The Fiser Saga: Power Amplifier Autopsy

 Yeah, it's been a few weeks, and a busy time it was with the gorgeous weather we've had for the last week.

And then I got clobbered with the cold/flu/whatever. Anywhoo...feeling much better.

Back to the Fisher RS-2010 Stereo Receiver I've now had almost a year. Yes, I'm embarrassed, but it turned out to be a LOT more work than I estimated. I'm now down to searching for ONE transistor, with all the rest having been sourced, and on-hand. The issue is that it's one of what's called a Complementary Pair, and while a sub for one of the two is readily available, it's compliment is not. Soooo....I have to dive into either finding a vendor with some in stock (NO eBay or Amazon....too many fake parts!), -OR- start going through the "sub-for-the-sub" routine, and find a complementary pair that's available. 

Bags of new transistors awaiting their long overdue compatriot.

It gets a bit tedious....

 

So what killed several transistors, and let the smoke out of three resistors? Who did the dirty, dastardly, despicable deed?

'Twas a shorted Zener Diode, it was.... that's what who dunnit! 

The diode in question is labeled "DO6" in this clip:

 In this application it's being used to develop a voltage which is 12 Volts less than the supply voltage, rather than developing "just" 12 Volts. They do this because the entire amplifier is isolated from "DC Ground", and floats between the +60 Volt and -60 Volt power supplies. The only transistor in this circuit connected to the chassis, is Q06. When the diode failed, it put Q04 into saturation (FULLY turned on), meaning it had only .65V drop across it, which in turn took out everybody connected to it by putting them into an over-voltage condition. A classic example of a Cascading Failure.

So that's where I'm at right now, changing transistors, and getting ready for a brief test when they're all replaced. After I verify all the voltages are back to where they should be, I'll be ready to run it at 50% power for a few hours while I monitor the temperatures to make sure nothing goes bonkers, and then run it Pedal-to-the-Metal for a couple of hours to see how it well it performs.

 

Gettin'n close, finally, to a resolution on this one.

Groan.....Got Clobbered By Something

 Been flat on my back since Monday. Aches like I've never had (even my knees hurt),  coughing, sneezing, going between sweats and chills, and all sorts of other things.

Finally got out of bed today for more than 30 minutes, and while I'm still a bit wobbly, at least I'm feeling somewhat better.

Fifteen Years of Wedded Life....

 

 Was celebrated yesterday. My Sweet Little Wife, knowing me rather well, decided to get married on Valentine's Day so I'd never forget what day it was.

Pretty smart cookie, she is! 

Her Maid of Honor and the Minister who married us will be out here in May for a Girl's Trip, so we'll renew our wedding vows while they're here.

I didn't really think I'd get married again, but then she came into my life and completed it.

I love you, honey, and I'd do it again in a femtosecond! 

Work continues on the Fisher RS-2010, and I finally sourced all the transistor needed to finish it. The parts should be here this coming week.

 

Have a pleasant weekend (snowing here and cold), stay in Condition Yellow, and stock up on popcorn. I think the next few months will be 'interesting'.....

The Fisher Saga....Chasing Classic Parts

 

With apologies to Wayne Carini.......

One of the things about working on old equipment is that sometimes the parts are hard to find, especially if they were declared obsolete years, or decades, ago.

Now while most passive components like resistors and capacitors are readily available, some of the semiconductors are not. Diodes are easy to find, and even the infamous "Darlington Power Packs" now have discrete versions available, but most of the Small Signal Transistors have been extinct for quite some time.

The issue then becomes finding suitable replacements, of modern manufacture. Most "US" transistors have a "2Nxxx" number on them, like one of my old favorites, the 2N2222.

