Marantz PM7200 Can't adjust Bias or DC Offset

[mossieurclo]

Thanks for reading me:

After i repaired the speaker protection board of that amplifier, all was ok.

So I then proceeded to adjust iddle current, got it, then DC offset. Oh, bboy.

At that stage, on the left channel, after first seeing normal readings while moving the trimpot to get to the correct setting, suddenly readings were all over the place and did not respond to my turning the trimpot.

I played music to earphones, to find that there was no sound on left channel. Only distorted crackles on peaks.

I took out the board and took out the trim pot, measured it as being ok from end to end and from the moving wiper. Cleaned it anyway and retested as OK.

And, i resoldered 7265 that had broken solder joints. That component was probably the one acting up (relays clicking) when i pushed away the supply wires (touching 7265?) to have access to the trimpot.

(BTW why do these (TO126?) stand so high on their legs, wrapped in shrink tubing??) I also reflowed the connector for the supply lines, just to be sure.

I reinstalled the board. The audio is now heard on left channel. Distortion is hard to qualify on cheap earphones and with my old ears and in haste not to blow anything up.

But: i can't adjust either Iddle Current (.2mv) or DC Offset (160mv). Trim pots have no effect. With power off i rechecked that DC offset trimpot and it was still behaving correctly.

I probed the supply lines as deep as i could, including around some transistors (results i should not quote now, but maybe of interest: 7261 is E 53.2V, B -52.7V, E -53V). All seemed normal in my limited understanding of those circuits. I tested in circuit the first components branching out of + and -VB, and double diode tested 7265 and 7267. I did not probe under the copper shield (housing 7251, 7253, 7255, 7257, 7259) yet.

I hope there is an obvious "classic case" component causing that anomaly, which some knowledgable and experienced person here will point out to me.

I saw that i can unsolder (update: not easily!) that copper shield and probe those components. If someone points me to what i should measure there?

BTW all i have is a digital multimeter. It can test semiconductors. It also has capacitance capability but seems unable to measure >20?uf . To find good units in my NOS stash i rely on reading DC volts, discharge, remeasure DC volts, do long Ohms test, and remeasure DC volts to see if it went up. Served me well to date ;-)


Thank you all for your insights and help.

service manual:
https://www.hifiengine.com/hfe_downloads/index.php?marantz/marantz_pm7200_service.pdf

[petehall347]

Re: Marantz PM7200 Can't adjust Bias or DC Offset

have you gone through class A and AB adjustments ? have you tested the switch ?

[tibimakai]

Re: Marantz PM7200 Can't adjust Bias or DC Offset

You may still have some bad component, in the amplifier that has caused the issue.

[mossieurclo]

Re: Marantz PM7200 Can't adjust Bias or DC Offset

have you gone through class A and AB adjustments ?

Yes i did adjust Bias for both Classes, before trying to adjust DC where it went crazy. And after resoldering 7265 i could not adjust either DC or Bias.

I did not go back to try and adjust Class A Bias because of fear of letting the blue smoke powering all electronics get out.

But as mentioned the 56V suplies are present as deep as i could probe.

Am now in the process of removing the copper shield (i'm just about ready to try with a torch) over the first stages to probe around there.

have you tested the switch ?

You mean the Class A switch? It's relay clicks and led lights up...But next powered tests will include that Bias. After i get the copper shield off and probe under there, and resolder 7267 that i took out to test.

[mossieurclo]

Re: Marantz PM7200 Can't adjust Bias or DC Offset

You may still have some bad component, in the amplifier that has caused the issue.

Ah, well... i am really looking for that bugger.

Would you have an idea of the more likely guilty party??

I did spot 2 slightly bulging caps, but on the right side! (yes they will get replaced), but the described trouble is on the left.

Will be probe happy for a few days...

[momaka]

Re: Marantz PM7200 Can't adjust Bias or DC Offset

Will be probe happy for a few days...

In that case... and in general when testing something that may not be stable or blow up components, wire the device to a DBT (Dim Bulb Tester).

Basically, you place a 60-100 Watt incandescent or halogen bulb is series with the AC line supply going to the device you want to test, like this:
https://www.badcaps.net/forum/showpo...4&postcount=70

What this will do: if something goes very wrong with your amplifier, the incandescent/halogen bulb will glow brightly but limit the maximum power (and current) going to the amplifier instead of a fuse or other part blowing up.

So the neat thing is you're a lot less likely to see major damage if you happen to short or mis-configure something inside the amplifier or if a part fails while you're probing.

The not-so-neat thing is your input power is limited, so you can't stress-test a device to use its full power while connected to the DBT. At best, you will be limited to half the rated power of the light bulb.

[petehall347]

Re: Marantz PM7200 Can't adjust Bias or DC Offset

In that case... and in general when testing something that may not be stable or blow up components, wire the device to a DBT (Dim Bulb Tester).

