Marantz 2270 dc offset problem- solved !

Scotteboy

Active Member
Hi all, haven't been posting for a while but this one has me stumped. I have done a bunch of 2270's and I have a problem I don't think I have had before. (maybe old age is kicking in....)

I just recapped and redid the power supply, relay new output transistors from Digikey and bourns 25 turn pots. Adjusted the voltage and the relay kicks in but on the right channel I have 0.5vdc when starting the dc offset adjustment. It does not change with adjusting the potentiometer either. This took me by surprise and I was wondering if anyone had any ideas. No smoke , just no glory right now.
 
Have you changed the differential pair of transistors? They are on the amp boards coupled together with shrink tubing with their flats together.

Double check all your work and soldering. Obvious I know but I have found my own issues more the once.;)

Would start there.
 
Yep , I did that but didn’t mention it in the post, also the next transistor but it is looking like this could be something to check overall, I just found the 2sa494 had severe leakage/ low gain so now they are all in line for a friendly check by my bk precision tester! Thanks for the input, I will report back! I often recommend the DP as well as they seem to cause a lot of problems, I just replace them while recapping
 
0.5Vdc sounds like a fairly big offset to adjust with a trimmer.

For the problematic channel measure the voltage at the R760/R767 node/junction and GND
repeat for R761/R768 node/junction and GND. voltages should be the same except +/-

Measure R760,R761, in circuit should be ok, amp powered off/unplugged, they'll be ok.

Maybe a problem with the zeners H762,3
 
Okay so resistors R760 and R761 measure about 6k each as expected in circuit. This matches the other channel all with power off, uplugged. Replaced the leaky transistor pair H758 and H759 with 2n3440 and 2n5416 as I have in other times they have failed. Measured node of R760/R767 = +15.15 volts dc and 1.6 vdc other side of resistor. Measured node of R761/R768 and measured -14.46 vdc and - 1.12 vdc other side of resistor all with respect to ground. Same result 0.5vdc at speaker terminal. What do you recommend I try next ? Is the 0.5 vdc difference due to a bad zener/ should i replace these H762 and H763?
 
Little surprised that R760,1 only measure 6k, could not see alternate paths, though happy
it matches the left channel.

Expected voltages at R760 (+15.15V) and R761 (-14.46V) to be closer, the difference 0.7V
is in the ballpark of your 0.5V offset, the trimmer compensating by 0.2V.

H763 (15V zener) may be failing, alternatively there might be a higher than expected
current drain on that negative rail, eg, R765, 768, 787 etc...

Suggest measure viltage at R761/R768 node on working left channel, expect closer to 15Vdc.
Replace H763 -15V zener
 
So checked other channel, +14.41vdc at R760, and -14.56vdc at R761 bias and dc offset measured out perfectly.

Ordered zener(s) will start with H763, already replaced H753 during recap but did not put in post .

Thanks all, just waiting on parts and will report back!
 
So checked other channel, +14.41vdc at R760, and -14.56vdc at R761 bias and dc offset measured out perfectly.
Looking at the voltages on the left channel is kinda suggesting the odd man out is the +15.15Vdc on the right channel, ie, the positive voltage rail.
So candidates become H762 (15V zener) and ??? I would have thought +15.15Vdc was within zener tolerances. Looking for reasons for lower than
expected current draw through R767->bigger volt drop.

Start with replacing the zeners.
 
So back to the effort, got parts in and replaced zener H762 and H763. Now voltage at R760 is -14.45 v and R761 is 0.863vdc, still 0.5vdc at speaker terminal. Checked R767 both channels and the left channel reads 47vdc incoming and a drop to 14.4vdc but on the right it reads 0,863vdc. I am going to replace the resistor next and see what happens. Any other thoughts would be appreciated ! Update, replaced R767 on right channel and still 0.863 vdc and 0.5vdc at speaker terminal.
 
Last edited:
Okay egg on my face I put H762 in backwards and now I have very nice balanced voltages at R760 at -14.45vdc and R761 reads +14.49vdc. Still -0.5vdc at right speaker terminal with no change by 2.2k potentiometer. Still trying to track this down will replace H753 next as it seems most likely.
 
Trying to come back up to speed on this one.

Zeners H762,3 replaced
before H762 +15.15Vdc After +14.4 (R767/R760 node)
H763 -14.46V (R768/R761 node)

Before After
R760
+15.15V and +1.6V (=13.55V) +14.45 and 0.863V (=13.6V)

R761
-14.46 and -1.12 (=13.34V)

The voltage at H762 is now in line with the negative rail and also the other channel at 14.4V, so
that's a start.

The dc offset trimmer (R762) controls/sets H752 Vb, controlling how hard it works. H752 then controls
H753. If you have a replacement for H753 you could try, alternatively you could measure b/c/e voltages
to GND then adjust the trimmer, maybe 1/4 turn then redo, all voltages, particularly the collector
should change
 
Working on now, it and thanks for your help so far, gotta take the thing apart again will report back in a few. Thanks to all who help , I am an organic chemist with basic understanding of this as a hobby but I am no EE and Marantz doesn't include voltages on the schematics often times so it can be trying just comparing boards.
 
Your doing ok for a chemist, learning along the way. Importantly you will have a nice sounding amp at the end of it...
 
So just replaced H753 and still -0.5 vdc at right speaker terminal. Starting to wonder if I have a problem with the potentiometer or speaker switch.
 
So just replaced H753 and still -0.5 vdc at right speaker terminal. Starting to wonder if I have a problem with the potentiometer or speaker switch.
About 1.3mA through R760. Does the offset move a little when you adjust the trimmer?
 
Back
Top Bottom