2270: No bias?!

quadtech

Well-Known Member
Hi All,

I just finished recapping all the boards in my 2270.
The receiver sounds and works great - and the FM and the phono
are working really well.

The issue is - I see hardly any bias voltage across the collectors (metal
body) of the driver transistors - with the volume turned down, I
see just about 0.5mV. Adjusting R763 seems to make little difference.
Just to be sure, I also measured it on one of the boards directly across J754 and J760 and
it's the same.

And I see the same thing on both channels! The heat sinks are cool to touch.
Both boards are clean, with no sign of any burnt resistors.

With the volume at a moderate listening level, I can see the voltage
varying to as much as 100mV with the music.

I cannot find the 2SC495/496 or the NTE295 here in India.
So I replaced the cylindrical 2SC984 with a MJE243G which
was posted here earlier. I put a small mica spacer to insulate the metal tab from
the heat sink.

The bias comp diode is in good shape and tested fine with a multimeter.

For the bias transistor H760, I also have a couple of
2SD669 on hand, as it was recommended here, but they
are PNP, unlike the original NPN. Should I try that
instead of the MJE243? Do I need to make any change for the PNP?

FWIW, I was able to adjust the DC offset from about 200mV down to zero.

Across the main filter caps, I have 43V-0-43V and the
J802 on the P800 power board is rock solid at 35V.

Appreciate your help and ideas.
 
Last edited:
2270 SN<3901 Idle current or bias adjust:
Meter set to read DC mV, connect from J753 to J754, set R763 for 5mV

2270 SN>3900 Idle current or bias adjust:
Meter set to read DC mV, connect from J754 to J760, set R763 for 10mV

For everything you ever wanted to know about bias and the 2270, read this excellent analysis by ecluser.
BTW, 2SA and 2SB types are PNP , 2SC and 2SD are NPN.

Tom
 
2270 SN<3901 Idle current or bias adjust:
Meter set to read DC mV, connect from J753 to J754, set R763 for 5mV

2270 SN>3900 Idle current or bias adjust:
Meter set to read DC mV, connect from J754 to J760, set R763 for 10mV

For everything you ever wanted to know about bias and the 2270, read this excellent analysis by ecluser.
BTW, 2SA and 2SB types are PNP , 2SC and 2SD are NPN.

Tom

Thanks, Tom. I was mistaken that 2SDs are PNP.

My receiver is serial # 21929, so I am checking the voltage between
J754 to J760 - which are the collectors on the two output transistors.
My multi is set to 200mV range.

Should I suspect the 0.2 ohm resistors? I'd be glad to try out
any other tests to narrow this down. I am kinda lost now.
 
I doubt that the .2 Ohm resistors are the problem.
For a start, set your meter to DC volts, connect the black lead to the chassis. Measure the voltages on H760. Use an insulated probe so that you don't short anything out.
H 760
B
E
C

Tom
 
Tom, here are the voltage for the H760 on the left and right channels -

Left

E -0.63V
B -4mV
C +0.55V

Right

E -0.53V
B +94mV
C +0.66V
 
Hmm, those readings look fine. I wonder if there is a problem with the way you are measuring the bias current.

Connect your meter, set to read DC volts to J753 and J755 and note the reading.
then connect your meter, set to read DC volts to J759 and J761 and note the reading.

Be sure to have the volume all the way down, use mini clips to avoid shorting anything out. Turn the unit off before connecting the meter, then turn it on to take the measurement. We are looking for around 600 mVDC which will tell us that there is a small amount of forward bias on the outputs.

J753-J755
J759-J761

Tom
 
Tom, thanks for stepping me through this.

I have been quite careful to take the measurements
in the proper range of the multimeter, with the volume turned
down all the way, and nothing shorted.

I also checked that the wiper of R763 on both channels is making contact, as I see the
end to end resistance (wiper is connected to one end) varying smoothly as I
turn the pot.

Here are the next set of readings -

Left
------
J753-755 150mV
J759-761 123mV

Right
------

J753-755 570mV
J759-761 36mV
 
Do any of these reading vary when you rotate the bias pot?
Why is the right channel's bias asymmetrical?
You did the offset adjustment, right?

Tom
 
I don't know if the readings vary if I turn the bias pot. Will check it out.
Yep, I did adjust the DC offset to zero without any issues. It is still
close to zero now.

When I first opened the amp, I found an old service sticker of a local
repair shop inside the amp.
While recapping and cleaning the boards, I saw that there are a
few parts on the left power amp board which
are non-original parts - there are some carbon comp resistors,
and a metal cap transistor at H755 (should be 2SC735?).
I am not sure if he swapped the output transistors out too - all 4
are unmarked and a bit rusty too.

