Hameg 203-5 analogue scope - minor faults - Page 1
Forum section: electronics / repair

author: Doggone
author_rank: Contributor
timestamp: September 29, 2025, 10:52:11 am
content: Just purchased this scope and initial tests show it to be generally in good shape and usable, however on all traces (A and B Channels) there is a short vertical line to the left-hand side of the screen which is joined on to the trace, this obviously should not be there and I suspect this is a fly back blanking issue - does anyone have any pointers to help me zoom in on this fault? also the B Channel trace is a bit jittery - so this also could be a possible component failure? The scope is roughly 1983 manufactured so forty years old plus - usual culprits will be checked - electrolytic caps etcI can post a photo of the trace showing this short vertical line if needs be?Regards Nick

author: andrewtaylor
author_rank: Contributor
timestamp: September 29, 2025, 11:36:05 am
content: Take a close look on the schmeatic, opto coupler is usually at its *endoflive* in these Hameg.A slight readjustment is also recommended due to age of scope.

author: Doggone
author_rank: Contributor
timestamp: September 29, 2025, 05:10:41 pm
content: Many Thanks Andrew, will do this and let you know how I get on,Regards Nick

author: David Hess
author_rank: Super Contributor
timestamp: September 30, 2025, 12:32:42 am
content: Quote from: Doggone on September 29, 2025, 10:52:11 amThat is likely a blanking issue. I would do the z-axis calibration which should include adjusting the grid bias.

author: GGMM
author_rank: Frequent Contributor
timestamp: September 30, 2025, 08:09:53 am
content: Hello,See herehttps://www.eevblog.com/forum/repair/hameg-312-8-spot-problem/msg4717508/#msg4717508cdtI had a similar problem with a Hameg 312 some time ago.

author: MathWizard
author_rank: Super Contributor
timestamp: September 30, 2025, 01:05:50 pm
content: Quote from: andrewtaylor on September 29, 2025, 11:36:05 amWhat's the typical working life of opto's ? Were these ones just not made to last all that long? First time I've heard anyone mention opto's wearing out or EOL, but yeah I'm sure it can happen.

author: Zenith
author_rank: Frequent Contributor
timestamp: September 30, 2025, 03:42:48 pm
content: There's a procedure in the manual for setting up the unblanking pulse, showing the waveform you ought to aim for. There's fairly wide range of positions for the adjusting pot to allow for temperature variations. I've found it can be set by eye, although I've also used another scope. You may need to download several versions of the manual, including different language versions. Some are missing sections. Go through the whole of the set up procedure. I recall it isn't lengthy.It's possible it can't be adjusted because of a fault somewhere else.Make sure the power supply voltages are the proper values and with low ripple before doing anything else. The electrolytic capacitors in the power supplies on those scopes are often faulty.

author: Doggone
author_rank: Contributor
timestamp: October 06, 2025, 01:38:04 pm
content: Re screen fault on this Hameg scope - I have tried adjusting VR51 on the trigger cct board but this reduces it only slightly - see photo, have also upgraded the 68pF 2kv caps C933,934 & 935 with 3kv caps, this has made no difference, so wondering if it is the opto-coupler or maybe one or more components on the trigger board closely associated with the unblanking pulse? The trigger works fine in adjusting the right hand tail of the sine wave but it's this left hand edge edge of the trace which is the problem. Incidentally if I feed a square -wave signal from My Rigol 1khz test point I get a bright dot on the left hand side of the screen next to a good square -wave trace - this is obviously not good for the screen so did not leave it there for too long! any pointers greatly appreciated.Nick

author: Doggone
author_rank: Contributor
timestamp: October 06, 2025, 04:23:11 pm
content: Footnote to today's post - also have a significant vertical jitter on channel 2 trace so possibly final amp problems?

author: David Hess
author_rank: Super Contributor
timestamp: October 07, 2025, 12:38:59 am
content: Quote from: Doggone on October 06, 2025, 04:23:11 pmIf it is only channel 2, then it is a channel 2 problem, usually a slightly open connection or dirty potentiometer. A problem in the CRT vertical amplifier would affect both channels.

author: TaylorD93
author_rank: Regular Contributor
timestamp: October 08, 2025, 08:56:40 pm
content: IIRC there is a BF440 and a second TO-92 package transistor, both fitted to a 6 pin DIP socket near the HV caps, opto etc in the un-blanking circuit. i have had issues here where the pins had tarnished and did not make good connection to the IC socket.The BF440 and the 2nd device are a common failure in my experience with the 20x series of scopes.hope that helps!

author: Greybeard
author_rank: Frequent Contributor
timestamp: October 13, 2025, 10:48:59 am
content: Try replacing the socketed BF199 near the optocoupler.

author: Doggone
author_rank: Contributor
timestamp: October 14, 2025, 08:50:26 pm
content: Thanks Greybeard,I will try that, have done some more troubleshooting today on the Hameg 203-5 and I think I may be getting close to the fault - just to re-cap, this fault is a small vertical line at the start of the trace, which I think is a timing issue, I have picked up an unusual sawtooth trace from pin 5 on the flip flop (74lS74) unblanking pulse output, there is a small horizontal pause just before the trace rises, I think it's this pause that is allowing the short vertical line to appear on the screen - I have attached a photo of trace as it appeared on my Rigol scope - any help in pinpointing the fault would be welcome. Nick

author: Greybeard
author_rank: Frequent Contributor
timestamp: October 15, 2025, 08:36:02 am
content: Quote from: Doggone on October 14, 2025, 08:50:26 pmThat's correct.Can you display the (un)blanking signal on R941 and the sawtooth signal simultaneously (chopped mode)?If no unblanking pulse is visible, show signal on R943.Be very careful, there are points in this circuit that carry up to 2000V.These can be fatal to you or your measuring instruments!

