#1
I thought I would post this as it seems there are people out there who may have dud preamps on their Hohners.

There is virtually no information available about this board which was faulty when I removed it from the instrument. Hohner sells complete kits for this unit including pots, switch, led etc, but the new one they sent me did not work.

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In order to install it in the Jack Bass it was necessary to unsolder all the wires from the board and make use of the longer, existing ones which run through a bore in the body. As a result Hohner had no interest in replacing it with a new one. No circuit diagram or documents are available and I was informed that the person who designed and organised the manufacture of the board was no longer with the company and therefore no tech help was available from them. Thus I would need to reverse engineer it if I was to repair it.

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#2
Here is my cleaned-up image of the underside of the board that clearly shows all the traces, plus a drawing of the top side of the board, as printed, showing the components. There are two modifications to the drawing, reflecting differences between the graphic on the board and the actual wiring:

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#3
The grey areas on the lower drawing were not printed on the PCB. They are a ghost image of the traces on the underside of the board above. From this it was possible to draw the circuit diagram.

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The following component list was compiled mainly from reading the values off the existing components, but also measuring components where this information was not available:

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#4
With this information it was then possible to follow the signal path and establish that the opamp had failed at U1a. I believe this was due to the battery shorting to ground with the guitar jack unplugged, which in turn was caused by poor design of the battery compartment and subsequent damage to the contacts. You have to look at the actual construction to see how this is possible.

Repair then involved changing the way the battery is connected and replacement of the opamp.

Although this was a huge amount of work, it was a fascinating learning curve and resulted in a properly functional instrument.

A brief note about the NJM072S OpAmp. This IC is symmetrical and can be put in either way round. Both pins 1 and 9 are power inputs, only one needing to be connected. Pin 5 is the ground. Pins 4 and 6 are +ve inputs. Pins 3 and 7 are the -ve inputs and pins 2 and 8 are the outputs.

I hope this is helpful to others on the way who may encounter the same lack of help from Hohner.
#5
Brilliant bit of research - thanks very much!
You are obviously knowledgeable about electronics, so perhaps you can answer this query about the parts list?
The volume pots are B500k right (yes they are, I'm looking at mine now)
B denotes linear?
I thought volume controls always used log?
Thanks.
#6
Hi Dave. You are right and they are probably replaceable for V1 and V2. Not sure how much difference you would notice with the active circuitry between them and the output - notice the 100K pots for treble and bass.

The log pots are really, as you probably know, to compensate for anomalies in the way we "hear" volume. Perhaps the opamp responds differently needing a linear level reduction? I didn't design the circuit and I am not enough of a genius to fathom that out. Maybe an expert can tell us.

Or you could experiment and see what happens? You could even try the standard 1meg and 250K options as well? Let us know.

My objective really was to get the axe working properly again. Never got a thanks from the owner btw. Maybe the shop took the credit. I did get paid though. Lol.
#7
And also btw I supplied my diagrams and info to Hohner free of charge so they should be able to help people with this now. If they say they can't, complain loudly here!!
#8
Hello,

Does any one happen to have access to the images of the Hohner preamp/eq PCB schematic mentioned in this thread or can direct me to a working location for the images rather than "invalid img"?

Cheers.