Tumble dryer fault - input PSU circuit?

[gutbag]

I am trying to repair a tumble dryer board that is producing no DC voltage to the micro. The mains voltage is about 240AC (UK). I have found two resistors at the power input stage that are clearly sick (one burned and open circuit, the other measuring about 2 ohms). They are R1 & R2 in the attached schematic. The schematic is just the input stage that I’ve been able to work out so far, with lots of tracks hidden under components. I’m pretty sure R1 is 62 ohms although it’s burned and the colours are not 100% clear. R2 is brown, black, violet (I think), silver, so 100M. Before I change both and fire it up, and as I’m out of my depth with this design of power stage, I have a couple of questions:

1. Are the values 62R and 100M plausible for R1 & R2?
2. Is it likely that one or other of these resistors is the root cause of the problem? There are no other signs of stress on other components and the diodes all measure OK.
3. Are there any other specific things I should measure?

Thanks!

[wasedadoc]

I doubt R2 was 100M.  Maybe 1k.  Looks like the diode at top right and the RC parallel pair connected to its cathode would be the flyback recovery components of a switched mode PSU. R!, D1 and R2 would need to pass the current used by the primary side of the SMPS and R2 being 100M is much, much too high resistance for that.

I would not be surprised if the SMPS chip U1 is dead.  Gone short and large current flow through R1, D1, R2, transformer primary, chip to neutral has cooked R2. You might be lucky and find that it was C2 that went shorted.

[floobydust]

Those parts don't burn up unless there is an overload due to another fault.
They have a safety function being flameproof, fusible resistor and the other looks like an inductor. So replacing them just leads to more smoke.
I would say it's likely the SMPS IC LNK304PN has maybe shorted and I would check that as well as the diodes.
Wild guess 6.2R and 1mH.

What make and model is this? 40005949, Bitron 30411814, Hoover VHC691B-80.
You can find other board pics on the web too.

[fmashockie]

As floobydust said, what you have labeled as R2 is probably an inductor.  They pretty much have the same color code as resistors.  This is also why its measuring ~2ohms.  When resistors fail, they typically fail open, not short. 

Floobydust also shared an application schematic for the LNK304 which is a AC/DC converter for the switch-mode power supply.  I have it attached as well.  If you look at it, you can see there's a 1.2mH inductor in parallel with the D2 rectifier diode.  The bands on 'R2' look like they are black, brown, red which would indicated a 1mH inductor - matching very closely to the application schematic for LNK304.  Also, you said R1 is 62 ohms.  This would also match the application schematic somewhat.  The manufacturer states to use a 8.2 ohm and 47ohm fusible resistors in series (~55ohm).  Looks like they may have settled for one fusible resistor at 62ohms.

I also agree with floobydust in that there's a good chance that the AC/DC converter is fried/shorted.  Fortunately, if it is, it is a very inexpensive chip.  Only $0.75 per unit.  You can get them here at mouser https://www.mouser.com/ProductDetail/Power-Integrations/LNK304PN?qs=Jph8NoUxIfXysUyZfWy7Lg%3D%3D

In my experience repairing switch-mode power supplies, if the fuse has blown (in this case a fusible resistor) there's a good chance the switching device is as well (in this case the LNK304).  But definitely check the filter caps (electrolytics) and rectifier diodes as well.  The filter cap(s) might have been what led to the failure in the first place, then creating a cascade of secondary failures.  Feel free to reach out if you need more help. 

[FIXITNOW2003]

what make model ? i may be able to track down circuit

[xavier60]

Referring to the diagram in post #3, C1/C2 failure is often the cause of the LNK failure. In some controllers, the 2 HVDC caps are separated by a small choke. If it's High-Side Buck Boost topology like in the post, the control circuity usually survives the LNK failure.

[wraper]

"R2" is certainly an inductor. While the difference in appearance may look subtle to untrained eye, resistors do not look like this.

[gutbag]

what make model ? i may be able to track down circuit

Thanks, it's a Hoover VHC 691B, bought in 2012. I haven't been able to find a schematic.

[floobydust]

Using the board number or dryer model you can find pictures of the boards, that might help confirm the resistor's value.
I would look for the short - the IC or diodes or capacitors etc. before solving the resistor question.
The LNK304 is weird and has no GND pin, so if it shorts D-S that could be very bad overvoltaging the loads

[gutbag]

Thanks for all the advice. The board is now working. I checked all the power-related caps and diodes and they were all OK. I replaced the fusible resistor R1 and the LNK304PN but it still didn’t work and blew the resistor again. Then I found this video in which a very similar board is repaired:



He treats what was believed to be an inductor as a 1k resistor. The colour code matched mine, although I had initially thought it was 100M because the red looked violet. Having run out of other ideas, I tried it, and it worked.

Thanks again for the advice. It helped a lot and I learned a few things.

[fmashockie]

Nice work! I think why so many people (including me) assumed that 1k resistor was an inductor was due to the fact that you initially reported that you measured it as being ~2 ohms.  As said before, when resistors fail, they fail open.  Just like in the video that you shared.