Unilab 032.602 oscilloscope

[mazfg1971]

Hi all,

I have dusted off this scope that I no longer need. On inital power up, it all worked fine and displayed a sine wave, with all functions as expected.

After replacing the brown cover and powering back up, it now appears that the Y-Axis is massively offset and with the Y-Shift fully CCW (and brightness turned up) I can just about see the trace at the top of the screen. With all functions seemingly still working....except it's not sitting in the middle of the screen.

I'm wondering if it needs degaussing. I did notice a heavy influence of trace movement when rubbing my hand over the front of the screen...whereby the trace dissappeared upwards and then slowly came back down

Any help appreciated.

[Chris55000]

Hi!

This type of fault might occur if one of the vertical final amplifier transistors, or even the Y deflector plate connections to the CRT itself, are open–circuited!

A large piece of metal electrode like an unconnected CRT deflector–plate will tend to collect stray charges on account of it's being in contact with the remaining plate via the beam of electrons on it's way to the screen, and will also be very liable to being affected by the static charges that collect on the CRT envelope from simply rubbing it!

The way to deal with type of fault is to trace the leads back from the CRT base–socket to the vertical amplifier – to find out which is the vertical amp, place the positive probe of your DVM, on 1000V DC., at the main pcb end of the connector to the CRT base, negative to chassis, and rotate the "Y Shift" (vertical double – ended arrow) control – if you get no change in voltage, either you're on the horizontal (X) amplifier or the Y amp is faulty!

There will always be two fairly large w.w. or carbon resistors connected near to the deflection transistors for the CRT plates, and also the CRT connections, and this is how you locate them!

Unfortunately I have neither a Unilab scope or Manual, but I think the Unilab 031.602 is probably an updated version of the 032.601, which you can get a diagram on this Forum for – download and print the 032.601 diagram off and compare the CRT base on the diagram with the one in your '602 – almost certainly the CRT will be the same type!

Chris Williams

[mazfg1971]

Many thanks Chris, some really helpful information there.
I shall endeavour to go through these points next week and will update this thread with my results.
Thankyou once again!

[hysteresis]

Originally Posted by Chris55000

... I think the Unilab 031.602 is probably an updated version of the 032.601, which you can get a diagram on this Forum ...

The diagram is here:
https://www.vintage-radio.net/forum/...lab#post838503

[Chris55000]

Hi!

Assuming the 031.602 is based on the circuit of the 032.601, you could hardly have anything more simpler!

The CRT Y1 and Y2 plates on pins 6 & 7 of the CRT are connected to tbe collectors of a differential long–tailed pair of transistors Tr2 andTr3 (ZTX342) which are medium power high–voltage transistors of the video–amplifier type.

The Y1 transistor Tr2 receives the signal under examination from the input FET Tr1, connected as a simple source–follower between the +9V and –9V l.t. rails–this provides no voltage amplification, being provided to provide a high impedance to the input socket.

Tr2 provides the only signal amplification in the oscilloscope, with it's collector connected to the +100V h.t. line via a 22k load resistor, the collector directly connected to the Y1 plate of the CRT.

The opposite plate Y2 is connected to the collector of the other half of the differential pair Tr3, which has it's operating–point set by the customer "Y–Shift" (vertical arrow labelled) control, so the trace is in tbe desired position on the screen.

Tr2/Tr3 act as a differential amplifier long–tailed pair by virtue of the common emitter tail coupling resistor R7 returning the emitters of Tr2 & Tr3 to the –9V rail, the effect of this being that the collectors of Tr2 and Tr3 behave like a "see–saw", so if you apply a signal of such a polarity to bias back Tr2 such that it moves the collector of Tr2 by +5V upwards towards h.t., the reduction in current thro' R7 will bias Tr3 on a little harder, so if the preset gain control R11 correctly adjusted, Tr3's collector will drop by –5V, giving an effective change in Y plate–to–plate voltage of 10V, which will move the spot upwards about 1 division.

Fault–finding in this oscilloscope isn't difficult! The first thing to do is to make sure the ±9V supplies are present and correct, low voltages can indicate the reservoir electrolytics C18 and C19 might be low in value or o/c, the approximate voltage across C18/C19 from the rectifiers is approx ±18–20V under normal working conditions.

Very low or zero voltage on either rail may indicate the zener stabilisers D17 or D18 are s/c, or their feed resistors R59/R61 high in value are o/c.

The +100V h.t. supply is another simple zener–stabilised supply, and the comments given above for fault finding the ±9V supplies apply.

The Y amplifier is equally simple to fault–find – make sure the Y–Shift Control wiper hasn't got a dud wiper (please do NOT use WD40 on it!) – a good quality DVM should enable you to check it's middle wiper tag varies between +9V & –9V as you rotate the knob – if either end is missing voltage you have a simple wiring fault between the ±9V lines and the Y shift pot!

If the pot slider shows the correct voltage variation from +9 to –9V, check Tr3's collector load resistor R18, the base feedback resistor R20 (470k) and R22 (100k) from the wiper of the shift control to the base of Tr3 – high value resistors connected to h.t. lines have been wandering up to an open–circuit ever since Marconi first sent waves across the Atlantic.

If the resistors all test OK, one of the transistors may be defective, and it's best to replace both Tr2 & Tr3 as a pair from the same source/batch if possible.

The FET source–follower Tr1 can easily be tested with one of those little Chinese LCR T4 component testers, unless a large signal has been inadvertently fed into the input, this should be OK. If you need to replace it, an ordinary JFET of the 2N3819 type will probably be OK.

Finally ensure the "Astigmatism" pot. connected across the 100V h.t. supply to the third anode/IPS of the tube is OK and the voltage is reaching pin 8 of the CRT – strange effects can occur if this is missing!

If all the above fails to resolve your fault, then unfortunately that only leaves the CRT itself possibly at fault, most likely an internal plate or anode disconnection!

Good luck, the notes above should help get it fixed for you!

Chris Williams

[Chris55000]

PS!

If you need to make any adjustments after repairing your Y amplifier, adjust R3 (Y Offset) to give a correctly centred trace on the mid–way graticule line of the CRT when the Y Shift is set to the centre of it's travel and the input is short–circuited with the "ac/dc" switch in "dc" position, whilst the Y Gain Preset R10 is adjusted by setting the knob of the "Y Gain" control to the '1' volt mark on the panel, then applying an accurate +1V d.c. from a psu to the input, then finally preset R10 is adjusted so that applying +1V to the input moves the trace upwards 1 div.

(Note – the graticule divisions are quite a bit finer than most oscilloscopes, so it does need a bit of care in measuring waveform amplitudes on it!)

Chris Williams

[Chris55000]

Hi!

In case anybody is wondering – I worked out the theory of operation and suggested adjustment procedure simply from printouts of the circuit and a front–panel image of one of these taken off the internet – somebody is obviously hiding a manual somewhere as that's where the circuit came from but I've never seen one myself!

All my training & experience has been in repair – I'm not an expert circuit–designer I'm afraid!

Chris Williams