Old Receiver

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Hello all!

I have an old receiver from my Dad and the right channel is heavily distorted. Anyway, I've measured the amps and regulators...nothing looks black or brown. Found a grounding problem and fixed that.

Now it doesn't go POP! when you turn it on.

However I found one of the chips, not exactly sure what it is yet, measuring out of spec with the others in the circuit. The following is written on the chip (B699 BL3). I'm hoping it'll ring a bell in someon's head.

I know the amp was built in the mid 70's. I just need to find some kind of replacement for the ONE bad chip.

TIA, Brodey Dover


EE student at Univeristy of Ottawa
 
Diode check from base to emitter, then from base to collector. Both should show a standard forward voltage drop...something like 0.4V to 0.7V. With an NPN transistor, positive on the base and negative on the emitter and collector. Then check collector to emitter, first with positive on the collector and negative on the emitter, then reverse...negative on collector and positive on emitter. Should read nothing measuring from collector to emitter.

Of course, check a PNP transistor with the leads reversed...negative on the base and positive on the collector and emitter... Same deal with the collector to emitter check, there should be no conducting with the leads in any configuration.
 
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Hi Bose(o),
Given that the measurements are temperature sensitive, possibly contact pressure sensitive and the meter is unknown ... I wouldn't worry one little bit about the spread of your readings. In servicing we are normally only worried that the junction is there within 0.1V (in your case, some meters measure other units).
If you are testing a channel where some drivers or outputs have shorted you might be more interested in C-E and C-B leakage. That is what will cause you grief. If they are in parallel, put the units with the closest gain figures together (per channel) to share current more evenly.
As echowars pointed out, 0.4V - 0.7V is what to look for in a quick check.
So, you may have an open base feed resistor, open driver transistor or shorted bias transistor. Never mind open capacitors and broken solder connections. Be patient and check these.
-Chris
 
So, you may have an open base feed resistor, open driver transistor or shorted bias transistor.

If I checked the voltages while the unit is on, but without the transistors in place on the PCB, would I be able to determine whether or not, any of the above is occuring?

Oh and by feed resistor, do you mean feeback resisitor?
 
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Hi Bose(o),
You can measure those things easily with the power off. Safer that way. The resistor I mentioned is sometimes is series with the output and/or driver transistor base. An open feedback resistor would give you really high gain with some DC offset most likely.
What is the make and model, an "old" guy like me might recognise it. Someone may even have a schematic for it! And because I'm curious, what type of meter are you using?
Oh, don't forget to measure the emitter resistors for continuity, they can open and you may still measure a ballpark voltage that you would expect! A 'scope tells the story here when under (light) load.
-Chris
 
I just measured everything I can on the board where the transistors live on. Nothing really jumped out. It seems that there are 4-channels living in this beast.

The receiver is by Bose, it is their spatial control receiver. A lot of hitachi components in this beast.

Alright I can if I need to, get a hold of an oscilloscope, so no worries there. Right now, I am using my RS DMM. It has a diode check, DC V, AC V, DC I, AC I.

Thank you for your help! I appreciate it a lot!
 
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Hi Brodey,
Okay, looking over your first posts again, I notice that you are measuring things to ground .. or were. One of the few things that you can measure in relation to ground is the DC offset voltage, also an AC waveform with an oscilloscope. In the output stage, voltages and resistance measurements are more meaningful referenced to the output terminal of the channel you are working on. Components are measured either across their leads (more meaningful out of circuit), or to one or the other power supplies that power that circuit or to the output terminal. You will get far more information that way, and the measurements won't lead you astray.
It sometimes helps to draw a picture of what is connected to what, with component values. Yup, you are drawing your own schematic. You will then get a feel for the circuit you are working on.
When you get access to a 'scope, feed in a sine wave and look at the INPUT to the amplifier channel. Do the same for the good channel. Look at both outputs as well. The waveform can tell you a fair amount about what is going on. By the way, you are looking at the input to confirm that channel is getting a good signal. I've seen techs work for days on a perfectly good amplifier when the fault lay in a previous stage. Pretty funny really as these guys do this for a living.
-Chris
 
Alright, I've got a shorting problem right now, I think I slipped up on soldering somewhere.

Anyway, I appreciate your help and will continue working on the receiver. I will refer back to your last post when I get the oscilloscope and update you when I get my results.

Reading PCB and making a circuit diagram sucks! I mean, is tedious and I enjoy it...
 
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Hi Brodey,
It gets easier when you recognise the basic circuit. You just draw it out and fill in the blanks. There is always a minor twist. This is a very good way to become familar with laying out PCB's. (Yah, I still do that by hand)
-Chris
 
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