Fix A Rare Tek-465

2018/12/18

I got these 2 tek-465 long time ago with good price,  of cause, it did not work any more. but its very rear version of tek-465, I even does not find any info about that. It's a tek-465 with a plugin Module, It's a Digital Store plus volt or time read out.

Bring Back the Trace

This Digital Store Module had a very big BB produced A/D converter. This not the same one as Tek-468.

The Digital Store Module is definitely dead on somewhere,  only few light get response for pushing switch. unluckily, all the supply test point's voltage is OK,  it seems not a typical short circuits problem. 

Beam finder could find  2 dots with a badly sweeping line between them. I try to pull out all Digital Storage plug out, so i can isolate the problem.  It turns out pulling one of the connector  ,might be the supply, did works, the sweep line recovered.

It works well on Mix mode with very bright line, but A sweep part is missing.  I'm be stupid for a while, after checking around again and gain, I find i just not turn the brightness on enough, so only the B sweep part is visible.  

CH1 Signal Amplitude Almost Zero

I pretty much give up fix the storage board, I can't even find the schematic. Pull out power plug out of the Digital Storage, the trace come up. CH2 is good, but CH1 almost got nothing, of course it's because the CH1 amplifier problem .

Power supply is Good, no shorted tantalum capacitor.  I  observe CH1 signal with another Scope, comparing it with CH2's same position.  It's obvious,  Q102,   waveform 5 is in good condition, but TP141, waveform 7 gone.  

So the CH1's Amplifier's problem about around Q122, Q132, Q124, Q134.

DC voltage on Q222's emitter is normal, 4.19V,  but Q222's  collector voltage is not 7.6V, it's drop down to 5.5V!!  The related transistor seems all good.  These Stages  DC coupled together, few stages after Q122  is not normal too, but before Q122, everything is good. This pretty much tell me the problem just between Q122  and Q132. 

I don't realized this point, all in my mind is, who pull the voltage so low, it's may be the Q123 is turned on so much. the key point here is, Q122, Q124's DC condition almost determine by R127, R128, if the supply here is fault, the phenomenon will be exactly same as Q123 is turned hardly ON. 

Pulling out Q132 out could isolating the problem here.  After Pull Out Q132, Q134, DC voltage still not recover. Now i can tell the problem is the +15V supply of this stage.  Measure voltage on VR129 tell me this VR129 seems dead. 

Then I try to use a 200 ohm resistor parallel with this VR129, in this way, if it is come alive, it's confirm here it is.

The lead of this diode is oxidized, I just clean it a little bit, everything just become Good! CH1 alive. 

Fault is gone just because I identified it, interesting.

Re think what happen here, this VR129 might bite me later, but i got that, let it be.

I actually found Ch1's top transistor had same behavior with lower one. This symmetry phenomenon actually suggested the fault must be somewhere affect the top transistor as much as the bottom one.  

Retrace is flickering Visible

Now the scope is almost back to alive. wired thing  is the retrace beam is flickering visible.

Indistinctly i thing it's some kind power supply's problem.  For tracking power supply problem, First of ALL is measuring DC voltage test point. They all seems pretty fine under DMM.

And second one, been told everywhere on WWW,  checking the voltage ripple.

Scary Power Supply and High voltage supply.  Components around 2 White Connector  is  A and B sweeping circuits.

To make life easier, I make this Marked photo. Inspire by Ron Childress, thanks. Reffer to  Tracking Tek 4xx Power Supply.

Immediately, I found this, 50 Vpp saw-tooth voltage on 110 V test point TP-1518. This is Z supply voltage. The Z amplifier output test point TP-1486 also had a low frequency waveform, come from here.

50Vpp sawtooth voltage back On TP1518

This Indicate 110V filter capacitor is almost dead, the saw tooth voltage drop is the Horizon Amplifier's effect.

First suspicious capacitor is C1512. I got a dangerous operation, don't do this:  hand hold a capacitor parallel with C1512, the problem gone.

De-Soldering the giant capacitor is very hard and could broken the PCB trace. I finally got it fix with jumpers.

But here this other options like this one, come from eevblog David Hess. This is genius.

Further more tips is, i can just use sharp scissor cut off the old capacitor's body and soldering new one on it. It is already a dry capacitor!

Much radical approach is just use wire jumper parallel the new one on the old one, it's would work just fine.