author: David Hess
author_rank: Super Contributor
timestamp: October 15, 2025, 02:00:43 pm
content: The aberration looks like the retrace to me, where the intensity is not fully blanked, making it a blanking level problem where the voltage at K-2 is not high enough during the retrace. At low frequencies this is controlled by VR95 through the optocoupler.Maybe R946 has drifted high.T932 is suppose to pull the cathode high at the start of the blanking pulse and is apparently working. It is driven by AC through C934, but it looks to me like biasing relies on leakage through D934. I question the reliability of the DC bias, but it is apparently working.

author: Doggone
author_rank: Contributor
timestamp: October 16, 2025, 11:14:29 am
content: Ok - progress so far - have checked R946 and R948 in-cct - R946 is 223k which is very slightly high, R948 is 51ohm. I have also replaced both The BF199 and the BF440 - made no difference to this fault. not sure where to go from here, have attached photo of trace clearly showing the fault. As this is on both channels it must be something in the final X-amp cct.

author: Zenith
author_rank: Frequent Contributor
timestamp: October 16, 2025, 05:31:00 pm
content: I'd use a scope to determine whether the sweep was what it should be, whether the unblanking pulse looks right and so on. It doesn't have to be a very good scope, and sometimes you can use the problem scope itself to diagnose its own problems.

author: Greybeard
author_rank: Frequent Contributor
timestamp: October 16, 2025, 05:53:10 pm
content: Check the blanking pulse relative to the x-sawtooth.

author: Doggone
author_rank: Contributor
timestamp: October 16, 2025, 07:15:06 pm
content: Greybeard - thanks - we are just about at this stage now - have done some scope traces to compare the unblocking pulse output trace with the X-Out signal but have more to do, had a few problems with my Rigol digi scope - the probes supplied with these are rubbish so have ordered some more probe leads today, will continue with tests tomorrow.There seems to be a very low voltage coming out of Pin 5 of the 74LS74 flip flop - 1.2v? as the supply to this chip is 5v so surely the unblanking output should either be 0v or 5v ish, maybe we have found the fault? a sick 74LS74? what do you think? have checked supply to the IC430 op-amp (11.62v) and the supply to the 74LS74 (4.6v)

author: David Hess
author_rank: Super Contributor
timestamp: October 16, 2025, 08:05:05 pm
content: Quote from: Doggone on October 16, 2025, 07:15:06 pm1.2 volts is unlikely with a good 74LS74. Usually the output will be below 0.6 volts or at least 2.4 volts, and only higher if a pull-up is present. The 74LS74 is inexpensive to replace, and there are plenty of comparable and higher performance alternatives.The problem now does look like the blanking is partially or completely missing.

author: Greybeard
author_rank: Frequent Contributor
timestamp: October 16, 2025, 08:46:48 pm
content: Quote from: Doggone on October 16, 2025, 07:15:06 pmDid you really mean - (minus) 1.2V?https://www.nutsvolts.com/uploads/wygwam/NV_0806_Marston_Figure15.jpghttps://www.nutsvolts.com/uploads/wygwam/NV_0806_Marston_Figure16.jpg

author: Doggone
author_rank: Contributor
timestamp: October 18, 2025 09:10:15 am
content: Good Morning Greybeard, thank you for the 74LS74 info - will re-check voltages today - another clue discovered - when the brightness is turned fully up the flyback trace is just visible.

author: Doggone
author_rank: Contributor
timestamp: October 18, 2025 09:40:49 am
content: Traces taken this morning as follows:Chan1: Unblanking pulse as taken from R500Chan2: X-Out as taken from R510Also did check on T930: 2.2vdc on Base, 2.93vdc on Emitter, 0vdc on Collector2.2vdc on pin 5 of the 74LS74

author: Greybeard
author_rank: Frequent Contributor
timestamp: October 18, 2025 11:52:59 am
content: The signal shape and phase actually look OK.What does the yellow 184 mV in the top right of the screen mean? Trigger level of the Rigol?I read 2.1 divs x 200 mV/div = 420 mVpp (CH I amplitude). That probably isn't enough.Can you replace the 74LS74 with a new one?

author: Doggone
author_rank: Contributor
timestamp: October 18, 2025 01:17:16 pm
content: Yes - the trigger voltage from pin 5 of the 74LS74 is only just above the 2v threshold - and this may be causing the flip flop to hesitate before changing state? but it does seem to be triggering, albeit late, got a new 74LS74 coming on Sunday, so will see if this makes any difference - re Rigol scope traces, which trace are you referring to when you say the 420mV is not enough? is an indication of a faulty 74LS74? or maybe a component further back in the chain?

author: Greybeard
author_rank: Frequent Contributor
timestamp: October 18, 2025 01:59:43 pm
content: Quote from: Doggone onYesterdayat 01:17:16 pmNo, the timing and shape of both traces is OK, but the amplitude at pin 5 (Q-output) of the 74LS74 should be significantly more than 420mVpp.The cause could be defective 74LS74 or overload at Pin 5.VOH-VOL=2.7V-0.5V=2.2Vpp (LS-TTL)

author: Zenith
author_rank: Frequent Contributor
timestamp: October 18, 2025 05:51:20 pm
content: It could be that the scope isn't set up for the sensitivity of the probe, say the probe is x1 and the scope x10. Just a thought.The scope waveforms look right, but it would be interesting to know what the scope does when this display is being produced. Is it displaying the fault? What happens when the timebase speed is increased?

author: Doggone
author_rank: Contributor
timestamp: October 18, 2025 07:04:46 pm
content: Thanks for post Zenith - its definitely a component fault and not something that can be adjusted out - we are getting on for a week now on this fault and gradually zooming in on the faulty component - it's certainly been a difficult one and we are not there yet.