Fisher 450T issues

[cmpat]

Thanks for the technical tips, very informative. I'm still learning my way around a schematic. I tested the voltage at the speakers and think I might have found a problem. Right channel was around 15-17mV but Left channel was 6.6 - 7 Volts (I didn't leave it on long enough to fully warm up). I tested with selector to AUX, no input, Volume at min, tone controls centered, no speakers connected and speaker switch on. But the numbers weren't too different when I connected speakers and tested again. The left channel is the side with the non-original output transistors, so could point toward a problem on that side?

I have a variac, but no DBT, though been meaning to build one. I ordered a manual, hopefully it has some info about idle current.

 

[fred soop]

Per the 500-TX power amplifier schematic (same part number), bias should be 20 mV across 0.47 Ω emitter resistors, or 42.5 mA.

Don't leave a speaker connected to the channel with 6-7 V output. This would be a continuous 6 watts into an 8 Ω load, but being DC would be more like 14 watts across the 3-4 Ω DC resistance of the speaker.

The non original output transistors are probably not the problem. The DC voltage at the output would more likely be a problem with the input differential amplifier. If the driver bases have something other than the + and - 0.62 V, or at least the same ± voltage, then the output is doing exactly what it is being told to do.

 

[cmpat]

Ok, been a little delayed because one of my kids brought a nasty bug home from preschool. Good hunch, Fred. Measuring the bases, the right channel pair tested ok, but the left were way off. Here are the numbers from left to right on the board:
Q805: -7V
Q807: -8V
Q808: -0.64V
Q806: +0.68V

Would that mean there's a problem upstream of there? Also, I did get the SM in and for kicks tried to set the idle current. Got the right channel in spec but the left channel read -0.2mV and moving the trimmer didn't change anything. Here are some pics of the schematic and also a pic of my actual board, left channel. I'll put the whole manual online when I figure out how to make a good scan.

 

[fred soop]

Can't really read the schematic well enough to recommend anything. Will probably need to wait for a better copy to be posted.

 

[larryderouin]

Fred: If you don't have it on your computer, grab IRFANVIEW. I have no problems copying the pics above and viewing them in original size on IRFANVIEW. They are quite readable. www.irfanview.com/

Larry

 

[fred soop]

It's an .exe file and I'm on a Mac without a Windoze partition. The original post is also out of focus, possibly the camera has a macro focus option for closeup photos.

 

[cmpat]

Ok, thanks for trying to have a look guys. I took the photos on a phone, and they look clear and zoom-able there, but maybe I lost something in the process of uploading. Will look into getting the whole manual up. In the meantime maybe I can check all the other voltages listed and see if I can narrow down the problem area.

 

[fred soop]

Doing a screen capture and blowing it up to the full 24 inch screen only produces larger blurred numbers. But, if I make a few assumptions, I think you are measuring the base voltage of the driver transistors. If that is correct, the 0.64, 0.68 V readings are good. The 7-8 V readings would indicate that the bias circuit is open and the transistors are being protected only by the 2 strings of series diodes. If so, this channel probably gets quite hot after a few minutes.

 

[cmpat]

Hey Fred. That's right-- I did measure the base of the driver transistors, and the outputs do get very hot. Should have time to do some more measuring in the next day or so. When you say the bias circuit is open, should I be looking for a failed component somewhere on that board?

 

[fred soop]

Yes, possibly the resistors between the driver base terminals. Looks like there might be a thermister and also a pot. Either would be a high potential for a problem.

 

[cmpat]

Hi Fred, Larry and others. Trying to pick this back up after a little delay. I had done some quick poking around from the top side of the power amp board back in February, without finding anything amiss. I put the 450T aside until I had time to pull the board and dig in further. I also ordered an inexpensive ESR in the meantime. Today I did pull the board, and found a potential problem. C805 (Mallory 50uf / 15V) measures ESR of 22, where its counterpart on the good right channel measures 0.92. So while I've got it pulled I'll just go ahead and recap the board--there don't seem to be many of them. Are these Mallory caps with the white plastic cases and green lettering just normal electrolytics? Also the little brown/gray caps listed as C803/804 (4uF / 35V) in the schematic? Should I replace them with anything special? Also, is there anything else you recommend to upgrade or refurbish the board while I've got it out? Thanks.

Chris

 

[larryderouin]

All 6 of them are normal E-lytics. Use Nichicon UKL series in there and get 47uf /50V and 4.7(4decimal7)uf/50v. They'll be smaller than the current caps so there won't be any problems with size. Bumping the voltage to 50v won't hurt anything and it makes it easier on you trying to find caps. There should be either a 1000uf or 1500uf/50v lytic off the board going to ground. Bump this up to a UPW Series 2200uf @ 63V. See 3rd listing. And you're all set.

47uf/50v 647-UKL1H470MPDANA $0.49Ea http://www.mouser.com/ProductDetail...=sGAEpiMZZMtZ1n0r9vR22cK9fic32uhg34e7xrNI3UE=

4.7uf 50v 647-UKL1H4R7MDDANA $0.43ea http://www.mouser.com/ProductDetail...=sGAEpiMZZMtZ1n0r9vR22cK9fic32uhgOGISXf9VQs0=

2200uf 63v UPW 647-UPW1J222MHD $2.50ea http://www.mouser.com/ProductDetail...khLA/OZ3d6ps7uo%2b16TrdtVpvqAs9Uw3FXNvq7z0g==

Use economy shipping and it's around $9.50 total + tax.

 

[cmpat]

Great, thanks Larry. I'd replaced all the large caps when I first got the receiver. That cap off the board is now a Nichicon 1200uf/63v. Do you think the extra capacitance will pay off here? If so I don't mind springing for another one.

 

[larryderouin]

If you've already replaced it no worries. Leave it. All you have to do is the 4.7's now.

 

[cmpat]

Took me a while to get back to this project, but I want to report that recapping the power amp board did fix the issue. I'm now able to set the bias current on both channels. The unit has some issues with the tuner that I'll have to address in the future, but at least it's back from the dead. Thanks, Larry and Fred, for the help. I most likely would've been stuck otherwise.

 

[larryderouin]

Nice recovery!