2245 Bias Transistor.

[Dearslayer]

I've just started the restoration on a working excellent condition 2245. I completed the 800 PCB last night and started the 750 Amp Module tonight and noticed the round type bias transistor. I remember reading about the 2270 that had a Service Bulletin that suggested to update and replace the transistor at this location but I can't find info if the same is required for the 2245. Given the similarities between the 2245 and 2270 would it be safe to assume I should do the same Service Bulletin update? If so is there a particular sub most have used? Having a problem loading the photos I took earlier.

GT.

 

[Steven Tate]

When I had a 2245 that would not set bias, someone (I think rBuckner) suggested I replace this with On Semi MJE243G. It solved my problem. I have seen threads since then questioning whether this is the best sub. But it’s chugging along fine in my 2245. Note, I was replacing the newer TO-220 type bias transistor. I don’t know if replacing the older round type is any different.
Steve

 

[Dearslayer]

Thanks Steven. Not sure there is a problem setting the Bias at this point. I guess I could at this point I could button it back up to see if I have a problem with the Bias. I should have checked that before even starting the recap. Even if the Bias sets without issue the question still remains if I should do the update or was the update ( if there is one for the 2245 with the round transistor ) only necessary if there is a bias problem?

GT

 

[catrafter]

I always replace the little round transistor (2SC984-B) whether it's in a 2245 or a 2270 with a 2SC495, Marantz SB M2270-1 mentions a 2SC496, the 2SC495 is easier to source and is the same except for having a higher Vcbo.
The bracket from the old transistor can be bent and trimmed to work with the new TO-126 transistor. Don't forget to insulate the bias transistor from the heat sink, I use a small mica washer. A little thermal compound wouldn't be a bad idea either.

If the bias won't set properly after, you can select a bias transistor with a higher or lower Vbe to get bias back into the range of the adjustment.
Type Vbe (Atlas DCA55)
2SD669=0.68V
MJE243G=0.70V
2SD612=0.70V
2SC495=0.72V
KSC3503=0.76V

Tom

 

[Dearslayer]

Thanks Tom for the reply. Only ones I have on hand are the KSC3503DSTU ( not sure about the DSTU suffix ) and the MJE243G. Is there one over the other you would suggest to use that might work best first time out of the gate? I'm gonna guess the MJ243G as Steven suggested seems as this one is mentioned in a number of threads or would it be best to order the 2SC495?

GT.

 

[Dearslayer]

Earlier today I decided to match the HFE on a pair of 992 transistors to swap,out the differential pair on the amp board. I'm really surprised to find the difference in measured values between the Peak Atlas DAC55 and my DMM. I measured about 13 just to see the difference or similaries in value. Why such a huge difference or am I missing a very simple explanation?

 

[petehall347]

Earlier today I decided to match the HFE on a pair of 992 transistors to swap,out the differential pair on the amp board. I'm really surprised to find the difference in measured values between the Peak Atlas DAC55 and my DMM. I measured about 13 just to see the difference or similaries in value. Why such a huge difference or am I missing a very simple explanation?

this is why you match them . simply put they rarely come matched .. also when matching let values settle especially if you touched them or sudden hot or cold draught happens . temp plays a big part in hfe . i do believe there are better ways to match them .

 

[Dearslayer]

I guess I'm more curious why the huge difference in value between devices used to measure with. For instance the HFE is on average higher by around a 100 HFE using the DAC55 as opposed to the DMM. I did let them settle using both devices and I also checked them over a second time. I found that the value had dropped on average about 10 HFE on both methods the second time round. Probably because the furnace kicked in at that time and the floor vent in the basement was directly above me having an effect on the surrounding temperature but still the values were relative ( dropped about 10 HFE ) . What is the better or best way to match them?

GT

 

[jd3rdcoast]

Thanks Steven. Not sure there is a problem setting the Bias at this point. I guess I could at this point I could button it back up to see if I have a problem with the Bias. I should have checked that before even starting the recap. Even if the Bias sets without issue the question still remains if I should do the update or was the update ( if there is one for the 2245 with the round transistor ) only necessary if there is a bias problem?

GT

Here is before and after pics replacing the bias transistor I did on a 2245 back in Aug. I cut the rolled section of brass bracket off with dikes and used to hold device per Tom’s suggestion. Note different ecb pinout may require moving a leg vs. original as I did.

 

[Dearslayer]

Thanks JD. Which transistor did you end up using as the replacement?

 

[jd3rdcoast]

Thanks JD. Which transistor did you end up using as the replacement?

I used the MJE243G from mouser. Can’t recall if was from reviewing posts on the subject or if Tom’s suggested 2SC495 was unavailable. Note it does have different ecb pinout. If the 2SC495 has same ecb pattern then I would use that and minimize chance of making mistake.

 

[Dearslayer]

It seems as though only the 3503 and 243 are the only ones available through Mouser or Digkey and I have those already so I’ll give the 243 a try. Thanks.

GT.

 

[catrafter]

I guess I'm more curious why the huge difference in value between devices used to measure with. For instance the HFE is on average higher by around a 100 HFE using the DAC55 as opposed to the DMM.

GT

Temperature can affect Hfe measurements (hotter usually increases Hfe), but the base current applied by the tester can also affect the reading.

Tom

 

[Dearslayer]

Thanks Tom... I knew temperature had an effect on the reading. I just didn't think the difference would be that much depending on the device used to measure. So I guess my best bet is to pick one device or the other and match two transistors as close as possible. the next question is.... for the differential pair how much of a difference in the two measurements is too much. Is there a margin of error that is acceptable.

G.T.

 

[catrafter]

I think a few percent difference is OK.

Tom

 

[Dearslayer]

Family matters took me away from working on this unit and I’m just getting back to it now , and I want to ensure I’m doing the Bias Mod correctly on this 2245. I’ve cut the brass cylindrical transistor retainer and drilled out the threaded hole to accommodate a longer and slightly wider screw as the original wasn’t long enough to go through the thickness of the Heat Sink and new MJ243G transistor. I’m now left with just the flat square with the hole that will hold the Diode and new transistor. My question is should I drill a hole in the cut off section ( I rolled it out flat ) and place it flat against the heatsink with the 243 sandwiched between it and the modded retainer …… or just toss the cut off piece and place a new Mica with heatsink compound then the 243 held in place with the modified retainer? Also I assume the flat side of the 243 with the metal face showing should be against the heatsink with the Mica?

G.T.

 

[Steven Tate]

The later units just screw the metal side of the transistor directly to the heat sink with a round mica washer and a dab of thermal compound between transistor and heat sink. Let me know if you need a mica washer. I have a bag of them - if I didn’t lose it in the move.

 

[Dearslayer]

Thanks Steven for the offer. I think I have some small square Mica that I can perhaps punch a hole in the center for the screw. If not I'll let you know. On night shift so will check when I get up tonight.

GT.

 

[Dearslayer]

Working long hours on night shift sure gets in the way of the fun stuff. Finally getting around to the Bias transistors and done both left and right 750 PCB's tonight. The bias seemed to set ok at 10 mV with leads on J754 and J760. Hope I have it done correctly. Photos taken before it was cleaned up. First time doing this Mod.

GT

 

[Rasputin47]

Very nice work Deerslayer!

I replaced those transistors in my 2245 and 2270 in a similar way using 243s. Works perfectly.