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Louie
Louie, Technician
Category: Home Theater-Stereo
Satisfied Customers: 11433
Experience:  Have been in the business 25+ yrs., down to component level.
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I need to chat with a tech who works at the component

Customer Question

I need to chat with a tech who works at the component level.I have spoken with Louie in the past and was satisfied with him.Is there some way to get him on line again.I have a onkyo tx-ds989 and it turns on but after a few seconds it says "thermal protect" and shuts down, and starts to flash in the standby mode.
Submitted: 5 months ago.
Category: Home Theater-Stereo
Expert:  Louie replied 5 months ago.

Hi again Basil and welcome back to JustAnswer,

Thank you for your patience. I am currently mobile with sluggish Internet connection hence the delays.

Stll, have you tried:

  • with the top cover off;
  • check if the blower fan works even momentarily on turn on?
Customer: replied 5 months ago.
Hi There I have taken the compete amp section out of the case and have checked out of circuit all 14 of the output transistors as well as the driver transistors and the 0.22ohm resistors, and found no shorts or open components.The unit does not stay on but for a few seconds.When I turn the unit on the fan starts.I noticed two things the first is that the words thermal protection flash and then the shutdown. I did this four or five times and then noticed that the heat sink on the side with the 4 outputs was very hot so this is why I took the whole amp section out of the case and started to check for shorts and opens. I do have the service manual for the unit but need you to tell me where to go from here.
Expert:  Louie replied 5 months ago.

If the outputs and their respective drivers are confirmed to be good, then check the amp circuit's front end starting with the differential pair (2 X 2SC1845). With these low level/high gain transistors, of particular attention to leaks/reverse checks even more than the forward test results.

Additionally, consider the BIAS adjustment as a possible cause for the above described heatsink heating up.

Please bear in mind that this is a DC amp; i.e. any DC offset at the front end including a faulty BIAS adjustment would result to the outputs being "virtually" driven.

Still on the road but expect to be back at my regular workstation in about 2 days.

Customer: replied 5 months ago.
you asked me if the blower fan came on when the power was turned on and it does not but was running when the heat sink got hot on the 4 output side after repeated attempts to turn on the unitI have 2 of these amps,so I took the whole front end board #6817 and put it in the second(working) amp and it worked just fineCould it have anything to do with the speaker relays stuck on
Expert:  Louie replied 5 months ago.

Appreciate the update Basil and the patience.

Even if "...speaker relays stuck on...", the described symptom of "... thermal protection flash...shutdown...heat sink...very hot..." would not be the result. This is particularly so if there are no speaker/wires connected to the speaker terminals or more so if any of these speaker/wires is/are shorted or very low Ω.

And yes as posted above; front end board (NAAF-6817) as it holds the differential pair and BIAS adjustment circuits.

Significantly and with this board(s) disconnected, it would be possible then to check which particular "area / transistor" is most likely faulty in the suspect board by using the other good board as reference/template. Connect the common ground of each board together and then perform continuity/resistance checks even in circuit of the transistors.

Confirmation of fault would be out-of-circuit / hFE test of these transistors.

Customer: replied 5 months ago.
I don't understand the disconnecting procedure to narrow down the area/transistor.The suspect board was put into a working machine and it worked flawless.Would you not agree that the suspect board #6817 is working properly and that all the transistors are working fine.
This leads me to think that there could still be a problem on one of the output boards even though I found that the output transistors and their drivers checked out good.
Expert:  Louie replied 5 months ago.

My bad; I misunderstood the prior post and agree.

Still, the above proposed side-by-side resistance check can also be applied to the amp board NAAF-6828. Since these boards are the same, then their common grounds can be tied together and resistanc e comparison can be easily done. Any deviation would point to the suspect area/transistor. Then confirmation with out-of-circuit forward/reverse tests.

Please note that this amp board design employs Darlington transistor pairs; i.e. two transistors that behaves as a single transistor but with a very high current gain with the collector of a transistor directly tied to the base of the next transistor. As such, it is this high current flow that causes the output transistor to heat up the heat sink.

If the output and its Darlington driver checks and is confirmed good out-of-circuit, then re-check the first stage pair of Q602 & Q603.

Similarly, re-check out-of-circuit BIAS transistor Q601.

