Harman Kardon Citation 12, bias value question

[Talon]

I'd replaced a damaged right channel differential pair and Q708 bias resistor. I had the thing dialed in at 40mA per the service instructions. It sounded decent. Two days in the thing made some loud crackling noises, hums, bumps etc like it was throwing major DC.

Going by the schematic values and checking for mVDC instead of mA, I'm getting 2.5mV on the right channel, 0v on the left. I didn't touch the pair on the left channel, or the bias transistor. What is going on with this thing?

 

[rayma]

Gonna have to check all the parts, connectors, and connections. Even the pots could be bad,
it's a half century old. Clean all the connectors and pots first.

 

[Talon]

I've already replaced every capacitor, every resistor, the output transistors, and the pots. The things I haven't replaced: left channel differential pair and bias transistor, pre-driver and driver transistors. Also, the transformers, the RCA inputs, the wire wound resistors, and the power cord. The amp was working just fine until it wasn't.

 

[rayma]

Then do the DC voltages comparison. Transistors are easy to check (often even in circuit)
with a DVM on its diode function. Check BE, BC, CE, all both ways for each transistor.
The CE should be open both ways, of course.

The output transistor emitter resistors can fail, so be sure to check those.
It's old, everything fails eventually.

 

[Talon]

Thanks Ray, I'll do that in the morning. I'm already late so I'll have to pick this up tomorrow.

 

[huggygood]

 

[Talon]

Right? Its always something.

I had a lousy night's "sleep" so its all I can do to answer posts right now. I'll have to get on the technical work tomorrow. Hopefully I can do about 10 hours of sleep tonight.

 

[rayma]

There's a lot of that going around.

 

[indianajo]

I operate equipment that is 55 & 60 years old. Hammond & wurlizer organs, dynaco amps & preamps. I don't find transistors, diodes, AB or sprague resistors, film caps to be particularly prone to failure. Output transistors are sensitive to heat problems by bad design or wire or speaker shorts, and some driver transistors also overheat without adequate heat sinks. Rubber sealed wet caps are failure prone, aluminum electrolytics, tantalums, and pots. Paper dielectric caps are suspicious but die gracefully by going off value instead of developing sudden pops. Ceramic disk caps that failed have usually had a void in the coating from day one. Or an oscillating amp melted them. Non-AB & sprague resistors leak water through the coating & go off value. Even AB & Sprague resistors prior to 1961 didn't have the water barrier paint. The brown body carbon comp resistors used ubiqutously in old quality products. Interboard connectors can be a problem, particularly the long abandoned PCB edge connector.
With all the work you've done, I would suspect pops to be from one or more bad solder joints. I make a few. Setting up a trap with a trash speaker protected by series minus to minus electrolytic capacitors, is a great way to detect bad joints. Set up a dvm parallel to show dc out speaker events. Play some music from a battery radio set to 1/3 volume or a CD player or MP3 player.
Then go pushing the ends of all components with a chopstick or dead ball point pen. Heat gun or circuit cool may be required in tough cases. Where the pop occurs, that is what is wrong.

 

[Talon]

Do you think that would cause both channels to lose bias adjustment? It seems possible but unlikely.

 

[rayma]

There could be a weak point in the design that happened to fail in both of the channels
in this particular unit. Assume that you have checked for presence of the +/-40V supply on the amp board.
Check for voltage across CR5 to start. Check the output 0.27R resistors for ohms continuity.

 

[Talon]

I checked for voltage at pins 4,9,21, 25 and I saw 42v at all of them. I previously removed the 4 wire wound resistors and I was going to replace them but the Dales I received were the wrong value, so I verified each resistor was 0.27R and soldered them back into place.

I'm going to do the diode check on all of those transistors now.

 

[Talon]

Then do the DC voltages comparison. Transistors are easy to check (often even in circuit)
with a DVM on its diode function. Check BE, BC, CE, all both ways for each transistor.
The CE should be open both ways, of course.

The output transistor emitter resistors can fail, so be sure to check those.
It's old, everything fails eventually.

I did the transistor tests, as best I understand them. All parts were tested on the board, with the board unplugged from the amp.

