EQD BOWS signal path schematic

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Manfred
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Post by Manfred »

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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Manfred
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Post by Manfred »

Here the schematic once again with the read voltages.
EQD_Bowe-SignalPartVoltages.JPG
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Post by Manfred »

Here the shot that I have made.
EQD_Bows_PCB_shot.jpg
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Seiche
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Post by Seiche »

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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Post by Manfred »

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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Post by Manfred »

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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Seiche
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Post by Seiche »

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

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Post by Manfred »

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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Post by Manfred »

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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Post by Manfred »

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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Post by Manfred »

In order to make a circuit comparison, I just ordered BC140-6 and BC140-10 transitors.

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Post by Manfred »

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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