Tektronix 475A & DM44 Repair

One more of those unfortunate souls, who was neglected. I got it off from someone who was selling  it saying its all working, but when I went there, there was not even trace on the scope. Anyways, since I am mad, and addicted to scopes, I rescued it and got it home.

More pictures of the 475A @  http://w140.com/tekwiki/wiki/475

Since there was no trace, I knew one of the power rails is bad and if you are familiar with Tektronix scopes, the 4XX series is well known for horrible tantalum caps, which shorts out. Some of them do burn out and expose themselves, some don’t.

I got it home, cleaned first. Like I always say, I don’t touch anything unless I clean it in and out, knobs, everything so that its free of dust, rust, labels, mud.

Safety Warning:

There are high voltages around the main A9 Board, including Line voltage. Be careful while working on this. Do not attempt repair unless you are qualified to do so and understands the risks and safety procedures involved  with working on devices containing  Line voltages.

Opened and didn’t power it on, as I know one rail is short. Did a quick check with multi-meter and here you go, 50V Rail is dead short.

Bit more explanation around how to isolate the shorted power rail. Typical symptoms are scope either powering off itself or not powering on at all. Reason as mentioned one of the tantalum in the power supply is shorted, shorting that power rail to ground. It cold be anywhere on the scope.

To troubleshoot, power off the scope, ensure all caps are discharged, and then check the resistance between each power rail and ground. The shorted rail can be identified by this method. Once you nail the shorted power rail, next step is to identify the capacitor in that rail which is shorted.

If you have a good multi-meter, which can show like 4+ digits, takes only few minutes to trace the shorted component on the board.Start taking resistance measurements  on the PCB trace for the rail and the resistance (lower digits) on a precision meter will come down as you get close to the shorted capacitor.

Check the resistance on the trace and see in which direction its coming down, and it will guide you to the shorted component. Remember, you need something like 4 digit or above precision to do this quick. My fluke 8842A helped me in minutes to point the culprit, a tantalum on the 50V rail, C1428, 4.7uf.

Here it is in the schematic

and the Culprit

and the replacement

Okay, nothing special here, boring, anyone could fix it, Checked 50V rail resistance and powered it, I could hear the HV coming on, and trace !!

ooops..dot !!..Now its interesting :)..There is something seriously wrong with this fella. I am happy and excited, as its really boring to replace a cap and bring something to life, I don’t learn anything there.

Lets get to business..issue is very simple, no trace, means, no Sweep or H-Amp failed. Easy to identify, Open it up, check A9 Board and verify H-AMP Input.

Here is the section in schematics and on the A9 Board.

CR1121 and 1122 is marked on the picture,

I do get the right sweep here ~ 150mv signal. Which points fault to H-Amp and means sweep and trigger is all good. Wonder why Tek always tests me with H-AMP and Sweep. 549 I struggled with Sweep and H-AMP, and here you go, H-Amp again !!

So Here the H-AMP is simple, but very brilliant transistor design. Biggest challenge : You can not remove the A9 Board. It so tough, I always prefer not to remove the A9 board on any 4XX series scopes. Good part – Transistors on sockets, easy to pull out and check, make life so easy to troubleshoot, side effect : transistors may come improperly seated, so first step is to RESEAT ALL TRANSISTORS.

Troubleshooting H-AMP is simple  due to the well documented service manual. Set the scope to the settings as specified in the service manual and verify the DC voltages at each section/point as identified in the schematic. Most of the time the offending section can be identified by just verifying the DC voltage against the schematic. Do remember to keep the scope in the settings as specified  in the service manual.

Second option which I typically use is to insert a external H signal – Saw-tooth-. I generate this from my function generator at ~500Hz triangle wave, and adjust duty cycle to get a saw-tooth ramp and use that as external H input. Then trace the signal through the H-AMP with an oscilloscope. Thus you can find-out where the signal is going dead.

Check all transistors, as you can – coz they are on sockets.

This tool is handy, the component tester. Remember, Chinese sell it, but the original was made by someone here – https://www.mikrocontroller.net/articles/AVR_Transistortester

Truly genius mind, to use a uC to automatically recognize and test components. Amazing work ! Here it showing cap, it can test and tell you transistor in few seconds, including leads, ß and PNP or NPN.

For me all transistors turned out to be good. Basic issue was the output was zero, rather the gain of the H-AMP was zero.

The section below is the H-AMP of 475A. All transistors test good.  Note:  output to CRT H-Plates are shown below in blue.