Japanese transistors use a completely different numbering system, as explained on Wikipedia:

"The Japanese technical standard JIS-C-7102 provides a method of developing part numbers for transistor devices.^[1] The part number has up to five fields, for example in the number 2SC82DA:

• The first digit "2" indicates this is a 3 lead device ( a diode would have a prefix numeral 1)
• The letters "S" is common for all EIAJ registered semiconductors
• The following letter designates polarity and general application of the device. For transistors:
  • A PNP high frequency
  • B PNP low frequency
  • C NPN high frequency
  • D NPN low frequency
  • E P-gate thyristor
  • F N-base unijunction transistor
  • J P-channel field effect transistor
  • K N-channel field effect transistor
  • M bidirectional triode thyristor
• The numerals following indicate the order in which the application was received, starting at 11
• A suffix letter indicates improved characteristics....."

OK, so that at least tells me about what they are, but to find out exactly what they are, I had to get the datasheets for each (obsolete) part number. The datasheets list all the parameters for the described part number, and from there, you can use the selection guides on the vendor websites (I like Mouser the best) to find currently available parts with matching specifications.

But what if you can't find a datasheet? Remember, these are obsolete parts, and sometimes a datasheet hasn't been scanned and posted somewhere. Well....then you're getting down to hand-to-hand combat. First you look at the schematic, and see what the applied voltages are, and make a best guesstimate as the the current and power dissipation required.

This is the schematic for the right channel of the power amp :

At this point I've found current replacements for all but three of the original transistors, but it's going to take a bit longer to decide what to use for the last three.....

Sixty-Seven Degrees....In February?

 And it might hit Seventy on Monday. And it's been pretty windy, with wind warnings of sustained winds of 50~60MPH, with gusts to 85+ up around the Colorado/Wyoming border.

The dog sure loves it as she can sit outside, pull her ears back, half close her eyes, sit into the wind, and sniff, sniff, sniff about as long as she wants to.

Picture from November:

 

Had the windows open to air out the house, and even did a bit of yard work, and admired the new fence. Yes, the installers used metal poles, like for a chain-link fence, dug the holes with a little tow-along auger, and set them in with QuickCrete, plumb-lines, and levels. The next day they hung the new panels, and cleaned up the remaining bits and pieces of the old fencing.

 

Looks nice, but I see I forgot to secure one of my radials to it.

And work continues on the Fisher receiver, with a big post to follow.

Hope you all had a blessed, peaceful Sunday, and stay in Condition Yellow.....

The Fisher Saga....Power Amplifier Autopsy

 

 Or should that be Odyssey?

You might be thinking that since this was the "Original Problem" that brought it here I would have tackled it first, and I did do some troubleshooting very early on this assembly.

BUT....(always one of those...) when I noticed the underrated capacitors, the extra parts on this board, the out-of-date "Service" Manual that doesn't match the "As Built" configuration, and the tuning mechanism that fell apart in my hand, et al, I knew I needed to address those issues before I continued on. 

So I took care of all the other issues this receiver had before going into the Power Amp. 

After carefully bringing it up on my "Variac" while monitoring the current (YAY! NO smoke!), I started checking the voltages on all the transistors. ALL of them were way off, indicating some other issues on the board. A search on the AudioKarma forum showed that this is commonly caused by some of the diodes used to bias the transistors were failing. Sure enough, two of the six diodes used were shorted, and both of the 12V Zener Diodes had failed; One shorted, and the other not regulating at 12 Volts. Unfortunately this took out a couple of small signal transistors on the board, so I have to make another trip down Substitution Lane to find some suitable, currently made transistors.

So the work continues, one step at a time......

Tonight's Weather from Fort Collins....

 And YOWZIR! It's gonna be COLD the next few days. We've had a night here and there that were below zero, but this week should break the record for the season

Started snowing around 1700, and now at 2140, we have a good 3" on the "Snow Table", and it's still kinda-sorta snowing. Looks like dry, fluffy snow, and it's going to stay that way. Should be easy going for the Ariens tomorrow.

Took advantage of the 60* ( ! ) weather the last few days and knocked out some yard work. Got the Big Back Gates secured properly for winter weather, and cut six more logs up and stacked them in the garage. After they finish drying (Reeeeal fast here with the low humidity) I'll split them up for the wood box. I'd really like to find a place where I could get some "logs" about 6~10" in diameter and 6~8' long. I can cut them to 18" lengths for the fireplace, rack 'em in the garage, and split-as-needed, which is what I've been doing with the huge limb that came down from out Ash tree in the back yard. Geez....We've gotten four years worth of firewood from that fall!

