Hameg HM8130 function generator teardown and repair

[anotherlin]

Hi,

My Hameg HM8130 function generator just stopped working. The main fuse has blown for no reason when I was using it.

The HM8130 is an arbitrary function generator from the beginning of the 90s. Mine seems to have been built in 1993 (see main board).
Even if Hameg is German, it seems to have been built in France, in Villejuif. That's a town not far from Paris, less than 10km from where I live.
The factory seems to have long closed.

Remote controllable using GPIB or RS-232 with an extension board, which mine doesn't have. There is a "hole" for that on the back (see picture).

It is a very capable unit. Sine waves, square waves, pulses. Up to 10MHz for sine wave. I've taken a quick a look at schematic, it doesn't seem to use DDS for sine wave generation or square. Seems to be purely analog.

I'm trying to repair it. To troubleshoot it, I disconnected the PSU from the "main board" and the fuse keeps blowing. So problem must be around PSU.
I've taken only a quick glance. Checking the filtering caps with my handheld DMM, ok. Rectification diodes also seem ok, but some allow voltage in reverse.

The schematics are available in the pdf of the french manual on hameg's website. The suppli voltages are -12/+12, -5/+5, and VCC (5V for digital). As you can see on pictures, there a big black radiators for them (probably overkill).
The -12/+12 supply both use two linear regulator IC in series : L200? followed by 7912/7812.

I'll keep you guys updated if you're interested.

Best regards,

[free_electron]

its a dds machine alright..
just not integrated in 1 chip

take the grey flatcable with black connector from the rear right pcb off the board and power up.
see if that blows the fuse. check the voltages on the regulators.

my suspicion is that a bridge rectifier died...

all those regulators have current limit and if there was a problem in the system the regulators would current limit but the fuse would not blow....

so i suspect the problem to be before the regulators. either a shorted out transformer winding or a toasted rectifier ...

[anotherlin]

its a dds machine alright..
just not integrated in 1 chip

I wouldn't be so sure. Check the schematics :

=download&tx_hmdownloads_pi1[uid]=1065]http://www.hameg.com/manuals.0.html?&no_cache=1&L=3&tx_hmdownloads_pi1[mode]=download&tx_hmdownloads_pi1[uid]=1065

It's at end of manual. It really looks like analog.

take the grey flatcable with black connector from the rear right pcb off the board and power up.
see if that blows the fuse. check the voltages on the regulators.

Already did that, and fuse keeps blowing.

my suspicion is that a bridge rectifier died...

all those regulators have current limit and if there was a problem in the system the regulators would current limit but the fuse would not blow....

so i suspect the problem to be before the regulators. either a shorted out transformer winding or a toasted rectifier ...

I just hope it's one of those bridge rectifiers or those regulators, they are easy to replace.

If it's the transformer, that's gonna be a big problem. I don't think I can find a replacement.

By the way, how can I test the transformer ? If I remove all cables to the rectifiers, wouldn't be a problem if the primary is not connected to any secondary load. Hence doing a short ?

[free_electron]

looks like an EREA clone transformer. its a standard of the shelf thing in europe. no big deal to find.

if you disconnect the transofrmer form the regulator boards and the fuse keeps popping its the transformer ..

[free_electron]

i just looked at the schematics. that thing has an integrated netfilter.... wouldn't be the first time a cap in there shorts out ... ( FL1 on page 16 )

[Bored@Work]

It's at end of manual. It really looks like analog.

The generator uses multiple techniques. From a quick look it has separate analog HF and LF sin generators, but a DDS for ARB (the phase generator board) , and yet another digital generator for pulses.

If it's the transformer, that's gonna be a big problem. I don't think I can find a replacement.

You could just ask Hameg if they still stock a spare.

By the way, how can I test the transformer ? If I remove all cables to the rectifiers, wouldn't be a problem if the primary is not connected to any secondary load. Hence doing a short ?

An open  (not loaded) secondary of such a transformer is not a problem. Just use an multimeter in AC mode to measure the output. You should expect that the secondary voltage under no load a good deal higher than you'd think it needs to be.

[anotherlin]

Thanks for your suggestions and comments.

Seems that I'll have to go a little bit deeper. The supply boards are all stacked above each other, and I'll have to unscrew the radiators.

I just hope it's not the transformer.

