I have aquired a Carver M500 amp that is in need of output transistors 2SD745/2SB705 made by NEC. I have found a substitute which is 2SC2921/2SA1215 made by Sanken. Has anybody ever used these substitutes in this situation? Did they hold up and was there any sacrifice to performance of the amp? Any help is appreciated . Thanks, Harry
Hi Harry,
Those should work fine as long as they fit. I once used a sub (could have been those) that I had to drill new mounting holes into the heatsink. They were close, but not close enough. If you have to do this, be sure to clean and sand the surface flat. My final sand is with #600 wet/dry. It's not as bad as it sounds.
As long as it doesn't oscillate and you get the bias right, the amp will meet all its specs. If the outputs were blown, be sure to replace the drivers as well. Also make sure you didn't lose any commutators or blocking diodes. You do know this is not a normal amp, right? I also think this has a mag coil, so the triac isn't a switch, it's a voltage regulator. DO NOT SHORT IT and apply over 1/2 full supply voltage! If you do, you will run everything at far too high a voltage (not to mention the high current draw and angry hum just before the fuse blows).
-Chris
Those should work fine as long as they fit. I once used a sub (could have been those) that I had to drill new mounting holes into the heatsink. They were close, but not close enough. If you have to do this, be sure to clean and sand the surface flat. My final sand is with #600 wet/dry. It's not as bad as it sounds.
As long as it doesn't oscillate and you get the bias right, the amp will meet all its specs. If the outputs were blown, be sure to replace the drivers as well. Also make sure you didn't lose any commutators or blocking diodes. You do know this is not a normal amp, right? I also think this has a mag coil, so the triac isn't a switch, it's a voltage regulator. DO NOT SHORT IT and apply over 1/2 full supply voltage! If you do, you will run everything at far too high a voltage (not to mention the high current draw and angry hum just before the fuse blows).
-Chris
Hi Chris. I may be biting off more than I can chew with this amp and I may possess just enough knowledge to be dangerous. First off I bought this amp broken from a self professed tech he stated it had many new parts that had been replaced. The filter caps in the power supply had wires soldered to them and were flopping around inside the case when I first opened it up. After anchoring the caps I used my variac to fire it up with an amp meter in place of the fuse current draw was what I thought to be accessive so I shut it down. I disconnected the output voltages from the power supply to the amp board and fired it again this time supply appears to be OK brought her up to ac line voltage and current draw was minimal. Have started testing components on amp board and so far have 1 pair of blown outputs and blown drivers 4 transistors total tested bad. I have replaced the baising resistors for the outputs with 1/2 watt units and changed every electrolytic on the amp board I intend to replace every diode as well. If I have done anything improper up to this point I would appreciate some guidance from you. There are other issues with this amp as well it has some sort of solid state timer wire tied inside that appears to energize the relay for the speakers. Oh well one thing at a time right now I need to order the transistors that I need. Remember my statement about limited knowledge I know what an electrical commutator so I am assuming it is another way to reference the blocking diode. Thanks, Harry
Hi Chris, Yes I have a schematic and I dont see any ECG or NTE replacement devices. What I do see is an SK 9446 replacing 2SD745 in the channel that I am assuming functions properly, all transistors in that channel tested OK. What I am proposing is to buy 4 Sanken replacements and use them for the blown channel replace the SK9446 with the 2sd745 and having all NEC devices in the other channel or do you feel it would be better to replace them all with the Sankens? Thanks, Harry
Hi Harry,
The SK's have to go, and the rest in that channel. Sorry, older bad repair job.
You can use different devices between channels. You can not reuse any part that was installed in a damaged channel. Replace drivers as well. Match the outputs in each channel.
The 12 and 15 V zeners will be fine. Many have a 5 % or 10 % tolerance. The differences are then well within the possible error band for each.
-Chris
The SK's have to go, and the rest in that channel. Sorry, older bad repair job.
You can use different devices between channels. You can not reuse any part that was installed in a damaged channel. Replace drivers as well. Match the outputs in each channel.
The 12 and 15 V zeners will be fine. Many have a 5 % or 10 % tolerance. The differences are then well within the possible error band for each.
