Please Help Harmon Kardon surround amp power problem

[jim]

Hi

Please help i have a harmon kardon AVR630 surround amp. Ive had the unit for about 12 months and today for the first time ever the fan at the back came on and the unit got very hot. I unplugged the power and let it cool down. When it now powers up it is fine for about 30 secs and then it switches off.

Ive tried to see if any components are overheating with freezer spray but no luck so far.

Does anyone have any ideas ?

Thanks Jim

[absorbentgnome]

The excessive heat, probably from the output stages is likely due to excess current through the driver transistor pairs on one or more channels. This could be due to excess bias, or leaky transistors, more likely the latter. Remove each driver pair until the fault clears, but do not leave the amp switched on for too long, since excess dissipation can occur in the earlier stages depending on the design.
Measuring the forward voltage drop using a multimeter for each transistor across BC and BE can identify duff transistors (usually around 0.5V for a power part), compare a number of identical components to root out the damaged part.
Good luck!

[Guest]

Thanks for your help ive tracked it down to a faulty transistor Toshiba C5358 and i have looked everywhere on the net for replacements and cant find any anywhere.

Do you know anywhere i can try to find them ?

Thanks Jim

[Morrowrj]

Here is one that i know has it in stock http://www.bdent.com./search/part.jsp?partnum=2SC5358
as does Newark but they dont have stock http://www.newark.com/NewarkWebCommerce/newark/en_US/endecaSearch/searchPage2.jsp?x=3&Ntt=2SC5358&newSearch=true&searchtype=mfg&Nty=1&N=0&Ntk=gensearch&y=9

good luck
morrowrj

[absorbentgnome]

The datasheet for your component is here:

http://www.alldatasheet.co.kr/datasheet-pdf/pdf_kor/TOSHIBA/2SC5358.html

I'd suggest obtaining an equivalent matched audio transistor pair. Match the specs with an audio pair you can get in the same case type. Most important to get similar or more are the maximum collector emitter voltage (Vceo), gain (hfe), power dissipation (Pc) and collector current (Ic).

[absorbentgnome]

Replace both parts in the pair 'cos if one goes wrong, the other quickly follows...

[Guest]

when i first started testing the transistors one three blocks down from where i was testing blew out so i assume this was the faulty transistor.

2 other resistors blew out which i have replaced along with the transistor now when i power on the amp it swithces back off instantly ( before it lasted for about 50 secs )

When the power plug from the transistor circuit is removed the amp display powers on so the power supply board must still be ok.

What components can i test to try to root out the problem

Thanks Jim

[absorbentgnome]

Which transistor blew? Was it a main output transistor or smaller one in the pre-amp stages? Why did it blow? Was it because you removed an output pair (both the NPN and PNP transistors) and briefly switched on (as I recommended)? Was it becuase of an 'accident' while switched on? Have you made sure the insulating pads and plastic washers are fitted correctly, ensuring the insulation of the transistor's metal tab from the heatsink?

Because components are blowing here, further damage could be done.

Test all of the output pairs again for shorts. Test the forward voltage drop across each junction. (remeber that the PNP transistor requires you to measure the 'other way around' you know what I mean)

Test the pre-amp transistors driving the output pairs as well. Note that you may have to remove the components to test correctly if you think you've found a short or anomaly.

If you find a duff transistor remember you MUST replace its complimentary transistor and check that any resistors in the circuit are intact. You must replace all damaged components before switching on.

When switching on, use a series load like a 60W bulb in series of the mains circuit, to try and limit damage. This method will be less effective for an amp than for a TV, but it is better than nothing.

Use a digital multimeter to measure the resistance of either supply rail to the amp at its lowest setting, it should start off at 0 ohms and begin rising as the current from the meter charges capacitors in the amplifier circuit. Depending of the size of these capacitors this may take some time. If the resistance stays very low, then you still have a problem. Because you can disconnect the PSU from the amp, this makes this process easier.

You MUST ensure that any insulating washers are reinstalled correctly, otherwise you will cause a short from a supply rail and ground. Only switch on when you are absolutely sure (by testing all supply rails with your multimeter) that you have removed all shorts.

[Guest]

The 1st time the replacement transistor was fitted the insulating washer must not of been fitted corrently and I think this is what has caused the short. The problem is that when the amp is powered on it only lasts for 1 second beofre powering off so it is difficult to test the circuit. I have taken some pictures as the transistors seem to be seperated into four blocks of four. It is the larger one of the two that has blown in the picture, a Toshiba C5358, do I need to replace the one on the left hand side which is a Toshiba A1986? Do you know where I would be able to get the service manual for the 630?

http://www.gadgitech.co.uk/product_thumb.php?img=images/amp1.jpg&h=480&w=640
http://www.gadgitech.co.uk/product_thumb.php?img=images/amp2.jpg&h=480&w=640

[absorbentgnome]

Testing the circuit is to be done with the power off!!
Do not apply power until you are sure you have cleared all the faults.

Posting those pictures helps a great deal.

There may a short between the back of the transistor (the Collector) to the heatsink, which is grounded. Test this with the continuity tester on your multimeter, with one probe on the heatsink and one on the tab of the transistor. If there is a short, you have incorrectly assembled it.

Making an educated guess without seeing the underside of the PCB or knowing exacly how many separate output there are, the two large transistors are the power output pair, and the smaller ones constitute the pre-amp stage for driving the output pair.

Remove all four transistors using soldawick or soldermop, noting their position and carefully preserving the mica insulating pads.

Test each transistor's junction forward voltage using the diode option on your multimeter, or the continuity tester (which on most DVMs reads forward voltage). For NPN transistors like the 2SC5358, put the positive lead on the Base lead, and measure the forward voltage for each junction by placing the negative lead on the Collector pin and then the Emitter pin (use the datasheet to identify the pins). The forward voltage will be between 450-600mV for each. If it is 0V it is shorted, if it below 350mV it is more than likely knackered, if it doesn't read anything it's open. For the PNP transistors like the 2SA1986 you will have to measure by putting the negative lead on the Base pin.

Do this for all four transistors and post the result back here, labelling which pins you measure between and what polarity.

Also, with the other transistors *in circuit* for the other channels do the same thing, measuring the forward junction voltage on each transistor with the power off, to ensure there are no other faults in these stages.
Write these down too, and post here.

A service manual will not help you if you do not have much knowledge of how audio amplifiers work. (In fact I rarely find service manuals useful unless a circuit diagram is genuinely required.)

Do not apply power at any time in this process.