HK Projector HF Limit light on constantly

[TheSoundGuy43]

Dave I can't thank you enough. Thats the kind of advice / pointers I was hoping for. Thanks for taking the time to put it all in writing. I'll work through what you have said and come back with an update for anyone whose interested.

 

Once again thanks for taking the time to offer help.

 

Craig

[TheSoundGuy43]

Hi Dave

 

Thank you so much for taking the time to offer me advice. I have done exactly what you stated. I used the Multimeter in continuity mode and measured the reading from each of the heatsinks to the speaker connection. Based on your advice and from what I've read, I think I have identified the problematic PCB. Firstly I have created an image to help describe what I've found so far.

 

 

As you can see I think that the erroneous multimeter readings on the upper PCB indicate that this is the damaged one. The continuity readings on the lower PCB seem to indicate that it is working fine. Am I correct to think this? Which then leads me to believe that the upper PCB controls the MF's and the lower one controls the HF's. As soon as I can confidently identify the damaged board I can order a new one from HK. (both seem to be in stock for a change). Im having trouble finding which part number refers to which board as the diagram on the HK website doesn't load. Any more advice on this would be great. Hopefully I will soon be in a position to replace the damaged part and fire it up.

 

Once again thanks for everyones help.

 

Craig

[sandall]

......leads me to believe that the upper PCB controls the MF's and the lower one controls the HF's.

 

Can you not confirm this by checking which wire goes to which speaker?

[DrV]

I assume that by "001" you mean you are seeing 1 ohm between the heatsink and the spade terminal in each case. If so then THIS is the faulty board. This resistance you are measuring is between the collector and emitter of each transistor. (The heatsink is the collector and the spade terminal is the connection between the emitters of the transistors - it's a conventional complimentary pair with an NPN transistor in the top leg and a PNP in the bottom. Actually they are Darlington transistors but that's not relevant any more as they are now a short circuited blob! As Sandall says, you should easily be able to confirm which speaker this is connected to by either following the wires or just connecting a battery between the speaker terminals and listening for the click (for an HF unit) or the thump (for an LF unit). You will find that at least one of the fuses on this board is blown as a pair of shorted output transistors just shorts the power supply rails together.

 

It's up to you whether you replace the whole board or repair this one but I have repaired at least twenty of them and it is easy to do. If you count the solder joints it is actually less work to repair than to replace! If you want any tips on repairing it then let me know.

 

As for the reason for the failure - almost all the units I have repaired have been the Elias range which use the same modules but not the same speakers so I can't tell you whether the speaker is likely to have gone short. You should test the speaker separately using a small amp if possible. (Ideally you use a speaker tester which would clearly show up shorted turns but just trying it out at low volume on a small amp will do.)

 

I have never actually seen a speaker cause the failure of one of these modules but I have seen lots of modules just fail for no apparent reason. I believe the cooling is a bit marginal, especially if the vents or the fan get full of fluff. This is exacerbated by the fact that the bias current specified in the service manual is somewhat higher than necessary. I always set this a little lower than the spec and it doesn't seem to give rise to any crossover distortion. I have never had one of these modules come back after a repair so perhaps this does help.

 

By the way, we've been here before!

[Jivemaster]

Don't be afraid to replace the cooling fan with one rated for more airflow. While the inside may be clean now, possibly the original failure was a cooling failure attributable to disco smoke and cigarette smoke.