HAMEG HM203-4 Oscilloscope repair - Page 1
Forum section: electronics / repair

author: MAG_1944
author_rank: Newbie
timestamp: March 21, 2021, 10:06:58 pm
content: Greeting everyone, I've been looking for a oscilloscope to help we with my vintage hifi repair work and have been able to get a Hameg HM203-4 complete with all the original manuals.https://www.allaboutcircuits.com/test-measurement/oscilloscopes/hm-203-series-hm-203-4/manual/link for PDF version of the manualBefore powering it up I had a look at the insides to look for any vented caps or obvious physical damage.I found a few things that I'd like some input on.On The XY Board for LV powerC2074 0.1uf is shown on the schematic as 400v rated but the actual part has 250V printed on it, as its connected to the +260V rail should this be replaced with the correct rating or was this a factory modification after the service manual was printed? it looks similar to to the other 0.1uf caps in that section of the boardR2084 a 22k also have a burn mark on it but does measure in spec, thinking I should probably replace this as a precaution, but would like to find why this has begun to over heat.There is a burn mark on the side of C2061, wonder if the bridge rectifier has got a bit toasty at some point, how would a test a discrete module like that?On the T-board for HV power there is a burnt and cracked resistor, R3052, removed it from the board and tested and its showing open. Parts list shows this should be a 1K 0.25w 1% with a TC of 50ppm/K° Any thoughts on why this might have burnt up, I'd like to track down the root cause before just replacing it for that to also burn up.Now for the interesting issue. Someone has done some work on the unit before, on the T-board HV Power they have lifted the anode of D3050 and connected it straight to the leg of R3050. On the underside of the board the traces that originally connected these have been scrapped off. The issue I can see is that D3050 should be connected to the output of C3051 which is now left unconnected.Is it possible that the burnt resistor R3052 which is connected to C3052 (C3051 twin) is linked to this?Can anyone think of a reason why this might have been done before I start making changes?Il need to order some replacement parts in, is there anything else on these units which is prone to failure after nearly 40 years I should think about replacing as I get this up and running. Appreciate any input you can give me

author: MAG_1944
author_rank: Newbie
timestamp: March 21, 2021, 10:09:12 pm
content: picture of the cut traces

author: lowimpedance
author_rank: Super Contributor
timestamp: March 21, 2021, 10:52:33 pm
content: While I have no experience with Hameg gear there are a few things that stand out in the photo apart from the obvious bodges showing a hack has been in there, is the use of sockets for the transistors as sell as the ICs and given the vintage and that those sockets are of budget quality there is a real possibility of connection issues.At the very least a clean and a check of the component lead for oxidation/tarnishing or scum build up, particularly as the PCB itself looks dirty/greasy (and needs a clean too).A full reverse engineering of the bodges to see what was done compared to the manual is in order as well.

author: MAG_1944
author_rank: Newbie
timestamp: March 22, 2021, 07:30:04 pm
content: Il pull most of the socketed items out and give them a clean with some IPA and P600 sandpaper if there is any surface corrosion on the leads.For the Bodge around D3050, with the old traces having been cut i'm thinking the best way to reinstate the correct path is to run a piece of wire from the joint of D3050 and R3050 down to the original connection point for D3050 so that C3051 is reconnected. what gauge of wire for the connection would be suitable as by that point is should be carrying -1900V!Il probably also need to put some heat shrink over all of it to prevent any shorts in the futureI know of audio equipment that controls often need a good dose of dexoit, which controls should be cleaned out and which should be avoided on a oscope?Looking at the service manual I was a bit confused by the reading on pin 4 of the voltage check strip it states "+3.1V (-12V antipole)" I've never come across "antipole" before and a google search didn't reveal anything obvious, is anyone able to shed any light on how I should check this once I don't have things running again.is there any draw back to replacing the carbon film R2084 with a metal film resistor providing it meets all the specs? Only carbon film I found on mouser was a custom order but lots of metal film options

author: MAG_1944
author_rank: Newbie
timestamp: March 22, 2021, 09:09:29 pm
content: would this film cap be a suitable replacement for the orange Philips MKT C2074 0.1uf which should be 400V. trying to find something that can be packaged right next to another MKT has been a bit difficult.Also found another one C2028 which should be 400V but is 250, hummm....https://www.mouser.co.uk/ProductDetail/594-222246728104

