mustang guy Posted March 22, 2016 Share Posted March 22, 2016 (edited) I think I got this, but since I don't actually have to get into this until tomorrow, I thought I might throw it out there for any of you who have more experience. I replaced all the amplifier transistors, the main power caps, and all the electrolytics on the main amp circuit board. Everything sounded fine until I turned the volume, and in particular the heavy bass then the R659 smoked and died. I put an older 390Ohm resistor in and turned the Variac slowly to see what would happen at low volume, and even with low input voltage and low volume, the replacement resistor got hot enough to smoke. I think the C leg of the shown transistor may have arced or something through the thermal pad I used as a replacement for the old mica and grease insulator. Perhaps I over tightened that transistor and cut through the pad. I have two extra thermal pads, but if that one went, then maybe I should go back to mica plus thermal paste. What are your thoughts? Should I start by checking continuity between the transistor casing and the heatsink? Here is the service manual for the Technics SA-5760 I am working on: https://drive.google.com/open?id=0B_-5nvuxy-j7MXhTUl92OWRxS2s Edited March 22, 2016 by mustang guy Quote Link to comment Share on other sites More sharing options...
babadono Posted March 22, 2016 Share Posted March 22, 2016 MG, Definitely make sure none of the electrodes(emitter,base and collector) are contacting the heatsink. Have you also replaced TR609 and 611? Quote Link to comment Share on other sites More sharing options...
mustang guy Posted March 23, 2016 Author Share Posted March 23, 2016 MG, Definitely make sure none of the electrodes(emitter,base and collector) are contacting the heatsink. Have you also replaced TR609 and 611? I removed those two transistors, and they both test bad. Great catch. I am testing the rest of the transistors now. Is there anything else I should look for? I tested the main amp for short to heatsink, and that's OK. thankfully... Quote Link to comment Share on other sites More sharing options...
mustang guy Posted March 23, 2016 Author Share Posted March 23, 2016 (edited) After testing those two transistors you told me about, I went through and tested every component on the amplifier board and was surprised that all 4 of the big 5 watt resistors (R669-672) were bad with readings of 1.3, 1.2, 1.5 Ohms, and finally 47.8KOhms. Also bad were TR615 and D605. I also discovered R629 and 630 show 137 Ohm in circuit where they are 270 Ohm resistors, but I think they are good. There are thermistors running parallel to each of them. I have no specs on the thermistor room temp rating. I think they must have the same resistance as the resistors they parallel, hence the almost exact halving of the reading (270 to 137) on each of those circuits. Edited March 23, 2016 by mustang guy Quote Link to comment Share on other sites More sharing options...
babadono Posted March 23, 2016 Share Posted March 23, 2016 So basically this thing was toasted to a crackly crunch. Well good luck going forward. Sounds like you're covering all the bases. When you get all the parts you have found bad replaced check the bias voltages before you just apply signal and turn it up. If you're really curious about R629 and 30 you can unsolder one end and measure out of circuit. Quote Link to comment Share on other sites More sharing options...
mustang guy Posted March 24, 2016 Author Share Posted March 24, 2016 So basically this thing was toasted to a crackly crunch. Well good luck going forward. Sounds like you're covering all the bases. When you get all the parts you have found bad replaced check the bias voltages before you just apply signal and turn it up. If you're really curious about R629 and 30 you can unsolder one end and measure out of circuit. Right you are. I only hope that whatever the cause was will go away by replacing all these bad parts. That's a good call unsoldering one end. That will allow me to test the thermistor too. Still, I have no idea what the ambient resistance is supposed to be. I couldn't find that info in the SM. I may call on you to help with bias. I assume that is all done using the VR's on the amplifier board too. There are a couple MOhm and a couple KOhm if I remember right. If you have a decent source for bias balancing, that would be a nice thing to study while I wait on parts from Mouser. Quote Link to comment Share on other sites More sharing options...
babadono Posted March 24, 2016 Share Posted March 24, 2016 MG, To clarify on measuring the bias, I am talking about the voltages that are called out in the little square boxes above the schematic. Quote Link to comment Share on other sites More sharing options...
Marvel Posted March 24, 2016 Share Posted March 24, 2016 If you're really curious about R629 and 30 you can unsolder one end and measure out of circuit. It is sometimes rally hard to get a good reading on parts when they are still in the circuit. Bruce Quote Link to comment Share on other sites More sharing options...
mustang guy Posted March 25, 2016 Author Share Posted March 25, 2016 New problem. Replacement transistors. I have used cross references and mouser searches to try to come up with the best replacements, and here is what I have found: TR615 2SD382A-L2 Drive amplifier in ranks L2 or M2 - KSC2073 TR609 2SC1509-R Pre drive amp current protection, voltage control and Relay driver ranks Q or R - KSC2316 TR611 2SC1573-Q Current protection ranks R or Q - Can't find anything in TO-92 with collector in center Will the ones I picked out work? Any ideas on the 573? bb, I will read up on how to measure and adjust the biases according to the BCE voltages. It shouldn't be too hard as long as the receiver isn't smoking. Quote Link to comment Share on other sites More sharing options...
