Problems with a 500-T

Hiya Folks, I picked up a 500-T as part of a package deal a few years back, serial number 11876A. It came with AR-4x speakers and an AR-XA turntable, so I couldn't resist. The PO told me that he had bought it new, but after many years of faithful service the left channel started to fade on him, so he threw it in hoping I would be able to get it back on its feet. And I told him I would certainly try.

He was right about the symptom. The right channel is crystal clear and gets to a good listening level at about 2 on the dial. The left channel needs 4 to be heard and is louder but distorted at 5. I am new to attempting repairs on SS units; thanks to the AK community I have re-worked several tube models, including a 500C. But my father down in FLA just got himself a new place and would like me to set him up with some tune-age, and this would be just the ticket. I remember him saying when I was a kid that in his neighborhood in Queens everyone wanted a Fisher unit, it was top of the line stuff. So he'll love this.

Any insights as to where I begin? What are the first diagnostic tests I should do? Does it sound like an output transistor, or would that be too easy? I will double check, I think I have the service manual around somewhere...btw, I've cleaned all the pots and switches, it isn't that.

Seriously doubt it's an output transistor. They tend to die completely on that side when one goes out. But you can test them anyway.

Has it been recapped at all??? @ 40+ it's definately needed. They respond nicely to recapping the power supply, Equalizer, tone control and the Amplifier boards. Switches tend to be problematic at this age, if they've never been cleaned or cleaned only once, they need at least 2-3 more applications. Especially the Tape Monitor, and Hi/Low Filter Switches. The Rotary Selector and speaker switches are also susceptable to causing dropout's. Clean these scrupulously. My Fisher's generally require cleaning at least once per year for some odd reason.

The transistors on these tend to be ok unless there is a catastropic failure. More likely cleaning and or recap. Try the Tone Control board 1st, then power supply, Equalizer board, and the amp. I can't remember if there are pre-out/ main in jumpers on the 500-T. If so, connect the preouts to another amp. Turn on both. If you get all functions, then the preamp side is fine. Then double check the amp. Connect an outboard preamp (or other receiver with the jumpers) to the main in Jumpers. You should get sound and changes dependent on the outboard preamp.

Larry

If you have the manual, (and you'll need it for adjusting the Center Voltage and Idle Current if you don't PM me your email address and the serial #, I have both versions.), Perform the POWER AMPLIFIER TEST as outlined on PAGE 5. If there is something you don't understand, post the question. If everything pan's out, then do the Center Voltage and Idle Current adjustments. Doubtful they are off by too much, but it's possible.

The neat thing about the 500-T, which is also a problem is most of the small signal transistors are socketed(which I actually forgot!). Dirty sockets can cause dropouts. So pull each and everyone, and with a Q-tip soaked in DeOxit, clean each pin. Then cycle them in and out of the socket (think electronic sex) about 10 times, or just enough for the socket to think it's having a good time, then stop! Insert final time and move on to the next one. Do this on ALL Socketed transistors in the unit. Cheap 1st date!

Now that you've cleaned up your monitor screen of the morning coffee.........Seriously, clean all the transistor pins and sockets. But concentrate on these 3 boards 1st. Amplifier(Power Driver), Pre-Driver, Tone Control(Audio Control Amp), and the Equalizer(Pre-Amplifier). don't mess with the tuner boards, or the multiplexer board right now. Pull each of the outputs, test, CLEAN, and reinstall using NEW Heat Sink Compound and good mica insulators (inspect the old ones for broken sections or thru and thru splits. If bad, get some TO-3 Mica insulators. The Heat sink Grease is the White Silicone Grease between the transistor and the mica and the heatsink. All Surfaces get some, but just enough to make a thin film. Be careful of the TO-3 Mounts. Make sure they bottom out in the holes. This centers up the pins so they can work without shorting to the heatsink.

Larry

Thanks Larry. I will open her up tomorrow for some gentle in-and-out of the sockets and let you know how it goes. I have to look through my files for the manual, if I don't have it I'll drop you a line.

Steve

Status update

Thanks to Larry I now have the right service manual. First I gave the plug-in transistors the old in and out with some cleaning spray, then I cleaned all the switches and pots real good. The Main/Remote switch seemed to give me the most trouble, but I think it's all clean and lubed now. And still only a very weak signal to the left speaker. Here's what I think is the odd thing - through a pair of headphones, once I got that remote switch clean I get a nice clean signal on both channels. It's very nice as far as a headphone amp is concerned. Does that mean anything as far as my bigger problem is concerned, or is the headphone jack on a different line that has nothing to do with the speaker outs? Could it mean that my pre-amp board is fine but there's a problem on the amp board?

From what i can tell the predriver board P1478-2 Pins 8J and 8K feed the headphones and also the driver (amp board) P-1489-2 @ 8M and 8Q. Check the transistors and caps on the predriver board. Also check the resistors on the board.

Have you done a recap at all on this unit??? Might be a good idea on a 50 year old unit.

But lets see if we can fix it first.

Let's make a Signal tracer. You need a .01uf 250V ceramic cap, a RCA Cable, and a receiver(Other than the one being tested.). Set up the receiver with speakers. The 500-T doesn't need them. Either set the 500-T for FM or use a CD with a tone (400hz or 1000Hz are good) on the whole CD.

Wrap one lead of the cap to the center pin of the RCA cable on one end. Solder it on. Don't short the outer clips to the center pin. Plug it in to the AUX input on the spare receiver. The free end on the cap is your probe. Insulate all but the extreme tip. with the500-t ON and receiveing, Use the tip of the cap, and touch it to the components in the signal path on the distorted side, and compare it to the good side mirror part. Start from the input on the 500-T. Work toward the output. When you get good sound on one side of a component and bad sound on the other, that is the bad part.

Larry