RFZ-V781 Trimmers Calibration

[smilan]

HI, I have 2 RFZ-V781 units that I serviced, now both units are working but there's a 1.2 db difference between the preamps.
I would like to calibrate the three trimmers (R102, R114 and R129).
According to the manual (with google translate): "The power supply circuit contains a pnp, transistor (T 113) as a filter resistor, so that the necessary ground voltage reduction is achieved in the non-earthed connection line. R129 is used to set the operating voltage (- 22 V to both)."
So I set the voltage on the negative side of C121 as close as possible to -22V (-22.45 was the lowest voltage I managed to get from the trimmer).
Now what about R102 and R114?
In all the schematics I found the voltages values on the bases of T102 and T107 are none readable.
I tried to search on the manual but couldn't find any reference to the base voltages.
In reality, the base voltage of T102 varies between -22 to -14 V and the base voltage of T107 varies between -22 to -17 V.
Maybe someone knows what should be the correct voltages?

 

[jokeramik]

You need an audio analyzer to adjust R 102 and R 114. This was so described in the manual.

Which measuring equipment do you use?

Be careful with measuring, because there is a resistor (R131) in the negativ rail.

Best regards!

 

[jokeramik]

You need also a oscilloscope to look at the waveforms after amp 1.
Best regards!
jokeramik

 

[smilan]

Thanks, I have a scope and a Boonton 1120 audio analyzer.
Can you please tell me on which page on the manual I should look for the instructions ( and use Google translate to know what to do)?
This is the manual thst I have:

 

[jokeramik]

Please send me a photo of your V 781. There are 3 existing versions.
Best regards!
jokeramik

 

[smilan]

I have the two earlier versions

 

[jokeramik]

Hello,

Left: This is a very early version in which all relevant npn signal transistors were fitted with BC 109 and BC 179B. SF127 transistors from the semiconductor factory in Frankfurt/Oder are only used in the output stage.

Right: This is a medium version. All transistors were equipped with products from the semiconductor factory in Frankfurt/Oder.

From point 3 in the device documentation, the adjustment of the V781 with its two discrete operational amplifiers is described.
Best regards!

 

[jokeramik]

Please change the fuseholders.
Best regads!

 

[smilan]

Please change the fuseholders.
Best regads!

Thanks, I've already done so.

 

[sodderboy]

Maybe it is the photo, but it looks like the left unit has a bad electrolytic, the silver axial under the top big 200/25, has the left end popped-off? Give that cap a good look. If they were mine, I would not hesitate to change all electrolytics to get them fresh before adjusting anything. If they were for a customer, I would recommend doing this, because chances are that they would be back on my bench in the future with one of those blue cans popped-out like a party favor, having potentially damaged silicon parts upstream in the circuit. You will have to up-voltage a 100/3 axial, but it is worth the time/$. Odd that they would have the same date code, but "TGL7198" could be 1971. They are definitely not from '98!
When it is adjustment time, rock those trims a bit around the point where they have been sitting for decades to clean the area and give a solid reading.
Mike