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Hitachi VC-7104, channels not working

kiril_nedev

New Member
Hello
I apologize for the dummy question, I realize it's pretty hard to troubleshoot something on the other side of the world, just wanted to give it a try here.
I recently got this oscilloscope - Hitachi VC-7104 (4-channel 150Mhz)
one of the channels is working fine (even though it's a little bit shifted from the central position, and there's some noise (visual inspections shows some of the capacitors looking bad))
The other three channels though have the following problem:
The beam on the first two is outside of the view (always shows at the top of the screen). No matter how much I scale, it's always outside.
The third channel is the same but shows at the bottom of the screen (even though for this one If I shift it up by some voltage I can see the line on the screen)
Visually most of the capacitors on channel 1 looked pretty bad so I replaced them, the issue of course persists.
It looks like the signal is short circuited somewhere to +V or something..
Can anyone think of a typical problem which may be causing this or..?
 

worldradio

New Member
Hey, here is tom.
I serviced 2 7104 in the past and the story is still ongoing.
The first thing you have to do is to renew ALL coils and capacitors at the digitizer PCB and at the Y amplifier, check at ebay with SMD types. Most of the fails will disapear and you might come to a valid calibration. If only ONE module is defective you will just hear clicking relais at cal procedure.
NEVER turn any potentiometer andturntable capacitors because they are very critical to readjust. Tell me your results after theat basic service i told you. nice regards, tom
 

kiril_nedev

New Member
Thank you Tom!
I had replaced all Cap's on one of the PCB's .. not sure if Digitizer PCB is the one where the actual input connector sits but that's what I replaced. I bought some Panasonic caps.
When I turne it on indeed I hear a lot of clicking relays... after 5-10 minutes it fails most of the calibration procedures and shows the normal screen .
 

worldradio

New Member
ok, this sounds good. Why. Built OUT all digitizer and Y ch 1-4, Choose only place ch4, ONLY put the digitizer and the Y ch in there and calibrate. do first the default procedure in cal menue. Then, in the cal menue see if you have high values, not more than 32000, if so hou have to calibrate DC and HF gain. Basically wait for warming up the Y ch to see if you reach the zero line, switching from GND to AC to DC in the 2mV range.(the Y ch is getting hot, dont worry, the relais are providing heat as most of them are activated) Hit the zero line key before. If there is no signifant moving when switching you can start a recal hitting the three buttons getting the cal menue. the cal procedure should not need more than 3 minutes for 4 ch. if he sees great values it will loop and loop again. also, on the digitizer change the 2 coils and all the caps !!! WHY ? These leaky capacitors hits down the supply voltage and are destryoing the DC filter coils, even when they loooks good. (measure the coil value in ohm, not more than 1 OHM is ok) good speed. tom
 
Last edited:

unclejed613

Well-Known Member
Most Helpful Member
i think what tom was meaning was to resolder everything in the bad channels and see if that fixes the problem. if it doesn't, you can use the good channel as a reference for measuring voltages in the bad channels, because the circuits for all 4 are identical. when you begin replacing parts, a few will be in locations where it doesn't affect the calibration of the scope, but there are many components that will require recalibration, mainly any components in the signal path will affect the calibration. components outside the signal path (such as in the power supply) won't have much of an effect. you can do a rough calibration by using the known good channel as a baseline, and adjusting the repaired channels to match, but if you really need to know that what you are measuring with the scope is accurate, you should get the scope calibrated. maintaining traceable calibration standards is a bit too expensive and time consuming to be in the realm of the hobbyist (i have seen some people that collect old cal lab equipment, and some of them make the effort to maintain it, but as a hobby, that's a very small one, and usually the hobbyist also works in metrology as a career)
 

kiril_nedev

New Member
Thanks for the help guys. I am currently out of my town and will check it in about a week when I get back. I will keep you posted
 

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