What are the voltage ratings of those caps on your board?
SAE 2400L Restoration
- Thread starter sabrefencer
- Start date
I have got the 2400L back together. I rebuilt pretty much everything. The Output module, the drive boards, relay board, and gain switch board have almost everything replaced. Everything is working as expected.
I am setting DC offset, QC, and bias now. DC offset and QC are dialed in pretty close 0vDC and 1.6vDC respectively. The bias is a bit trickier. I started with it set to 1.2v measuring across the 62 ohm resistor near the bottom of each drive board. This is R41 on the MusicParts schematic for the 2400L.
Is this the correct place to be measuring bias?
I also have the output connected to an HP334A distortion analyzer and am feeding 20kHz at 1w per the service manual. But when I turn the bias pot, I don't get much, if any, deflection of the distortion meter. I am measuring about 0.5% distortion, which is too high.
Any suggestion on getting the bias correctly adjusted? I measured across this 62ohm resistor successfully on my 2200, but not sure if it is correct for the 2400L.
I am setting DC offset, QC, and bias now. DC offset and QC are dialed in pretty close 0vDC and 1.6vDC respectively. The bias is a bit trickier. I started with it set to 1.2v measuring across the 62 ohm resistor near the bottom of each drive board. This is R41 on the MusicParts schematic for the 2400L.
Is this the correct place to be measuring bias?
I also have the output connected to an HP334A distortion analyzer and am feeding 20kHz at 1w per the service manual. But when I turn the bias pot, I don't get much, if any, deflection of the distortion meter. I am measuring about 0.5% distortion, which is too high.
Any suggestion on getting the bias correctly adjusted? I measured across this 62ohm resistor successfully on my 2200, but not sure if it is correct for the 2400L.
Regarding the signal generator, its a Heathkit IG-18 rebuilt with modern mods. At about 0.5v output from this generator, distortion is about 0.04%. However, I have to drop the output level of the generator when driving the 2400L to 2.8v output. The distortion of this generator goes up when the output is very low, so yes I am introducing distortion to the measurement. But that does not explain why the distortion level does not change when I rotate the bias pot. The range of the voltage measurement at the 62ohm resistor when rotating the pot from stop to stop provides a range from about 0.8v to about 1.8vDC. A setting of 2.1 or 2.2v does not seem possible.
I find that when I can't get a measurement result that makes sense, It's usually operator error with the measurement equipment, in this case me. So, I want to start with the simplest approach possible to set the bias -- without distortion measurement.
What is the simplest method of setting the bias? If I can get this rough baseline correct, then I can apply the the distortion method to see if that improves my results.
I find that when I can't get a measurement result that makes sense, It's usually operator error with the measurement equipment, in this case me. So, I want to start with the simplest approach possible to set the bias -- without distortion measurement.
What is the simplest method of setting the bias? If I can get this rough baseline correct, then I can apply the the distortion method to see if that improves my results.
For each base-emitter PN junction there will be an approx. .6VDC drop. If measuring across the 62 Ohm resistor you will have the PN junctions of Q19 and Q21 for 1.2VDC. If you go back to the collectors of Q7 and Q8 add two more junctions for 2.4VDC. This is where some of the SAE SMs have you measure. The 2500 and 2600 SM have you measure at the collectors for 2.1VDC which I think is low. The SM also wants 2.1VDC for the QC which is way too high.
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Craig,
This helps a lot. If I read my schematic correctly, and it matches my boards (and I already know there are differences), the collector of Q7 is connected to the base of Q12, and the same for Q8 and Q13. It is easier to clip on to Q12/Q13, and indeed I am getting about 2.3v here. I am also clipped on to the 62ohm resistor which is emitter of Q12/Q13, and I am getting about 1.2v at this point. The actual values are slightly below 2.4v and slightly above 1.2v, so, I think it is in the ballpark.
Outputs are connected to 8ohm dummy loads, and there is nothing connected to the inputs.
Here is a photo of my test leads and meter readings. Let me know if this does not look right.
DC offset (with shorted inputs) is within 3mV of 0vDC, and QC is very close to 1.6vDC on both channels. I'm still observing and tweaking, but I think it is pretty close.
This helps a lot. If I read my schematic correctly, and it matches my boards (and I already know there are differences), the collector of Q7 is connected to the base of Q12, and the same for Q8 and Q13. It is easier to clip on to Q12/Q13, and indeed I am getting about 2.3v here. I am also clipped on to the 62ohm resistor which is emitter of Q12/Q13, and I am getting about 1.2v at this point. The actual values are slightly below 2.4v and slightly above 1.2v, so, I think it is in the ballpark.
