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2270 BIAS will not Adjust below 45 mV...

Whaleman

Whaleman

Hi-Fi Rescue Mission
Kind of stuck here, just replaced most of the amp board transistors and the outputs but I can not get BIAS below 45 mV :sigh: with the adjustment fully counterclockwise. Also, the heat sink is very warm to touch.

What gives? H760 (2sc984) is still original. :scratch2:
 
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Have you ever been able to adjust the bias to spec?

Does the bias adjust pot do anything?

H760 could be leaky, but if the amp was previously blown, then the money is on opened or partially burnt resistors. The usual suspects here would be R773 & R774.
 
I was able to set bias before the amp started doing the static noise and then the protection kicking in. The bias go up when going turning the pot clockwise but I only let it go to 49 mV, too chicken to go further.

Thanks Echo, will check those resistors.
 
I put in another 2sc984 that I had laying around, the one that was in there tested OK, R773 and R774 test OK. However, BIAS still not coming down below 43 mV.

So back to digging and searching.
 
Greetings, try changing out the bias diode H761 with a new unused diode such as a 1N4001 or something like that. I had a similiar problem with a 2270 3 years ago and it was the diode. If I remember right I could not get it down to about 10 millivolts across the collector resistors. Swapped out the diode with a larger size one and worked fine.

H761 is the diode thats thermally connected to the power amp heatsink. Just solder in the diode temporarily to see if its the problem.

Later, Marcus
 
Put in an 1N4001 (new 2sc984 too), same results, BIAS in the 45 mV or so:

60323176_o.jpg


New input pair and other driver pairs using MPSA06 and MPSA56, and MPSA20, note input pair rigged together with a piece of phone wire and tacky glue:

60324941_o.jpg


60323221_o.jpg


Took out R762 and R763 2.2K Ohm trimpots and cleaned them, they measure identically from end to end. Also checked new outputs, not blown:

60323118_o.jpg


So what gives, BIAS still will not come down below 43 mV.
 
Do you have a schematic with voltages listed on it? With it put back together, check all the locations on the board where they show a voltage on the schematic. You may find the problem that way.
Good luck.

Rob
 
Please pardon if this is a stupid question to you but are you taking the voltage measurement for the bias from J754 to J760? If you measure .010 volts or 10 millivolts across these two points using Ohms law .01 volts divided by .4 ohms which is the total of the 2 collector resistors you should get a bias of .025 amps or 25 milliamps which is the setting from the service manual.
According to your info you have 112.5 milliamps bias through your outputs. Is this right? Please check the value of R788 and R789 to see if they are .2 ohms apiece.
 
One more question for ya. Why did you change out the transistors in the amp in the first place? A problem or just wanted to change things for new?
 
Supplementing EchoWars' remark, bandwidth is very important when you "retransistor" a circuit. The original closed-loose compensation was based on specific bandwidth (f sub t) characteristics of the original amplifier transistors -- largely a function of internal collector-to-base capacitance. Each capacitance caused a frequency-dependent phase shift and the phase shifts summed to yield an aggregate frequency-dependent phase shift. The feedback loop was carefully tailored to generate an opposing phase shift so that, at all frequencies in the amplifier's passband, the feedback would remain negative -- and thus, oscillation would not occur.

Use of transistors with substantially different bandwidth can upset this apple-cart.

If you hook a scope to the amplifier at the output point (the collectors of the output transistors) you should be able to see any oscillations using a low-capacitance probe. Note that a typical AC voltmeter does not have sufficient measurement bandwidth to "see" an oscillation, say, at 700kHz.

Fred
 
Thanks folks, I am armed with my neighbors oscilloscope. I will try to measure at points 754 and 760, the output collector points. Bias reading was done at 754 and 756. R788 an R789 measure .8 ohms in the circuit. When I get bias down, I will follow EchoWars advise to set bias with 754 and 760 at 10 mV.

Putting this thing back in is a PITA, may extend wires out of the unit so I don't have to reassemble the board into the unit and be able to get the scope on it.

I guess if the board is oscillating, I am looking at a disharmony of the replaced transistors.
 
Some 2270s did not use J754 and J760 for bias measurement.
A quick check across one of the emitter resistors should show approx. 1/2 of the forward bias voltage, if all else is operating properly.
 
A quick check across one of the emitter resistors should show approx. 1/2 of the forward bias voltage, if all else is operating properly.

Yup, thats right. I use the measurement across J754 and J760 cuz' its a larger voltage than 5 millivolts. What worrys me is that he measured 45 millivolts across one resistor and according to my calculations he has double the 112 for a total of 224 milliamps, OUCH!!

No wonder his heatsinks are warm........
 
I think I found my goof, when I came up with the list of what transistors to put in, it was very late. I am NOT sure where I came up with MPSA20 and MPSA06 and MPSA56, but these total 5 transistors.

I have removed them all and found good original replacements, 2sc945, 2sc735, and 2sa562. All are in and ready to go. I will reply later when I get it all put back together. The new outputs are in also.

I have also rigged a wire to 754 and 760 to aim for 10 mV accross these points when I set the BIAS.
 
Well I am moving on to the 9090DB from this POS.

Put everything back to the original state except for the daisy heat sink transistors, the bias diode that I replaced with a 1N4001, and the new input pair. Ran a wire to 754 & 760.

BIAS still at 95-100 mV. Hooked up speakers and sounds great.

At this point I am ready put the covers on it, run it and let it self destruct to a good bit of Rock n Roll and then sell it for parts.

Too bad it was a nice unit.

How handy should I keep the fire extinguisher? :D
 
Before you blow it up, maybe there is someone here that could rebuild the board for you. Can you swap boards? Right to left? Have you? I do not have the SM, but I have to believe someone will help you. I just went through a board on a 4300. Talk about a pain. It's got to be something.

Rob
 
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