Output transistors keep blowing on SX-2500

I've been trying to figure out why the output transistors keep blowing on an SX-2500 I have. In the process, originally swapped out all the transistors on the amplifier board, and I was getting a single channel working, but I couldn't get anything out of the left channel. I decided to check the transistors again, and all but one of the output transistors had blown. I checked the driver transistors, and they seemed fine, so I put new output transistors in, and turned it back on, but when I did that, R40 caught fire, and R38 got so hot that it discolored the resistor. I replaced the two resistors, and decided to check all the transistors on the board, and found that Q1 and Q2 had blown, so I replaced them again, and pulled every other transistor off the board, tested them, and verified that they were inserted into the board in the correct orientation. When I turned the receiver on this time, I didn't supply any audio signal to them. I just let it stay on for about a minute, then turned it off and tested the output transistors again. They had blown. At this point, I decided to put a signal into the main inputs, and I was able to get weak sound, suggesting to me that the driver transistors are still working.

I'm not sure how these problems are related, or if they are related.

Does anyone have any idea what might be causing this, and how I can prevent it from happening? I haven't been able to bias the output transistors, because they die before I get a chance. I'm wondering if they might be so badly adjusted that they damage the transistors, even with no sound going through them. I'm not sure what I should do at this point. Any advice would be helpful.

 

tell us what you replaced what with ?

 

Ditto

 

I replaced the output transistors with Motorola branded MJ21194 transistors.

 

I originally replaced them with 2sc1079 transistors I had salvaged from another receiver, but the second set were the MJ21194's.

 

They must be very old since Motorola sold the business to Onsemi in the 90's.
I wonder if they are real or fake, i'd try genuine Onsemi parts from Mouser, digikey or other authorized dealer.
esp if both channels are doing the same thing.

 

Did you (probably didn't) record the resistances of the 4 variable resistors on the board before replacing them?

Without reverse engineering the q5 / q6 Vbe multiplier circuit that sets the idle current, I don't know which END to set the VR3 and VR4 pot wiper to.

WHERE are VR3 and VR4 set now?

 

Yeah, they are probably pretty old. Not sure how old. I got a whole stash of parts from an old engineer that passed away. He had stuff ranging in dates from the '60s to about 2016, when he stopped being able to do anything. I only have 1 of them left though, that I can see, so unless I find something else, I'll probably have to order new ones. I worry that they will blow too. That's $20+ a try. LOL

 

I haven't touched those resistors, so they are set where they were originally. I am assuming when you said "replacing them" you meant the transistors, and not the resistors. If you meant the resistors, I didn't replace them.

 

Are you using proper bench procedures?
DBT?
Idle trimmers check out?
Just wondering.

 

Number one imperative - DO NOT TURN ON THIS UNIT UNTIL WE SAY TO!!!
There's a lot of talking and testing with power off before we even DREAM of turning on the power!!

I'm gonna try to talk the guys through this:

Some of this stuff isn't intuitive. This unit uses an unusual for Pioneer method of idle current control. If on the USUAL method of control the pots go open circuit or a thermal diode wire breaks, the idle current goes INFINITE and immediately blows the outputs, and sometimes the drivers.
I say this because your symptoms are remarkably similar.

We replace old trim pots in thermal bias circuit because of the extreme danger of them going open circuit and EATING their outputs and drivers. The circuit says TURN ON HARD both sides at the same time - resulting in HUGE idle currents. The pots have a wiper that slides on the resistance element on one end, but the OTHER end slides on a (usually circular) copper contact that when corroded (like a bad switch) goes open circuit as the other end of the wiper hits the corrosion.
BOOM!!!! BLOOIE!!!! ZZZAAAPP!!!!

This circuit is a Vbe multiplier, and as the temperature of the Q5(Q6) junction increases the voltage between the collector and emitter will increase proportionately.

Looking at this unit's circuit: we have a diode, 470 ohm resistor, 220 ohm pot and a 330 ohm resistor.
The pot wiper is connected to the base of Q5(Q6). Until Q5(Q6) is turned on by having about 0.6v from emitter to base, it will NOT conduct and will LOOK like an open circuit. That's BAAAAD. Lots of idle current will flow.
Thus, having between 0.6v and 5 volts at the base of Q5(Q6) is OK. At 5v the transistor will be damaged from too much base current - IF it could get it through the resistors in the way.

One thing that could upset it is EXTREME centerpoint voltage set by VR1(VR2) - leave that pot alone for now - it had been set and should be close enough to get us going - then later after we have better idle current control we'll futz with that adjustment.


Q5 (and Q6 separately) should have about 2.4 volts (needed to set idle current through two diode junctions and 2 sides of push pull amp stages) across them
LESS voltage is the preferred startup - which means they should be conducting at power up - that means Q5(Q6) should be biased on.

Thus VR3 and VR4 should have MAXIMUM voltage at the wiper which supplies the base voltage to turn ON Q5 (and Q6 separately). Then the base voltage is adjusted DOWN to reduce the conductivity of Q5 (and Q6 separately) and INCREASE the Collector to Emitter voltage of Q5 (and Q6 separately).

 

I'm sure I have suitable pots, but I have yet to organize all the stuff I got from this guy. It filled an entire 20 foot Uhaul truck. At this point though, it will probably be faster for me to buy replacements.

It appears to me that I should buy a handful of new MJ21194s, and some 100K and 220 ohm potentiometers, and get back to this thread when those are replaced to get more advice at that point.

Anything else I should be concerned about replacing, as long as I'm making order? Electrolytics have been replaced.

 

As long as it's just BUY and not REPLACE - DON'T TOUCH THE 100K center point voltage POTS

I'm going to assume that you didn't see that thing about waiting to post... I'm trying to make a "reference post" with all the needed information in one place rather than vital stuff being scattered.

 

Okay. I caught the part where you said not to power it up.

 

You should make a dim bulb tester.

 

I have a dim bulb tester. In fact, I have a variac plugged into an isolation transformer, with a dim bulb tester, but I don't see that helping this situation. It isn't even blowing the three amp fuses that are directly connected.

I use that rig extensively on tube equipment, but find its usefulness to be limited mostly to power supply issues when used on solid state devices.

 

First rule of electronics, transistors blow to protect fuses. Realistically, fuses are there to prevent house fires.

3 amps on the primary side of the transformer is 360 watts, the power available is the power of the saturated e-core of the transformer, plus the charge stored in the capacitors. Considering the safe operating area of the power transistors, it's over before one powerline cycle completes itself.

 

I normally don't do repairs on receivers. Mostly, the things I repair that plug into walls are televisions and two way radio equipment. I have tried using the DBT for solid state stuff, and it is kind of hit and miss. Mostly miss. For instance, going through a switch mode power supply tends to be an exercise in futility. I have seen people claim that a DBT saved their transistors, and I am skeptical. Especially since I have had transistors blow while the DUT was connected to a DBT. I can see it preventing a fire if a transistor does blow short, but you can blow a transistor without getting extremely low impedance first. It really doesn't take much current at all to blow a transistor. Just enough current flowing in the wrong direction, and that amount of current is different for every transistor.

Anyway, I don't want to make this a debate about the usefulness of a DBT, but in this situation, I don't think it's going to be of much use.

 

Selection of the proper wattage lamp is critical.

As you indicated, a DBT does not guaranty protection, but it is added insurance against excessive current.

I liken it to when I played competitive hockey for about 20+ years. I didn't always get hit, kicked, or take a stick or puck to the groin, but I ALWAYS wore a cup. Use the DBT.