Outputs are connected to 8ohm dummy loads, and there is nothing connected to the inputs.
Here is a photo of my test leads and meter readings. Let me know if this does not look right.
DC offset (with shorted inputs) is within 3mV of 0vDC, and QC is very close to 1.6vDC on both channels. I'm still observing and tweaking, but I think it is pretty close.
Attachments
Yesterday I re-connected my distortion measurement set up and was able to set the bias, but instead of 20kHz, I made the adjustment to minimize distortion at 1kHz. The measured distortion was about 0.05% and the bias setting at 62 ohm resistor was about 1.3 to 1.4 v on each channel. Today I decided to run more distortion measurements across the power range at 1kHz. I started by checking visible clipping on the scope which occurred at 44V or 242 watts on each channel. I then began distortion measurements starting at 1v (driving both channels into dummy loads) and working up. I was getting results at or near 0.05% up to about 200W.
I did not get to finish testing the first channel. Something went "POP" and flashed.
The flash came from the output assy, so I pulled it and checked the output transistors. Sure enough on Ch A one of the NPN outputs shorted, base to collector. I checked the output and driver boards carefully for visible signs of carbon or flash, checked resistor values, checked driver transistors for shorts, and found no issues. I did not remove the heatsink from the output board, I only removed the small section of the heat sink necessary to get to the output transistors. Since I had used silpads (on your advice in another post) it was quick and easy to swap in another transistor.
I put to back together, and brought it up on a dim bulb, no short indicated, brought it up on the variac, relays pulled in. So after letting it warm up, I rechecked the DC offset, QC current and bias, and they were all very close to the the settings before the transistor blew. I tweaked the settings a bit and then started the distortion tests again. At 1 watt, 1kHz, distortion measured about 0.05%. But is was getting pretty warm after perhaps 20 minutes running at idle during adjustments and the two distortion tests at 1W. I then tried to bring it up to full power to check clipping on the scope, and again "Pop"/flash. I have not yet pulled the output section apart again, but it seems like the same issue. I assume there is be something causing the outputs to short, but on the first round of investigation I did not find anything suspect.
I am now way better at pulling this thing apart and putting it back together than I wanted to be.
The outputs I used are Toshiba 2SD424 and 2SB554. I replaced what I found in the unit which were also Toshibas, but noticed they had different gain ratings (a mix of "O" and "R"). I had a QSC Audio power amp module from the mid-80s which has 10ea of the Toshiba 2SD424 and 2SB554 all with "O" gain ratings. I used these in my build and for the replacement of the one that blew. I tested them on my Atlas DCA55 before installing them and they tested fine and I had enough to match gain.
At this point I question if I should have used new On Semi MJs instead, especially since another one blew. But not sure if the outputs are the cause or the symptom. If they are the symptom, I definitely want to find the cause before putting new outputs in.
I'll pull it apart again tomorrow and check everything, but would like to get any thoughts what to look for that could be possible causes for shorted outputs.
Two steps forward, one step back (or maybe two or three).
Gary
I did not get to finish testing the first channel. Something went "POP" and flashed.
The flash came from the output assy, so I pulled it and checked the output transistors. Sure enough on Ch A one of the NPN outputs shorted, base to collector. I checked the output and driver boards carefully for visible signs of carbon or flash, checked resistor values, checked driver transistors for shorts, and found no issues. I did not remove the heatsink from the output board, I only removed the small section of the heat sink necessary to get to the output transistors. Since I had used silpads (on your advice in another post) it was quick and easy to swap in another transistor.
I put to back together, and brought it up on a dim bulb, no short indicated, brought it up on the variac, relays pulled in. So after letting it warm up, I rechecked the DC offset, QC current and bias, and they were all very close to the the settings before the transistor blew. I tweaked the settings a bit and then started the distortion tests again. At 1 watt, 1kHz, distortion measured about 0.05%. But is was getting pretty warm after perhaps 20 minutes running at idle during adjustments and the two distortion tests at 1W. I then tried to bring it up to full power to check clipping on the scope, and again "Pop"/flash. I have not yet pulled the output section apart again, but it seems like the same issue. I assume there is be something causing the outputs to short, but on the first round of investigation I did not find anything suspect.
I am now way better at pulling this thing apart and putting it back together than I wanted to be.
