Marantz 1200

[tonebells]

Now I get it! Good luck.

Tony

 

[ocdecio]

I just find it odd that this modification is done only on the Q510 side:



No such on the other side:

 

[ocdecio]

Now I get it! Good luck.

Tony

Thanks! Now on to the R553 modification.

 

[catrafter]

I don't like to replace TO-66 devices with TO-220 devices, still, ya gotta do what you gotta do.

Tom

 

[ocdecio]

I don't like to replace TO-66 devices with TO-220 devices, still, ya gotta do what you gotta do.

Tom

I hear you and I thought a lot about it - it would be really nice to keep them stock. However these are high quality Sanyo high audio power transistors (2SC2344 / 2SA1011), with very close hFE that are working well on my 240. I'd rather use those than some dubious TO-66 devices out of fleabay. Tough choices, I know.

BTW I see people using MJE's for this application, I tried and could not make them work. They don't have a fT high enough and the hFE between the pair is far apart at least in the batches I have. Aren't they meant for power supplies?

 

[catrafter]

I hear you and I thought a lot about it - it would be really nice to keep them stock. However these are high quality Sanyo high audio power transistors (2SC2344 / 2SA1011), with very close hFE that are working well on my 240. I'd rather use those than some dubious TO-66 devices out of fleabay. Tough choices, I know.

BTW I see people using MJE's for this application, I tried and could not make them work. They don't have a fT high enough and the hFE between the pair is far apart at least in the batches I have. Aren't they meant for power supplies?

I have had bad experiences, too.Fleabay is a crap shoot, it is getting very difficult to get 'real' TO-66 parts any more.

I echo your MJE experiences, I like them in power supply applications and many folks have had good luck using them as driver or output devices.
My testing has indicated that they (the MJEs) don't match the original parts as far as collector current vs. base current very well.

Tom

 

[ocdecio]

Besides the ones I cited above I sourced the following parts to try as drivers:

SC5171/SA1930 ;Toshiba
SC2238/SC4793
2SA1837/2SC4793 Toshiba

These seem all to be good candidates for their hFE proximity (NPN-PNP), current characteristics, and fT. Hard to find though.

-Otavio

 

[tonebells]

I have had bad experiences, too.Fleabay is a crap shoot, it is getting very difficult to get 'real' TO-66 parts any more.

I echo your MJE experiences, I like them in power supply applications and many folks have had good luck using them as driver or output devices.
My testing has indicated that they (the MJEs) don't match the original parts as far as collector current vs. base current very well.

Tom

Hi Tom

Do you have a sub you have tried and liked in this spot?

Tony

 

[catrafter]

I am not the authority on semi conductor substitution, you are probably better off listening to someone with more hands on experience than I, I'm not comfortable 'experimenting' with other people's gear.
Always get semiconductors from a reputable supplier that you trust.

For TO-39 or TO-5 predrivers, 2N3440 and 2N5415 (or 2N5416) ST Micro brand is my choice, IF you can find them. Central Semi work OK.
For TO-66 drivers many use 2SC783 (NPN) and 2SA483 (PNP), or 2N3584 (NPN) and 2N6211 (PNP). I'm on the fence with these. I have been told that Motorola 2N3584/2N6211 parts are the 'same' as the SJ parts as long as you stick with vintage Motorola brand. Who knows?
For TO3 outputs I use MJ21193G and 94G or 95G and 96G from ON Semi.
All 'complementary pairs' matched for Hfe at a specific base current.

I haven't had to sub TO-66 parts for TO-220 yet, but I am leaning towards 2SA1011 and 2SC2344 (KSA, KSC), YMMV

Again, I am not the authority on semi conductor substitution, you are probably better off listening to someone with more hands on experience than I, I'm not comfortable 'experimenting' with other people's gear.

Tom

 

[ocdecio]

So I rebuilt the entire left channel amplifier, most all transistors were blown and I had resistors that exploded and the PCB is a little warped. I replaced the output sockets, and tested everything with an octopus before turning it on to make sure there weren't any shorts.

Under a DBT the amp comes up with very low DC, and DC is adjustable without any issues.

Now to my very stupid question - when I check bias under the DBT I get a flat 0.00 and I didn't want to go all the way CW to find out if I could make it go up - should I get at least SOME bias when connected to the DBT?

Since the DBT is dimming, should I just go ahead and connect it directly to see if I can get a bias? I checked the epoxy transistor and it seems OK with an hFE of 470 or so. The germanium diode is brand new, laying across Q507.

BTW I do get a waveform at the output (yellow wire) when I connect it to a signal generator.

 

[ocdecio]

I guess it is safe to take it off the DBT? CH 1 = output, CH2 = input. I'd say more than 10x amplification?

Still, bias = 0.00V - I imagine I will be able to adjust that only out of the DBT?

 

[ocdecio]

Continuing on to other parts of the amp (I still need to sort out the bias issue but hey I get good sound so until it explodes...)

Some poor soul did this on the Tone Board (two of these mind you) :



I put a square wave in, bass/treble sliders up and down - nothing.

So here's what I found:



Original honest to goodness 2N4250 (these ones from Micro-Electronics).

Result: tone board working again!

 

[catrafter]

Tom

 

[ocdecio]

Brand new exact replacements for the rectifiers