HAMEG oscilloscope HM400 can't adjust Y position  - Page 1
Forum section: electronics / testgear

author: Francisco.s
author_rank: Contributor
timestamp: December 27, 2021, 06:27:42 am
content: Hi, my not too old HAMEG HM400 oscilloscope finally has broken.Since a few weeks ago the trace line disappear from the screen randomly, and can not center it.Now the random issue became permanent and the trace is always at the top of the screen.I'm trying to get the schematic, but seems it is not available.I've attached a photo from the part of the circuit that I think the problem is located.There are two bipolar BFG135 NPN transistors that seems to drive the final "Y" amplifier.The voltage at the base and collector from this transistors changes when moving the "Y-pos" knob, but these voltage are not similar on both transistors with knob set at its center point.The output of this transistors goes from this board to the final power amplifier trough a 3 pins connector, seen at the bottom of the attached photo. (with this 3 pin connector removed, the trace is in the middle of the screen)This circuit is similar to the one in HM205-3 model, but with many differences. The tiny 8 pin chip at the center of the photo seems to drive the two BFG135 transistors, and probably powered by the +12v and -12v, but only have +11v and -3v in those ceramic capacitors that you can see near the tiny 8 pin chip.Can someone identify this tiny 8 pin chip?Need a similar schematic or an explanation of this circuit if someone knows about it.Thanks in advance.

author: Francisco.s
author_rank: Contributor
timestamp: December 27, 2021, 11:01:45 am
content: QuoteSeems that circuit from hameg HM605 is better for compared than HM205.BFG135 transistors in HM400 are comparable in its function as T503 and T504 in HM605.QuoteThis happen in component tester function as well.

author: techneut
author_rank: Regular Contributor
timestamp: December 27, 2021, 01:19:54 pm
content: The ic could be a AD8132, 8 pin msop marking HMA

author: Francisco.s
author_rank: Contributor
timestamp: December 27, 2021, 10:01:47 pm
content: Thanks, I think you are right. I will check the circuit of HM400 and compare it with AD8132 datasheet.I will post my findings.

author: Francisco.s
author_rank: Contributor
timestamp: December 28, 2021, 04:20:55 pm
content: QuoteI have connected the input 1 of HM400 to a 1 khz signal from a function generator, and I can mesure it at the output of AD8132 with another oscilloscope. The output level of pin 4 and pin 5 are different in dc level and amplitud and are dependents of the position of center knob adust.Pin 3 of AD8132 have +7.9v and pin 6 have -3.1v and an associated 1 khz ripple, It is not extrange?I will appreciate your comments.Thanks.

author: Francisco.s
author_rank: Contributor
timestamp: December 28, 2021, 09:39:43 pm
content: QuoteHi, I've found a damaged diode in this circuit. It is one of the two diodes near electrolitic capacitor, marked as D1. (the one whith the catode to the left)I don't know what type of diode are, but the good one seems to be a 2.7v zener diode.Will get one 2.7v zener and check if it solves the issue.I hope it won't get worse.

author: Francisco.s
author_rank: Contributor
timestamp: December 30, 2021, 10:01:26 pm
content: QuoteI don't know how it happened but after change the zener diode nothing seemed to work, then I made a short circuit trying to measure in the connector coming from power supply, and some smoke gets out from the circuit near this connector.Now the problem is that another chip is damaged, in this case it's a OPA354. (250-MHz, Rail-to-Rail I/O, CMOS Operational Amplifiers)This OPA354 seems to work in the "X" channel amplifier.In a few days this chip will arrive and I will check how deep is the damage now.Sometimes this things happen.

author: Francisco.s
author_rank: Contributor
timestamp: January 14, 2022, 10:09:19 pm
content: QuoteHi, replaced the opa354 but there is no signal deflection in the "X" channel amplifier.Only when component tester is selected a horizontal line is visible in the screen.What I was able to solve is the Y channel. (one of the output power transistors was bad (one of the two 2SC3953))This 2SC3953 transistors are mounted with nylon screws to the aluminum plate in order to dissipate the heat generated.This nylon screws are good for insulating the transistor but seems a bit fragile.I will highly recommend you to replace this nylon screws when you are servicing a client's Hameg oscilloscope.

author: Francisco.s
author_rank: Contributor
timestamp: January 14, 2022, 10:13:48 pm
content: Forgot to attach a photo from final Y power amplifier.Still working on X channel.

author: TaylorD93
author_rank: Regular Contributor
timestamp: January 14, 2022, 10:27:06 pm
content: Try finding another HAMEG model which is of a similar era. Usually there are some date code marking on the boards.I have a HAMEG 1004-3 which im trying to repair. The only schematics i can find online are for a 1996 model, mine is 2001. Rohde & Schwarz UK were no help so ive found another Hameg model, think it is for a 1500-3, but the PSU matched my scope and was a god send.It can be tricky without the documentation but is possible

author: Francisco.s
author_rank: Contributor
timestamp: January 15, 2022, 10:30:52 pm
content: QuoteYea, it is possible. I have removed a smd resistor and have partially solved the problem with X amplifier.But I can find a similar circuit in other Hameg oscilloscope. (the way opa354 works in this circuit)The resistor removed is located near opa354. This smd resistor is connected to -6 volts in one of this pins and the other pin is connected to pin 1 of opa354 (output of op. amp.) via a complicated network. (at least for me)Attached a photo from the mentioned circuit.The black arrow points to the resistor removed.

author: TaylorD93
author_rank: Regular Contributor
timestamp: January 15, 2022, 10:45:14 pm
content: Does your HAMEG have a component tester?Typically i have found on other HAMEG Models, if there an issue with the Y attenuator or Pre-amp, using the Component tester it drives the Y stage in a different point which can help narrow down the issues.

author: Francisco.s
author_rank: Contributor
timestamp: January 15, 2022, 10:50:37 pm
content: QuoteIt is partially solved because it only traces 1.5 cm on the screen and I have to to push the x-magnifier to see the trace in all the screen.At least I can see a correct waveform but can not use it for accurate measures.I will study the circuit and try to find out how it works.Of course I will need a lot of help.

author: Francisco.s
author_rank: Contributor
timestamp: January 15, 2022, 11:00:25 pm
content: QuoteYes I mention it in Reply #7.I am using schematics from old Hameg scopes but seems to be many differences.I hope someone in the forum can figure out how this circuit works only watching the photos.

author: TaylorD93
author_rank: Regular Contributor
timestamp: January 15, 2022, 11:16:24 pm
content: What year is the unit dated?Alot of the boards or CRT may have a sticker with a date code.

author: Francisco.s
author_rank: Contributor
timestamp: January 16, 2022, 06:58:39 pm
content: QuoteI bought it in 2012 and user manual mentions 01. 06. 2007 (really I don't know when this model were manufactured)I have to re-open it and see the stickers. (I am using it, in fact I use it more than my dso)

author: Francisco.s
author_rank: Contributor
timestamp: January 23, 2022, 06:13:58 pm
content: QuoteI have found a sticker wit the date. You can see the date in the photo attached.The date in the sticker shows 23.06.2010 and what seems the code of the damaged board. (50-1MBA-1110)I have searched for the code of this board and found nothing.QuoteThe resistor removed seems to set the polarization voltage for the double diode "M3BZ". (I think this x2 diode works in the switching circuit for component tester/ ramp generator)What is weird is that there is a sawtooth waveform in one pin of the diode, only when the resistor is removed.Can someone explain this to me?