Tektronix 585A Voltage Diagnoses Problem edited

#159316

Hello All,

I have a 585A scope that has me stumped.. the LV voltages are out of spec and I cannot find the problem.  Here are the voltage readings;

-150  =  -182
-100  =  -115
 225  =   263
 350  =  409
 500  =  587

The -150v adjustment pot does not change the voltage value at all. All the tubes were pulled and tested for shorts and emission. Not being very good at diagnosis I shotgunned all the electrolytics and filter caps although most of the tested good in circuit I found a couple of bad ones.  No improvement. I have the schematic and can upload that if it helps.  Can anyone suggest a way I can isolate the circuit causing me grief?  The scope is clean and intact otherwise.

Thanks in advance.

Roy Morgan

#159321

I no longer have my 585 or a manual, but I am pretty sure that all the other B+ voltages depend on the -150 supply.  I suggest you focus on that one - you may find that all the others come to correct voltage if that one is right. I suggest you read the 585 manual section about the B+ supplies. 

“Typical OscilloscopeCircuitry” confirms this on pg 12-22.  Fig 12-16 shows the supply for the 531A and 545A.  The 585 supply is likely very similar. 

Your report below about the -150 volt adjustment pot having no effect is the smoking gun!!

There are 4 capacitors .01 uF in the 531/545 supply shown. If ANY of them is a “black beauty” cap, replace them ALL.  Check for reasonable voltages on all 3 terminals of that adjustment pot. 

Note: the calibration of the entire scope depends on the -150 and the others to be right. 

I assume you can rely on your voltmeter to be right. 

Roy sends.

Show quoted text

On Aug 23, 2019, at 6:43 PM, randolphbeebe@... wrote:

Hello All,

I have a 585A scope that has me stumped.. the LV voltages are out of spec and I cannot find the problem.  Here are the voltage readings;

-150  =  -182
...

The -150v adjustment pot does not change the voltage value at all.

Chuck Harris

#159346

Everything is referenced to the -150V supply.  It must
be right, or the others will behave truly weird... like
you are seeing.

Start there.

Also, the 585A has DC filaments.  If it doesn't have a plugin, it
switches off the -100V supply that is used to power the filaments
on the plugin... Or, it has an internal dummy load... can't remember
right now.

-Chuck Harris

randolphbeebe@... wrote:

Show quoted text

Hello All,

I have a 585A scope that has me stumped.. the LV voltages are out of spec and I cannot find the problem.  Here are the voltage readings;

-150  =  -182
-100  =  -115
 225  =   263
 350  =  409
 500  =  587

The -150v adjustment pot does not change the voltage value at all. All the tubes were pulled and tested for shorts and emission. Not being very good at diagnosis I shotgunned all the electrolytics and filter caps although most of the tested good in circuit I found a couple of bad ones.  No improvement. I have the schematic and can upload that if it helps.  Can anyone suggest a way I can isolate the circuit causing me grief?  The scope is clean and intact otherwise.

Thanks in advance.

#159382

Hi Chuck,

The scope has a CA plug-in right now with a type 81A adapter. I am pretty sure all is good with the plug-in because the scope responds to the vertical position and input etc.  I have a couple of the Type 81A adapters, I wonder if the present one may be causing problems.

Thanks for the

#159383

Thanks Roy,

My 585A does not have the black beauties but the red Erie caps that tested good in circuit.  I replaced them all anyway this morning just in case there was cumulative leakage problems.  No improvement yet so I will keep digging.  I will post a schematic when I figure out how to.

Randy

Chuck Harris

#159385

Hi Randolph,

There is no reason to expect the plugin would pull the voltages off
their mark.

Get the -150V right and everything else should fall into line.

-Chuck Harris

randolphbeebe@... wrote:

Show quoted text

Hi Chuck,

The scope has a CA plug-in right now with a type 81A adapter. I am pretty sure all is good with the plug-in because the scope responds to the vertical position and input etc.  I have a couple of the Type 81A adapters, I wonder if the present one may be causing problems.

Thanks for the 

#159391

Hi Randolph,

You checked all the tubes, but also the voltage reference tube V609? Should measure about 85  V.
The grid voltages of V 624 should be approximately equal, well within 1 V or so.
The voltage at the grids should be what you expect from the divider R615/616/617 with "your" -150V.
If the fault is elsewhere then likely you will find grid pin 7 far too negative and pin 6 (anode) far too positive, almost at "your" +100V.  Then continue with V634.
Is the screen voltage about normal (not very negative)?
And so on (for now).

Albert

#159407

Checking these voltages now.  Thanks Albert

#159409

Albert,

I checked the voltages and here is what I measured after a 10 minute warm up;

Tube          Pin          Voltage

V609          7              -107.4

V624          6              +1.5
                  7               -107.6
                  2               -104.0

V634         6               -38.0
                 7 and 2      -87.0
                 1                -87.1
                 4                -2.4

V627         9                +69.0
V637         same
V647         same


As you mentioned in you previous reply I found that pins 6 and 7 of V624 vary widely.  I changed that 12AX7 with no change.

Thanks Again,

Randy

#159410

Albert,

I am not sure if this is related but I seem to get a short between Ground and the positive terminals of C790-791-792 ( All have been replaced now) of the regulated heater supply (Upper Left portion of the LV Power Supply Schematic).  It appears to be isolated between the Emitter of Q797 and Diode 793 and Diode 792 somewhere.  Is this normal?

Thanks

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Tektronix 585A Voltage Diagnoses Problem edited