Hitachi V-509 Cathode Ray Oscilloscope

Thread Starter

Roj.

Joined Oct 4, 2023
5
Hello VCFers



I'm a retired serviceman from the scientific instrument field.

I have a seldom-used portable Hitachi V-509 CRO which I find, after a long period of dormancy, has a Ch2 vertical problem.

This CRO was seldom needed but always carried into the field so has done a lot of miles in my service vehicle and become dusty and cosmetically, a little wanting.

I can't recall how long ago I used 2 channels but it would have been years ago (I have a Kikusui 60MHz bench CRO so that the infrequent needs was supplied by it).

The attached waveforms are generated by the 0.5V square wave, inbuilt cal signal with a serviceable X1 probe connected to it and each channel.

Ch1 (upper trace) is barely OK but Ch2 (lower trace) is unusable.

Additionally, operating the AC/Gnd/DC switch of the faulty channel (Ch2) into the DC position, the trace 'sits' upon ~+8vDC rather than on 0vDC.

Ch1 behaves as expected.

Save a couple of DVMs and the CROs, I have no test equipment, virtually none of that called upon by the Hitachi V-509 Service Manual.

I'm looking for any guidance before I take off the covers.

Thanks in anticipation.

Roj.
 

Attachments

Ya’akov

Joined Jan 27, 2019
9,169
Welcome to AAC. (not to be confused with the Vintage Computer Federation forum)

With only the briefest of glances at the attachments, the first thing I would do is check the electrolytic caps in the channel two amplifier circuit. The +8V offset seems no coincidence since the supply for that circuit is a doubled ended 8V.

Electrolytic caps tend to dry out over time and stop being caps. It may well be as simple as replacing the few electrolytics involved. Visual inspection may show you something—look for swelling, leaking, and discoloration. But, even in the absence of these things, drying out is the silent killer of caps and very common in older gear.

Someone may happen along willing to do a more thorough analysis of the circuit in light of your problem, but I wanted to give you something possibly fruitful to work on. New caps will not hurt, and might improve the life of the scope overall.

Good luck, again—welcome. It‘s nice to have you join us.
 

MrChips

Joined Oct 2, 2009
30,821
Use your Kikusui oscilloscope and compare voltages at same nodes on CH1 and CH2 vertical amplifiers. Start off by confirming all DC supply voltages at the amplifiers.
 

Thread Starter

Roj.

Joined Oct 4, 2023
5
Thanks Both (AACers)

(Yes, guilty of asking widely for advice and, to whom I am addressing, not sufficiently vigilant )

I have made a fairly close inspection of the electros and see no leakage or swelling. Access to fit replacement caps will involve a fair bit of work so will first 'look' with the Kikusui CRO and my ESR device (forgot to mention that item and my blue Atlas DCA55 semicon tester).

Will advise on my findings but retirement can get hectic, so don't wait up.

Roj.
 

MrChips

Joined Oct 2, 2009
30,821
In order to measure capacitor ESR, you have to remove the capacitor from the circuit. I suggest you probe the circuit first with your working oscilloscope. We can guide you along with where to look.
 

MrChips

Joined Oct 2, 2009
30,821
The first place to start is check that +8V and -8V power rails are ok on both channels.

Next, with square wave input applied, check and compare waveforms at R44 and R144.

1696491349880.png
 

Thread Starter

Roj.

Joined Oct 4, 2023
5
Thanks MrChips

I agree that the CRO alternative will probably indicate the problem area. I'll give that a Red Hot Go and advise what I find

Now to clear enough bench space for two CROs.....

(My ESR meter is of the Bob Parker design that uses 100kHz signals, small enough to avoid turning on semicons but doesn't avoid misinterpretations when there is more than one cap paralleling measuring points.)

Regards
Roj.
 

Thread Starter

Roj.

Joined Oct 4, 2023
5
Mixed "results" (fault is intermittent and can sometimes be induced by freezing of input components):

1. +/_ 8Vdc supplies found present
2. Found a significant Vdc difference at the gates of the input FETs:
"good" CH1, TR1, Vg = 650mVdc + noisy small square wave signal
"bad" CH2, TR101, Vg = 200mVdc + same noisy small square wave signal.
(Both Drains are 8Vdc, both Sources (1.3Vdc + small square wave))
3. using spray freeze on CH2 input stage was able to "fix" problem.
4. Voltages in 2. above, remain the same when no fault is present
5. Compared freezing effects on good CH1. This appeared more sensitive to freezing of a couple of spots. The same fault was NOT induced in this channel.
6. Fault returned during freeze application but unable to locate faulty device with certainty.
7. As might be expected, the components at the inputs of the FET stage and the Op Amp seemed to produce the most DC reaction to freezing. Condensate on the pwb took time to dry and was probably contributing to the DC drift.
8. There wasn't any consistency in what reintroduced the fault.
9. removed Vert Preamp pwb, inspected and resoldered all connections in the input area. Removed flux with IPA. Reconnected the pwb and the fault is NOT present. Will leave it to stew a while and give it the freeze treatment again.
 

Thread Starter

Roj.

Joined Oct 4, 2023
5
I also reflowed the accessible connections on the little pwb that supports the AC/DC switch. I wasn't game to reflow the wired connection to the rear of the BNC as I predicted too much work if the wire 'jumped' away from the connector. There's too little space there!

Further thermal shock i.e. freeze at input cap C123 and resistor R120; causes a momentary -ve voltage shift after which the waveform assumes its previous, correct position. Same for DC and AC input selection.
Since the reflowing exercises, the freezer treatment causes both channels respond the same.
No heating or this freezing has caused the fault to reappear.
The CRO was on for several hours late yesterday and has been on all today and there has been no reversion to the CH2 fault condition.
Condition 2. above remains and is the only odd condition that I've found. This may have been pre-existing but it seems too odd not to be connected with the fault.
However, as long as CH2 remains normal, I'm not going to attempt sourcing parts or do any more (famous last words?).

Thanks for your interest and suggestions.

Roj.

The pics:

No fault at present (CRO is still upside down to expose Vert preamp pwb).

Removing the case/chassis part which supports the front rubber feet improves access (2 screws) but not for the BNC connector
 

Attachments

MrChips

Joined Oct 2, 2009
30,821
So the problem was intermittent and has resolved itself. This is one of the most difficult type of problem to troubleshoot.

In any case, glad to know that you have it working now.
 
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