Hameg HM806-1 cal board issues - Page 1
Forum section: electronics / repair

author: agent_power
author_rank: Regular Contributor
timestamp: June 09, 2022, 11:39:57 am
content: Hi communityI recently started tinkering with old electronics to learn a bit about it. I've got a Hameg HM806-1 80MHz 2Ch scope off eBay, which I managed to get working, but I'm stuck with two things and decided to seek advice here:1. the cal boardIt's a HM205 CAL-02 PCB which is supposed to output 2V / .2V 1KHz / 1 MHz square wave. While I can get the amplitude right with adjusting the trim pot, the frequency is always wrong (too low, never 1 KHz or 1 MHz).I tried replacing the 10uF capacitor which had a bit of a high ESR (I think?), cleaned contacts and sockets and tried swapping the ICs, but nothing helped.What else could it be? I measured the resistors but I'm not sure about the results. They all seem ok except the low value ones, they're higher than what's on the schematic (R904 and R913 measured 57 Ohm instead of 51 Ohm, and R914 7,5 Ohm instead of 6,8 Ohm).The VCC voltages are 2.65V for the 74HC132 and 2.85V for the 74HC00, which seems kind of low.Attached is the schematic for reference.2. calibration procedureI can't find the calibration procedure for this scope. The only service document is herehttps://www.sky-messtechnik.de/reparaturdienst-service/downloads(this company is now in charge of Hameg products, but they could only point me to this downloads page), but there is are no procedures, only schematics and a description of the pots. Without the procedure, I have no idea how to do it properly.Thanks for your help

author: jonpaul
author_rank: Super Contributor
timestamp: June 09, 2022, 12:45:04 pm
content: Bonjour cher Monsieurthese vintage Hameg analog scopes are wonderful 1970s..1980sWe have HM 103, and HM203 found for a low cost at a neighborhood sale.Scope vertical accuracy perhaps 5..10%horizontal accuracy better but 2..4% is good.the calibrator is designed for adjustment of a 10x scope probe...attach probe tip to CAL, use 1 kHz, adjust probe compensation for best squarewave,nether 1 MHz nor 1 kHz CAL is a precision time or voltage reference!If your scope has good traces, suggesting that you do not "fix" nor "calibrate"!The internal trim pot and caps need adjust only after repair!EnjoyJon

author: agent_power
author_rank: Regular Contributor
timestamp: June 09, 2022, 01:24:20 pm
content: Hello Jean-PaulThanks for your advice. Unfortunately, the traces were off by about 20%, so it's kind of useless without getting it to at least the percentages you're suggesting.Have a look at the square wave it produces (yellow trace, compared to RIGOLs cal signal in blue). Does it look the same on your Hameg? I don't expect to be precise, but at least somewhat correct.I have never seen a scope with such a "broken" calibration output, but if you say that this is acceptable then I'll focus on the rest of the scope.

author: jonpaul
author_rank: Super Contributor
timestamp: June 09, 2022, 06:00:13 pm
content: RebonjourI do not comprehend the exact problem.both frequency and duty of CAL are normal.Do you suspect that the Hameg CAL is somehow off or the CAL is OK and Hameg Scopes CH1 or CH2 are not in cal?your Rigol yellow trace confirm 2.0 V P-P correct Amplitude of CAL.SVP Send scope photos of the Hameg connected to the CAL.beware all scopes CAL out are in peak to peak V not RMS, etc.bon chancejon

author: agent_power
author_rank: Regular Contributor
timestamp: June 09, 2022, 10:37:09 pm
content: Bon jour (or bon soir)Quote from: jonpaul on June 09, 2022, 06:00:13 pmIn the beginning, I thought both were off. I now adjusted the main +12V, X and Y amplification so it looks correct. I'm restraining myself to touch any of the other pots without proper instructions.Now, the Rigol calibration signal looks what I would expect it to look like, filling the divisions according to the scope's settings. The HM806 calibration signal just looks so random. But if yours looks like this I'll accept it. I just thought that 1KHz means 1KHz and not something else like 930~Hz.Here are some pictures, sorry about the upside down scope:

author: shakalnokturn
author_rank: Super Contributor
timestamp: June 09, 2022, 11:12:03 pm
content: The "calibration" output is only really any use for probe compensation, that's what it's for.Of course I can understand one wanting it to have good enough frequency and amplitude to "sanity check" the scope.The only thing I'd worry about on your waveforms is the slight overshoot, just make sure that's not coming from the "calibration" output itself.Other than that, if your DSO and Hameg are agreeing in time and amplitude on all settings you should be good to go.

