Hameg 7042-2 Service Manual

[rvalente]

Hello Mates,

is the service manual for the Hameg 7042-2 available? I'm looking to get this power supply but the low voltage channel has the display off.

Greetings, Renan

[KevinA]

Electrotanya

[dl6lr]

The voltage and current displays are each driven by ICL7107 in MQFP on the front board. As far as I remember: the voltage divider is on the main board, as it displays the voltage even when the output is off. The chips may be driven by one of the auxiliary voltages of the 8 pin ribbon cable at the edgae of the main board. From the edge there are two pairs of +/-9V for the channels up to 30v and the last pair may be the one driving auxiliary voltages of the 5V channel. I have't checked that.

Anyone has the schematics as well? The SM does not include the schematics. I have got one of these for very cheap with the second 0-30V channel defective. The main problem was solved (blown TIP33C and blown current shunts) but I am struggeling with more nasty problems of the channel (like voltage spiking upon switching the output on and off).

This PSU has some bad habits by design too: The voltage is overshooting upon switching from CC to CV mode (confirmed by one with a fully working unit). So beware for delicate equipment.

Bernd

[rvalente]

Nah, due to the lack os schematics and service manual I'll probably pass this one.

I'm doig wrong? The supply is beeing offered by R$700

[dl6lr]

R$700 will be about US$120? For a defective device I would not spend that much. Mine was €10,- (about R$67,-).

The display is fed from an auxiliary voltage and I doubt the voltage and current regulation will perform if the aux supply is missing. However this could be easy to fix.

I am currently drawing parts of the schematics to hunt down my issue. This is all SMD besides the power parts.

[rvalente]

R$700 will be about US$120? For a defective device I would not spend that much. Mine was €10,- (about R$67,-).

The display is fed from an auxiliary voltage and I doubt the voltage and current regulation will perform if the aux supply is missing. However this could be easy to fix.

I am currently drawing parts of the schematics to hunt down my issue. This is all SMD besides the power parts.

Yeap 120 Trumps, by the other hand getting T&M equipment as bargain prices is almost impossible here, so R$700 is yet cheap 
I own a few HP and Agilent supplies from 70's to 2000's so I do not realy need another bench power supply although, I dont have any tracked/simetrical power supply.

I try not to own any vintagem equipment which has no full SM manual available. Does this HAMEG have any custom ICs or is just jelly beam stuff?

[dl6lr]

As far as I saw it has only standard components, but all SMD except the power stage and the rectifier board. Conservative design, the most complex chip is the ICL7107. The regulation is done with LM358 and some BCX53/BCX56. The two channels have a  UC3842 SMPS each, but I haven't figured out yet what this is for.
The two channels are not identical despite having the same technical data. They are using a TIP33C with a BCX53 as a Sziklai pair for load regulation. The left channel however has a BUZ271 MOSFET preregulation.

To give you an idea, here is a photo of the regulation main board with the front panel board attached (you can see the single LED digits). R-click and display image to get full resolution.



Regards, Bernd

[kripton2035]

this is the 7042-5 version of the service manual : https://elektrotanya.com/hameg_hm7042-5_power_supply_sm.pdf/download.html
and they contains the schematic, as well as component placement on pcb.

[dl6lr]

But the 7042-5 is a totally different design, it has nearly nothing to do besides the outer size with the 7042-2 or -3.

[kripton2035]

But the 7042-5 is a totally different design, it has nearly nothing to do besides the outer size with the 7042-2 or -3.

sorry did not know that !

[dl6lr]

OK, mine is now working again without the spikes. I still have to order a new relay and replace it before closing the cabinet.

@Renan: If you want to repair the PSU, I will try to help you as much as possible, at least I have a working unit now and can give you some measurements.

First I would check the voltage of the aux supply. They are carried from the rectifier board to the mainboard with the ribbon cable (in the photo top right), the aux voltage of the low voltage channel should be the two left most cables seen from the top. If you turn the unit around and look from the bottom side, the aux voltages are as follows: channel3: GND, -9V, +9V, channel1: GND, -9V, +9V, channel2: -8V, +14V, all measured against ground of the according channel (1..3 from left to right on the front face, 2 being the low voltage channel). All channels are isolated against each other. The aux supply is a simple bridge rectifier with a storage capacitor and some bypass caps. If you are *very* unlucky, the transformer could be damaged. In that case there is some spare space in the cabinet to install another transformer with about 16V AC.