-Chris
Chris, Additonal parts are ordered will post more when I Install them. As a trouble shooter my skills are still in the developmental stage. It is my intention to replace all defective parts and perform comparison resistance readings between the two channels which should indicate any problem areas. Is this a logical approach? Thanks, Harry
Hi Harry,
What is far more useful is determining what different operating voltages and currents are. From there you look at the schematic and use logic. When checking output stages, connect your negative meter lead to the output of the affected channel (reference). From there you can measure base - emitter drops easily as well as judge current through the resistors that reference to the output.
So to determine current, measure across a handy resistor in the series path you are curious about. Emitter - base voltages can be very helpful as you see reverse biased transistors very quickly. Keep in mind that you may see the odd open E-B junction. Once I find the problem circuit I will power off and start checking parts.
Keep in mind that some amps will not have any bias if the current generator for the diff pair is off.
-Chris
I seldom use that approach as it's use is very limited.
What is far more useful is determining what different operating voltages and currents are. From there you look at the schematic and use logic. When checking output stages, connect your negative meter lead to the output of the affected channel (reference). From there you can measure base - emitter drops easily as well as judge current through the resistors that reference to the output.
So to determine current, measure across a handy resistor in the series path you are curious about. Emitter - base voltages can be very helpful as you see reverse biased transistors very quickly. Keep in mind that you may see the odd open E-B junction. Once I find the problem circuit I will power off and start checking parts.
Keep in mind that some amps will not have any bias if the current generator for the diff pair is off.
-Chris
M500 repair
Harry,
Where are you loated in PA? I'm about 5 min from Hersheypark area. If your close enough you can pop down sometime and I can help you with your carver M500 repair. I have quite a few Carvers (and M500s) so you can get an idea of what a proper one looks like inside, you can also take some measurements on a working one. As far as outputs, I'm down to my last 2 pairs of B705/D745, Carver had a service bulletin to replace the originals with the 2SA1302/2AC3281 (now obsolete so we have the 2SA1943/2SC5200 pair now). You can still buy all the PS caps that fit properly where the old ones were, no reason to "wire" other ones in.
I buy a lot of broken Carvers to repair and resell, I know what you mean about "self professed techs" that can really mess up these amps. Anatech is the only other person I've found that knows these amps inside and out, I'm sure better then I do.
tjshopp
Harry,
Where are you loated in PA? I'm about 5 min from Hersheypark area. If your close enough you can pop down sometime and I can help you with your carver M500 repair. I have quite a few Carvers (and M500s) so you can get an idea of what a proper one looks like inside, you can also take some measurements on a working one. As far as outputs, I'm down to my last 2 pairs of B705/D745, Carver had a service bulletin to replace the originals with the 2SA1302/2AC3281 (now obsolete so we have the 2SA1943/2SC5200 pair now). You can still buy all the PS caps that fit properly where the old ones were, no reason to "wire" other ones in.
I buy a lot of broken Carvers to repair and resell, I know what you mean about "self professed techs" that can really mess up these amps. Anatech is the only other person I've found that knows these amps inside and out, I'm sure better then I do.
tjshopp
Tj, I am located in Mount Pocono. Hershey is about a two hour ride which does not matter to me. I appreciate your offer to help with the amp. Possibly you could provide me with your phone # and I could call you for some guidance which might rule out the need for a trip . Email is hewlew1@netzero.com Thanks, Harry
Chris, I am not out of the woods yet .Hopefully you will still guide and advise me through this endeavor to its conclusion. Between yourself and Tj I may just pull this off. I just wanted to again thank you both for your assistance with my problem. You guys are an assest to this forum. Harry
Hi Harry,
Possibly. I would use power diodes to simulate the E-B junction of your power transistors. You may still smoke the drivers, but at least it will not take out a set of outputs.
Don't drive it past headphone level and make sure the bias control has ranger (that it works). Set the bias control close to the low end.
-Chris
Possibly. I would use power diodes to simulate the E-B junction of your power transistors. You may still smoke the drivers, but at least it will not take out a set of outputs.
Don't drive it past headphone level and make sure the bias control has ranger (that it works). Set the bias control close to the low end.
-Chris
Chris,
At 1:20. this morning I fired it up successfully I stayed very reserved with the volume control. The next problem is to set the Idle or bias current since I backed that adjustment off per your instructions. Would appreciate some guidance on this procedure as I have never done this before. Thanks, Harry
At 1:20. this morning I fired it up successfully I stayed very reserved with the volume control. The next problem is to set the Idle or bias current since I backed that adjustment off per your instructions. Would appreciate some guidance on this procedure as I have never done this before. Thanks, Harry
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