Marantz PM7200 Can't adjust Idle Current and 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. 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 easily unsolder 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 ;-)

So with that multimeter, can i test transistors IN circuit with the double diode test? i'm afraid to kill them with heat from the iron, but will manage a heat sink (like, my wife will hold the longnose pliers ;-)) if necessary.

Thank you all for your insights and help.

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

 

[mossieurclo]

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, 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:

 

[Lavane]

If this amplifier topology uses differential pairs, one may be way off from the other in that channel. These must be matched transistors. Sometimes you find them shrink tubed together. Pull them and check how well they are HF matched. If they aren't matched it can be very difficult or impossible to adjust offset. Its a place to start. Ive run across enough old trimmers that won't stay put when adjusted i usually just replace them if they are the older open type, usually with multi turn pots if they will fit.

 

[mossieurclo]

If this amplifier topology uses differential pairs, one may be way off from the other in that channel. These must be matched transistors. Sometimes you find them shrink tubed together. Pull them and check how well they are HF matched. If they aren't matched it can be very difficult or impossible to adjust offset. Its a place to start. Ive run across enough old trimmers that won't stay put when adjusted i usually just replace them if they are the older open type, usually with multi turn pots if they will fit.

yEP there are these. And now i can get to them, because:

 

[mossieurclo]

And now i can get to them, because:

igottheshieldsoff igottheshieldsoff igottheshieldsoff igottheshieldsoff

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

 

[mossieurclo]

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/mega328_transistor_tester_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]

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]

check out ksa992

 

[mossieurclo]

check out ksa992

Will do, thanks. What about its litle brother 2SC2240?

So i should stay away from ECG/NTE? i read somewhere that they take the strongest of a family to supply replacement?

 

[petehall347]

KSC1845 might do it .

 

[yellowfox]

Will do, thanks. What about its litle brother 2SC2240?

So i should stay away from ECG/NTE? i read somewhere that they take the strongest of a family to supply replacement?

NTE = Nth Time Explodes, Not Too Effective, Naughty and Terribly Expensive

 

[mossieurclo]

NTE = Nth Time Explodes, Not Too Effective, Naughty and Terribly Expensive

Shall i gather you had troubles with NTE replacements?

Expensive, yes (i was selling them when i was working in an electronics shop, a century ago). But if one is faced with a no equivalent unobtanium component, isn't NTE a suitable avenue? They even have matched pairs!

And, anagram for anagram (yours are funny anyway) i came up with this one for guesswho:
"Thou Will Immolate That Thought Extremely Reduced"

Needless to say, i don't have an account there. Nor on FaceCrook.

Oups, Out of Topic.