I bought this non-working amplifier, mostly out of nostalgia, because I can remember one of my friends having one a few years ago and I really like the sound and simple design. Doesn't hurt to have 5-pairs of outputs and be able to drive almost any load very loudly.
I included some pics, which of course only vaguely alluded to the damage on the website I bid on. That said, I still think I can get it up and running and luckily I have one good channel to compare to.
I have compared all the resistors and capacitors channel to channel and I have 6 resistors and at least three small signal transistors along with what appears to be the entire bank of NPN transistors. They are measuring oddly with the diode and very low ohms in resistor check (like 3-5 ohms, BE, CE BC)
Someone tried to repair it - because one MJL3281A and one MJL1302A (should be original Motorola) have been replaced with ON Semi versions. Weird thing is they didn't replace the same NPN/PNP pair, they are diagonal to each other.
1 - Is there a good substitute for the drivers, 2SC3298B/2SA1306A? I have seen where a suggested substitute is 2SC4793/2SA1943.
Is that a good sub? If not - does anyone know of a reputable seller for the 2SC3298B on eBay or other site? I see high priced ones on eBay, but unsure if they are real.
2 - Can I just replace individual blown power transistors, or would I be required to replace the entire bank of 5? If I could find a place to get original Motorola parts? I plan to pull any that measure weird and test them out of circuit, so I should know the one(s) that are bad.
Trying to get all the parts from one supplier that I might need, since it is a fairly simple design, not a whole lot of parts to replace. Will cost me a lot more if I have to pay twice for shipping.
Now for the pics - first couple are how I got it, then a couple after I cleaned the board up, then a couple of the actual amp together and finally the marketing pamphlet.
A schematic would be a huge help too - in case someone has one. The amp is supposed to be made by Coda and rated at 200/400 at 8r/4r, with a 1200VA toroidal supplying +/-75Vdc.
I included some pics, which of course only vaguely alluded to the damage on the website I bid on. That said, I still think I can get it up and running and luckily I have one good channel to compare to.
I have compared all the resistors and capacitors channel to channel and I have 6 resistors and at least three small signal transistors along with what appears to be the entire bank of NPN transistors. They are measuring oddly with the diode and very low ohms in resistor check (like 3-5 ohms, BE, CE BC)
Someone tried to repair it - because one MJL3281A and one MJL1302A (should be original Motorola) have been replaced with ON Semi versions. Weird thing is they didn't replace the same NPN/PNP pair, they are diagonal to each other.
1 - Is there a good substitute for the drivers, 2SC3298B/2SA1306A? I have seen where a suggested substitute is 2SC4793/2SA1943.
Is that a good sub? If not - does anyone know of a reputable seller for the 2SC3298B on eBay or other site? I see high priced ones on eBay, but unsure if they are real.
2 - Can I just replace individual blown power transistors, or would I be required to replace the entire bank of 5? If I could find a place to get original Motorola parts? I plan to pull any that measure weird and test them out of circuit, so I should know the one(s) that are bad.
Trying to get all the parts from one supplier that I might need, since it is a fairly simple design, not a whole lot of parts to replace. Will cost me a lot more if I have to pay twice for shipping.
Now for the pics - first couple are how I got it, then a couple after I cleaned the board up, then a couple of the actual amp together and finally the marketing pamphlet.
A schematic would be a huge help too - in case someone has one. The amp is supposed to be made by Coda and rated at 200/400 at 8r/4r, with a 1200VA toroidal supplying +/-75Vdc.
Attachments
OnSemi is the former Motorola semiconductor divison, so no problems there. I would test all of the output transistors though.
You need to check the board to see how badly burned it is - burned PCB usually becomes conductive, causing further problems.
Avoid buying transistors from eBay, especially obsolete types - they will almost certainly be fakes. The 2SA1387/2SC1943 drivers from Toshiba are good devices.
You need to check the board to see how badly burned it is - burned PCB usually becomes conductive, causing further problems.
Avoid buying transistors from eBay, especially obsolete types - they will almost certainly be fakes. The 2SA1387/2SC1943 drivers from Toshiba are good devices.
Here's a datasheet for the C3298B:
C3298B Datasheet PDF - Datasheet4U.com
Looks like 1.5A 200V 20W, Ft 100 MHz, COB 25 pF
DC SOA of about .2A at 50V, without derating.
The A parts are probably 160V and the B 200V, 160 should do for both.
C3298B Datasheet PDF - Datasheet4U.com
Looks like 1.5A 200V 20W, Ft 100 MHz, COB 25 pF
DC SOA of about .2A at 50V, without derating.
The A parts are probably 160V and the B 200V, 160 should do for both.
I'd use 2SC4883A / 2SA1859A devices that are available, cheap at Digikey and perhaps others.