Basically, you place a 60-100 Watt incandescent or halogen bulb is series with the AC line supply going to the device you want to test, like this:
https://www.badcaps.net/forum/showpo...4&postcount=70

What this will do: if something goes very wrong with your amplifier, the incandescent/halogen bulb will glow brightly but limit the maximum power (and current) going to the amplifier instead of a fuse or other part blowing up.

So the neat thing is you're a lot less likely to see major damage if you happen to short or mis-configure something inside the amplifier or if a part fails while you're probing.

The not-so-neat thing is your input power is limited, so you can't stress-test a device to use its full power while connected to the DBT. At best, you will be limited to half the rated power of the light bulb.

just remember to make sure you are not working on a VFET amp as it will blow up if not on full mains .

[momaka]

Re: Marantz PM7200 Can't adjust Bias or DC Offset

just remember to make sure you are not working on a VFET amp as it will blow up if not on full mains .

Thanks for the heads up, will keep it in mind.

Yes, there are devices where DBT is not always suitable. Some switching PSUs (especially ones with APFC) won't work too well with a DBT or any high-impedance on the line. But for those, I find that a 450-600 Watt heating element instead of the light bulb will usually work OK. Obviously, of course, the downside is that the higher power rating of the heating element will allow a lot higher current to go through, so certain parts inside the device may not get protected from blowing up as they would with the the lower power incandescent / halogen bulb. But in many cases, it's still better than directly plugging the device in the wall and relying on the built-in fuse(s) to save whatever is downstream. Also, the heating element doesn't give any visual clues if the device is shorting or not (well, you will get a glow from the heating element after a while, if the device is badly shorted - but that's not really desirable ). So to get around that, a multimter or current clamp must be used on the AC line after the heating element to check the input AC voltage / current for anomalies.

[mossieurclo]

Re: Marantz PM7200 Can't adjust Bias or DC Offset

(...)

(...)

Yeah, i read about the light bulb safety device. I did not expect to go look for incandescent bulbs again ...

And there are no FETs around. I kind of scrutinized that schematic trying to figure things out, i would have noticed

Ordered an ESR/component tester today, and a solder sucker to replace the shopvac

Next steps are: finish removing that damn copper shield, testing the fusibles, and probing that first stage.

thank you both. To be continued...

[mossieurclo]

Re: Marantz PM7200 Can't adjust Bias or DC Offset

I ordered a mega328 esr meter, and a solder sucker. 1 week?

I tested a lot of a lot of diodes and fusible resistors, those i thought could be tested in circuit. All tested ok.

A lot more tests to do, especially after i remove the copper shields. I at least succeeded in tinning the old butane irons (ECG J-500 and big tipped Portasol T1), those could help.

So i plan to annotate a .pdf with my readings and post it, hoping someone more experienced than i will be there to read it and advise. :scared:

[mossieurclo]

Re: Marantz PM7200 Can't adjust Bias or DC Offset

igottheshieldsoff igottheshieldsoff igottheshieldsoff igottheshieldsoff

! ;0 GOT THE ;O SHIELDS ;D OFF ;"!!!!!!!

[mossieurclo]

Re: Marantz PM7200 Can't adjust Bias or DC Offset

I ordered a mega328 esr meter, and a solder sucker. 1 week?

So i plan to annotate a .pdf with my readings and post it, hoping someone more experienced than i will be there to read it and advise. :scared:

I got the meter and solder pump today.

I calibrated the meter, rigged pins and alligators to extend the ZIF socket, tested a few cans in my NOS and 7267 of the Marantz that i had taken out (it's good :-().

That puny meter looks useful.

For Google customers searching, a pretty good manual (not supplied with the lower priced item from Amazon.ca) for "LCR-T4 Mega328 Component Tester" is here:

https://kookye.com/download/manual/m...er_user_guide-...

Now brace yourselves, friends. Data storm coming soon. Thanks for weathering it.

(i should not play with those words, sorry for those who live through those very real weather storms down south)

[mossieurclo]

Re: Marantz PM7200 Can't adjust Bias or DC Offset

After i repaired the speaker protection board of that amplifier, all was ok.

So I then proceeded to adjust iddle current, got it, then DC offset. Oh, bboy.
[...]

Summing up how that problem was resolved (thank you to all who contributed!!):

I replaced fusible resistor 3315 that had gone very high.

And lowered 3301 (and 3302) to 220R to bring DC back within adjustable range (7267 and 7268 having drifted?)

Now will be replacing lots of electrolytics, all other fusibles, and putting back 7289 that i blew up (remember: discharge those Caps!), to reenable Protection. Looking for that 2SA970 and other through hole legged critters i discovered that these are now really scarce, like in hard to find.

Are NTE replacements viable??

[petehall347]

Re: Marantz PM7200 Can't adjust Bias or DC Offset

check out ksa992