Hitachi V1150 (1986) scope with no vertical deflection ch1-2

[Dmaloneys]

Picked up this old scope at a thrift store for $10. Couldn’t resist. Powers up, but does not give a trace on either ch1 or 2. Ch 3 and 4 work seem to be normal, but have limited controls c/w ch 1-2.  Have full range on the screen and show a trace that has vertical adjustments.  Displays signals from both channels.

On ch 1/2 pressing “trace finder” shows the trace at the very bottom of the display. “Inverting” ch2, and pressing “trace finder” shows that the trace is now above the very top of the display. Vertical adjustment has no effect. The selection voltage control seems to work. Selecting all 4 traces only shows ch3 and 4. “Trace finder” shows the missing ch below the screen. Inputting  a signal does not change anything- “trace finder” shows the same flat trace.

Have the service man and checked out all of the power supply voltages which are all within range. I do not see any obvious damage or leaking caps.  Not sure where to start looking (and I am sure I am in over my head...)

Should I put a signal in and try to trace it?

Thanks if anyone has any thoughts.

Stay safe.

D

[cnqhdszq]

Are you Proficient in using oscilloscope?
Please check the sweep  mode  ,it should be turned to  "auto" 
Do you have service manual ?

here is the link :
https://elektrotanya.com/hitachi_v-1150_sm.pdf/download.html

[shakalnokturn]

If yours is built the same as the Hitachi I have the shielding around the CRT has two oval holes for the connections to the deflection plates. Check hat the CRT hasn't slipped backwards a little allowing one of the deflection pins to touch the shielding.

[Wallace Gasiewicz]

I have not looked at the manual.
However here is my 2 cents:
The usual suspect in old scopes is the first input transistor/FET. Usually someone puts too much power on the input and blows the first amplifying transistor. These are usually dual FETs or Dual transistors. Six leads. Some really old scopes have two matched separate transistors.
Happens all the time. I knew a guy who used to buy broken scopes and repair them. he told me he made a good profit because of this particular problem which was responsible for over half of the scope malfunctions. The other common thing is the input attenuator, can be blown by a similar power overload.
Seems like most of the scope is intact if two of the inputs work.
Nice buy for a few bucks.

[Wallace Gasiewicz]

OK I was bored. Looked at the manual. It appears that the Channel ONE and TWO input preamplifiers are different from the other two channels. It is a simple layout and uses only ONE FET and TWO transistors and an OP Amp. The OP Amp is for DC bias. Look at page  43-44 of Service Manual.
My money is on the FET TR12. Remove one and check it. However, rebuilding the entire preamp looks pretty easy.
Alternately, you can use the calibrator signal in the scope and use another scope to check the signal at the attenuator and at and after the preamp.
There is a mistake in the block outline however. Channel one preamp is identified in both the channel one and channel two path. I think it should read Ch2 in Channel 2 path

It is not unusual for someone to blow both channels.

[Dmaloneys]

Thanks all for the suggestions.

I do not have another scope at this time that I can use, but *Could I use one of the working channels (ch 3 or 4) to analyze a signal coming in through ch1 or ch2?- ie use the scope to analyze itself???

As was pointed out.  Ch 1 and 2 are on a separate board (#734).  I have checked that the supply voltages are correct and arriving at the board. As noted earlier I do not see any physical damage to this pre-amp board, but have not pulled it out of the machine completely.

Thanks Wallace for looking at the schematic.  I have been reading those pages (43-44) in the the manual. Will focus on locating FET TR12 on the board.

I was surprised that ch1 and ch2 would both be "blown" and was thinking that the problem would be down stream. With your helpful comments I will focus on the early signal getting through the pre-amp board.

Thanks.

d

[Wallace Gasiewicz]

Go ahead and try to use the good channels. Should work.
Nice idea.