Hoping for some ideas on what can cause the bias transistor Q309 to fail open circuit after a few minutes. Before it fails it's possible to adjust bias and the bias current stays stable until it breaks. The position of VR301 is as to be expected and there are no smoke or visible signs of overload on the transistor.
Is there a typical reason for a transistor to fail as open or short, for example will a collector current overload typically short the transistor (melting the junction) and a base voltage overload leave it in an open state?
Is there a typical reason for a transistor to fail as open or short, for example will a collector current overload typically short the transistor (melting the junction) and a base voltage overload leave it in an open state?
Hi A 8,
I recommend you show the entire PA schematic. What's the model number of the amp?
If Q309 opens, I would expect the output stages to conduct massively, perhaps destructively. What happens to the rest of the amp?
Is Q309 mounted to the output heatsink so that it's thermally connected to the output stages?
Do you have a scope? If so, look for oscillation that might lead to failure. Be sure to use x10 scope probes to ensure minimal probing capacitance.
I can't think of any typical cause for failure of the bias spreader--- it sees relatively light duty. You might confirm viability of C305 and associated PC traces, though I doubt it's the culprit. More often the spreader is a collateral casualty of failing/damaged output stages. Hence, my question above about the rest of the amp when Q309 opens.
Good luck.
I recommend you show the entire PA schematic. What's the model number of the amp?
If Q309 opens, I would expect the output stages to conduct massively, perhaps destructively. What happens to the rest of the amp?
Is Q309 mounted to the output heatsink so that it's thermally connected to the output stages?
Do you have a scope? If so, look for oscillation that might lead to failure. Be sure to use x10 scope probes to ensure minimal probing capacitance.
I can't think of any typical cause for failure of the bias spreader--- it sees relatively light duty. You might confirm viability of C305 and associated PC traces, though I doubt it's the culprit. More often the spreader is a collateral casualty of failing/damaged output stages. Hence, my question above about the rest of the amp when Q309 opens.
Good luck.
It's a Rotel 985, 5 channel amp and here is a schematic including the output stage (the input stage is on another page), mine don't have Q318 and associated components. The bias transistor Q309 is a Darlington with an inbuilt diod, picture below. Not sure how that effects things when it breaks.
I've run out of the 2SD1953 so need to find and order a replacement before I can do more testing.
I've run out of the 2SD1953 so need to find and order a replacement before I can do more testing.
Thanks BSST, when it breaks there is a bit of cracking and then mostly silence. One can still hear traces of the music but in a cracking manner. There is also a small dc offset on the output. Q309 is mounted on the heatsink.
I measured the voltages over Q309 when it first happened as I suspected it was the problem and iirc the voltages were lower (maybe 0.7volts) than on a working channel which as you say indicates its more shorted rather than open but on the other hand if the diod in Q309 is still ok perhaps that's why the output stage stays calm.
I was in the amp testing different values for the hum breaking resistors before this happened so for now I will assume Q309 breaking is a collateral causality and check the PC traces before I put a new Q309 in and test/measure again.
I used a new silpad for the second Q309 I tested so should not be the issue even though its a good point as the old one was falling apart.
I’m going to assume that Q309 didn’t fail open, since you didn’t complain of blown output transistors. I’m taking a very wild guess that Q309 may not be the fault. With scant evidence, I’m suspicious that the bias pot’s wiper may be opening as it warms.
The output stage is a cascade of three emitter followers, so the voltage across the bias spreader should be roughly 3.6V when in a working state. When the amp enters the fault mode, what voltage appears across Q309 c-e?
Any chance if you let the amp thoroughly cool that it works briefly when powered on?
The output stage is a cascade of three emitter followers, so the voltage across the bias spreader should be roughly 3.6V when in a working state. When the amp enters the fault mode, what voltage appears across Q309 c-e?
Any chance if you let the amp thoroughly cool that it works briefly when powered on?
Wouldn’t that just put the output stage into Class B, i.e. moderate decline in sound and quality, rather than almost no signal at all?
Good point. I'm way out on a limb with this guess. 🙂
A 8 could test the Darlington out of circuit to confirm failure. I think shorting base to collector and biasing through 470R to a 9V battery would be a reasonable test. Should present about 1.2V c-e, dropping voltage at the part warms, I think.
A 8 could test the Darlington out of circuit to confirm failure. I think shorting base to collector and biasing through 470R to a 9V battery would be a reasonable test. Should present about 1.2V c-e, dropping voltage at the part warms, I think.
One of BSST original ideas was correct and indeed Q309 was not broken. Turns out there was some solder paste residue on the pcb and a small damage in the lacquer protection right under it thus "semi" shorting one of the board traces after a few minutes of running the amp.
Always very helpful getting new perspectives when one gets stuck on the wrong track, thanks all!
Always very helpful getting new perspectives when one gets stuck on the wrong track, thanks all!