The outputs I used are Toshiba 2SD424 and 2SB554. I replaced what I found in the unit which were also Toshibas, but noticed they had different gain ratings (a mix of "O" and "R"). I had a QSC Audio power amp module from the mid-80s which has 10ea of the Toshiba 2SD424 and 2SB554 all with "O" gain ratings. I used these in my build and for the replacement of the one that blew. I tested them on my Atlas DCA55 before installing them and they tested fine and I had enough to match gain.
At this point I question if I should have used new On Semi MJs instead, especially since another one blew. But not sure if the outputs are the cause or the symptom. If they are the symptom, I definitely want to find the cause before putting new outputs in.
I'll pull it apart again tomorrow and check everything, but would like to get any thoughts what to look for that could be possible causes for shorted outputs.
Two steps forward, one step back (or maybe two or three).
Gary
I have the output module out of the unit, and tested all 8 outputs (in place) with the DCA55. All tested OK. The bias transistors both test faulty (in place). Bias resistors (1k and 2.2k) are OK. Next step is to pull the outputs and separate the output board from the heat sink assy.
Photos are of the bias transistor installation and the output board before initial assy. Note the Dow 340 on the MPSA06 bias transistors, which made good contact to the bottom of the heat sink. At bottom left is a small perf board I used to mount the diode and cap (not yet installed when this photo was taken) for the power down modification.
Photos are of the bias transistor installation and the output board before initial assy. Note the Dow 340 on the MPSA06 bias transistors, which made good contact to the bottom of the heat sink. At bottom left is a small perf board I used to mount the diode and cap (not yet installed when this photo was taken) for the power down modification.
Attachments
I pulled the output module apart. I thoroughly checked the output board, driver boards and relay boards, component by component with no visible signs of any problems, carbon, arcing etc. I pulled the bias transistors which did not test in-circuit and checked them out of circuit and they tested fine.
I checked every diode and resistor, and the driver transistors for shorts, on both drive boards and the relay board -- no detectable issues. No visible problems. I did not pull any transistors from the drive boards, but did test for shorts.
THEN . . . I saw it.
I had checked the outputs before I pulled them from the heat sinks and they all tested fine. But I decided to test them now that they were removed from the heat sink. Then I saw a tiny black spot on the edge of one of the 2SD424s - the one next to the one that shorted. The case (collector) shorted to the heat sink and made a hole in the silpad, and made a tiny arc hole on the edge of the transistor package, and the heat sink, where it left a carbon flash. See photos.
Could the failed component be the silpad in both the internally and externally shorted transistors? Could excessive current have caused the silpad to fail? If so what may have caused the excessive current. I was monitoring bias, and QC when it went, and they were right where they should be. I still have not found any evidence of what else could have caused the failures.
This is my first time using silpads. Could they be too "thin" or otherwise defective? Did I use the wrong type (see last photo)? Could I have installed them incorrectly? Seems pretty simple. Put the silpad on the clean heatsink, install the transistor, carefully tighten screws. No I don't have a torque screwdriver, but was careful to use my finger tips to turn the screwdriver, and alternate across the screws and not over tighten.
I think that when I put it back together, despite the convenience of the silpads, I will use mica insulators and paste. Even though the transistor that burned through the silpad tested OK, I don't plan on reusing it.
For the relay release, I removed the jumper from the output board. Instead of running the new wire (from the AC side of the bridge rectifier) to the board and cutting the trace, I added a small piece of perf board and mounted the diode on it and used it as a tie point for the feed wire, which connects to the anode of a 4N4004. The cathode is connected to the pin (two actually) that was fed by the jumper, and the new cap (+). The (-) side of the cap is soldered to a lug that replaced a lock washer in the hardware that connects the heat sink to the board. While physically different from they way Craig described it, I think it is the same electrically. The relay was cutting out immediately on power down while the amp was functioning properly. I did notice that during both shorting events, the relays clicked, but I was reaching for the off switch and did not notice if they cut out and stayed out. They did not kill the power - but don't think they were designed to do that.
I checked every diode and resistor, and the driver transistors for shorts, on both drive boards and the relay board -- no detectable issues. No visible problems. I did not pull any transistors from the drive boards, but did test for shorts.
THEN . . . I saw it.
I had checked the outputs before I pulled them from the heat sinks and they all tested fine. But I decided to test them now that they were removed from the heat sink. Then I saw a tiny black spot on the edge of one of the 2SD424s - the one next to the one that shorted. The case (collector) shorted to the heat sink and made a hole in the silpad, and made a tiny arc hole on the edge of the transistor package, and the heat sink, where it left a carbon flash. See photos.
Could the failed component be the silpad in both the internally and externally shorted transistors? Could excessive current have caused the silpad to fail? If so what may have caused the excessive current. I was monitoring bias, and QC when it went, and they were right where they should be. I still have not found any evidence of what else could have caused the failures.
