EQD BOWS signal path schematic

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EQD BOWS signal path schematic

Postby Manfred » 05 Jun 2019, 12:56

Before longer time I had got an EQD bows for repair, now I found my handwritten drawings from the signal path of it.
Here the redrewn schematic:
EQD_Bows-SignalPart.JPG
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Re: EQD BOWS signal path schematic

Postby Manfred » 06 Jun 2019, 08:08

Here the schematic once again with the read voltages.
EQD_Bowe-SignalPartVoltages.JPG
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Re: EQD BOWS signal path schematic

Postby Manfred » 06 Jun 2019, 19:29

Here the shot that I have made.
EQD_Bows_PCB_shot.jpg
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Re: EQD BOWS signal path schematic

Postby Seiche » 08 Jun 2019, 11:06

So this is an NPN rangemaster with full range mod?

Only the transistors emitter seems to be biased differently (100R instead of 3k9).
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Re: EQD BOWS signal path schematic

Postby Manfred » 10 Jun 2019, 01:00

Seiche wrote:So this is an NPN rangemaster with full range mod?

Only the transistors emitter seems to be biased differently (100R instead of 3k9).


The BOWS has a more symmetrical clipping than the NPN rangmaster.

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Re: EQD BOWS signal path schematic

Postby Manfred » 10 Jun 2019, 14:44

I built the circuit on breadboard for measuring the values.
It was hard to find a OC139 of a bunch of 100, which matches nearly the read DC-values of the orignal device,
there fore is necessary to set the bias setting for the rest of the bunch.
EQD_Bows-SignalPartTestCircuit.JPG

BOWS_CurveTraces.jpg
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Re: EQD BOWS signal path schematic

Postby Seiche » 10 Jun 2019, 16:39

So they are probably using a transistor with Hfe = 35, am I understanding this correctly?
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Re: EQD BOWS signal path schematic

Postby Manfred » 10 Jun 2019, 19:55

Seiche wrote:So they are probably using a transistor with Hfe = 35, am I understanding this correctly?


Yes, it is.
I took out the transistor of the circuit after the measurement and then I read its values by means of my semiconductor analyzer.
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Re: EQD BOWS signal path schematic

Postby Manfred » 11 Jun 2019, 00:44

I remembered having a bunch of BF128C transistors which have a lower Hfe, so I have replace the OC139 by it for a test.
The input seriell circuit made of the 10k resistor and the 100n capacitor was to be adjusted to maintain the gain and the frequency response.
I replaced 10k by 27k and 100n by 47n.
The result looks not bad, but maybe a bias adaption is needed to obtain better results.
EQD_Bows-SignalPart Test BF128C.JPG

BOWS_CurveTraces BF128C OC139.jpg

Frequency response Compare BOWS OC139 BF128C.jpg
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Re: EQD BOWS signal path schematic

Postby Manfred » 13 Jun 2019, 21:57

I must correct the transistor type, I had mistakenly read B instead S.
It is the SF128 transistor made in GDR.
According to sources in the Internet it could be can be replaced by a BC140.
EQD_Bows-SignalPart SF128.JPG
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Re: EQD BOWS signal path schematic

Postby Manfred » 14 Jun 2019, 05:59

In order to make a circuit comparison, I just ordered BC140-6 and BC140-10 transitors.
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Re: EQD BOWS signal path schematic

Postby Manfred » 20 Jun 2019, 14:55

As anounced, the comparation trace graphs using the BC140-6 and the SF128C transistors.
The resulst are very similar.
EQD_Bows-SignalPart BC140-6.JPG

BOWS_CurveTraces Compare BC-140 SF128C.jpg
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