Magic smoke released from my Yamaha CR-820

[nakmandan]

Yesterday I was listening to some music through my recently recapped and retransistored Yamaha CR-820. It had been operating fine for several weeks and then the right channel suddenly went silent. A second later there was some loud hum coming from the right channel and I as I lept to turn it off, smoke started to pour out of the vents.

Unplugged it to put it on the bench. Luckily, I recently overhauled a Yamaha CR-840 so I was able to swap it in my garage system and fire up the tunes again.

Well, popped the hood and discovered a couple of resistors were toast. One of them was of the fusible type. The output transistors I had just replaced were still good but both pre-output transistors were fried. Fortunately, I had ordered double transistors when I recapped the unit so I still have replacements. I'm still checking the surrounding components for damage but so far the casualty list is two transistors and three resistors. I've read several threads here that recommend replacing all fusible resistors with the same value, higher wattage metal film resistors. So I'll go ahead and do that as part of this fix.

I'll post a pic or two later after I assess the full extent of the damage.

 

[Tinman]

Make sure you didn't end up with counterfeit parts. This is usually the symptom when they go.

 

[nakmandan]

That thought had crossed my mind. I ordered the parts from B&D, Mouser and Digikey, all good companies that have histories of being recommended here. No swap meet or 'Bay items were used.

Still not out of the question though.

 

[merrylander]

Replace those fusistors with 68 ohm 1/4 watt, they have a bad habit of going offf value and overloading the drivers.

 

[nakmandan]

Replace those fusistors with 68 ohm 1/4 watt, they have a bad habit of going offf value and overloading the drivers.

I'll definitely heed your expert advice! I read in other threads that you are not a fan of fusistors and replace them no questions asked.

I would have thought to go bigger than 1/4 watt as they are like 2 watt size. I guess that's because of the fuse portion?

I was also planning on replacing all of the other 1W and 2W metal film resistors because while I was checking for more collateral damage after this blowup, I found one that had gone towards the low side. So cheap insurance?

 

[Jon_Logan]

....2W? I don't think them are the fuse resistors.

 

[Nakdoc]

No, keep the fuse resistors at proper wattage! The problem is NOT that the resistors burn, but that too much current is required to drive the outputs you chose (I assume you are using substitutes.) Check and compare gain (hfe) on the original and subs...I'll bet you have lost some current gain with the new set. CR820s had no margin for low impedance, so do not use 2 speaker pairs or 4 or 6 ohm speakers. Yamaha speakers of that era had very flat impedance curves, but pair the CR820 with Advent or ADS or Infinity and you ask for more smoke.

 

[nakmandan]

I was running Infinity VRS-4's at the time at moderate levels. I believe these are a 6 ohm nominal speaker. I've never run 2 pairs of speakers at once except on dedicated 4 channel amps or surround receivers obviously.

The outputs I'm running are OnSemi's recommended substitutes, the rest of the transistors are exact replacements from supposedly reputable sources.

 

[merrylander]

Never had a problem using 1/4 watt. The 2sa814 and the 2sc1624 are no longer available but B&D will get you good subs. Yamaha also used that same pair in the CR-2020 and I have used B&D's suggested subs with good results.

 

[Nakdoc]

I'm addressing the second failure specifically. The 814/1625 pair have very low current specs. Try a 2SB649/ 2SD669 pair as subs. These have 50% higher collector current. I like to use GMA fuse clips as small heatsinks.

 

[nakmandan]

Piggybacking on what Rob said, I used B+D's recommended sustitutes for the 814/1624 pair which were: 2SA1011 and 2SC2344.

By the way, the OnSemi replacements for the original 2SB557/2SD427 output transistors are the MJ15015G/MJ15016G pair. These are the ones I used on both channels.

 

[nakmandan]

Here are some pics of the damaged components. The transistors are the B+D recommended replacements. The large resistor is actually the 68 ohm 85mA fusistor

 

[ecluser]

Looking at the schematic, there is only ~1.2V across the 68 Ohm fusistor, if the drivers are good. The current in this 68 Ohm resistor is less than 20 mA, and the power dissipated by this resistor is approximately 20mW. This fusistor can not fail if the drivers are working properly.

But, there is approximately 750mW dissipated by each driver and they are free standing without heat sink, as I can see in the picture.

I think the drivers are the culprits, if not the first components to fail (thermal runaway), then the fusistor fails and the 1.5 kOhm resistor between the driver's emitter and the output node fails also.

It would be safe to add a heat sink to the drivers, or better yet, replace the 68 Ohm resistor by a larger resistor. A 330 Ohm resistor would cool down the drivers to a much more comfortable temperature. The power dissipated by each driver would be only 150 mW.