Grundig Klirrfaktormesser MGL 1451a

[HRmeteohub]

Hello!

A friend of mine got a Grundig MGL1451a from e-bay last year.





Unfortunately, it seems that the device is not really working as it should. Only info I could find is a copy of ebay listing, with only two images. One can see that the device was tested in 1978, and once more in 1986. The components inside are from 1972. There is no other data, manual or service manual to be found. At least, I could not find it. Anyway...

I am feeding the input with 1 kHz, 0.7 - 5 Vpp (sinus wave with 3% THD) from signal generator, but the device shows about 0.3 V, and I can not get it to show actual value. E.g. When I apply 3 Vpp, it shows about 0.8 Veff. (edit.I managed to set the THD within of 10% of the value I got with my oscilloscope.) Using the 1k trimer pot, I can't get any usefull change, although the voltage over the 22uF capacitor gets up to 2V, the voltage for indicator is in range of 10s of mV (see image below, see down, indicator marked "napon").

I hoped to see the that there is a rejection of 1kHz by at least 60dB after exiting first board, as I would place the T-filer right at the connector. But, I can clearly see that the first board always has a 100mV signal on the output.
In moment of desperation, I made some drawings of the board that drives both instruments (section down, sorry for some errors), and started to draw the constant-output amp board (top left). There is a image of (presumably) filter board (left).



Could someone give me some advice?

 

[floyd]

The black square component in the first pic on the left looks to have some leakage at the base.

 

[BoomboxLover48]

The black square component in the first pic on the left looks to have some leakage at the base.

In some cases they put a glue under that electrolytic cap. No matter what those large caps needs replacement.
Full electrolytic cap replacement is the first thing to do before powering on the unit.

 

[HRmeteohub]

Since I do not have any data about the unit, I started backwards - from instrument to sig.gen.
In first block of voltmeter (top section of bottom image) I discovered one crystal diode turned to resistor, and input cap had capacity but also a leakage. Other caps and resistors are ok. I am kind a hesitating to test transistors and OP-amp especially, since I do not have a replacement for op-amps, nor I can find them. To be sincere, I do have one uA709 and LM301, and finally This is how the indicator board looks like now. Now, I believe that is not uA 725 on board, because internet search of this strange mark (fu5f7715393) leads to even stranger replacement op-amp.



Also... The voltmeter reads now 0.55 V for 2 V input. However, the voltage at the 100uF capacitor before the diode bridge is about 2Vpp. Now...the resistance of the instrument is about 350 ohm, and it is connected in series with 1k pot and 2k2, but rotating the pot from 1k down to 100 ohm gives no difference in readout whatsoever. I get more response from multi-turn pots at the range selector. The voltage and current on the indicator itself are consistent with the read out (I have placed 0.5mA constant current source and it indicated 50% (0.5).
I'll have to check and see what happens during the power-up, because than I see the full swing on the indicator.

Finally, the black block in up-left image is definitely not a cap. It has several connections, as I have drawn in the schema. I guess it could be some kind of coil or transformer? I'll tear it down guys, just for our pleasure.

When BL48 says "large caps", I tend to smile, since these caps are large for todays standard caps. Most of them are only up to 220uF. I also have to say, the boards are of supreme quality compared to my last repair (Thule Spirit amp).

Thank you people for your interest.

 

[HRmeteohub]

I was just wondering...

I could rip everything from inside, make a brand-new analog hardware. After all, this was built with op-amps such as uA709, LM302 and such. One can get much much better silicon today and probably make it better (and smaller).