author: jdragoset
author_rank: Frequent Contributor
timestamp: March 22, 2021, 09:38:14 pm
content: Looking at the service manual I was a bit confused by the reading on pin 4 of the voltage check strip it states "+3.1V (-12V antipole)" I've never come across "antipole" before and a google search didn't reveal anything obvious, is anyone able to shed any light on how I should check this once I don't have things running again.Measure that pin (positive lead) with the negative lead on the -12 volt pin

author: MAG_1944
author_rank: Newbie
timestamp: March 22, 2021, 10:20:24 pm
content: thats my new things learnt for the dayil get the bits on order from mouser tomorrow and keep my fingers crossed they turn up by the weekend so I can have a crack at getting it powered on and see what else crawls out at that pointappreciate all the help thus far

author: MAG_1944
author_rank: Newbie
timestamp: April 08, 2021, 10:07:32 am
content: So the parts from mouser have arrived and i've got them installed.I've checked all the DC voltages on the check strip and they match the spec's. I've also checked for ripple on each pin using a 400V ceramic cap inline with the meter probe, details are belowPin Spec Measured mvAC1 -12 -11.94 5.42 5 5 5.73 24 23.6 64 3.1 3.08 6.65 260 263 7.26 140 140 67 3.75 3.75 6.38 NA9 190010 1900I've not got the equipment to check the HV 1.9Kv into the CRT so have left it alone for now. I was able to check the PCB on the CRT's socket and the pin's listed as needing approx. +85V are getting +87.6VThe scope is now displaying a trace but I've noticed a few things that are a bit off. Using the 0.2v calibration signal the trace is displaying but on 50mv/div its giving a height of 3.5cm rather than the 4cm listed in the manual, says +/-1mm is acceptable. Not sure if the calibration signal is off or the scope???I'd did put my DMM with ground on the chassis and the positive pin onto the calibration hook, showed 100.3mV, its listed as 0.2v, should I adjust this with the pot?When I switch to 20µs or below on the time base the trace noticeably dims, on all the other settings the trace is bright and the intensity dial can be adjusted correctly.At the same time the trace lengthens and there is a bright spot showing on the LHS of the display, the trace is about 10cm long at this point. According the the manuals in the 50µs positions the trace should be adjusted with a pot to reach a 10cm length, currently its about 6cm long and adjusting the pot doesn't increase its length but moves it left to right.. I'm not sure if this indicates an issue with teh sweep rate or possibly with the unblanking pulse for the CRT.Any input would be greatly appreciatedThanks

author: Smith
author_rank: Frequent Contributor
timestamp: April 08, 2021, 04:15:10 pm
content: Quote from: MAG_1944 on April 08, 2021, 10:07:32 amCheck your multi meters impedance. Normally it is 10Meg (at least at the higher ranges). Set it to the highest range (which should be 10 Meg impedance) and use an HV 90 / 100Meg divider resistor in series to measure the 1,9kV. Works great. Do remember that most resistors from 100Meg and up have quite bad tolerances. Do be careful though. If you don't feel comfortable and the screen works great just let it be.Quote from: MAG_1944 on April 08, 2021, 10:07:32 amYou can use a RMS multimeter for lower frequencies to check the output voltage (use the calculator, depending on your signal shape). Most scopes I repaired did need calibration on at least a few ranges.Quote from: MAG_1944 on April 08, 2021, 10:07:32 amThis is normal for analog scopes. Some may have some circuitry to compensate for this, but I've never seen one.Quote from: MAG_1944 on April 08, 2021, 10:07:32 amCheck the pot for the in and outputs. Probably some voltage will be off. Unfortunately the easiest way to fix a scope is to have another scope (guess this is why I have 4 of them).

author: BreakingOhmsLaw
author_rank: Frequent Contributor
timestamp: April 08, 2021, 04:30:34 pm
content: Quote from: MAG_1944 on March 21, 2021, 10:06:58 pmThe input stage on Hameg scopes have a matched N-Channel JFET Pair from Vishay called U440.That tends to be busted a lot if the scope has seen abuse. These are getting harder and harder to get, so if you are serious about keeping this baby running, get one.But seriously, get an entry level digital scope. For under €200 you will get a scopewith probes includedthat will blow this thing out of the water.Schematic:https://www.sky-messtechnik.de/images/PDFs/HM203-4.pdf

author: shakalnokturn
author_rank: Super Contributor
timestamp: April 10, 2021, 12:15:24 pm
content: For checking the amplitude is correct a DMM and battery should be enough.Is the trace length about 8.5cm when switching to Component Tester? This should point you to either the timebase or the X final amplifier.