Kenwood KR-3090 transistor and resistor replacement

and a dumb question....why are you replacing the emitter resistors? are they bad? I know most DVMs wont accurately resolve to less than 2-5 ohms so you need other means but a 2 or so watt low value resistor is pretty hard to pop without a lot of stuff popping sooner and shuttin off the juice if'n ya know what I mean...

when you reassemble this, Id disco the outputs and listen via headphones using just the drivers (which someone gave you the replacements for) use that to look for noisy inputs or mirrors or whatever and peek at the fuse resistors (the 4.7-10ohm things at 1/4-1/2 watt) if the inputs/mirrors are broken, I have a 4070 thread somewhere where I worked that part out (4070 lower stages are about the same)
 
First of all thank you for all your help. I am almost back to where I was before I shorted the left channel while poking around. After replacing all of the left channel transistors in the input, sec, driver, and output stages in the left channel, now I have sound again but it is completely distorted at all volume levels. Headphones are also completely distorted from the left channel.

I checked the bias current. I measured the right channel at ~12mA as a reference. The left channel measured ~1.5mA. The service manual bias current spec range is ~15mA to ~100mA so the left is way out of spec. There's a bias resistor (currently 160 Ohm) not a pot so it little difficult to adjust the bias since I dont have stock of various resistors on hand.

Here's where my head is on this:
1) One or more of new transistors in the driver stage or before that are bad. Possible, but all tested fine before installation.
2) The low bias current is causing the driver and power transistors to switch on and off.

#2 makes the most sense, but would like to get some thoughts before I take the shotgun approach. I am not ready to toss it in garbage yet. I really like the receiver, and I am enjoying the troubleshooting and repair process.

Thanks.
 
when I did the very similar 4070 I matched the inputs AND mirrors to eliminate any nonsense. a lot of the resistors on the thing are etched into the board. so how to the drivers stack up to each other?

what are you measuring across each emitter?
 
I didn't match the transistors. I just ordered what I needed and slapped in there. I will pull and measure them tonight.
 
Getting closer. Out of curiosity I pulled Rm33, the bias adjustment resistor, and powered up the receiver - nothing changed. Output is still distorted from the left channel. I am not going to pretend I know what I am talking about here, but looking at the schematic, with Rm33 missing OR in-place, it appears that the circuit is open between Qm7 and Qm13 which I am pretty sure means Qm13 and Qm17 are not conducting. So, I am looking at Dm1 as the culprit.

Here's what I've done:
1) I tested Rm33 - it measured at 159 Ohms
2) To eliminate a bad trace between Dm1 and Rm33, I connect the Rm33 lead directly to the cathode side of Dm1 - no dice
3) I pulled Dm1 to test out of circuit and it measured open with my ohmmeter

Thoughts? Thanks.


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Uh no. I just looked up the part # for Dm1. It's a varistor STV-4H(W) which are no longer available.
STV-4H is four diode junctions - Most meters don't put out high enough voltages to measure them, and good ones usually measure open. However, I doubt that is your problem, because if it were open, channel would be overheating and drawing too much current.
Measuring voltages around the bias circuit will give us (armchair techs) more useful information.
Best way to measure an STV4 is with a battery and appropriate resistor. (9 volt with a 4.7k ohm resistor should be about right) and to see what kind of voltage drop you get across the device (somewhere around 2 volts or so)
 
if I may, I think you have underneath, the same problem I had on the 4070 I did and a pioneer SX5 (in both cases I think the outputs were technically fine but it was the other stuff)

Inject a 1khz 200mv signal and scope the bad channel output. I betcha the distortion is because only one output is running or more likely one driver is working.

FWIW, I found on those 2 recently done machines that not only did I change the outputs and drivers, but the inputs, mirrors, zeners and all the safety resistors (the low wattage, low value 4.7, 10 etc ohm ones)

AND, you should be able to listen to this with the outputs out of the machine, on headphones or light speakers at low volume levels to test your work. if you pull the outputs and use headphones and its still distorted - there ya go.

and as I mentioned on the 4070 thread, I matched ALL the little guys. there is a thread somewhere around here by echo that states 4 UF4004s (or something real close to that) can make a STV4H purty close.

Edited to add: just out of curiosity...what is the 'ASO' portion of that channel doing?
 
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I am happy to report the problem has been resolved. I pulled all of the newly installed transistors last night and while testing them I noticed that all of the new transistors have the same pin configuration EXCEPT the Qm9 replacement which is an NTE123AP. I reinstalled Qm9 in the proper configuration and BOOM! This was my first electronic repair attempt and can say that is has been educational and satisfying. Thanks everyone.
 
Qm9, if I am reading it right is part of what the SCM calls the 'ASO' - whatever that is. replace all parts of it both sides.

ps: dont use NTE, Qm9 and 10 are 2SC945 - KSC945CBGU works well.
 
Qm9, if I am reading it right is part of what the SCM calls the 'ASO' - whatever that is. replace all parts of it both sides.

ps: dont use NTE, Qm9 and 10 are 2SC945 - KSC945CBGU works well.

+1 on the KSC945CBGU. Make sure you get this exact part number with "C" type center collector. NTE are mystery parts.
 
Quick update and question.

The receiver is sounding great paired with my KLH model seventeens which I am going to recap the crossovers tonight since the capacitors are original. I cant seem to get receiver to pick up FM stereo transmission, but I am not too hung up on that issue. I am working through it slowly as time affords. I have a question regarding the bias current. I measured the bias current on the left 35mA and the right at 9mA. The spec calls for 15 - 100mA. Should I try to adjust the right channel to get the bias current to match the left channel at 35mA? Or, should I just get with 15 - 100mA? Thanks.
 
is the spec really wide ranging from 15-100? typically its 15ish on other stuff, at the very least, make em match.

now, when its not going stereo...is the case that it tunes well but the stereo light never goes on?

nearly EVERY 4070 I ever did (see a theme here, 4070s have been numerous to me) there is a VR for the VCO that gets cruddy. mark it, spray it, twirl it and put it back where it was. in one and only case did I have to replace the sanyo IC (still have 1-2 in stock!)

and of course make sure the stereo light is ok, but if this VR is losing contact, it wont decode and stays in mono
 
Yep, it's actually 15 -110. I'll try to get both channels as close I can. They really should've put VR's instead of fixed resistors. Kinda of pain since I dont have a variety of resistors on hand.

I'll try clearing the VCO pot tonight (after I check the bulb of course).
 
is the spec really wide ranging from 15-100? typically its 15ish on other stuff, at the very least, make em match.

now, when its not going stereo...is the case that it tunes well but the stereo light never goes on?

nearly EVERY 4070 I ever did (see a theme here, 4070s have been numerous to me) there is a VR for the VCO that gets cruddy. mark it, spray it, twirl it and put it back where it was. in one and only case did I have to replace the sanyo IC (still have 1-2 in stock!)

and of course make sure the stereo light is ok, but if this VR is losing contact, it wont decode and stays in mono

I changed the bulb - still didn't work. I then marked the VR, gave it a few twist back and forth and the light popped on and went into stereo. Thanks!
 
F5 that pot and twirl it again...if it corroded once, it will again, the F5 will lube and somewhat protect it. this area gets freely precipitated on with dust and space crud, be nice to have a plastic cover of some sorts...nothing gets hot in this section
 
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