Marantz PM-80mkII issues (I'm sorta stuck)

[epicyclic]

Biasing has always been the side issue , the main issue is premature protection activation .

The next step will be to see if its over current or erroneous DC detection that is causing protect mode to kick in , whilst trying to avoid waiting 20 minutes for a result , which is why I suggested cranking the volume up early to determine if it is generally over protecting itself although I am not advocating ridiculously high sound levels . Just to dispel a minor thought that I have had …. are you using loudspeakers that have a difficult impedance to drive .

 

[epicyclic]

Biasing has always been the side issue

May I retract this statement and just say that with a mostly working amp it was
an issue that needed resolving but was unlikely to be the main problem .

 

[Lalaland]

Tested again as you suggested and amp tripped protect mode pretty immediately on loudish volume, but not any louder than I would listen to for a good while. Amp plays nicely at low volume now 20 min later.

My thought about the dropping bias voltage was if there is some component that drains/shorts the circuitry when heated

Regarding DC detection I'm inclined to believe there would be some audible pop or something but I can't say there is any abnormality when it trips

 

[epicyclic]

If CN06 on the power amp board ( connecting to TA7317P ) has dried out so not shunting AC then the DC protection circuitry could be wrongfully detecting . CN06 is a polarised electrolytic handling a reasonable amount of AC with next to no polarising voltage . Removing RN05 and RN06 ( both 68K ) would help us get closer to a solution by disabling the DC side of protection just leaving over current protection in place . If that makes no difference then removing RN08 would disable the over current detection , all a process of elimination .

 

[Lalaland]

Amp played fine at low level for over an hour before I had to turn it off. Forgot to mention earlier what speakers I'm driving - XTZ 93.26 mkII witch should not be too demanding. I have been using a tiny NAD 310 as backup since the Marantz would not give, and my tube preamp and power amp combo got smacked by lighting a few weeks ago. The NAD never complained. Damn, a lot of broken components lately - my Hegel DAC as well


Anyways, I'm really liking your systematic approach in fault finding, and I want to thank you for sharing your knowledge. I'm really learning a lot here.
Time to start de-soldering as you de scribe and do some more testing. I'll get back as soon as possible, but looks like the weekend could be quite busy so might take a few days

 

[Lalaland]

Ok, so I pulled out the P851 board to do some visual inspection and it was littered with bad solder joints. With some small pliers I could easily wiggle many of them completely off the trace. Some had already loosened without touching them. I'm gonna start re-soldering all the joints on this board and test again. P701 joints I checked early in the process and it looks good - perhaps give it a new look just in case

 

[Rick PA Stadel]

Good catch on the P851 board -- hopefully that makes it happy. I have to second JonSnell Electronic's suggestion to replace C711 and C712, though. That's a challenging job for an electrolytic anyway, and those are no longer new. Also hopefully, somebody on here smarter than me will address the polarity issue: There may be a good reason (e.g. power on sequence causing a higher applied voltage) for it to be reverse-biased to -0.2V, but I would seriously consider flipping it (even though -0.2V isn't enough to make it really angry). Or perhaps install a pair of them, back-to-back, since that is a widely applied solution.

Regards

 

[Lalaland]

really [/I]angry). Or perhaps install a pair of them, back-to-back, since that is a widely applied solution.

Regards

Thanks for chiming in. Was wondering if you could elaborate a bit perhaps on the above. Do you mean flipping the polarity of the caps itself? And back to back, as in parallel two of them? Sorry, I need somethings dumbed down a bit to understand Sadly I kind of forgot about JonSnell's post as I got more and more into the other stuff about versions of amp boards and stuff - but I will follow up on this advice and order some caps.

Anyways, here is an update after re-soldering all joints on P851:
Amp works fine in AB at reasonably loud volume for about 5-6min then trips, Class A only works a minute or so. So it feels like it is slightly happier now but still... One observation though - before P851 fix protection mode meant full blackout (power led and all), now a relay clicks, but all leds stays on. I'm thinking full blackout was a abnormal behavior.

epicyclic; I want to continue with your suggested elimination method but have one concern though. If i disable DC detection and that is the case (DC), will that not pose a threat to damaging my speakers? I have read briefly about dummy speaker loads - would that be a safer route?

Thanks, and regards

 

[epicyclic]

Yes dummy loads would be safer . Just had a thought , try the procedure without speakers before removing resistors and see what happens it should help eliminate over current detection .

Yes the power led should have stayed on . Check the rest of the boards for doggy joints .

 

[Lalaland]

Yes dummy loads would be safer . Just had a thought , try the procedure without speakers before removing resistors and see what happens it should help eliminate over current detection .

Yes the power led should have stayed on . Check the rest of the boards for doggy joints .

