Vintage Teardown and hopefully repair: RFT G-1001.500 Benchmultimeter

[hli]

It says Common Anode there...

The site says that, but the data sheet clearly shows the common cathodes (K1 and K2). I think I have four at home (in an ADC setup which might quite as old as this meter...). So when I'm back home at the weekend, I will try to look at them more closely.

[Thilo78]

The site says that, but the data sheet clearly shows the common cathodes (K1 and K2).

No, I fell for that one at first, as well.
K1 and K2 are described as Anodes underneath the sectional view.
A..H are the Cathodes as per the pic.
That I could confirm by measurement on the units in the G-1001.500 and the VQB71 I have as spares.

I think I have four at home (in an ADC setup which might quite as old as this meter...). So when I'm back home at the weekend, I will try to look at them more closely.

That sounds great. Let me know the result 

[hli]

Btw: as far as I can tell from the images, there are no russian ICs or packages in there. Everything seems to be made in the former GDR, with some parts from Tesla in Czechoslovakia (like the MAA725). The A109 is the east german version of the uA709. All the ICs named D1xx are 74xx parts, following the same naming schema.

For most of the discrete components, use this cross-reference: http://www-user.tu-chemnitz.de/~heha/bastelecke/Konsumg%C3%BCter-Bastelei/DDR-Halbleiter/ to find out what the equivalent is. For smaller transistors, the first letter is missing on the package (so the transistors on #53 are SC239E). The diodes sometimes have just a color code on their top. The date code is typically just a letter-number combination (in #53 looks like a K8 or K9). Zener diodes have the voltage on them, and the date code below.

Also, the company where schwarz-brots unit comes from was manufacturing industrial ceramic stuff, including resistors (see http://www.robotrontechnik.de/html/standorte/kwh.htm ). Some of the resistors in this DMM actually come from there! (as can be seen by the logos on them)

[hli]

K1 and K2 are described as Anodes underneath the sectional view.
A..H are the Cathodes as per the pic.

Oh you're right. Datasheets don't lie, except when they do

[Thilo78]

Oh you're right. Datasheets don't lie, except when they do

 
At least measurements don't lie.

Those I did yesterday confirm the (latest) version I quoted here.
So I wondered about K1/K2 being anodes, but that was okay for me as it's backed with measurements.

[Thilo78]

Any news about the G-1001?

Mine is doing well. I only had to replace one of the 7-segment-displays. (the right-most)

[kblue]

Gonna use this "old" topic, because pictures and schematics already exist here. I have two of these units - one works 100% fine, the second one has some burned shunts, resistors and shorted caps. Shunts are already replaced as are the caps (except the filtercaps which seems to be fine), meter does "work" in most ranges perfectly fine. While checking parameters from the service manual, the manual wishes for 0 Volts between "LO" (shorted together with HI) and RX2 Input. The fully working G-1001.500 only shows about 4-10 mV at worst, but the second unit shows 0,5 V from start slowly rising to about 0,8 and sometimes close to 1 Volt.

The basics don't seem to be affected ... 1 Volt DC is 1,000 VDC, 1k Ohm precision resistor shows as 1k, 1 mA is 1 mA and 1 VAC (1kHz/Sine) shows as 1 VAC in AC-Mode. So i guess it kinda works, but still the book wants 0 Volts between LO/HI and RX2 Input-Terminals. R287 does change the measured voltage between LO and Rx2 (as stated in the manual) but the "Zero @display" will be way off scale. Right now display is perfectly zeroed and any change with R287 (which should be used to lower the voltage between LO/HI and Rx2 throws of the ZERO). I wonder if this is "from factory" and they just "compensated" for it further down the line.

So in short: If i change R287, R276 or R277 i could achieve "0 Volts" between Hi/Lo and Rx2 Inputs but the Zero will be waaaaaay off.



Best wishes!


By the way, my unit has original Nippon-Chemicon filtercaps and HP-parts build in and it seems this was done "from factory" even with sanctions in place in the old days.