Restoration Tektronix 464

Aug/28 2017

while i received this Tek464, it's totally blind, but with several light still glow. it looks like new outside, with working DM44, even the front cover does not lost. i get it 38$ with shipment(Not in US).

Power Supply

There is no trace. only few light glow. A simple check Power/Interface Board, ie, on the bottom of 464, find out the HV fuse is broken.

HV fuse F1487 failure means the HV circuits might got shorted. but worth a try with a new fuse. luckily, the fuse not burn out immediately. still no trace, but with beam finder, 2 beam point get back.

the next step is checking the all power supply voltage on this board. all supply voltage test-points located this board's, near the shield of HV circuits. and obviously 15V is zero.

Q1756 is the biggest power transistor on the very end, under the screw-bar heat sink. Q1754 shutdown the Q1752 while there is a short in the load, and this is what the Tantalum capacitor is famous for.

blind identify the shorted capacitor is hard. the key to identify where is it is isolated the board supply one by one. the entrance of the 15V power on every board, had a inductor for filtering. there is the good place to broken the whole board from 15V power rail. and other minor power line with DuPont style line been pulled out (plug only one pin for remembering it's direction).

in the middle of 2 white connector, the time/div board is not easy to access, luckily, 15V rail(on J2) had a jumper( like a white resistor). pull that out is handy.

And, finally, got this busted capacitor on the storage board.

After the 15V rail back, trace appear ! Although it is hard to get stable triggering.

Trigger

The trigger board:

I can not get a stable trigger. debug it with the trigger view, it's really useful, i got this.

Obviously the rigger signal is distortion, and it seems coupling switch problem. Trigger system is a kind of loop, input signal get sampled from pre-amplifier, send to control rigger amplifier board to triggering a tunnel diode, but the tunnel diode is controlled by HOLD OFF from the control board, this hold-off is not only use the the knob named hold-off, it's a sweeping status: trigger is hold off after the sweeping start to prevent the signal triggering the tunnel diode during the sweeping .

Control board get input from the tunnel diode, different triggering setting, Auto Normal, One time, B trigger, Hold Off, determine the correcting timing to sent out the trigger event to sweep circuits, and further, the controlling board get sweeping start and End pulse to Z amplifier switch on/off the trace.

Marking the trigger amplifier board on photo is very useful trick. Following photo marked several testing point, make life so easy to narrow down fault.

I can tel, after doing some test and view the signal waveform in different points, the trigger coupling switch: AC/DC/LF/HF and the trigger selecting switch: AUTO/CH1/CH2/Ext, is definitely need to be cleaned up. It's hard to disassemble all the board to reach that switch. but lucky, all those switches is gold plated, the problem most likely caused by dusty.

It does, after use the 95% alcohol wash the switch several time, all triggering problem gone.

HV Power

the happiness fixing it does not last too long, it run into the biggest problem. the first time i notice the problem is the HV fuse blow up again, after a long time run.

general suggestion for HV circuits is dig around all high voltage stuff looking for leaking or dead short. short circuits should be most easy thing to fix, cause somewhere might had zero ohm to ground. leakage is hard, but by remove the loading from transformer, there is way to isolating problem.

HV section PCB, the chip resistor in middle of following photo providing HV feadback.

HV drive transistor Primary/feed back winding joint ==== R1507 (HV 65V reference ground)

Don't disconnecting this diode from HV 1470V, this is provide the feedback, without this, drive transistor will run into huge current blow up the fuse.

an noticeable phenomenon is the current flow via the HV fuse increased gradually, 0.X mA per-seconds, continue going up until reach huge volume of current. dig into web, this is a typical HV transformer fault: it's leaking or shorted some turns. this is BAD news. this kind of thing is no replacement, and had huge turns to winding.

To make sure it's definitely the HV transformer fault, it's a hard thing to prove. i disconnecting the 7kV multiplier, even disconnecting all tube socket. nothing stopping the current increasing with time.

monitoring the R1482,R1483's joint's, with scope, this point's voltage change a little bit higher while the current volume getting huge.

Beside all of these, if i use a powerful fan blow the HV transformer, take it's heat away, It will stop increasing. All of these suggrest the HV transformer is faulty It's heated, and sensitive to temperature change. Finally, i got HV transformer removed from PCB, it's hard without proper tool.

I built a similar oscillator, run this transformer alone, confirmed its broken, definitely. Voltage going down while no feedback increase the current...., and heating the transformer.

Tek 464 HV Regulator

Tek scope widely use this High voltage regulator.

Q1486 with T1501 form a sine wave self-excited oscillator, bias current controlled the oscillating amplitude,then control the output high voltage. Q1472, Q1476, Q1484 control the bias of Power transistor Q1486.

at boot time, R1483 provide part of bias, and Q1472 open by R1425C via CR1454 ( the schematic mark there 0.4V, but it's actually 65.6 volt) this voltage open Q1472, sequentially, Q1484 Open, it's almost add another bias resistor R1486(via Q1484), this ensure a grace start.

The 1525C, 1.25M, provide a bias current around 52 uA current to turn on the Q1472, while the HV side get it's finally voltage 1470V(TIP1501), R1525D ,the 28.3609 Mohm will pull (1470-0.5)/28.3609 = 51.8 uA current, this will turn off Q1472, reduce the High voltage output, finnally stable at this stage, define the output voltage by R1525 network.

Rewinding HV transformer

Figure out the original HV transformer winding Data is key to rewind replica one. Use a signal generator to find out the turns is reasonable and somehow easy.

I don't have fancy signal sources, so building a oscillator according Tek 464's HV schematic.

The luck thing is the heater's winding is countable and it's in a separated coil form.

Here is the Oscillator I built to verify the HV transformer.

The original Tek 464 HV transformer.

The heater winding in the white plastic form, is exactly 3 Turns . That's all we had to begin our journey.

By use 10:1 probe, attach to the primary winding(CH2) and heater winding (CH1), notice the signal displayed is both on 50 mV/Div , we got Primary:Heater = 7.4/4.4=1.68.

We know heater is 3T, so Primary is: 3T * 1.68 = 5.04 Turns, it's obviously 5 Turns for Primary.

Here is the final Tek 464 HV transformer turns Data:

TEK464 HV Transformer:

Primary(5-6) 5 Turns 30Vpp (typical working condition)

Feedback 2 Turns

Heater 3 Turns 18Vpp 9V peak 6.36 Vrms

600V(1-2) 200 Turns 1200Vpp 600Vpeak

1470V 500 Turns 3000Vpp 1500Vpeak

7 kv 1200 Turns 7200Vpp 3600Vpeak