I think I'll take a little time to get the boards out again, and
go over them carefully and get back.
 

Attachments

  • 2270-1.jpg
    2270-1.jpg
    55.8 KB · Views: 158
  • 2270-2.jpg
    2270-2.jpg
    84.3 KB · Views: 179
  • 2270-3.jpg
    2270-3.jpg
    73.5 KB · Views: 308
Last edited:
I'd clean the rust off the output transistor, for sure.
I find it odd that you can't measure the bias on either channel.
Perhaps the previous 'tech' has modified the power amps so that you aren't able to meter the bias on the usual J numbers.
Why are there two terminal strips and big green resistors mounted on the rear panel?
I would trace out the modifications and determine what is needed to get it back to stock form.

Yes, H755 should be a 2SC735.
On-Semi MJ21193G (pnp) and MJ21194G (npn) are good replacements for the output transistors.

Tom
 
Oh....the terminal strips and green resistors looked original.
I'll trace out where they are connected.
This unit has a single voltage (110V only) transformer.

All signs indicate this amp had it's output shorted at some time.
The P800 board was messed up bad too by the previous tech - broken
relay cover, badly soldered wires, relay "hardwired" to be always on.

I put in a new relay and replaced the associated capacitors and
transistors (H808, H809, H810) to fix the P800 relay circuit.
(in case it is useful to someone - I used BC546 at H808 and BC639
at H809, H810. Caution - the pinouts of these subtitutes are not
the same as the original transistors in those positions)

I do have replacements for the outputs - MJ21195G and MJ21196G from
Mouser, but I'll put them in only after sorting out the bias issues.

Tom , would the BC337 be a good sub for the 2SC735?
And the BC327 as a sub for 2SA562?
The BCxxx transistors are easier to source here, and I plan to pick up
a few of the suitable subsitutes while tracing the amp boards, in
case I need to replace any.

-----------
Transistor 2SC735 Parameters
Si npn
maximum allowable power dissipation (Pc): 300mW
Allowable collector-base voltage (UCB): 35V
Maximum allowable collector-emitter voltage (UCE): 30V
Allowable base-emitter voltage (ueb): 5V
Maximum collector current (Ic max): 400mA
pn-junction temperature (Tj): 125 º C
Maximum frequency (Ft): 150MHz
Collector capacitance (Cc), Pf: 14
Current amplification factor (HFE), min / max: 40/400
Manufacturer: TOSHIBA
BODY: TO98-1
 
I have no experience with BC transistors.
You already know the pinout is different, EIAJ transistors have the collector in the center.

Tom
 
I looked at the 2N3904, but it has lower collector current Ic rating (200mA)
than the 2SC735 (400mA).
Both BC and 2N types are easier to come by than the Japanese
numbered parts (2SC/2SA...), which are likely to be fakes anyway.
 
I don't know if the readings vary if I turn the bias pot. Will check it out.

This is fundamental to check it out. Apparently from your first post you did. Maybe H760 was replaced before. If the Vbe drop in H760 is too low you won't be able to have enough voltage between C and E of H760 to properly bias the output stage. From what I see your actual voltage between C and E of H760 is too low. You can correct this if you add a resistor (4.7 to 10 Ohms) in series with the emitter of H760.

You should measure the resistance of R786 on the right channel. If it is open it could explain why you have 570mV across R786 and 36mV across R787.

The BC327/BC337, and the 2N3904/2N3906 will work for the current limiter circuit. There will be less than 20mA in these transistors, if they ever conduct.
 
Thanks, Ecluser. I'll stock up on a few of those transistors.
It's the later readings which Tom asked for, which I didn't check
if they vary with R763.

Time to take the P750 boards out again and check all transistors and
resistors (noted your point about R786), and all connections, including
the wires to the driver transistors.
 
You are in great hands. Ecluser's circuit analysis skills are top notch and far superior to mine.

Tom
 
You are in great hands. Ecluser's circuit analysis skills are top notch and far superior to mine.

Tom

Hey, don't abandon me.
Your skills and experience are far superior to mine.
I'd never have made it this far without all the information you have
posted in these forums.
Am I glad there are people like you, Ecluser, EW and so many others
here to help!
 
I'm not going anywhere. I just want you to have the best help available.
BTW, ecluser said to let you know that a 10 Ohm resistor added in series with the emitter of H760 should get the bias up.

Tom
 
Last edited:
Quadtech is right. You have a huge experience with these units, and I have never worked on a 2270. A kind of wedding between practice and theory is in order here. I need you.
 
Back
Top Bottom