Hameg HM8027 question

[RuudNL]

Because I need a distortion analyser (I don't want a PC based analyser!), I was looking for a Hameg HM8027.
It seems they are hard to find, because they are already out of production for some years.
So, I decided to create one myself; I've got the schematics.;D
Unfortunately, some of the resistor values are not given, maybe they are selected during production.
I would be very interested to know the values of: R112, R113, R114 and R141, R142, R143. They are marked 'V1'.
They are located around IC101 and IC104 (CA3054).
(I expect the values will be around 100K or so.)
Also the value of R160 is hard to read, it looks like 178K but this seems a strange value to me.
Any help or a clear picture of the PCB would be highly appreciated!

 

[radiance]

Can't you contact Hameg about this?
Tell them you're troubleshooting a broken unit or so...Hope they don't ask for a serial or so... :-\

 

[RuudNL]

I've tried that, but they say that they don't have further information, because the 8027 is out of production for some years.
So I hoped that somebody who owned a 8027 could take a peek inside, or make a detailled picture of the PCB.
Otherwise I will have to do some experiments to determine the right values, but I would have preferred to have an indication as a starting point.

 

[radiance]

Sorry I can't help you but it's a very interesting project none the less...

 

[Walrus]

I read those resistors as 1M (one megohm). And yes R160 appears to 178k, which is an actual resistor value.

 

[RuudNL]

Thanks!
I had a different version of the PDF file. I expected that the PDF I linked to was the same version, so I did not take a closer look.
But indeed, the missing values are readable as 1M! (I already expected them to be 100K+) So now I'm ready to go...
At the moment I am busy drawing the PCB design. Although Hameg says they can't produce the HM8027 anymore because some of the components are obsolete, I did not have very much problems finding them. So I hope I will have my 'homemade HM8027' soon!
The only hard to find parts now are the 3 interlocking switches with the right spacing. (Frequency selection)

 

[radiance]

The only hard to find parts now are the 3 interlocking switches with the right spacing. (Frequency selection)

Can't this be done with a rotary? Or are you making an exact copy?

 

[radiance]

So, what goes on inside a distortion meter really?

...does it inject a sine wave and looks for harmonics or so?

 

[RuudNL]

The idea is to make a '3-D copy' of the original, but of course a rotary switch could be an alternative.
Most distiortion meters use the same working principle: you inject a low distortion sinewave into the object to measure (I have a HM8037 low distortion generator, less than 0.01% distortion) and then cancel out the fundamental frequency. What remains are harmonics, noise etc. Usually there is an output for this signal, so you can use a scope to compare the distortion with the original signal, or listen to the distortion signal.
Here is the complete service manual for more information.

 

[radiance]

The idea is to make a '3-D copy' of the original, ...

I guess this means multiple pcb's right?
I would smack it all on one big pcb to avoid wiring...hm..might do one as well.
But first see how you get on with it ;-)  and find some time for it  :-(

 

[RuudNL]

Because I have the Hameg mainframe (can take two Hameg 80xx modules and has the power supply) and a Hameg HM8037 low distortion generator module, I would prefer to build it in an empty module. (Hameg offers empty module boxes for your own creations.)
The whole thing consists of a main PCB and two small PCBs, for the displays and the input amplifier.
I will let you know how I get on with it. If it works as expected, I can send you the layout for home etching, or (maybe!) produce some extra PCB's.

 

[radiance]

Ah, I see. Yes, that's an elegant solution indeed. And those frames alone do cost a fortune already....

 

[abbey road d enfer]

So, what goes on inside a distortion meter really?

...does it inject a sine wave and looks for harmonics or so?

All distortion meters operate on the same principle: a notch filter tuned at the exact fundamental frequency removes ...the fundamental. What's left is the sum of all harmonics and whatever noise is present in the signal. that's why the measurement is called THD+N.
There are also distortion analysers that operate by selecting and measuring each and every harmonic present in the signal.

 

[RuudNL]

In the meantime I have completed the motherboard and the results are excellent, even better than expected.
In fact the suppression of the fundamental frequency is better than measured on a commercially produced HM-8027!
I have used 1% resistors throughout, because I think this is important. The only problem I had was the matching of the capacitors in the filter; if they are not closely matched (I could not find 1% capacitors when I tested the board the first time), the suppression will be poor. Also the potentiometer (100K stereo, reverse log!) should have a good tracking between the sections.
Luckily I found that Farnell could deliver 7.5 nF / 1% capacitors, not a very common value IMHO. (I thought about using 2 x 15 nF in series.)
The other 'difficult' parts I found on ebay.
For those who are interested: I have a PCB design for home etching. I do not have the intention to produce and sell those PCBs, but if you would like to have the layout: feel free to send me a PM!

 

[harrimansat]

In the meantime I have completed the motherboard and the results are excellent, even better than expected.
In fact the suppression of the fundamental frequency is better than measured on a commercially produced HM-8027!
I have used 1% resistors throughout, because I think this is important. The only problem I had was the matching of the capacitors in the filter; if they are not closely matched (I could not find 1% capacitors when I tested the board the first time), the suppression will be poor. Also the potentiometer (100K stereo, reverse log!) should have a good tracking between the sections.
Luckily I found that Farnell could deliver 7.5 nF / 1% capacitors, not a very common value IMHO. (I thought about using 2 x 15 nF in series.)
The other 'difficult' parts I found on ebay.
For those who are interested: I have a PCB design for home etching. I do not have the intention to produce and sell those PCBs, but if you would like to have the layout: feel free to send me a PM!

Do you have any photo of your distortion anlyzer? I want to make one