That makes me a little more confident about the 424's I ordered from them. I have found them to be a responsive company in the past, but I haven't bought many semis from them. We'll see...
I've been using MCM for 20 years or so, that's usually the first place I go for Japanese type transistors. This is the first time I've had a problem. This is the first amp I've blown up on the bench after repair, that's been many many amps.
Craig
Craig
Hi llwhtt,
Maybe after this MCM will take measures to make sure there parts are originals.
David.
Maybe after this MCM will take measures to make sure there parts are originals.
David.
Hi David,
The entire industry is now talking measures to ensure the parts supply chain isn't contaminated. The stuff in the system already may take some time to shake out.
One thing I really like about On Semi's new parts is that the differences between NPN and PNP beta have been greatly reduced. The short-lived MJW0281A and MJW0302A were excellent parts where the beta was extremely tight, and also between polarities. I have never seen anything that good before or since. If you ever see some available and have a need for these type parts, try them out. I built a Symasym with them and had a low bias (22 mA) for minimum distortion. That and the distortion was very low.
Hi Craig,
Yes, a new thread for your amp would be best. Have you opened one up yet, or is there one existing?
Hi Fred,
You have a couple excellent courses of action to follow. Either direction is fine.
As for the beta matcher, anyone is welcome to start a thread and talk about it. I'll respond to questions.
-Chris
The entire industry is now talking measures to ensure the parts supply chain isn't contaminated. The stuff in the system already may take some time to shake out.
One thing I really like about On Semi's new parts is that the differences between NPN and PNP beta have been greatly reduced. The short-lived MJW0281A and MJW0302A were excellent parts where the beta was extremely tight, and also between polarities. I have never seen anything that good before or since. If you ever see some available and have a need for these type parts, try them out. I built a Symasym with them and had a low bias (22 mA) for minimum distortion. That and the distortion was very low.
Hi Craig,
Yes, a new thread for your amp would be best. Have you opened one up yet, or is there one existing?
Hi Fred,
You have a couple excellent courses of action to follow. Either direction is fine.
As for the beta matcher, anyone is welcome to start a thread and talk about it. I'll respond to questions.
-Chris
I'm trying out some of onsemi NJW0281G and NJW0302G from their sustained beta series. They claim to match them to 10%. I've got 6 pair paralleled arranged in EF and a double diff input stage. I like symmetry. It works well in simulation, but the thing is so blazingly fast I fear it may turn into ash on my bench.
David.
Hi David,
I can vouch for the claim of matching. These devices are the best outputs I have ever seen in their power class. In fact, I would rather parallel a bunch of these than go with fewer higher powered output transistors.
These are really not a problem to use. One common mistake that is often made is that experimenters forget to ground the heat sink. Most times this will cause the output stage to oscillate magnificently. Adding local HF bypass capacitors is only good practice.
I'm glad someone else has tried these new parts. The ones I have are the 2SB817 type size. When I search on the On Semi site, there are no matches found for your numbers. Did you buy them from Digikey? I think they still had some left. There may be a Thermal Track version of these parts.
-Chris
I can vouch for the claim of matching. These devices are the best outputs I have ever seen in their power class. In fact, I would rather parallel a bunch of these than go with fewer higher powered output transistors.
These are really not a problem to use. One common mistake that is often made is that experimenters forget to ground the heat sink. Most times this will cause the output stage to oscillate magnificently. Adding local HF bypass capacitors is only good practice.
I'm glad someone else has tried these new parts. The ones I have are the 2SB817 type size. When I search on the On Semi site, there are no matches found for your numbers. Did you buy them from Digikey? I think they still had some left. There may be a Thermal Track version of these parts.
-Chris
Hi Chris,
I think the M has been replaced with N at the beginning of onsemi part numbers. This would make it hard to search for them.
David.
I think the M has been replaced with N at the beginning of onsemi part numbers. This would make it hard to search for them.
David.
Okay, was able to do some work on this today. Not very encouraging, I'm afraid.
Current state of the amp:
* All caps replaced.
* All transistors on the input board replaced, (with same p/n, no subs) except:
* Diff pair (Q1 and Q2) pulled.
* T1 pulled and jumped.
* Only one pair of output transistors connected, Q17 removed outright, the others are out of circuit by removing their screws and lifting one leg of the emitter resistor.
* Poorly matched 30/31 drivers still installed.
* Bias pot at original setting
When I power it up in this state, no load, no input, I can raise the power to get -75vdc at the speaker out with no problems.
When I connect C of Q7 to the positive rail with 1k resistor (Pete's DC test) it freakin' starts to oscillate again when I get to about 30vdc at the speaker outs.
If I disconnect the jumper across T1 (leaving C of Q7 connected to the + rail), I can increase voltage to about 50vdc, but then voltage suddenly drops out, current draw spikes and the 1k resistor connected to the PS cap and C of Q7 smokes. 🙁
In other words, it's worse than it was before.
I could go through and get low-voltage readings, but something is really not right.
I'm close to saying Uncle...
Current state of the amp:
* All caps replaced.
* All transistors on the input board replaced, (with same p/n, no subs) except:
* Diff pair (Q1 and Q2) pulled.
* T1 pulled and jumped.
* Only one pair of output transistors connected, Q17 removed outright, the others are out of circuit by removing their screws and lifting one leg of the emitter resistor.
* Poorly matched 30/31 drivers still installed.