For a control group I verified that both the original pair at Q701/711 as well as the new pair at Q702/712 tested satisfactory. I then tested Q703 and Q704. Both of these devices are original RCA parts.
Starting with Q703, with the Fluke 115 set to diode and the black mini grabber at the base, I saw .285 at the emitter and OL at the collector. Moving the red grabber to the base, I saw .285 at the emitter and .574 at the collector. With red at the emitter and black at the collector, I saw .641, reversed I saw OL.
Moving to Q704, with black clamped to the base I had 0.284 at the emitter and OL at the collector. Switching to red clamped at the base, I received .284 at the emitter and .579 at the base. With red on the emitter and black at the collector I saw 0.639, reversed I saw OL.


For Q707 the results were more interesting. Black on base I hooked to the emitter and the DVM yelled like I was testing for continuity. The display read 0.050. Red at the collector gave me a value of 0.850. Placing red at the collector I hooked black to the emitter, and saw 0.830. Placing red at the base, I checked the emitter with the black lead and the DVM yelled at me again, with 0.050 on the display. I saw 0.852 at the collector. This device is original, the RCA40408. Probing E-C with red at the emitter I see 0.830, and reversed I get 0.829.

Moving to Q708, with black at the base and red at the emitter, I saw 0.373, with 1.137 at the collector. With red at the base I got 0.373 at the emitter and 0.615 at the collector. Red at the emitter and black at the collector gets 0.944, and reversed I see 0.669. This is a modern 2N5232 replacing the original RCA part.

I didn't see OL in every case where you suggested I should see it, so do I assume these are bad parts? Or is testing them in circuit affecting things, and I should pull the parts to test them?

 

[Talon]

I wasn't happy with those test results so I pulled Q704 and Q708 out of the board to test them separately. Note to self: I should have just done it that way to begin with. Too bad I can't edit my last post because now if anyone thought I was a n00b, now that will stand as proof. So, I have learned and let this stand as a lesson to everyone else who knows less than I do.

Out of circuit, both transistors show a proper OL on the CE/EC test, both test as proper PNP with OL at all the tests with black on the base lead. On Q704 with the red lead on the base I saw 0.602 at the emitter and 0.589 at the collector. On Q708, with red at the base I saw 0.681 emitter and 0.675 collector.

I'm going to pull and test the two driver transistors next.

 

[huggygood]

That's how we learn.
Buy yourself a little Chinese transistor tester(AtMega type) for $15, life will be cooler with it

 

[Talon]

I already have the Peak Atlas DCA55. It doesn't work in circuit, so I pulled out the drivers on the left channel and they both tested defective. I replaced them with new 2N5320/2N5322. Now the left side shows a bias value. I tweaked it to 40mA and it held rock solid so I shut the amp down and prepared the card for right channel driver replacements. With those installed I still had a 0.0 mA value on the right. I pulled the bias transistor and replaced that with a new 2N5232A. Still no bias value on the meter. Whats left? The pre-driver and the differential pair. Since the pair should effect bias I guess its time to replace that predriver.
One other thing - after doing the right side service I re-checked the left side bias and it was nearly half an amp. I cranked it back down to 40mA but it was unstable, it kept dancing around the 40mA number by 3 or 4 mA in each direction.

This amp is haunted.

 

[rayma]

I wasn't happy with those test results so I pulled Q704 and Q708 out of the board to test them separately.

Good work, sometimes you can test in circuit, sometimes not. The DVM is just fine for your purposes.
While you are doing this, inspect the boards for broken traces, bad solder joints, etc.

Remember to do all 6 tests for each transistor, and both tests for each diode.
However the double diode bias device probably won't work with the DVM diode test,
due to not enough voltage in the DVM. You could just check the DC voltage drop across it
when powered instead.

 

[huggygood]

Always pull the components to test them, afterwards, when you are used to it, you can take shortcuts

 

[Talon]

Yeah I think I'm going to skip the shortcut thing for a while.

I tested the Q703 pre-driver, the Peak showed it as a "diode junction", with one lead not showing a value. So that thing was bad. I've it, still no bias on the right chanel. Something tells me one of the drivers I just installed on that side might be bad. And I'm out of them. Mouser has done a great job of getting things to me in 2 days so I have to place an order.

I forgot to mention that I bought heat sinks for the new drivers but I was able to reuse the old ones by simply twisting a square file through the center hole until it was big enough to accept the TO-39 body.

 

[rayma]

Always test new parts before installation, just to be certain that they're ok.
Most of the time new parts will be ok, but you never know.