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 when I noticed the underrated capacitors, the extra parts on this board, the out-of-date "Service" Manual that doesn't match the "As Built" configuration, and the tuning mechanism that fell apart in my hand, et al, I knew I needed to address those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