2270 Idle current adjustment?
- Thread starter MikeED
- Start date
Thank you Tom. All very interesting.
pcmacd
New Member
I am horribly confused here.
I've a 2270 in the 32900 s/n range, and measuring DCV betwixt J753 & J754 gives me dozens of volts, around 46 VDC.
Measuring betwixt J754 and J760 allows me to set the idle current to 5 mv, per the manual, page 10.
Here I see a 10 mv recommendation for bias. What gives? I'm an engineer but mechanical..... what difference would that bias from 5 mv to 10mv make?
For that matter, what does the bias accomplish here?
Manual is undated but purchased around the time of the receiver in late summer of 1974.
pcmacd
New Member
Catrafter:
This image "2270 bias.jpg" shows J753 and J754 for the bias current measurement, as does my undated service manual purchased in late 1970.
I get around 46 VDC here, but at J754 and J760 (as suggested later in this thread) I get voltages in the millivolt range and can adjust bias to 5 or 10 mv, not sure which is correct.
Please advise.
tanks
Steven Tate
CEO Flat Forehead Soc.
Measure between J754 and J760. The 5mV setting was for measuring across one emitter resister. J754 to J760 measures across both emitter resistors, so 10mV is the proper setting. Of course, it’s kind of a moving target as the voltage will change gradually over 20 minutes or so. To set it, clip your leads onto the proper pins and adjust now and then to keep it near 10mV until it settles in and stops drifting. My meager understanding of bias is that it sets an environment that allows the transistors to operate in as close to a linear fashion across the frequency spectrum as possible. But I’m not an engineer. All I know is that it is designed into the circuit, and if it varies too much from the designated setting, you can get distortion or overheating or both.
pcmacd
New Member
Trying to adjust the bias current. I happen to have a pair of quality DMMs, so I can monitor both channels simultaneously.
It's about 100F in the garage (Greater Phoenix) and the amp has been idling for 24 hours.
The biases are all over the map. For a while this afternoon I could not even adjust the left channel down below 25 mv. What's that all about?
I set them both for a nominal 10 mV using J760 and J754 about an hour ago, and they are drifting up to 12 mV and down to nine in a minute.
Is this typical? Is it a problem?
Would it be better to locate the thermistors to the power transistors? (Did I get this right?)
It's been two hours, ambient temp same. Power transistors on heat sinks around 108F.
I've got 12.7 mV and 14.7 mV on my right/left channels. The bias is wandering all over the map, over minutes and not hours or days.
I'm going to leave it overnight again and come back in the morning to check.
It's about 100F in the garage (Greater Phoenix) and the amp has been idling for 24 hours.
The biases are all over the map. For a while this afternoon I could not even adjust the left channel down below 25 mv. What's that all about?
I set them both for a nominal 10 mV using J760 and J754 about an hour ago, and they are drifting up to 12 mV and down to nine in a minute.
Is this typical? Is it a problem?
Would it be better to locate the thermistors to the power transistors? (Did I get this right?)
It's been two hours, ambient temp same. Power transistors on heat sinks around 108F.
I've got 12.7 mV and 14.7 mV on my right/left channels. The bias is wandering all over the map, over minutes and not hours or days.
I'm going to leave it overnight again and come back in the morning to check.
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Steven Tate
CEO Flat Forehead Soc.
It should dial in and stay pretty close to 10 mV - say +/- 0.5mV. The hot garage could be a contributing factor. I would try to measure in a cool place. If it’s much outside of that range after a 20-30 minute warm up, you are going to have to track it down. One common source of unstable bias is a bad differential pair of transistors. Most of us routinely replace the diff pair with hFE matched transistors when we do a rebuild of an amp board. There can be other causes. I’m sure others will chime in with suggestions. You are checking with balance centered and volume zero - correct?
pcmacd
New Member
In particular, which transistors are you referring to? By the schematic callout, if you please?
And a source for "matched pair of hFE transistors"?
This amplifier has been a trooper since I bought it at 21 years of age, 46 years ago. A couple years ago I noticed a "wooshing" sound in both channels, and correctly determined that the filter caps were sucking hind teat. Directly after replacement, they started to leak electrolyte.
Replaced them. Decided to tweak all of the easy power amp stuff (I only use this as an amplifier) and now I have bias running all over the map, in a steady state temperature condition.
I do not understand your assertion that the hot environment is causing this problem, as these suckers get LOTS HOTTER on significant load. Driving transistors around 108F.
Left and right channels do not track in their drifts.
This has got to be a hardware problem.
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Steven Tate
CEO Flat Forehead Soc.
The diff pair are H751/H752. Replacements in the 2270 are KSA992FBU. To get a matched pair, you buy 20 or so and test them on a DMM with hFE capability (usually the cheap ones) or on some other component tester with that capability. eBay has a bunch of Chinese component testers in the $15 range that will work. It takes seconds to test one, and you find the two with the closest reading. The flat sides of the two transistors face each other and are shrink wrapped together with some thermal compound between them. There are also vendors who will sell matched pairs. I’ve never used them, so don’t know who. Someone here might know.
