EEVblog Electronics Community Forum
Products => Test Equipment => Topic started by: snoozer on February 07, 2021, 09:27:39 pm
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Hello,
I have recently purchased a used Karasch linear bench power supply as shown in the attached pictures. I had been in touch with the manufacturer to request any information they might have on the supply and also asked if they would share a schematic. They would have done so but it appears to be from a long time ago and no documentation exists apparently and nobody there is even familiar with the product. I had seen pictures of the PSU on the internet without any further details showing the same model with analogue meters, so I guess the design must be VERY old. I can not make out date codes on the ICs inside. There are only model numbers and some cryptic letter codes which mean nothing to me. I have been searching the internet with the model and part number as well as manufacturer and IC numbers and power devices number in my search but came up with nothing. Here a list of devices I identified inside:
Power devices: 2x IRFP240
Op amps: 2x LM224N, 1x TL084CN
Opto coupler: CNY17-3
Thyristor: BT152
NPN Transistor: BC141 (Fan control)
The PCB top side is all through hole devices, the bottom is SMD but only resistors, caps and diodes. The Transformer has 2 taps and the relay switches at above 8.4V and again below 7.5V. The panel meters are both the same, I measure 9V DC supply voltage and the input is 1mV @ 1V PSU output and 1mV @ 1A PSU output. The LCD displays show 2 digits before and 1 digit after the decimal point. The decimal point is fixed and does not move at low readings. There is active cooling which is speed controlled, a temperature sensor is mounted to the heat sink. One channel of one LM224N is used to control the BC141 for fan speed. The transformer has a extra tap on the secondary side with tiny flimsy wires. I am not sure if that is a thermal protection or just a "helper" supply, I measured 1.7 Ohm and if powered up 26V AC. On the PCB is a 2nd mini PCB mounted encapsuled transformer. 2x 9V AC, 2x0.25VA. The controls are 4 potentiometers, 2x Coarse for U and I as well as 2x fine for U and I. A green CV LED, red CC LED and a red Error LED. I have not seen the Error LED nin action, I don't know what condition it would indicate. There is also a OVP Trim Por externally accessible with a tiny screw driver between the output terminals. The outputs are floating, a chassis ground terminal is between both outputs tied to the chassis and PE from the mains supply. The PSU is for 230V AC supply non switchable.
At the back is what has me intreagued, a 9-pin Sub-D. There is not a bit of digital tech inside so I assumed it would be readouts. I have found 2 pins that connect to the Positive and Negative output terminals. All other pins show some voltages during operation but I would not know what they actually show me or if they are intended as red out or analogue remote control of some kind.
The reason I am looking for schematics and all information possible is that I like to see if I can make use of the Sub-D connector at the back somehow and I also like to replace the panel meters with higher resolution meters to have two decimals instead of just one. Also general curiosity and in case of a fault I suppose it would be nice to have a schematic.
Thank you and kind regards
Jan P.