Wow, good call! It trips with no load just like it does with load. Tried in both A and AB. So over current is not the case we can assume then

 

[epicyclic]

Ah progress , the next step would be to remove RN05 and RN06 and see if that stops the tripping . If it does then reinsert just one of the resistors and try again . I want to eliminate the vague possibility of either of the channels putting out spurious DC , so you will need to then remove that resistor and reinsert the other one and try again . All testing still without speakers .

 

[Lalaland]

Update: Moved amp from living room to hobby room, and tried no-load test again, because I wanted to check with no input as well - amp started in protect mode and never activated the speaker relays.


Had a look and wiggled some joints on amp board - found a few bad ones - fixed them - removed RN05 and RN06 - turned amp on main glass fuse totally splattered immediately

 

[epicyclic]

amp started in protect mode and never activated the speaker relays.

This is a new fault symptom . After fixing these additional bad joints did the amp then power up OK before you removed RN05 and RN06 . Removal of these two resistors only disables DC detection on the outputs which is there to protect the speakers only .

It sounds like this amp is littered with doggy joints .

Check you haven't bridges two joints with solder by mistake .

 

[Lalaland]

I removed RN05 and 06 while fixing the joint I found bad, so I can't know. Offcourse I might have brigded two joints as you point out, so need to have a closer look


Only thing I had the time to check is if the glass fuse holds with P701 disconnected - it did. All voltages seems ok, but class A voltage does not drop, but relay activates. Suspecting the relay have burnt/welded contacts after the blown fuse last time


Connected J705 to P701 - no problem. J704 to P701 - Glass fuse splattered. There is no continuety measured between pin 1-2-3 on J704 when powered off though

 

[Hamish119]

...but this amp belonged to my late father so it has some sentimental value.

In that case, please stop what you're doing and build this before you destroy the amplifier. You're scaring me (and possibly) other readers!

Watch this.

********* WARNING/CAUTION/ADVARSEL ***********
********* MAINS VOLTAGE CAN KILL YOU ***********

You'll require a minimum of 1x RCD, 2x 60W Incandescent bulbs and electrical knowledge.

Good Luck!

 

[epicyclic]

The above is good advice when you come to repower the amp up , given that it is now blowing the main fuse .

You can do a lot of checking WITHOUT power though .

Use your multi meter to check for a short circuit on the 3 pins of J704 on the P701 amp board ( all combinations of pins )and also check the left and right outputs resistance to a ground pin ( amp side of relay contacts ) .

I have checked , double checked and triple checked that my advise to remove RN05 and RN06 as a diagnostic aid is sound , but I still feel bad about the situation you are now in .

When you disconnected the P701 board and checked the class A voltage change you wont be able to see it because the 15000 uF caps will stay charged at the higher voltage as you disconnected their load .

 

[Lalaland]

Thank you for good advice. This project stopped beeing interresting and started feeling a bit risky, so yes I should stop.... Just to clarify since I seem to be scaring people. I feel like what I'm writing does not always "paint the whole pictute" as regards to my own safety working on this amp. I'm not completly lost on the dangers of mains voltages, so I'm not just unplugging/replugging and doing all this with power on, and offcourse am very aware of caps holding a charge after powering off


All that beeing said - I totally get your point, and will not continue work before I have some safety measures in order


epicyclic - you should not feel bad. I'm pretty shure this amp is littered with small faults that makes this a complicated and unpredictable operation - and above my own abilities. You have been a great source of help, and I have learned many things.
I did check the pins on J704 for short circuit, as I mentioned in my post the other day, but there is not a short


I guess it's time to build a current limiting device....

 

[epicyclic]

You have been a great source of help, and I have learned many things.
I did check the pins on J704 for short circuit, as I mentioned in my post the other day, but there is not a short

Thanks .

Yes I missed that . If the high current flow is through a transistor then you wont see a short circuit with digital meters unlike the old analogue . If you have a diode test position on your multi meter you can test the base/emitter base/collector etc junctions of the transistors , expect to see approx. 0.65V when the BE and BC junctions are forward biased and open circuit when reverse biased ( PNP and NPN have opposite polarities to each other ) . Please note this is a function of the multi meter to use WITHOUT POWER on the amp , both leads need to be applied to the transistor being tested . I am suggesting here testing the transistors in circuit so you might see some anomalies and comparison between left and right channels is a useful ploy . If you try this start with the output transistors first and move backwards , you can also test diodes .

 

[epicyclic]

PS to the above .

Test the EC junctions also , out of circuit they should test open circuit both polarity directions but in circuit you a likely to encounter a lot of anomalies that's where comparison can help .

 

[Lalaland]

Thanks for further advice. I will try that and report back