Vintage Graphic EQ with static rumble sound

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AnalogPackrat said:
Have you traced any of the circuit around the burned areas?
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hey...i hope you guys are well. I did recheck again on this EQ board a few things. That NPN transistor you've mentioned earlier "AnalogPackrat" is attached. MPS U05. One of their leg was kind of lose and by moving the transistor still inside the circuit, made a few changes. Decided to remove it. I did test this with my DMM on continuity mode, and + on base -ve on emitter reads an open circuit. This mean this transistor is bad right?

The other transistor opposite is a PNP , MPS U55 also reads odd...

I did check every single dual and single opamps, and jFETs. All of them are OK. Also, the PSU had two uA748TC they are also tested fine.

Removing these transistors, the PCB is also burned underneath.

Could you please let me know if there is any better alternatives for these? they are rare one now days it seems.

Many many thanks for you help with this!
 

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Hard to say, but most meters should show continuity in the forward junction orientation you used unless the voltage applied isn't high enough. What meter are you using?

As for substitutes, look at the datasheet and search Mouser or Digikey. I doubt these were used anywhere near their max ratings of 80V and 10W (more like 1W w/o heatsink) in your EQ. But, again, tracing out the circuit where they're used will help determine what they're doing and what to substitute. I suspect something like BD139/140 would be ok, but that depends on the circuit. They have half the hFE and a bit higher Veb0. Watch the pinout on any substitution you do.
 
AnalogPackrat said:
Hard to say, but most meters should show continuity in the forward junction orientation you used unless the voltage applied isn't high enough. What meter are you using?

As for substitutes, look at the datasheet and search Mouser or Digikey. I doubt these were used anywhere near their max ratings of 80V and 10W (more like 1W w/o heatsink) in your EQ. But, again, tracing out the circuit where they're used will help determine what they're doing and what to substitute. I suspect something like BD139/140 would be ok, but that depends on the circuit. They have half the hFE and a bit higher Veb0. Watch the pinout on any substitution you do.
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Thanks! My DMM is not the best nor the cheapest one either...Tenma, quiet solid but basic. Both transistor showed me no reading on base/emitter testing. I will replace them, wont hurt having fresh ones... I could get hold of something similar, but lower rated ones as you've mentioned more in the range of 1A and 6W ...but not 170Mhz. More like 120Mhz frequency. Would that be an issue? Thanks again!
 

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Put the meter on diode test and get the leads oriented correctly. A functioning PN junction should conduct one direction and not the other ("open circuit").

NPN transistors have P base and N collector and emitter. PNP are opposite. Polarity matters.
 
AnalogPackrat said:
Put the meter on diode test and get the leads oriented correctly. A functioning PN junction should conduct one direction and not the other ("open circuit").

NPN transistors have P base and N collector and emitter. PNP are opposite. Polarity matters.
Click to expand...
OK... attached. First the NPN transistor.

First pic: red probe on base, black probe on emitter. I have OL and no beep. Nothing. I should see some readings here if the transistor good?

Second pic: red probe on base black probe on collector. I get a reading.

Am I correct saying that the first reading is wrong? as I should have the same reading on both sides?
 

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Measuring B-E on NPN is across a PN junction. Forward biased is with higher voltage on base (P) than the emitter (N). Diode test mode puts the higher voltage on the red jack on the meter. My meters show the forward voltage when forward biased and OL when reversed.

Measuring B-E on PNP is across an NP junction, so forward biased is with the emitter at a higher voltage than the base.
 
AnalogPackrat said:
Are both images are from the same transistor with just BE leads reversed? Do you have any other known good transistors (of any bipolar type) to compare with?
Click to expand...

Yes, same transistor, BE reversed. I did try the PNP (U55) transistor opposite to this NPN and I have readings on both sides, quiet similar readings by swapping the probes, seems that PNP is good?...and NPN is bad?
 
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AnalogPackrat said:
You should get OL when reverse biased (red on N, black on P) and something else (read your meter manual) when forward biased (red on P, black on N).
Click to expand...
Yes, but I only get reading on one side of N, the collector with red on P and black on N.

I don't get any reading with emitter N with black probe and red probe on base P .

I thought I should read very close to each sides of Ns, but i could be wrong here....
 
The B-C and B-E junctions won't have the same characteristics (Vf, for example). If you aren't getting normal diode like readings on one, then the device is likely damaged.

Other components or something connected to the unit may have caused the failure which is why you should trace that part of the circuit and draw a schematic so you can understand what it's supposed to be doing.
 
AnalogPackrat said:
The B-C and B-E junctions won't have the same characteristics (Vf, for example). If you aren't getting normal diode like readings on one, then the device is likely damaged.

Other components or something connected to the unit may have caused the failure which is why you should trace that part of the circuit and draw a schematic so you can understand what it's supposed to be doing.
Click to expand...
Yes.. i think i am getting closer to just drop this off to a techy who actually understands electronics more than me...and has a scope. My last resort is to swap these two transistors to a new one and see what happens. Thanks so much for your time and help with this!
 
Studiogearlover said:
hey...i hope you guys are well. I did recheck again on this EQ board a few things. That NPN transistor you've mentioned earlier "AnalogPackrat" is attached. MPS U05. One of their leg was kind of lose and by moving the transistor still inside the circuit, made a few changes. Decided to remove it. I did test this with my DMM on continuity mode, and + on base -ve on emitter reads an open circuit. This mean this transistor is bad right?

The other transistor opposite is a PNP , MPS U55 also reads odd...

I did check every single dual and single opamps, and jFETs. All of them are OK. Also, the PSU had two uA748TC they are also tested fine.

Removing these transistors, the PCB is also burned underneath.

Could you please let me know if there is any better alternatives for these? they are rare one now days it seems.

Many many thanks for you help with this!
Click to expand...
The mps u05 and u55 were general purpose complementary transistors... I used lots of A05/ A55 in lower power to92 packages back in the day.

Look at their specs and find a modern device with similar specs in same package.

JR
 
Thanks guys... very thankful for all of your inputs...once is fixed, hopefully..... I will share what was wrong with it...
 

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