They are often needed to prevent ground loops which show up as increased hum and noise and distortion. It should all at least still work without them but yes, they are important.
Thank you much all, I’m working one channel at a time. The two channels are fully isolated from one another so they won’t effect each other with their current draw.
So far in the right channel I have found one open resistor and Q613 is dead. I also replaced the zener diodes just because
They marked it as a 2SA970, but it’s actually a 2SA872E. I have a bunch of KSA992F, but thinking maybe I should use the 2SA992E I have. I’m just wondering about the complementary device as I only have KSC1845F. Do you all think gain in this portion of the power supply is critical? I mean we’re only talking 500-520 for the E variant vs 440-470 for the F. Curious as to what your opinions on it are.
I too suspected Q612 and possibly Q609, but they are fine.
Dan
That’s the thing that kind of sucks, a lot of the component legs are extremely difficult to get to. And you can’t unscrew the board from the chassis and pull it out while having it powered at the same time, there’s no way for the wires to stay hooked up. It’s pretty cramped in the area with larger heat sinks, and I don’t know if I would entirely trust myself tonight slip with my lead. But I guess those new tips will make it hard to mess up. I did find the open resistor, actually the same resistor in the other side was open as well, so I pulled the board because I knew I’d have to at least replace that resistor, it was towards the front-ish of the circuit.
Dan
Bummer. I’m guessing trying to source those will be nearly impossible. What a shame.
So I will need to the complement as well because Q610 is also toast. It’s a 2SC1775E. Should I just go with the F variants? I have a good 2SC1775E I just pulled and it measures at about 580 for an hFE. Probably too much voltage for a BC550C/560C combo. Any other recommendations? Like 2N5551C/5401C? Or should I stop worrying about the gain here and use the KSC1845/KSA992?
Dan
Insulated RCA sockets are common and should be readily available but whether they will physically be OK is another thin. Maybe you could take the insulating kits off them.
Q612 could likely be replaced with an MJE350
A 2N5401 should work fine for that. Are you absolutely sure it is faulty?
Q612 could likely be replaced with an MJE350
Insulated RCA sockets are common and should be readily available but whether they will physically be OK is another thin. Maybe you could take the insulating kits off them.
A 2N5401 should work fine for that. Are you absolutely sure it is faulty?
Q612 could likely be replaced with an MJE350
Okay, I have some hopefully if anything the sleeves will fit. The RCAs on it are really nice.
Q610 and Q613 are both definitely bad. They measure as diodes on a Peak DCA75 and I confirmed one junction is open in each.
Are you thinking I need to change Q612? It measured okay as I mentioned and I do have many KSA1220s. Are you thinking they should be beefed up a bit?
Dan
I just realized that the 2N5401/2N5551 I have much lower gain, only 150 or so compared to the 450-550.
Another option, what do you think of ZTX694b and ZTX795a? High gain and far more robust. And low noise, but that doesn’t really matter.
Dan
Another option, what do you think of ZTX694b and ZTX795a? High gain and far more robust. And low noise, but that doesn’t really matter.
Dan
Have you checked the transistors with a simple diode test on a DVM?
Red lead on the base for the NPN and black lead on the emitter and it should read as a forward biased diode (around 0.600 on the meter). Same for base to collector. For the PNP reverse the lead polarity and the results should be the same.
I'll be honest and say I would be more than surprised if both had an open junction. Check them on a meter.
I keep saying about gathering evidence 🙂 If the voltages out of these regulators was measured as OK then all the transistors are OK.
Red lead on the base for the NPN and black lead on the emitter and it should read as a forward biased diode (around 0.600 on the meter). Same for base to collector. For the PNP reverse the lead polarity and the results should be the same.
I'll be honest and say I would be more than surprised if both had an open junction. Check them on a meter.
Doesn't matter. Its a classic text book regulator and should work with pretty much anything of suitable voltage and current rating.
I keep saying about gathering evidence 🙂 If the voltages out of these regulators was measured as OK then all the transistors are OK.
Failure of Q612 is the only viable current path into Q613 but it would more than likely cause Q613 to fail short circuit, not open.
You must check them on your meter the old fashioned way.
I found the exact same transistor for the other channel Q713, also a 2SA872 also dead. I’d was measuring as a diode junction on the DCA75. By the way, when I say that I measuring these, I am doing it out of circuit, I’m pulling the device and measuring on its own. But here you can see the NPN, Q610 I believe it was.
