fun.  I just wish that it wasn't deadly.

Onwards and upwards and once again thanks for all the help.

Richard

[Paul Birkeland]

I would suggest checking the resistance between the chassis and a known power supply ground.  Something just isn't adding up here.

-PB

[rockpassion]

Paul,

This may show my ignorance but are you suggesting I find a power supply ground within the amp.  Would that be a ground associated with the Power Transformer T3.  Also, I see on the schematic a number of System Ground points off the Power Transformer vs Chassis Ground would it be one of these?  Or, there is a black wire on the wiring schematic that is listed as a System Ground.  I currently am using that wire for my negative speaker connection, could I use that to check for the resistance you are suggesting.

This is where I am confused. There is one wire listed on the wiring schematic as Out.  I traced it back to the Output Transformers 8/16/32 ohm taps and am using that for my positive lead (red) to my speaker.  The Common tap, listed as 0, goes through a resistor R34, and a couple of capacitors C9 and C10.  From there a wire shows going out and is listed as the System Ground, I am using that for my negative lead (black) to my speaker.  Am I using the wrong wire for my speaker.   And if so will I have to run a wire from the O tap to my speaker.     

Thanks for the long distance help.

Richard

[Paul Birkeland]

C11 looks to be a multi-section lytic, it should have a single negative lug that is grounded.  Start there, then see where else it goes.  For convenience, you could head to the drug store, grab some green nail polish (should be an interesting checkout process), then dab a green dot where all the grounds are.

-PB

[rockpassion]

Paul,

It actually has two lugs and both of them are showing OL on my DMV.  What am I looking for with regards to a reading. 

One thing on the capacitor.  When I unmounted the old capacitor it had a phenolic plate that went between the capacitor and the chassis body.  The replacement capacitor had a phenolic plate and a metal plate.  Should this capacitor be grounded to the chassis? 

I will look for additional points tomorrow. 

Richard

[Paul Birkeland]

I'd take some photos.

You are looking for the ground leg of your capacitor.  This is power supply negative, and should be ground.  You should have power supply voltage (DC) across this capacitor.  This is the place to start, to be able to actually measure your DC power supply voltage (not AC). 

Once you can measure the power supply voltage, you can leave your black probe where it is and measure other DC voltages in the circuit.

So far, your voltage measurements have been erratic enough that this needs to be nailed down before looking for any issues.

-PB

[rockpassion]

I hope these photos are clear enough. 

Richard

[rockpassion]

Paul,

A little confused on what you are saying. 

When I look at the C11 capacitor there are three sections A, B, C, plus three lugs on the outer part of the capacitor can.  Are these the power supply negatives you are referring to.  Of the three only two are being used, one between A & B and one between A & C. 

When I trace the wires from these two lugs there are three wires attached.  The wire from the AB lug goes to the C8 capacitor, a second wire goes from the AC lug to the C10 capacitor, and a third wire goes from the AC lug to the V6 voltage regulator tube. 

So am I measuring DC voltage here.  If so am I leaving my black probe attached to the chassis and measuring these points or am I attaching my black probe to these points and measuring somewhere else?

I have been reading but I am still very confused on what a System Ground is.  Sorry for the ignorance and I appreciate your patience.  I hope you don't mind being a teacher here. 

Thanks

Richard

[Paul Birkeland]

Start by measuring the DC voltage across the first capacitor in the power supply.  I think you swapped that out for a Solen?

[rockpassion]

Paul,

I was able to take some measurements.  I measured all the Solen replacement capacitors.  I had my black probe attached to the Chassis and used my red probe on each capacitor.

C8 - two red wires that are attached to R33 & R22 and one black wire that is attached to the ground lug on C!!.  The readings were; on the red wire it started at 18.4 VDC and dropped to and stabilized at 14.3 VDC.  On the black wire it started at -2.18 VDC and dropped to -1.37 VDC.

C9 - two red wires that are attached to C10 and T2 and the black wire attached to R4 and a bridge connection to C10.  The readings were on the red wires; start at 40.4 VDC drops to 19.9 VDC.  On the black wire starts at -3.06 VDC and drops to -1.35 VDC. 

C10 - two red wires attached to C9 & L1 and the black wire connected to R34 and the same bridge connection that C9 .  The readings were on the red wires start at 30.86 VDC dropping to 22.45 VDC.  On the black wire starts at -2.96 VDC dropping to -1.82 VDC. 

Richard