Test Equipment Anonymous

This came from a rally. It had a sticker on it saying £10. At first I thought it was an Hitachi 20MHz scope. It looked clean, with no missing knobs or signs of being knocked about, and I haven't had a Hitachi scope before. Then I spotted it was 50MHz, so I parted with the tenner without haggling. It turns out they were sold by RS at one time and it has an RS label on the back - not rebadging but one of their preferred standard products.

When I got it home I powered it up and it produced a display with cursors. I fed each channel a square wave. Channel 2 was what was expected, but Channel 1 had what looked like a horrible compensation problem.
I found an operation manual with cct diagrams on the web, but no proper service manual. There is a service manual for the V425 and the circuits are quite similar. I checked the supply line voltages 75V, 8V, 5V and -8V for level and ripple. All pretty much spot on and with about 15mV PTP noise and ripple on the low voltage lines. I was wondering what could cause that sort of overshoot, maybe a dry joint.
The square wave from the calibrator was fed into each channel via a 10X probe and it was checked with a scope at various points through the two channels. Channel 2 produced a square wave as expected. Channel produced a square wave at the BNC connector and at the connector P2, but at the junction of R15 and C16 and thereafter it was distorted. Switching the attenuator stages made no difference to the shape of the waveform.

I wondered what the LF411 was doing. Then it dawned that at DC and low frequency C16 would pass nothing and the LF411 took care of things.

I tried with a DC signal on both channels. Channel 2 behaved normally but Channel 1 showed a third of the expected reading. I measured the resistance from the BNC input to earth on both channels. Channel 2 was 1 MegOhm, Channel 1 was over 3 MegOhms. The culprit had to be R2, a 47R 1206 SMD resistor on the back of the board (it's a single-sided board).

I removed the Y-amp board, and sure enough R2 was blackened. There's a plastic sheet to insulate the bottom of the circuit board from a metal plate forming part of the body of the scope. There was a small brown mark on the plastic corresponding to the position of R2. It looks as if it had overheated.
R2 was replaced and the Y-amp board was checked with a square wave and scope, outside the V-525. There was no need to have the board powered up for that check. Each channel showed a square wave at C16/C116. I cleaned up the timebase rotary switch with IPA and then contact cleaner, because it was fairly easy. It seemed pointless to clean up other switches and controls. It would have involved a lot of dismantling and they were showing no problems.

The scope was reassembled and the problem had gone. It was set up adapting the calibration instructions for the V-425. I thought of re-capping it, or replacing just the main smoothing caps, but decided not to. There were no problems with ripple and I have the impression the scope hasn't had much use.

R2 must have been overheated, but I really can't understand how. Nothing else in that area showed any sign of damage or drifted values. It's hard to see how it could be made to pass enough current to overheat. Possibly with a high voltage applied to the BNC connector, one end could have shorted through the plastic sheet, but the plastic sheet seems sound and there are taller points on the bottom of the board.

Thoughts...

A very nice analogue scope which seems to be a genuine 50MHz. Good build quality and it uses a minimum of special parts. Good quality Alps switches. It has a very sharp display at all points on the screen, which is unusual. Removing the board involved disconnecting plugs but no de-soldering.

The knobs have internal splines which engage with splines on the control shafts. They are a push fit. Some took more force to remove than I was comfortable applying.

I don't much like the cursor display. It's a thing I find only occasionally useful and there's no switch to turn it off. It can be dimmed away using a screwdriver to turn a front panel brightness preset pot. It also disappears when using the strange Ch1 Alt Mag feature.

Ch1 Alt Mag produces the Ch1 display normally and with a 10X expanded time base three divisions below. The two displays cannot be shifted independently.

All in all a nice 50MHz analogue scope and I don't begrudge the tenner.

Here it is pictured in the classic ebay "working" pose. Note that other items of test equipment, cables, leads, probes, manuals, jigs, tools, manuals, blueprints, spares etc are NOT included.

Thats a real nice win, I used to have the same scope but decided to get rid of it when I got my 200mhz fluke, dead handy with the cursors and measurements as well.

Nice catch, Zenith. Thanks for walking us through your debugging. It does sound odd that the resistor succumb to so much current. Maybe just a bad one?

The 47R resistor looked as if it was overloaded and the mark on the plastic supports that idea. It had over a 2 MOhm resistance. But it's in series with two 500K resistors, another small 47R wire ended resistor (looks about 1/8 W) and whatever attenuator section was in series when it died. None of that shows the slightest sign of drifting or damage.

I suppose it could have been damaged by corrosive solder which wasn't washed off, but there's no sign of that anywhere else. It seems a dubious explanation.

With enough voltage put into the BNC input, I suppose the attenuator switch could have flashed over. Maybe someone thought they were using a high voltage probe but it was an ordinary probe on the X1 setting?

It just seems strange.

While I haven't been actively seeking one out an Hitachi scope is on my "bucket" list for restoration. They were a worthy rival to Tek low end scopes.

Full of alien 2SC, 2SD and 2SA stuff. Spit. Yuck