Marantz SR7007 - not starting - rapidly blinking LED - defect transistor MMBT5551 in ASO protection circuit

I always wanted to peek into an AV receiver even though I know that these devices are notoriously difficult to service due to their tight packing of boards. But sometimes, we just need challenges, right?

The SR7007 caught my eye on eBay. It is still a quite young model (2012) and is technically mostly identical to the 08 Series of 2013. I need preamp outputs and this one had all channels. Nice.

When I switched it on, I heard one click, the display went on and then after about three seconds it switched into protection and the LED blinked rapidly about twice per second.

I suspected a problem with the power amplifier because after the delay normally the speaker relais should switch. Knowing nothing about this device I first tried my luck by isolating (aka plugging off) parts of the power amp board. I got lucky by pulling the high voltage supply. The receiver started and switched the relais after 3 sec. I found nothing suspicious on the power transistors. They all measured identical. Neither did I see any burned parts.

I studied the thing a little more and identified a cable with five lines as the protection signal cable, which goes to the main digital board.

When I pulled that, the receiver would again go into protection, albeit with a different blink code (1x per second).

Now it was time to get the service manual, which is only available for money. 12$ is not a bad price, so I got one and it proved to be worth every penny.

With the high voltage unplugged from the power amp board, the device could be booted in a diagnostic mode where it displayed the last protection error. After the steps in 3.2. shown below, you have to press the Status button on the device. The description is somewhat confusing.

I did that and I got this:

For ASO (no idea what this means), the manual states the following:

Now that did not make any sense. The receiver had never switched the speakers on and neither was there any speaker connected. However, it pointed me to the direction of the ASO protection circuit. Something had to be wrong with it.

Q7001 does a logical OR on all ASO signals supplied by the 7 power channels ASO circuits. The collector signal goes straight to the digital board:

The red line is high voltage plus. It made perfect sense that when this voltage is missing, the ASO protect signal would not go to the digital board. I confirmed through measurement that the transistor switched on during boot. So at least one of the channels must have had a problem.

Now about the actual ASO protection circuit (here: surround back right channel). The marked transistor senses excessive current and when switched on, R7744 passes a plus signal on to Q7001 on the previous image. This circuit is identical in all 7 channels.

The inconvenience was that all the parts for the protection circuit were on the underside of the board. The unit is easy to remove, that was not bad at all. I decided to bend the legs of the power transistors to get access to the underside. Better than unscrewing all of them (horror!)

Now I checked all those 22k resistors, which go from the sensing transistor to the ASO switching transistor's base. And lo and behold, one was off by 3kOhms. The surround back right channel had a problem.

The only possible failure could have been Q7739, the sensing transistor. It measured ok with the diode tester, but it showed a lowered resistance between C and E. I unsoldered it and presto, the resistor R7744 measured normal again. My transistor tester did not recognize the MMBT5551 transistor at all. It showed me two resistors instead.

It is the little guy in the center of the image:

I replaced that sucker and the device went back to normal. A cheap fix, time-intensive and very rewarding. I learned a lot. I always like to improve things after I have fixed them, but in this case I didn't come up with anything. It is hard to tell what happened here as the power stage was still in good shape.

While I was at it I took the chance to adjust the idle current of the power stage as well. It is nicely documented in the manual. The SBR channel was off the most. Was that a coincidence or some further symptom of  the incident? I don't know.

About the Marantz SR7007 in general

I never had an AV receiver and I was curious how it worked. Well, the sound is considerably worse than my highly modified Benchmark DAC2, which feeds Focal SM9 active speakers. That was no surprise at all. I have seen its guts and I knew.

What did disappoint me was the image quality via HDMI. One might think that digital signals will not get compromised in digital processing. Not true. The passed-through image definitely lost sharpness to a degree I don't accept it. So that was no option. I am watching movies via my PC and its optical output worked well with the receiver.

The serviceability is not so bad, really. The service manual is very good. The boards, which have the highest expected failure rate (digital board and power amp board), are easy to remove. I think none of the receivers have a bottom lid anymore for an easy checking of the power transistors. They replace whole boards these days and don't mess around.