Measuring as a diode, not sensing the green lead, the emitter, as being hooked up to anything.
And measuring from emitter to base it’s open in diode mode. Correct?
That’s 3 bad transistors between the two channels so far in addition to the open resistors. In your opinion, would you swap out Q612 and Q609? With these other failing components, is it possible they were under stress? Put in parts rated for higher current? Or should they be fine if they measured fine?
Dan
I just checked Q613 the exact same way, the 2SA872 and it is showing with an open emitter as well. I did double check it with my multimeter and diode mode in it doesn’t matter what I arrange the leads, I can’t get any measurement between the emitter and the other two leads, that’s with either the positive or negative on the emitter.
Dan
Dan
Its bizarre 🙂
The picture with the Fluke does seem to show it is open. Junctions going open are not a normal failure mode for small plastic devices like these and certainly not multiple ones failing the same way. Overcurrent (an overload) would normally see the pass transistors going short and if one also failed short to the base then it might take out one of the small ones but again, normally by failing short circuit from C to E.
All I can say is replace them all and for your own curiosity make sure all the replacements read as you would expect on the DVM. Q610, Q611 and Q613 handle very tiny currents. The pass transistors Q609 and Q612 pass all the load current but even that is low as witnessed by the 3.3 ohm 0.5 watt fusible resistor in each rail.
MJE340/350 for the NPN/PNP pass transistors should be fine and 2N5551 or 2N5401 for the others if you can still get those.
The picture with the Fluke does seem to show it is open. Junctions going open are not a normal failure mode for small plastic devices like these and certainly not multiple ones failing the same way. Overcurrent (an overload) would normally see the pass transistors going short and if one also failed short to the base then it might take out one of the small ones but again, normally by failing short circuit from C to E.
All I can say is replace them all and for your own curiosity make sure all the replacements read as you would expect on the DVM. Q610, Q611 and Q613 handle very tiny currents. The pass transistors Q609 and Q612 pass all the load current but even that is low as witnessed by the 3.3 ohm 0.5 watt fusible resistor in each rail.
MJE340/350 for the NPN/PNP pass transistors should be fine and 2N5551 or 2N5401 for the others if you can still get those.
Awesome, I’ll get them swapped. The smaller ones have been swapped, I’ll take care of the KSA1220 and KSC2690s. I’m just finishing up reflowing some joints and then I’ll apply power to the power supply.
Dan
So I’m not meaning to second guess you, you clearly know more than me, but just wanted to confirm. You say that Q609 and Q612 pass all the current through them and a good replacement would be MJE340/350 which are rated at less than half the current of the originals? I’m sure it’s good, but I just wanted to double check. Also I have a couple hundred each of the KSC2690 and KSA1220, but still should go for the MJE parts?
Dan
Dan
Regulators die charging capacitors, use higher rated transistors and increase R622,623,722,723 to 100 ohm 1Watt for slow start
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Thanks for the suggestion. Are you referring to the LM317 and LM377? Or are you referring to the KSC2690 and KSA1220 being discussed above? Recommend for collector resistor value and size?
On a side note, all four (Q609, Q612, Q709, Q712) all have a 4.7 ohm resistor soldered to the emitter which isn’t on the schematic, is this a similar method?
And when you say higher rated, I’m guessing you’re referring to current and dissipation rating?
I’m looking through what I have and not paying attention to Ft for higher rating I have:
2N4923G/2N4920G - 3A, 80V, 30W
2N5192G/2N5195G - 4A, 80V, 40W
Thought these last two are slower devices as 3 MHz and 2 MHz.
MJE243G/MJE253G - 4A, 100V, 15W
TTC004b/TTA004b gain a little in current capability at the expense of dissipation.
And then of course it was recommended earlier MJE340/MJE350, still waiting to hear back to confirm since they’re rated for only 500 mA vs the 1.2A of the originals.
If those weren’t the ones you were speaking of then disregard, if they were, any recommendations?
Dan
Those transistors do pass all the load current but the 3.3 ohm 0.5 watt fusible resistors put limits on what that current might be. The square root of P/R give the current so we have 0.39 amps as a maximum. The fusible will be run well below that current value. So this puts limits on the expected load current.
If you have genuine originals then you can of course fit those.
That is typical of an 'In Production' mod that is done when issues come to light in service. It may be reliability related or might be to stop some unwanted behaviour such as a stability issue.