* Bias pot at original setting
When I power it up in this state, no load, no input, I can raise the power to get -75vdc at the speaker out with no problems.
When I connect C of Q7 to the positive rail with 1k resistor (Pete's DC test) it freakin' starts to oscillate again when I get to about 30vdc at the speaker outs.
If I disconnect the jumper across T1 (leaving C of Q7 connected to the + rail), I can increase voltage to about 50vdc, but then voltage suddenly drops out, current draw spikes and the 1k resistor connected to the PS cap and C of Q7 smokes. 🙁
In other words, it's worse than it was before.
I could go through and get low-voltage readings, but something is really not right.
I'm close to saying Uncle...
Last edited:
Fred,
You want T1 open so that the current source is turned off - for now.
And P1 turned to a short or jumpered so that there is no bias current.
Sorry for the confusion.
You want T1 open so that the current source is turned off - for now.
And P1 turned to a short or jumpered so that there is no bias current.
Sorry for the confusion.
Maybe the logic behind this will help:
We want to isolate the output stage from the VAS and all upstream.
We do it by opening T1 which turns off the current source.
We want to check for leakage, so we need bias to be zero.
We do this by turning down the bias pot.
We then want to pull it low, which you can either do with the 1K resistor
or by connecting T1 again.
And high with the 1K from the VAS collector to the pos supply.
Measure the voltages across all connected emitter resistors with
each test. i (current) = v/r = v/.82
I thought last time you opened T1 you were able to go rail to rail
without oscillation. Now if you can get it to work again, measure
the emitter resistor voltages.
We want to isolate the output stage from the VAS and all upstream.
We do it by opening T1 which turns off the current source.
We want to check for leakage, so we need bias to be zero.
We do this by turning down the bias pot.
We then want to pull it low, which you can either do with the 1K resistor
or by connecting T1 again.
And high with the 1K from the VAS collector to the pos supply.
Measure the voltages across all connected emitter resistors with
each test. i (current) = v/r = v/.82
I thought last time you opened T1 you were able to go rail to rail
without oscillation. Now if you can get it to work again, measure
the emitter resistor voltages.
Last edited:
Last time when I opened T1, I was able to go to full + voltage. Now what happens is strange: I get to about 62 vdc okay, but then as I turn up the variac, the voltage at the speaker outs actually starts to go down and the current draw goes up to about .2-.3a. On the scope you can see it break into oscillation right around 62.
This happens whether the bias pot is at zero or not (which makes sense, since T1 is open, right?).
So yes, now it is oscillating even with T1 open. Here's what changed:
Replaced Q3, Q5, Q6. All the olds tested good.
Replaced C1, C2, C4. All the olds tested good.
Replaced R5, R6. All the olds tested good.
It does seem stable up to 60vdc, is that enough voltage to take readings? And if so, where should I start?
Q5 is there to drive the led. It doesn't have any other purpose and with the diff trans out there is no bias on it.
David.
David.
Hi Fred,
Just a suggestion. Put the amplifier into it's normal running condition and try to power it up. Make sure the bias control is set for minimum bias current. The original bias setting is normally a pretty safe bet also. Leave your oscilloscope connected to the output that oscillates and remove the fuses for the good channel. Now we only have one circuit to worry about.
There are times when a test may really suck up time and effort. What may work with one amplifier type may cause havoc in another. I'm not sure about this situation, but it seems to be reasonable to test the amp with the signal stages complete. You can still use one pair of output transistors and add the others once you know everything is fine.
-Chris
Just a suggestion. Put the amplifier into it's normal running condition and try to power it up. Make sure the bias control is set for minimum bias current. The original bias setting is normally a pretty safe bet also. Leave your oscilloscope connected to the output that oscillates and remove the fuses for the good channel. Now we only have one circuit to worry about.
There are times when a test may really suck up time and effort. What may work with one amplifier type may cause havoc in another. I'm not sure about this situation, but it seems to be reasonable to test the amp with the signal stages complete. You can still use one pair of output transistors and add the others once you know everything is fine.
-Chris
HI Fred,
Have you tested all the small caps on the input board for shorts?
I have in the past found shorted ceramic disc caps. Also test the bias pot again to make sure it is functioning properly. I agree with Chris on his last comments.
There nothing wrong with checking components a second time. Sometimes components can get damaged during a repair. I don't mean to get to crazy with that just have a quick look around.
David.
Have you tested all the small caps on the input board for shorts?
I have in the past found shorted ceramic disc caps. Also test the bias pot again to make sure it is functioning properly. I agree with Chris on his last comments.
There nothing wrong with checking components a second time. Sometimes components can get damaged during a repair. I don't mean to get to crazy with that just have a quick look around.
David.
Hi Fred,
I don't know if anyone has mentioned this to you.
It's a good idea to removed the mica insulators and inspect them for small burn holes from arcing. I have found this manly on the big work horse PA power amps but it is possible on high power hifi amps. What you are looking for is small brown or black marks on the surface of the mica. The arcing is caused from large amounts of back emf off the
speaker at high power output.
David.
I don't know if anyone has mentioned this to you.
It's a good idea to removed the mica insulators and inspect them for small burn holes from arcing. I have found this manly on the big work horse PA power amps but it is possible on high power hifi amps. What you are looking for is small brown or black marks on the surface of the mica. The arcing is caused from large amounts of back emf off the
speaker at high power output.
David.