[anotherlin]

Ok, it was quite difficult to unscrew everything and get to the transformer PCB.

I've checked the line filtering. It seems ok, not shorted.

I've checked the resistance of both primaries (110v parallel them, 220v serial them). And it seems there's a problem : I measure 43 ohm for one and 10k for the other.

43 ohm is a little bit high, I've heard primary resistance for 110v are around 25 ohms. But 10k ohm is way to high.

However, I still don't understand why the fuse would blow. As a 10k ohm primary would act as an open circuit, not a short.

The secondaries are fine. I also check isolation between primaries and secondaries, they measure as open circuit.

Should I go and contact Hameg if they have spare parts ?

[Nermash]

I would first disconnect the CN4 from CN5; and CN6 from CN7. Replace the fuse, fire it up. If it blows then it is either line filter or transformer. If it remains OK, then start checking the rectifiers and regulators on the two power supply boards.

[anotherlin]

I would first disconnect the CN4 from CN5; and CN6 from CN7. Replace the fuse, fire it up. If it blows then it is either line filter or transformer. If it remains OK, then start checking the rectifiers and regulators on the two power supply boards.

Already did that. The line filter seems ok. The transformer seems to have problems with its primaries.

[T4P]

The easiest way to check with the resistances? No actual way.
The resistances might tell a story but not really,
for a step down the primary is higher but yeah that 10k figure is a blown transformer
unless it's a 22mA transformer 

[anotherlin]

Good and bad news. Good one: Hameg still have spare transformers, 66 euros, which is an ok price considering this one has quite a lot secondaries. Bad one: 38 euros for shipping !

[poorchava]

Looking at schematic the transformer is for sure a custom one, but there's nothing sophisticated, just 4x9.5V 4x 18V secondaries. I think there are two ways to fix this:
1) buy a small toroidal transformer with 2x18V secondaries (primary 2x110 or 1x220 - doesn't really matter) and then wind additional secondaries by hand. For stepdown to 9.5V not very many turns will be needed. Coated magnet wire would be the best, but you can also use ordinary isolated wire (either solid or braided).

2) re-do whole supply. I wonder what was the reason for not using negative voltage regulators along with positive types to achieve correct bipolar voltages. That would save them like 3 rectification bridges. It can perfectly be done without much effort, as this is only simple multi-voltage linear supply without any sophisticated analog tricks. PCB redesign may be necessary anyway because it's quite possible that new traosformer won't fit.

In Poland we have a company which without any problem supplies toroidal transformers capable of powers up to 2kVA, and custom winding configurations don't cost significantly more than standard ones (and what is even more important they are not making any problem for single quantities). IIRC like 2 years ago I bought a 400VA transformer with 4x18V high current windings, 4x15V center tapped low current and additional 1x8V for miscelaneous stuff and paid what would translate to about 30euros . I guess smaller transformer would be like 1/3...1/2 cheaper.

[anotherlin]

Hi everyone,

Just received an MP asking me for the photos which are no longer there on imageshack.
I'll try to find them and re-upload them.

In the meantime, here's a follow-up.
I actually managed to fix this signal generator !
And the transformer was really busted.

At the beginning of this year, I wanted to get rid of this signal generator. It was broken and I wasn't using it.
I decided to take the risk of buying a replacement transformer Hameg as they have the spare in stock.
Picking it up at R&S office near Paris is free, so that would only cost around 70 euros.
I figure out that even if that failed, I would be able to sell it as "broken AS IS" on ebay for about a 100 euros.
So it seemed a reasonnable bet.

Got the transformer, soldered it and it worked !

I'm still wondering why the transformer suffered a "sudden death" in the first place ! 
But hey, it works now  O0

[rob040]

Hi,

Good to read that the instrument is up and running again. What about the temperature of the transformer when the generator is on? A possible reason for the sudden death can be a current on secundary side that is too high...

BR

[anotherlin]

Good to read that the instrument is up and running again. What about the temperature of the transformer when the generator is on? A possible reason for the sudden death can be a current on secundary side that is too high...

It was running at room temperature and only for about 15 minutes, so I don't think it's temperature.

Unfortunately, I'm unable to find the photos I've taken (old hard drive), but the service manual/schematics is fully available.

[rob040]

Unfortunately, I'm unable to find the photos I've taken (old hard drive), but the service manual/schematics is fully available.

Ok, thanks for searching.