Customer: replied 5 months ago.
Hi
I think you will have to walk me through the steps of narrowing down the suspect area - that is to which one of the amp boards is in
trouble
How do I apply the side-by-side resistance check to NAFF-6828 and do a resistance comparison
I don't know where to tie the common grounds together
where are the points that I would put my meter leads and what kind of reading to expect to get
Expert:  Louie replied 5 months ago.

I understand Basil.

To serve as visual reference for both of us, please post back with a clear picture of the amp circuit board with the heat sink the heats up. Click HERE.

Thanks.

Customer: replied 5 months ago.
let me know if you got the 2 files (pictures)
Expert:  Louie replied 5 months ago.

Got the two (2) pictures; thanks.

It would help to make use of alligator clips with wires similar to the product shown HERE.

Join all the first pins of all the P602s together and then do the same to all the last pins of P602s. This would now simulate that these 6828 boards have a common reference. Do not connect together the center pins.

It would also help to write down the readings in a sort of table/list.

With the multi-meter set to low Ω range and black probe to a first pin of a P602:

  • * touch the red probe to the base of a Q601 of 1 of the 6828 and note the Ω reading. Then move/touch the red probe to the next 6828/Q601/base and note the Ω reading. Repeat until all 6828s/Q601s/bases have been tested. Compare the readings. If faulty, a deviation would be clearly evident;
  • * touch the red probe to the collector of a Q601 of 1 of the 6828 and note the Ω reading. Then move/touch the red probe to the next 6828/Q601/collector and note the Ω reading. Repeat until all 6828s/Q601s/collectors have been tested. Compare the readings;
  • * touch the red probe to the emitter of a Q601 of 1 of the 6828 and note the Ω reading. Then move/touch the red probe to the next 6828/Q601/emitter and note the Ω reading. Repeat until all 6828s/Q601s/emitters have been tested. Compare the readings;
  • * repeat to Q602, then Q603........until Q610.

Repeat but this time, black probe to a third pin of a P602.

Customer: replied 5 months ago.
when I took out the heat sink with all the amp boards attached, I discovered that there were 3 one watt resistors that were well
cooked on board #6824(speaker relay circuit board) R6535,R6545,R6555 they are 18ohms 5% metal oxide all 3 of them
I don't have a complete service manual and I am missing the schematic for board #6824 but I have the parts list page.
I am pretty sure that this is the problem because the three wire harness goes directly to P602 on the #6828 amp board which
feeds the back right speaker
Would you tell me why you think that these resistors got burned up--Is it possible that the back right speaker terminals were shorted
Could I go ahead and replace these 3 resistors (2 are open and the other reads 311ohms instead of 18 ohms)
Expert:  Louie replied 5 months ago.

Nice find Basil!

And yes please; replace with the same type/specs resistors.

These three (3) are tied to the output of the amp and together with C6535 (to ground) serves as a damper load of sorts for spikes and transients. Actually, it helps protect the SURR BACK RIGHT speaker. Might as well check this capacitor.

Please click HERE for the schematics of NAAF-6824.

Customer: replied 5 months ago.
thanks for the schematic I really appreciate that
I still would like your view on the reason why these resistors were cooked
Expert:  Louie replied 5 months ago.

Possibilities; there was a spike/transient amplified by the SURR BACK RIGHT's Q606 & 608 and Q607 & 609 (likely) and/or C6535 has failed (most likely). A least likely possibility (but has to be checked even if just to eliminate as a suspect) is that any 1 transistor of the SURR BACK RIGHT's NAAF-6828 is faulty.

Customer: replied 5 months ago.
I had a little problem finding and ordering the #6535 capacitor. I finally ordered this from Mouser # *****
could you check and tell me if it will work in my application.
It is going to be a few days before I get the parts and after I install them I'll let you know what the results are
once again thanks
Expert:  Louie replied 5 months ago.

Hi again Basil,

Mouser 80-PHE426DJ5470JR17T will do the job BUT may not last long causing the 3 resistors as well as itself to fail (again). 80-PHE426DJ5470JR17T even if 0.047µF (473J) is only rated for 63VAC. The schematics calls for 100V capacity.

Customer: replied 5 months ago.
Hi there
I was able to get these caps instead and they are rated at 160v AC and 250v DC
how important is the dielectric in this type of cap,these are Polypropylene
505-MKP20.047/250/5
Expert:  Louie replied 5 months ago.

Polypropylene film capacitors are commonly used for EMI suppression thus very suitable in this particular application against spikes and transients.