Replacement of Panasonic Plasma parts (DAF30, 30F131, RFUH25, DG302, RJP30H2A, RF1501N)

The original Panasonic parts for the NeoPlasma series 30 and 50 are slowly going extinct. As I am a big fan of those devices, I was spending some time to find alternatives. I was able to source all parts either from AliExpress or DigiKey.

The parts listed here are susceptible to failure on SC or SN boards, respectively, and SS boards.

Use this information at your own risk! This is my result of doing research and experiments. So far, all of the Plasmas I have fixed are still running fine after 1-2 years. If the parts were fake, they would have blown up after a short time.

You can never be sure whether a chinese seller has the same quality parts on stock after some years. Take my tipps as a starting point. I wrote this article in early 2017.

New Kid in Town? TGD40N40P

I found yet another potential alternative to all the IGBT transistors: TGD30N40P.

This type is manufactured in Korea and is around since 2012 according to the datasheet. I will test them soon. Stay tuned.

I think that the small number of sources on Aliexpress is a good sign. The market does not yet seem to be swamped with crap copies.

IGBT Transistor GT31F131

Can be replaced with FGD4536. That I know for sure, because I fixed a TX-P50GT30 and a TX-P55VT30 with those successfully. Alas, this device is also discontinued.
My source has dried out. Stephen Foxall found a legit source HERE.

IGBT Transistor DG302

Datasheet
This transistor seems to be the strongest in the bunch. 250A peak and 40A continuous current. It's good to have a bunch of them on stock. I don't know any exact replacement for this one. It's the gold nugget of the circuit.  The FDG4536 might be a viable candidate. It is difficult to tell because the interval for the 250A peak current is not specified in the minimal data, which is available for the DG302. The FGD goes up to 220A for half-sine, pulse-width 1µsec and its switching times are even lower than the DG's.

In the meantime I have tested a 42VT30 with the FGD instead of DG and it runs perfectly fine. The FGD run at around 55°C with the original heat sink glued on top and that is totally normal.

I am happy to finally share a source of legit DG302 on Aliexpress! They measure exactly like the originals and one did work flawlessly recently in a 42VT30. 

IGBT Transistor RJP30H2A

Datasheet
Again, I fail to see the distinctive difference to the DG302 und 31F131 besides the 5A less collector current. I recon they could have built the whole thing with DGs exclusively. This type isn't used in the 50 series anymore. I have replaced them successfully with FGD4536. Even Panasonic uses an 31F131 instead in the scan board of the TX-P55VT30.

Generally, the problem with the data given in the sheets for the maximum  pulse current is difficult to compare as the manufactures use different pulse lengths. The F131 is rated for 3µsec, the DG302 datasheet doesn't tell anything about it, and the FGD4536 is rated for <1µsec. This leaves the hobbyist with trial and error as the only option.

Diode DAF30 (DA3DF30A)

Datasheet
Can be replaced with STTH20R04G. The datasheets are a perfect match. If you want more juice, the STTH30R04G will deliver it, it's a monster diode. I recently used STTH20 twice as substitute and it runs perfectly fine with normal temperature.

A STTH30 recently also worked fine in a 42VT30.

Diode RFUH25

Datasheet
Why Panasonic is using this diode alongside the DAF30 is a mystery to me. The specs read the same. Maybe there is some subtle detail I don't understand. I think the STTH will fit here, too.

Diode RF1501N

Datasheet

And yet another diode, which looks the same as the others. From the specs I cannot see any significant difference to the DAF30 and RFUH25. The DAF is a few nanoseconds quicker at recovering. They are dirt cheap and available from DigiKey or Mouser or even cheaper HERE on Aliexpress. The chinese source is legit. I have tested and used the diodes successfully.

Control Chip BD8639FVM (SMD Marking D86)

Bought some from THIS source without problems. Update: has been reported to have become a bad source.

Driver Transistors CPH5524 (SMD Marking 3Y)

Bought some from THIS source without problems.