author: jonpaul
author_rank: Super Contributor
timestamp: June 10, 2022, 06:46:26 am
content: agent_power ciao di nuovo signor!A few points to help to see if any supposed problem is in the CAL out voltage or the scope Vertical CAL.Do both channels have the same voltage error or do they differ? If same reading, less likely the scope Vert Cal is bad.\Today, I have setup both HM103 and HM203-5 side by side, using a BNC cable (not a probe) on each scope calibrator on each channel. and crossing the two scopes inputs and CAL outs.1/ Both scopes have good CAL voltage, 0.2 and (on HM 203) 2.0 V P-P these check within a few % reading on an AC DVM.2/ Both scopes CAL out have the same asymmetry of duty cycle you see!And some frequency/time error vs 1 kHz:The Hameg CAL circuit was NOT designed for an exact frequency nor 50% duty cycle!3/RE probes and connections (in your photos), probes can cause errors...we connect the CAL points direct to the scope BNC with a wire or BNC to clip lead cable.4/ HM103 is older, condition OK but a bit dirty inside...CH1 V cal is indeed a bit low, perhaps 5% error.5/ HM 205-5 is a later 1980s model mint condition, hardly used. CAL is as good as can be on CH1, CH2.6/ The Hameg VERT Variable controls can be scratchy, affecting VERT cal.....suggest to use a control cleaner on those.7/ You can check the VCAL of each scope channel: Use an external DC V, eg a lab PSU or battery, check on DVM, then compare to scope on DC input.8/For HOR cal of timebase: place unshielded wire to the scope input BNC, to pickup mains stray hum. Mains freq is precise: EU 50 Hz = 20 mS (USA 60 Hz mains = 16.66 mS).We have checked both Hameg time bases this way and both are within 1% of CAL.Spero che questo risolva tutto !Ciao~Jon

author: agent_power
author_rank: Regular Contributor
timestamp: June 10, 2022, 10:42:58 am
content: Quote from: shakalnokturn on June 09, 2022, 11:12:03 pmIt is exactly that, sanity check before doing anything. The overshoot came from the calibration output, it looks better on other signal sources.Quote from: jonpaul on June 10, 2022, 06:46:26 amThanks for checking with your scopes! I have cleaned the contacts and measured as you did, everyting checks out regarding amplitude. RE 2, if your Hamegs are like that too, then it's ok for me. I was not aware that it is not designed for exact cycles and frequencies.I could not use the wire method, there is a lot of other interference in my room, possibly caused by the LED lamps. I have a function generator (older than the scope probably), it is good enough for low frequency applications and I used it with a 50Ohm termination to adjust the calibration values.See the attachment for a 50 Hz sine wave, 1,5 V pk-pk into 50 Ohms.Tutto risolto per ora, mille grazieSaluti!PS: If someone in the future finds this thread and has more info on the calibration procdure, feel free to add it here

author: shakalnokturn
author_rank: Super Contributor
timestamp: June 10, 2022, 12:06:41 pm
content: If you're certain the overshoot comes from the "calibration" output itself and not from badly adjusted probe it could do with correcting.Just to be sure... In previous waveforms on Rigol DSO when showing the Hameg output, the probe used had previously been hooked to the Rigol's "calibration" output and adjusted for "flatness"? (Frequency response)At the risk of repeating myself the whole point of the "calibration" output is being a nicesquarewave that you have at hand to compensate the probes you hook to the scope with.

author: agent_power
author_rank: Regular Contributor
timestamp: June 10, 2022, 01:22:29 pm
content: Ok, I checked againThere is some visible overshoot on CH2 of the Hameg that is different on CH1 using the same .2V cal output at 5mv/div.I can compesate the probe on CH1 and it looks almost good, trying to compensate on CH2 doesn't work. "Best case" regarding flatness attached.

author: jonpaul
author_rank: Super Contributor
timestamp: June 10, 2022, 02:09:58 pm
content: abberation is well in spec.Expect a few %j

author: shakalnokturn
author_rank: Super Contributor
timestamp: June 10, 2022, 03:15:31 pm
content: Agreed, it's decent. The Rigol screenshot was making me expect something wrong...

author: agent_power
author_rank: Regular Contributor
timestamp: June 10, 2022, 09:32:20 pm
content: Thanks guys, appreciate the help!