I mentioned temperature because bias is very temperature dependent. If you spray the pair with cold spray while measuring bias, the reading will instantly nosedive. It stands to reason that if is too hot, the bias would rise. The designers surely assumed the unit would be run at room temperature and designed the bias circuit to work in a certain temperature range. If you run it in an environment 25 degrees higher than the designers expected, it might affect the bias circuit. I would say this is not at the top of my list of suspects, but it’s worth eliminating. Also on my list would be dirty/faulty trimmers. They often need cleaning or replacing.
I mentioned temperature because bias is very temperature dependent. If you spray the pair with cold spray while measuring bias, the reading will instantly nosedive. It stands to reason that if is too hot, the bias would rise. The designers surely assumed the unit would be run at room temperature and designed the bias circuit to work in a certain temperature range. If you run it in an environment 25 degrees higher than the designers expected, it might affect the bias circuit. I would say this is not at the top of my list of suspects, but it’s worth eliminating. Also on my list would be dirty/faulty trimmers. They often need cleaning or replacing.
catrafter
Marantz Specialist
pcmacd
New Member
Thanks.
This IS at room temperature. This garage is where the receiver is used.
Over three days of continuous operation with both biases initially set at 10 mV, the right bias has roamed from +14 mV to -0.2, and the left from 10 mV to 20 when checked late afternoon and evenings.
Balance mid, volume zero.
Do y'all agree with Mr. Tate that the culprit is the H751/H752 pairs?
If so, I'd rather pay a premium for matched pairs than fool around with sorting on a cheap DMM. I've two lab grade HPs, two premium grade Hp/Agilent, and Fluke and Agilent hand held DMMs.
And I wonder why the thermistors were not attached to that pair?
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Steven Tate
CEO Flat Forehead Soc.
While waiting for more input (a very good idea in this case), I recommend reading this thread that digs into the theory of 2270 bias management. AK member @ecluser gives a very thorough explanation of the design and its shortcomings. Pay special attention to his posts.
https://audiokarma.org/forums/index.php?threads/marantz-2270-bias-wont-go-below-15mv.322206/
https://audiokarma.org/forums/index.php?threads/marantz-2270-bias-wont-go-below-15mv.322206/
pcmacd
New Member
Good reading. Tanks for the tip.
biggal76
Well-Known Member
Hi, I'm restoring mine now (high serial number unit). Did right side power amp (elytics, transistor and diode that are off the board, and the two bigger trim pots). Bought the Ultimate restoration kit from Marblehead199 on Eb-y). Also replaced thermal compound on the output transistors. The left channel is still untouched (my control). Measuring J754 to 760 with pot all the way counterclockwise, I'm at around 20mv. The service manual adendum above says to measure from J754 to J756. Across those, I can't get down to 5mv but can get it down to about 8mv. The left (control) side I measured J754 to J760 and it read a little over 10mv. Not sure what's up. Any suggestions?
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If you are trying to adjust bias check the base to emmiter voltage of the output transistors. I set for 250 ish mv DC. That is typical value most units.
Edit what I meant was that is typically minimum setting for most units I’ve worked on to lower the bias some and run cooler. It probably is around 500mv from the factory.
Edit what I meant was that is typically minimum setting for most units I’ve worked on to lower the bias some and run cooler. It probably is around 500mv from the factory.
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If it was me, I'd look over the thread cross-referenced in post #32 of this thread.
Eh? 250mV? This might be OK for "pro" amps where the output stage is not intended to run any idle current, but I can't imagine that being described as typical, and I can't imagine such a procedure working out well for a 2270.
I’m expecting the factory setting to be around 500 mv if you verified it. But I dont have a 2270 on the bench. Lowering the bias some will not affect thd I’ve found and it will run cooler.
biggal76
Well-Known Member
"If it was me, I'd look over the thread cross-referenced in post #32 of this thread."- I did. A lot of it is over my head but I may try the resistor swap R784,785 if I can't think of anything else. I don't have them on hand so would have to order them or maybe try 2x 10ohm in parallel. I'd rather not have to swap out transistors. First I'm going to look back over everything and make sure I didn't cross a wire replacing transistor 760 or put a cap in backwards and recheck my soldering with magnification. Then, I'm thinking of restoring the left channel and see if I have the same issue.
biggal76
Well-Known Member
I'm not having luck. Rebuilt the other power amp. Went to set DC offset and it keeps drifting upward until I could not get it to 0 even with pot fully clockwise so I went back to check my power supply voltage (which I set last night easily) and for some reason, it's reading 6.7 volts no matter how the trim pot is turned. I kept trying and eventually shorted out my probe and blew a small transistor (hopefully just the transistor) so now I'm waiting for that to come. Not sure why I'm having so much trouble setting these voltages. I've done it before on other units. Anyway, it's dead until I get the transistor. Will work on the case and faceplate in the meantime.
Have you measured voltages vs the schematic? That is how I find issues. Furthest off from expected value will be the clue as to what component has an issue.