They are fast 2A parts with excellent SOA, discussed here:
https://www.diyaudio.com/forums/sol...lls-power-amplifier-book-820.html#post5014343
https://www.diyaudio.com/forums/sol...lls-power-amplifier-book-820.html#post5014381
They are fast 2A parts with excellent SOA, discussed here:
https://www.diyaudio.com/forums/sol...lls-power-amplifier-book-820.html#post5014343
https://www.diyaudio.com/forums/sol...lls-power-amplifier-book-820.html#post5014381
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I'd also measure all the EBC voltages on the transistors in the good channel, no input, no load.
Measure the output stage bias current in the good channel.
Do it before that channel fails for whatever reason.
Have you determined the basic topology, double or triple EF, CFP?
Try contacting Legacy for a schematic if they are still in business.
Measure the output stage bias current in the good channel.
Do it before that channel fails for whatever reason.
Have you determined the basic topology, double or triple EF, CFP?
Try contacting Legacy for a schematic if they are still in business.
Thanks for the info @PB2 and suggestion. I will be removing a lot of parts and testing them over the next week and then make final parts order.
You listed 2SC4793/2SA1943 this is not the proper pairing. The 2SC4793/2SA1837 are the correct complimentary pair. This part is many times used in place of the older 2SC3298B/2SA1306A. They can be purchased for < $1ea at bdent.com. From what I've seen bdent has a good track record for supplying authentic parts.
In response to question #2, If one of the output transistors fails then its prudent to always replace ALL of the output transistors in the affected channel. If there are hodge podge obviously non-original output transistors mixed in with the original ones then it was improperly repaired in the past. If you want this repair to last then plan to replace them all.
You are correct I mis-typed the substitute parts - thanks for clarifying!
Also, appreciate you confirming those parts and I do plan to replace all the power transistors - adds a lot of cost to the repair but I want to repair and have it back to as “new’ish” as I can.
Also, appreciate you confirming those parts and I do plan to replace all the power transistors - adds a lot of cost to the repair but I want to repair and have it back to as “new’ish” as I can.
Ok pulled out all the transistors and have tested them. Appears I have 2 bad MJL3281A, one bad 2SA1306B four bad resistors, and also the 2N5566 is open across all leads.
I have a couple options for the 2SA1306B thanks to you guys on here.
Any suggestions for a replacement for the 2N5566? I can find on eBay but I’m afraid it will be a fake.
Other question, the 0.33r emitter resistors (one measures 68r) - any idea about the wattage. In a metal film, I can only find 1.5-2 w versions. Most of the schematics I looked at they are 3-5w?
I have a couple options for the 2SA1306B thanks to you guys on here.
Any suggestions for a replacement for the 2N5566? I can find on eBay but I’m afraid it will be a fake.
Other question, the 0.33r emitter resistors (one measures 68r) - any idea about the wattage. In a metal film, I can only find 1.5-2 w versions. Most of the schematics I looked at they are 3-5w?
Panasonic 3W metal films are decent, but I just checked and Mouser was out of them in the .33ohm 3W variety, they have .36ohm right here though.
You could also look for .33r wirewounds, which will usually stand up to a bit more abuse.
As far as the 5566, you may need to go to discrete jfets for this. I'll let some others recommend something they've used in the past. Whatever you do don't buy them from Ebay or you'll be sorry.
You could also look for .33r wirewounds, which will usually stand up to a bit more abuse.
As far as the 5566, you may need to go to discrete jfets for this. I'll let some others recommend something they've used in the past. Whatever you do don't buy them from Ebay or you'll be sorry.
2N5566 in this case which can be replaced by LSK389A.
Stay away ebay, taobao etc... most of them are fake.
Stay away ebay, taobao etc... most of them are fake.
Ok-ordered/received all my parts.
- is it a good idea to replace the small signal transistors, resistors, zeners and drivers only? Then I can power on and check the voltages (w/ DBT)?
- Or should I install everything (incl. power transistors) and power-up (w/ DBT)?
Obviously the power transistors and LSK389 are the largest outlay on this repair so far.
I don’t want to blow them if I can avoid it!
- is it a good idea to replace the small signal transistors, resistors, zeners and drivers only? Then I can power on and check the voltages (w/ DBT)?
- Or should I install everything (incl. power transistors) and power-up (w/ DBT)?
Obviously the power transistors and LSK389 are the largest outlay on this repair so far.
I don’t want to blow them if I can avoid it!
I wouldn't put new parts in and turn it on first! i would spend some time studying the one "good" channel to make sure it is really good. then, try to understand what caused the parts in the bad channel to blow and make sure that gets addressed - those parts failed for a reason; no need to give that reason some new victims!
mlloyd1
mlloyd1
Thanks for the information. I have spent two weeks studying, comparing resistance and verifying parts and placement channel to channel.
I can’t do any voltages checks one to the other until I power up the repaired channel and can do some comparisons. Or am I missing something?
That’s why I was asking about powering up without the MJL large transistors, in case I missed something (which I won’t know until I give it some power - of course a limited amount).
You still have some of the original MJL’s right? Just install ONE npn/pnp set in each channel and get it running with the DBT first. When you’re satisfied that you have fixed everything, and it properly drives a light load (say 20 ohms) off one transistor pair, install all the new ones.
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