This is my first time using silpads. Could they be too "thin" or otherwise defective? Did I use the wrong type (see last photo)? Could I have installed them incorrectly? Seems pretty simple. Put the silpad on the clean heatsink, install the transistor, carefully tighten screws. No I don't have a torque screwdriver, but was careful to use my finger tips to turn the screwdriver, and alternate across the screws and not over tighten.
I think that when I put it back together, despite the convenience of the silpads, I will use mica insulators and paste. Even though the transistor that burned through the silpad tested OK, I don't plan on reusing it.
For the relay release, I removed the jumper from the output board. Instead of running the new wire (from the AC side of the bridge rectifier) to the board and cutting the trace, I added a small piece of perf board and mounted the diode on it and used it as a tie point for the feed wire, which connects to the anode of a 4N4004. The cathode is connected to the pin (two actually) that was fed by the jumper, and the new cap (+). The (-) side of the cap is soldered to a lug that replaced a lock washer in the hardware that connects the heat sink to the board. While physically different from they way Craig described it, I think it is the same electrically. The relay was cutting out immediately on power down while the amp was functioning properly. I did notice that during both shorting events, the relays clicked, but I was reaching for the off switch and did not notice if they cut out and stayed out. They did not kill the power - but don't think they were designed to do that.
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I just mentioned the relay mod since everything else seemed fine.
Shorting through the insulator, that has been my biggest issue but usually through a driver. I’ve been getting mine for the T03’s from a guy on the auction site, have not had an issue, they’re very nice, Bergquist brand.
Shorting through the insulator, that has been my biggest issue but usually through a driver. I’ve been getting mine for the T03’s from a guy on the auction site, have not had an issue, they’re very nice, Bergquist brand.
Could have been a tiny spec of something or maybe a tiny flaw. Obviously tiny since it took full power to break through.
That explains the second event. But the first one shorted internally, so I'm not sure if I can blame the silpad. I looked at it again, and don't see any holes or burns, but the transistor is definitely shorted emitter to collector. I did put a new pad in when I changed it, and it did not blow in the short time I ran the unit before the second spark.
Do I go for it and put it back together, or are there more things I should check? Should I stick with the Toshibas? I checked Mouser and they are back ordered on MJ21193/94, so I am tempted to stick with the Toshibas, since I have so many of them.
I ordered a bunch of those Bergquist insulators. They look more substantial than the ones I used. If I have not already greased it up by the time they arrive, I will give them a try.
It was running great before the fireworks started, and I was very close to being done. I guess the SAE Gods wanted me to keep working on it and learning about it for a bit longer.
Do I go for it and put it back together, or are there more things I should check? Should I stick with the Toshibas? I checked Mouser and they are back ordered on MJ21193/94, so I am tempted to stick with the Toshibas, since I have so many of them.
I ordered a bunch of those Bergquist insulators. They look more substantial than the ones I used. If I have not already greased it up by the time they arrive, I will give them a try.
It was running great before the fireworks started, and I was very close to being done. I guess the SAE Gods wanted me to keep working on it and learning about it for a bit longer.
Myself, I’d wait on the pads and use the Toshiba’s. I just hate greasing up mica.
These P500’s I have/had no choice for the drivers but to use mica.
After re-reading your posts it could be that the first transistor was weak and bought it under full loading and the second one was the silpad as the transistor still checked out ok on the tester, although as you said I wouldn’t use it.
These P500’s I have/had no choice for the drivers but to use mica.
After re-reading your posts it could be that the first transistor was weak and bought it under full loading and the second one was the silpad as the transistor still checked out ok on the tester, although as you said I wouldn’t use it.
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I got the new pads Friday, and put the output module back together over the weekend. When I went to power it up through the my Variac connected Dim Bulb Tester (200W bulb), the bulb lit brightly. No fan no relay clicks. I stopped. I pulled the primary (black) wires from the line input, and the secondary (red) wires from the rectifier. Black measured 0.4 ohms across the leads, and infinite (no continuity) to chassis from either lead. Red also measured 0.4 ohms across the leads, but each lead also measures 0.4 ohm to the chassis.
I'm not sure exactly what these readings should be, but the secondary (red) to chassis does not seem correct. I'm not done checking, but did not want to spin my wheels if I've got a transformer issue.
I'm not sure exactly what these readings should be, but the secondary (red) to chassis does not seem correct. I'm not done checking, but did not want to spin my wheels if I've got a transformer issue.