Para-phase Q1124/1134 is working fine as I do see inverted sweep  the collector of 1123 and 1134.  Since Tek Service manual is really good, they show the reference voltages, so time to check all voltage levels.

Next check found the issue, -3.8v  bias for collector of Q1152/Q1162- which is the Gain Setting-Amp stage – is at +14.9V. Obviously something wrong. I found R1169 of 160Ohm was overheating, which is natural, as its sitting across +15 and -8 (23V).  Since its driving the resistor hot. Tested all resistors and caps, all good. Looks like some dead short down the line.

Now the last step, start lifting resistors, R1163 first, I still see +14.9, Next was R1153, there you go, it was feeding the voltage, which was traced further down to R1152 of 120Ohms. Removed all transistors from Gain-setting amp to final output transistor in H-AMP and bagged them with number to isolate anything coming from the output section.

You can see old V Numbers, the same covers were used while fixing the Tek 549 to keep Tubes 🙂

Even without transistors, checked voltage again, still I see +14.9 @ R1152.

Could be a short underneath the board, but honestly I don’t wanna remove this board. I have to remove V-Preamp, Timing Board, disconnect all Pots in front and that is a project by itself.

Here is the exact location I spotted the short to.  You can see lifted  resistors

Tried an old trick (sorry not a professional option), to suck all the solder off that part of the trace. May be too much solder went down and causing short ?..so literally sucked all the solder off the PTH. Checked voltage again..and WOW !!..it disappeared..pointing to -4.9V

Looks good, soldered all resistors back, installed all transistors and Powered up.

Here is the result :). I don’t know what caused the short, may be a solder bridge under the board. Tested this beauty, all good and alive and working.

Here are some behind the scenes pictures from the project, and the gear I used to troubleshoot- two DMMs and one 465. Two DMM coz one for Voltage, one for Ohm.

 

Is it over ? Nah..LOL..

check the DMM..the DM44 – is showing weird readings :). 59 is the constant reading on Volts. Time and  1/Time (frequency) is working. Resistance is also broken.

Check  Tekwiki – http://w140.com/tekwiki/wiki/DM44 for more reference pictures of  DM44.

So now heading to fix the multi-meter Module. First task is to take it off the scope.

 

The push button switches in the front are  troublemakers. I have seen many HP3466A/3465Bs multimeters with clogged switch contacts being sold as faulty and comes to life just after a spray of contact cleaner.  So did the same attempt here,  and cleaned all  switches.  No luck. Reading is still same.

Here is the symptom. On first power up, the meter reads okay, fine, then the voltage swings to 56- 80. No matter which range you are in, it will show the same value. Tried applying 2v Input, no difference in reading,

Time to trace the schematic,

Input goes through the switches and resistor divider to end up in front of ADC Pin #15.

This is also marked as TP3167. Connected my fluke there and saw the good news, there is always 59mV there, that’s why ADC is reporting 59 in the display. No matter what input, its still 59mv.

 

Note: They lifted up and soldered pin #15 of the ADC for you to check it easily, in addition to the test point TP3167.

So now question is who is injecting and 59mv and draining the input. Time for Schematic again  and easy to find  the culprit, the low pass filter.

which is sitting right in front of ADC and TP3167. I was suspecting c3167, but its a ceramic cap, no need to investigate further with that.

Physical verification of the section on the board  provided next tip, its not the OpAmp, definitely its the Caps which are broken, lifted them off the board – as its a flow pass filter , the circuit will work without them –

 

and there you go, all back to life !!

replaced both caps and finally, all over for this boy.

 

Time for some stress test on the volt meter – from my Heathkit 2717 HV Power Supply ~ ~400V DC

Now, don’t ignore this scope thinking its vintage. its incredibly powerful. You can see the delayed sweep in action above, pasting it here again

Now with Integrated DM44, I can measure the time difference between two points on the trace.  I am feeding ~2MHz input to the scope and see the DM44 reporting the Time  between each cycle

.489µs, which is 1/.480µs =  2Mhz.

Now you don’t even need to calculate this, push the 1/Time Button on the scope it can do that for you

I didn’t calibrate  DM44 , so there is slight error, down to few decimal points.

Now, if you want to check delay between two points in the wave, you can move the Δ Time knob on the DM44 and measure that, Here it is on a 2Mhz Saw tooth.

0.736µS, which is 1 ½ Wave, which is ~  489µs + (489µs/2)

well, you can go totally anywhere, across entire screen to measure time gap/frequency

Doesn’t stop there. You can do it across two different waves across two channels. Remember this was 1970 when they did it 🙂

and ending this, with a “different kind of trace” 😉

 

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