Yamaha CR-220 output transistors overheating

[Cameron Ewart]

Hello All,

I have been attempting to restore a Yamaha CR-220 audio amplifier
and all is going well except for one problem - I get great sound
through headphones and speakers but the output transistors are getting
VERY hot, even when only the headphones are hooked up.

I had to replace most of the transistors in the output section.
The feedback pairs and the push-pull pairs on the output. The
transistors in the differential amplifier section were fine though.
The transistors I replaced them with were NTE. I compared the specs
and they were as good or better. I found the original power
transistors for the output section (2SC1986 and 2SA771).

I used the following transistors to replace the originals:

2SA844 - NTE 234
2SA659 - NTE 290A
2SC1175 - NTE 289A
2SC1918 - NTE 199

I have checked all of the diodes, resistors, and most of the
capacitors and all seem fine using my multimeter. It has a diode
check, resistance, and capacitance meter. It also measures Hfe of
bipolar transistors.

I tried to adjust the idling current using the pots that are in
place but the transistors still heat up very hot no matter where I set
them. If they are set too low it burns up the transistors, and above
a certain point it seems to make no difference.

I also have tried to match the output transistors as best I could
(Hfe and Vbe, Hfe is off by about 25 or so). Considering that in the
factory, this would probably be considered a fairly close match.

The power supplies seem to be the correct voltages but one of them
takes about 30 seconds to get up to it's rated voltage. There is no
significant ripple on either supply either.

I was hoping that someone may have experience with this type of
problem with this receiver or another type Any suggestions would be
appreciated.

Thanks,

Sincerely,

Cameron Ewart

[Joe]

The them NTE's are not the same selected for a high Hfe,
I always had problems with overheating when crossing Yamaha as well as
Panasonic's old RAMSA stuff.
I avoid them like the plague.
That bias pot may be fried and open at one end

Cameron Ewart wrote in message
<875ebf24.02012...@posting.google.com>...

[Cameron Ewart]

What are you implying by this message, that the NTE transistors have a
higher Hfe and that is causing the problem? That would make sense. I
did check the data sheets for the old transistors and the Hfe of the
NTE transistors I replaced them with are in the correct range.

I did check the path from the positive rail to the base of the
feedback pair which sets the idling current and it checked out fine.
I adjusted the pot and watched the meter move. It was dirty though.
Thanks for the suggestion.

Perhaps a higher value of pot is required since the gain may be higher
due to a higher Hfe in the tranistors... what do you think?

Cameron

"Joe" <joe...@NOSPAMverizon.net> wrote in message news:<X3q58.3813$pb....@nwrddc01.gnilink.net>...

[Joe]

I find the NTE's speck are not tight enough as originals, the Hfe could be
anything...leakage to.
Just like biasing for RCA 6L6 then using, Chinese or Czech 6L6 tube in a
audio amp
The bias usually does needs rescaling. (vintage guitar amp restoration here)
I would get see if Mouser www.mouser.com has originals, they are cheap
Always a problem with Yamaha stuff
-Joe

Cameron Ewart wrote in message

<875ebf24.0201...@posting.google.com>...

[Cameron Ewart]

Thanks for the information Joe,

I did find that the current gain of these replacement transistors
was indeed higher than the originals so I scaled back a couple of the
resistors in the circuit and all is working fine now.....

I live in Canada and I could not find the originals in Canada. If
I order from the US I get hit first off with the poor exchange rate as
of late, then duty, then shipping. They would have ended up costing
me about $2 each for each transistor after all of that! Unfortunate
but a fact of life.

But it was the biasing circuitry, but hopefully all is solved now.

Thanks,

Cameron

"Joe" <joe...@NOSPAMverizon.net> wrote in message news:<aXy58.3819$0V6....@nwrddc02.gnilink.net>...

[Stephen Sank]

I think it much more likely that you have an intermittent film capacitor or
two on the output line RC networks on either channel. This was a problem
usually confined to the CR-620, causing an inaudible 1mHz oscillation that
would either cook an amp channel or fry the power transformer's thermal
cutoff. The caps in question are the ones that are in series with a 5 or 10
ohm resistor directly from each channel's speaker output line to ground,
before the relay contacts.

--
Stephen Sank, Owner & RCA Ribbon Mic Restorer
Champlain Valley Speaker Company, aka Talking Dog Transducer Co.
http://stephensank.com
1624-B Eubank Boulevard N.E.
Albuquerque, New Mexico p.z.[ 87112 ]
505-332-0336 phone / Back-up email: steph...@yahoo.com
mc/visa accepted via Paypal.com:
https://secure.paypal.x.com/refer/pal=bk11%40thuntek.net
Authorised McIntosh/Nakamichi Servicer

"Cameron Ewart" <camero...@hotmail.com> wrote in message
news:875ebf24.02012...@posting.google.com...

[Stephen Shaw]

On 29 Jan 2002 19:22:05 -0800, Cameron Ewart wrote:

> Thanks for the information Joe,
>
> I did find that the current gain of these replacement transistors
> was indeed higher than the originals so I scaled back a couple of the
> resistors in the circuit and all is working fine now.....
>
> I live in Canada and I could not find the originals in Canada. If
> I order from the US I get hit first off with the poor exchange rate as
> of late, then duty, then shipping. They would have ended up costing
> me about $2 each for each transistor after all of that! Unfortunate
> but a fact of life.

<snip>

I thought with NAFTA there was no more customs duty between Canada, Mexico and
the U.S.?

--
Some drink at the fountain of knowledge, others just gargle