John Warren Posted March 25, 2016 Share Posted March 25, 2016 (edited) If you're not picky, NTE has subs for all of the devices you list-check packages MCM has the Panasonic 573 at $5 each(!!!). If you want to get into this a bit deeper, here's a niffy little tool (will set you back about $400): Edited March 25, 2016 by John Warren Quote Link to comment Share on other sites More sharing options...
mustang guy Posted March 25, 2016 Author Share Posted March 25, 2016 If you're not picky, NTE has subs for all of the devices you list-check packages MCM has the Panasonic 573 at $5 each(!!!). If you want to get into this a bit deeper, here's a niffy little tool (will set you back about $400): I'm still not sure about this, and help?: TR615 2SD382A-L2 Drive amplifier in ranks L2 or M2 - KSC2073 But I found the rest I needed outside digikey and mouser: Quote Link to comment Share on other sites More sharing options...
babadono Posted March 25, 2016 Share Posted March 25, 2016 (edited) NTE says this will work for 2SC1573: http://www.nteinc.com/specs/300to399/pdf/nte399.pdf and this for 2SD382: http://www.nteinc.com/specs/200to299/pdf/nte291.pdf Thanks for the suggestion to check NTE John MG, Did you go ahead and purchase from Ebay? Edited March 25, 2016 by babadono Quote Link to comment Share on other sites More sharing options...
mustang guy Posted March 25, 2016 Author Share Posted March 25, 2016 Yup, went ahead and purchased them. MCM didn't actually have any, and the shipping of the reseller was like 10 days till they ship. I was concerned about the aftermarket NTE's. This guy had NOS in stock and the prices weren't all that much more. I am still skeptical about the TR615 2SD382A-L2 Drive amplifier in ranks L2 or M2 - KSC2073. I went ahead and ordered them anyway as well as the complimentary KSA940 and the diode I thought I needed. Here's that order: I also ordered QTY 5 of every resistor I needed. Since they are 10 cents each and shipping is $7.99 minimum, it's better to have them. Quote Link to comment Share on other sites More sharing options...
Marvel Posted March 25, 2016 Share Posted March 25, 2016 There are a lot of fakes out there, if you find someone reliable that would be really good. Quote Link to comment Share on other sites More sharing options...
mustang guy Posted March 25, 2016 Author Share Posted March 25, 2016 There are a lot of fakes out there, if you find someone reliable that would be really good. Consolidated Electronics has pretty good feedback, and they seem to specialize in obsolete stuff. I kinda figure they buy out old electric shops and add them to the inventory. That would be a tedious but good business. Here is their store: http://stores.ebay.com/ceitron/ Mostly I buy from Mouser though. Quote Link to comment Share on other sites More sharing options...
John Warren Posted March 26, 2016 Share Posted March 26, 2016 I am still skeptical about the TR615 2SD382A-L2 Drive amplifier in ranks L2 or M2 - KSC2073. Higher GBWP MJE15030G, MJE15031G (NPN/PNP) Quote Link to comment Share on other sites More sharing options...
djk Posted March 26, 2016 Share Posted March 26, 2016 The MJE parts make great driver transistors, especially for the Adcom GFA555. Quote Link to comment Share on other sites More sharing options...
mustang guy Posted March 27, 2016 Author Share Posted March 27, 2016 I am still skeptical about the TR615 2SD382A-L2 Drive amplifier in ranks L2 or M2 - KSC2073. Higher GBWP MJE15030G, MJE15031G (NPN/PNP) Crap I already ordered. Should I go ahead and buy the MJE's and use the ones I get as fishing lures? Quote Link to comment Share on other sites More sharing options...
mustang guy Posted March 28, 2016 Author Share Posted March 28, 2016 (edited) PS: I was going on what I could understand about the Ft, and that the minimum GWBP was 3Mhz. Will 30MHz make a difference over the 4MHz of the one I ordered? Edited March 28, 2016 by mustang guy Quote Link to comment Share on other sites More sharing options...
mustang guy Posted March 28, 2016 Author Share Posted March 28, 2016 (edited) The Muses op-amps found in the highest end sound cards are 3.3MHz GB and Ft 3MHz. http://www.njr.com/semicon/PDF/MUSES01_E.pdf Please get back to me so I know which way to turn on this... Edited March 28, 2016 by mustang guy Quote Link to comment Share on other sites More sharing options...
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