7555 PWM wrong behaviour

[m.m.m]

Hi, I've made a 7555 pwm generator from

schematic and it worked well on the breadboard but it doesn't work well on the PCB when soldered. when I adjust the potentiometer the fan at first is at full speed and then when the potentiometer is on the half way it's on low speed and when it's on the other end it's again at full speed. what should I do?
thanks a lot.

[Simon]

check your diodes orientation sounds like they are both the same way round.

[TerminalJack505]

Check to see if you swapped a couple of the wires on the pot as well.

[m.m.m]

no, the diodes are OK and I replaced the potentiometer but no sense.
this pic might make sense.
thanks

[Simon]

Check your bread board very carefully maybe with a multimeter to check that all the tracks that need to be brocken are brocken and that there are no invisible solder bridges.

[m.m.m]

I checked the resistance between the two not-moving terminals of the potentiometer (i.e. the resistance of the whole potentiometer) and it was about 6k, but it should be around 80~100kOHM. It's definitely the diodes, and when I pull the potentiometer off it's place in the circuit it's resistance gets back to normal. And the resistance around the variable terminal of the potentiometer is about 20k when the knob is at the middle and around 10 ohms when it's on the ends of the potentiometer (even in the other end of the potentiometer where it should be around its maximum value).
BTW I used 1N5819 diodes (as suggested in this link)

[Simon]

if it's a 100K pot and half way it measures 20K it could be a log one, what is the resistance in the place of the potentiometer, if you think the diodes are affecting the result then reverse the meter probes

[m.m.m]

no, it's not log scale, it's 50kOhm when not in circuit when the knob's half way, reversing the probes makes no sense (maybe because there is two diodes in both directions).

[m.m.m]

the problem was with the diodes, they were soldered like parallel but the had to be like series.
my bad, sorry.