Annnnnnd.....Work continues on the Fisher, with another post in the works. That thing has some very poor design and layout issues, other than the low cost capacitors they used.

And it takes some and digging around when the Service Manual is wrong, the parts called out on the schematic don't match the parts in the Parts List, and some other things.

Take care of yourself, take care of each other, and keep that head on a swivel!

Took a Week, But I'm Feeling OK....

 Thought I was feeling better, but then it smacked me again on it's way out the door...

ANYWHOO....work continues on numerous things. I'm making up an "Electrical Experiments Board" for grandson #!, although I'm sure his younger brother will enjoy it, too.

It will use one of the LiMH batteries from the small R/C trucks we have, and an assortment of switches, lights, bells, buzzers, and motors. Kind of like an "Electrical LEGO Set" that they can use to learn how to hook things together using wire and "Fahnestock Clips" for the connections.

And of course I'm rolling along on the Fisher RS-2010 receiver. The next post about it will be on the rebuild of the Power Amplifier Board. Had some bad diodes that threw all the voltages off, and clobbered a couple of transistors. Looks like another trip down "Substitution Lane" to see what modern parts are suitable.

Oh, Lord.....What Hit Me?.....

 Picked up SLW about half-past midnight at DIA on (early) Friday morning from a trip to see one of her friends in SoCal. She'd been sick her last day there, and was wiped out the next two days "going at both ends", as it were.

Then it hit me Sunday night.

Flat on my back all day Monday, and even now that I'm up and moving, I'm still kinda 'shaky'.

We got our first measurable snowfall last night.  Looks like about 3~4", about 22* outside right now, with an expected low of 5* by early morning.

Time for a mug of hot cocoa, a blanket, and some TV watching time with a nice fire going in the fireplace.

The Fisher Saga Continues....It's ALIVE!

 Well, at least it didn't go up in smoke when I brought it up on the variac!

And now we go into troubleshooting the Power Amplifier Board, the original problem that brought the receiver here.

This board has six undocumented changes from the schematics in the Service Manual.

1 - Four 10uF capacitors, two each on the STK-0080 "Darlington Power Packs".

2 - Two small inductors on the bottom of the board.

3- An "extra" transistor on each driver transistor

4- An extra resistor in each channel.

5 - A resistor that doesn't go to the designated pad in each channel.

6 - A capacitor/resistor network mounted to the chassis, with "flying leads" going to the board. The capacitor is the out-of-focus blue cylinder handing in midair.

Things like this, along with the haphazard layout of the board (parts are scattered everywhere, part numbering is odd, etc) makes troubleshooting tedious.

This shows "Q17" with an extra resistor, and an extra transistor stuck into some of the holes for Q17. Resistor "R03" is also not going to where it looks like it should go. These are most of the major changes Sanyo made that I have to figure out.

Yes, it needed a lot more work than I anticipated, but that's why people bring me these things to fix. At least I'm down to one circuit board, which to me is the Light At The End Of The Tunnel.

Happy New Year!

 I'll be among the legions of those who will go to bed early. Just getting too old to stay up as late as I used to.

Been very windy here, and the fence on the North side of the backyard has come loose from the rotted posts it was attached to. Looks like we'll be getting together with our neighbors to split the cost of getting it replaced. The fence is the original one from the late 1970's when the houses were built. Our previous neighbor and I replaced a few of the old posts, but several of the others have since rotted away at ground level, and now the fence panels are loose. I rigged them up with some paracord and some "MILSPEC" antenna stakes, but it's time to rebuild that section of fence.

Been cold, too, with the low last night in the high teens, and very dry. We've had two light dustings of snow, and about 1/4" of rain since Halloween way below normal. Looks like the Great Push to get the snowblower serviced has kept the snow away.

Be safe, checketh thy six regularly, and hope 2025 will be what we need.