Marantz 4270 Help...

[Ray122]

Alright, so I've got a Marantz 4270 with a bad channel, spitting out DC and throwing it into protection mode. Only one of the output transistors was shot, I believe it was the one that handles the negative swing. Otherwise, all the semiconductors on the board and otherwise are testing OK.

I can get the unit to come out of protection mode when I remove the output transistors on the bad channel, un-surprisingly, but with that, I can see I've got around 5.8V DC on pin J712 (the base of the output transistor). On the working channel, this is around .8V

(I should also note that I'm taking all my measurements while using a DBT so all the voltages are lower than normal.)

I can trace that 5.8V down to the emitter of the driver transistor (H712), through the base of H712 where it's now around 6V. Following that to what I'm guessing is the voltage divider that set the bias for the drivers (R723 and 725) at which point I have 7V. For comparison the working channel at that same point gives me .99V.

And this is where I'm stumped. Not really sure how exactly this circuit works here or if it's even a "voltage divider at all. Either way not at all sure why I'd have 7V on one side and .99V on the other if all of the components are testing OK. I did try replacing R723 and R725, as well as C710 but no luck. Would love some insight into what I may be missing here.

Thanks in advance!

 

[Ray122]

Quick update for anyone who may find it useful!

I narrowed down the issue to D010. Which is an unusual suspect. But considering, it's the only logical path to "ground" or in this case the negative voltage line, that was my next place to look. It tested ok as a diode in the circuit, but it was preventing the 7V from traveling to the base of H714 which isn't what I expected it would do... With that being somewhat of a special package diode, I figured my best bet was to swap it with the one from the working channel to see if the issue would follow the diode... Which it didn't... but both channels are working now! Go figure! Maybe a cracked solder joint that just needed to be reflowed or something. Kind of a hollow victory, but we'll see what happens next. I've yet to take it off the DBT or try putting the four amps into stereo mode.

So I'll let you know if I manage to blow it up again!

 

[blhagstrom]

Does sound like a trace crack. Bad solder joint perhaps or funny ground path.

Great job tracking it. Backtracking voltage from the output is the trick. Trying to explain that to people online here is a challenge.

FWIW, those 4270 work their power supply hard. If you really want a puzzle, recap that thing.

 

[Lavane]

Not a hollow victory at all if it's working now.

 

[Ray122]

Turns out the saga continues!... Unfortunately...

So after swapping those diodes, the unit powered up on the DBT no problem. Everything was looking great. So I went about my day, saving the final tests for this evening. Went to power up the unit, and it didn't want to come out of protection mode again. The DBT wasn't particularly bright, so I assumed maybe the larger current draw was too much for the 45W bulb I was using. Swapped it over to a 75W bulb, and although it took some time, the unit powered up and came out of protection mode.

At this point, my biggest problem was the protection mode trigger when I brought the volume up. Right about 1/3 was all the volume it'd take while in the 70-watt stereo mode. This made me a little cautious to take it off the DBT, but I figured that would be the next logical step considering everything seemed to be ok. At that point, I stopped for dinner. Came back to it after that and now, the 75W bulb is coming on bright stealing all the power away from the receiver.

No dead shorts across any of the output transistor pins, but now the only thing to do is go back to square one, remove all the output transistors from the heatsink, and power it up to check on those pesky diodes again.

It seemed to me like the issue got worse each time I powered the unit up. My guess is maybe one or both of those diodes are intermittently leaky. But at this point, that's next week's headache. I'll pick it back up on Monday and keep you all posted!

In the meantime, does anyone have any part suggestions for replacing H010?... which according to the service manual is a varistor... (Marantz part number HV0000308 SV-02)

 

[Ray122]

I know I said I was going to wait until next week, but there are some projects I just can't walk away from! Plus, it's raining today, so it's a shop day.

Did some research on that SV-02 and this guy had some luck using a pair of 1N4148 diodes in series as a replacement.
https://audiokarma.org/forums/index...id-state-troubles-marantz-2235b.404648/page-2

First things first were to confirm the issue was still present. Powering up the unit still on the DBT, and yes, that bulb was still bright as ever. Just like I left it.
Pulled all the power transistors out and check the two diodes (Varistors). Although I was a little fuzzy, on the reading I got from the good diode/varistor, it was apparent my issue did follow to the other channel when I swapped the bad one onto the other channel.

To confirm this, I poped the output transistors back into the channel that didn't have the bad diode/varistor in it and powered it up. The bulb went dark, the unit clicked on. Looks like it's definitely that diode/varistor causing all of my headaches.

Swapped out the bad diode/varistor for a pair of those 1N4148 diodes, put all of the output transistors back in, powered it up again, and success!! While in 4 channel 25W mode, all channels working with little to no glow from a 25W bulb!

For what it's worth anyone scouring AK looking for a replacement for SV-02, two 1n4148 diodes in series are working great for me!

Switching over to 2 channel 70 watt mode, and the unit clicks on and off as it draws too much current across the bulb. Switching over to a 75W bulb solves that problem. I was really pushing my luck there since the whole receiver is rated at 400W power consumption.

75W bulb seems to be handling the 2 channel mode just fine providing I don't push the volume the past 11 o'clock position. In that event, the bulb gets really bright really quickly, and the unit powers off. I know I'm still pushing my luck with a 75W bulb on a 400W device, but I'm still a bit concerned. In my experience, the bulb usually gets brighter and brighter as you bring the volume up before the unit clicks off. Is a sudden current draw from the unit like I've described typical for these amps? Or should it slowly be getting brighter, then powering off? I'm nervous to take it off the DBT and risk any of the output transistors. Before I run to the hardware store and get a 100W bulb, would anyone else be worried here? Would love any thoughts or insight anyone has to share.

Thanks!

 

[Ray122]

Hopefully, this will be my last update on this tread, because long story short, we're good.

I picked up a 150W light bulb, and although the unit still suddenly draw a bunch of currents as it clicks off before I can get the volume up halfway, I was able to get the volume up, which is proof enough for me it's just the DBT doing its thing and not the amplifier trying to self destruct. So I took it off the DBT and it's running fantastic now! Honestly, I'm a little surprised that those two 10 cent diodes are all it took to bring this thing back to life. I would have liked to replace that diode/varistor with something closer to the original part, but hey, I'll take it. It is running great, and I was able to adjust the bias and the offset with no problem. Also good to know for the future. Hopefully, someone finds this thread helpful for troubleshooting their own Marantz or for finding a replacement for that SV-02 diode.

Already looking forward to the next challenge!