HAMEG 208 Oscilloscope - Page 1
Forum section: electronics / repair

author: Urbanus
author_rank: Newbie
timestamp: January 29, 2025, 01:15:37 pm
content: Hi,I'm new to the forum so firstly, hello to all.Now to my problem.... I have had the above scope for well over 20 years and it has behaved impeccably until very recently when I dug it out to help diagnose a non working 1980s Arcade Space Invaders I was given in exchange for some work I did on a friends camper van.When I came to use it I found that, after a period of time the trace disappeared. This initially happened when I had glanced away from the scope so I thought the lead had become disconnected but after some investigation it was obvious that a component is starting to approach the end of its life.To try and get more information to help diagnose it, I firstly removed the case. This caused the fault timeframe to increase from around 3 minutes to around 9 minutes so this makes me think that the extra cooling provided when the case is removed means it takes longer for the component to reach critical levels.I then turned on the second trace to see if was common across the lines and yes, both traces disappear at the same time. With both traces on I could see that what appears to happen is the trigger led briefly flashes followed by both traces slowly vertically rising whilst dimming before going completely. If, during this period of time (2 or 3 seconds) I adjust the intensity, I can bring the trace back into view before it finally dies. I also think the trace oscillates at this point but cannot be 100% sure.I do still have the original manual for the scope and, with what I have noticed as detailed above,I started to wonder if the problem was somewhere in the HV line to the CRT etc. I don't have the necessary equipment ( and I have reached an age where I would eventually like to be alive to get whats left of my state pension), nor the skills, to mess around near the HV side of a CRT but a quick check with an temp probe shows me that a capacitor on the bridge rectifier (C949/C950 if anybody has the circuit) is much hotter than the other capacitor. They both start off around 18C but over a period of approx 9 minutes (case off) it rises to 48C before the trace dies. The other capacitor adjacent to it stays at 18C. They are both 0.1uF/1kvI should mention this is the CRT-Circuit, Unblanking, HV Supply board.Before I set to with the soldering iron, do any of the more enlightened souls on here want to point me in a different direction or point out some obvious error I have made?All and any advise gratefully accepted.CheersUrbanus

author: Urbanus
author_rank: Newbie
timestamp: January 29, 2025, 01:49:41 pm
content: Quick update,Just rigged up a quick down and dirty fan to blow ambient air across the capacitor and both traces were stable for 15 minutes and counting. Maximum temperature of the capacitor was 30C. I turned off the fan and within a couple of minutes the temperature had hit 40C and the traces disappeared.That does seem very low for a capacitor breakdown though???

author: m k
author_rank: Super Contributor
timestamp: February 02, 2025, 06:51:40 am
content: Cooling a faulty component can be very sensitive, blowing air is not very much so.But temperature difference is not very vague thing, and who known how a faulty component decides to operate.If they are equal you can swap them.A bit nasty voltages for regular meter, but you can always make a divider.

author: TaylorD93
author_rank: Regular Contributor
timestamp: February 28, 2025, 09:50:00 pm
content: Hello Urbaus,The Hameg 208 and 605 share a rear HV PCB so you should be able to find the schematics for either to help you.i would suggest that you start by checking the power supplies on the main board first, if i recall correctly the 12V rail is then used on the HV Board, there is a small transformer with a 741 based oscillator which drives a step-up transformer, generating the high voltage, one tap of the transformer goes into the HV Cascade to the CRT Anode, the other side of the transformer goes to the Cathode voltage around -2kVDC.The -2kVDC rails is attenuated down to the 741 opamp input.In normal operation, volts should measure the same voltage on both inverting an non inverting inputs of the 741. so providing your low voltage rails are correct, then i would check the feedback from the HV circuit. there are also a number of resistors cascaded to attenuate the voltage down from -2kVDC, i have seen these fail in either Open or Short circuit.Hope that helps.

author: jdragoset
author_rank: Frequent Contributor
timestamp: February 28, 2025, 10:33:43 pm
content: Get two 0.1 uf x 1kv caps and change the one that is over-heating.Canned air upside down makes a good spot cooler, liquid (-20 C) is released so a very tiny release with the trigger is best.