Uneven temperature on Bosch KSR2701 fridge

[IC WG]

Although it’s 20 years old I’d still like to keep this Bosch fridge serviceable. It’s not the main fridge, but is useful when a bit of extra storage is needed at Christmas, BBQ season, etc.

Problem is that things are getting frozen lower down, and my DMM themocouple measures -8C there the other day. If the temperature control is turned down to compensate it seems too warm at the top. Overall there seems to be a temperature gradient from top to bottom, rather than even all the way. Since it’s 150cm tall there’s quite a distance for that to build up over. I’m looking at getting some temp loggers to plot whether this really is happening and by how much. In the meantime I’m looking at other bits.

I have the electrical workings safely exposed and am using a multi-meter to check things. I can see how the temperature control knob is adjusting the reference voltage to the control op-amp between ~1.5V-2.5V. The thermistor is buried in the shell, but if I disconnect it I get a full supply 10V on the op-amps sensor pin. At the random (test bowl of water in fridge frozen) temperature that I tested it the thermistor read about 16kOhm, which when connected was getting ~3V on that sensor pin (which exceeds the 2.5V so compressor not running). I’m going to to try to test thermistor resistance vs. temperature better tomorrow. Also planning to test control board with some fixed value resistors to get a sense of what resistance range is needed for on / off each of the 1-5 control settings, and any hysteresis there. I might then be able to find a suitable replacement thermistor to position inside the cabinet.

Alongside all that I’m wondering if the evaporator coil is just not cooling the whole back of the fridge, and so a fully working thermistor higher up its never getting cooled while the compressor keeps running and freezes the lower part. The evaporator looks like a single pipe / double skin arrangement. If it’s something like lack of refrigerant circulation I’m considering adding a fan inside the fridge on the switched compressor supply (after checking various current ratings) to churn air around and get a more even temperature might work, especially if I switch the glass shelves for wire ones.

Does anyone have experience of these models, or general ideas to help?

[kwatt]

Sounds like the compressor is on the ay out and, after 20 years that’s no great surprise. That being the case, if you did want to repair it it’d cost a small fortune as that will be using R12 or R134a so will be difficult to repair.

K.

[IC WG]

kwatt wrote:Sounds like the compressor is on the ay out and, after 20 years that’s no great surprise. That being the case, if you did want to repair it it’d cost a small fortune as that will be using R12 or R134a so will be difficult to repair.

K.

Many thanks. I’ll stop looking at the conrtol board then as it seems to be OK based on my tests so far and that would fit your diagnosis.

It’s R600a gas, but I doubt that’ll change the economics of a renovation very much :(.

I’ll try my idea of a circulation fan and see if it evens things up. Any initial bets on whether it’ll work or not? Is a regular 240V fan going to grumble running at 5C? Should I put lighter oil in the bearings.

[electrofix]

if your running a fan inside a fridge you also have condensation issues unless the fan is designed to work in the environment
plus a potential fire issue

Dave

[IC WG]

electrofix wrote:if your running a fan inside a fridge you also have condensation issues unless the fan is designed to work in the environment
plus a potential fire issue

Dave

Hmmm :(.

Do the condensation and fire risk go together? Or separate issues? If the fire risk is from higher current consumption I’d take care of that by running additional / larger wires.

I can see parts like Miele Dynacool fans with housing that would presumably be OK, as long as they are 240V.

[electrofix]

as long as you use a fan that’s designed to be inside a fridge and the connectors are waterproof then its ok

you could find a 12v fan and run it of a small power supply. 12v dc power supplies are easy to get

possibly ?
https://www.ebay.co.uk/itm/233387219972

and low voltage means total electrical safety

Dave

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