Home Theater Questions? Ask a Technician for Answers ASAP
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:
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.
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.
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.
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.
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:
Repeat but this time, black probe to a third pin of a P602.
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.
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.
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.
Polypropylene film capacitors are commonly used for EMI suppression thus very suitable in this particular application against spikes and transients.