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 Post subject: Fisher 800-T and Thermistors...
PostPosted: Oct Thu 18, 2018 5:59 pm 
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Location: Rochester, NY 14425
I'm fixing up a Fisher 800-T for a friend and had a question about the thermsitors used here.

There are three used - two on the amplifier board and one on the power supply board. All three are referenced as NTC 150 in the service manual. I am taking a leap and assuming these are neg temp, 150 ohm thermistors. The old ones cold (room temp, no handling) ready about 220 ohms. The new thermistors I mention below read about 330 ohms cold.

(1) I have a small pile of B57164K0151J000 150 Ohm, 3% thermistors. Looking to validate these are proper replacements?

(2) Is there a specific location the thermistors should be located to monitor a temp on the amp board? Hover over the emitter resistors? As I received this unit, the original thermistors were at odd angles/locations. As seen on the couple of pics on the net, one seems to be over the emitter resistors, other pics have them dangling all over.

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Pic from the net:
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 Post subject: Re: Fisher 800-T and Thermistors...
PostPosted: Oct Thu 18, 2018 6:43 pm 
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I would not replace them unless bias could not be set correctly or changes greatly under operation.


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 Post subject: Re: Fisher 800-T and Thermistors...
PostPosted: Oct Thu 18, 2018 10:28 pm 
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Thanks - I put the originals back in for now. After a little testing, the B57164K0151J000 sent me test like PTC on the bench (raise in value with temp increase) so leaving originals alone for now.


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 Post subject: Re: Fisher 800-T and Thermistors...
PostPosted: Oct Fri 19, 2018 1:26 am 
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These thermistors seldom fails and are rather critical in stabilizing the (idle) bias current against temperature changes. I would not change them (nor change their initial factory location), but the (visibly) leaking electrolytic cap at the right of the board should be replaced before it fails completely.


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 Post subject: Re: Fisher 800-T and Thermistors...
PostPosted: Oct Fri 19, 2018 2:17 am 
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Thanks - that pic is not mine and all electrolytics have been changed. I am now reusing the original thermistors and only need to validate the factory locations for them to sense heat. As it came in, they were floating in different directions/locations. One pic on the web pic shows them neatly over the emitter resistors, another 5 pics have them dangling all over.


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 Post subject: Re: Fisher 800-T and Thermistors...
PostPosted: Oct Sun 21, 2018 5:50 pm 
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Update and questions...

This early SN 800-T is now all re-capped and having a problem with bias/operation on the right channel. All caps in the PS and Pre/Driver board have been recapped and I also did the zener's and any out of tolerance resistors. Power supply tests good on all voltages, etc. And as one of the output transistors was very leaky, I replaced all four with MJ21193G/MJ21143G transistors. Also replaced - the 1k bias pots for new multi-turn pots.

On the driver board, the left channel bias's up nicely and remains steady at 15mV per spec. The right side however, starts out ok (drawing current and playing music), but after about 1 minute - and at about 7mV, the idle current suddenly drops off to near 0. And of course the sound fades and distorts in that one channel.

I have tested the thermistor, along with the two multi-section diodes. I also did some testing by replacing the two diodes CR801/803 with a string of four 1N4148 diodes in series. And re-checked the new OT's on the right side - all good there, but same issue. Nothing is heating up on the right channel that would suggest a thermal/heat issue. Obviously a current draw issue? But the way it shuts off current to the right channel indicates something is monitoring/sensing an issue - either correctly, or via a faulty part in the sensing circuit. Right?

I should mention, in that first minute or so (before bias current is even close to the recommended 15mV) sound in both channels is good.

Left channel remains strong and steady. Any thoughts or advice?

This is an early 800-T and the schematic below (500-TX) is dead on with the exception of CR809 (top left diodes) is not present, and CR801/803 (center) uses a 4 diode component as opposed to the shown 6. Otherwise all is the same.
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 Post subject: Re: Fisher 800-T and Thermistors...
PostPosted: Oct Tue 23, 2018 6:14 am 
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Location: Houston, TX
If it walks like a duck and talks like a duck er.. or something like that.. then it's a duck.
If all is normal until it warms up, then it's a thermal issue.

Sounds like you've shotgunned just about everything in there, but I didn't see any mention of thoroughly checking for bad solder joints or microscopic hairline fracture(s) in PCB traces. These can open up from heating.

You might consider getting a can of freeze spray and give a shot to the usual (and even not so usual) suspects. Or, with the unit cold, you can try the reverse with hot air and see if you can force it to act up.
You might try tapping on various components (and areas of PCB) and see if it behaves, however briefly. My homebrew tap tool is an audio cassette pinch roller pushed onto the end of a flimsy plastic hex alignment driver; makes a nice little low mass, low impact hammer. A pencil eraser might work.

Mechanical and thermal shock can be helpful in pinpointing intermittents.

You might try pushing on the PCB (and components) with a plastic or wood stick from both sides of the board and see whether flexing the board restores operation.


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 Post subject: Re: Fisher 800-T and Thermistors...
PostPosted: Oct Tue 23, 2018 1:49 pm 
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And that is what finally fixed it. I pulled the board again and re-flowed all the connections. Up and steady now.

Thanks


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 Post subject: Re: Fisher 800-T and Thermistors...
PostPosted: Oct Wed 24, 2018 2:22 pm 
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Another question...

Bias is solid as mentioned above. At cold startup, both speakers make the expected soft 'thump', but followed immediately by a 2 - 3 second somewhat loud/annoying 'Fvvvvt' noise primarily in the right channel (how's that for a technical description?). After coming to full temp, a power off (with a normal power down thump), then power back on, there is no startup noise.

As mentioned above, all caps have been replaced and any OOT resistors including new emitter resistors. Board has been re-flowed, etc. MJ21193G/MJ21194G replacement are installed for the output transistors.

Also, the power supply board has been completely rebuilt - all caps, zeners, fast recover diodes, etc. The only deviation here from stock is 3300uf caps replacing the original 2000uf. There is also a CL40 thermistor on AC primary. All voltages are spot on (+/- 5%).

Any thoughts on where to start looking?


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 Post subject: Re: Fisher 800-T and Thermistors...
PostPosted: Oct Wed 24, 2018 2:30 pm 
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One more add; all on-board transistors were pulled and tested on a Cricket for leakage.


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 Post subject: Re: Fisher 800-T and Thermistors...
PostPosted: Oct Sat 27, 2018 6:15 am 
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mbear2k wrote:
At cold startup, both speakers make the expected soft 'thump', but followed immediately by a 2 - 3 second somewhat loud/annoying 'Fvvvvt' noise primarily in the right channel (how's that for a technical description?). After coming to full temp, a power off (with a normal power down thump), then power back on, there is no startup noise.


How long did you wait after power down before power up again? It probably wasn't long. I suggest unplugging the unit and waiting much longer (minutes, even hours) and see if that noise recurs on subsequent power up. You can probably surmise where I'm going with this...


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 Post subject: Re: Fisher 800-T and Thermistors...
PostPosted: Oct Sat 27, 2018 4:19 pm 
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for deletion...


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 Post subject: Re: Fisher 800-T and Thermistors...
PostPosted: Oct Sat 27, 2018 5:47 pm 
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Yes - I tested specifically to see if it would still make the noise while still at/near operating temp and it does not. Let it cool down again and the noise is back. So I'm assuming a thermal issue remains - but wanted to see if the noise might be related to something else someone has run across.

IE, the manual says to let the unit warm up for 30 min and set bias. Cold, this starts at 1mv and very slowly climbs to the target 15mv and only after a good 20 min. Other amps I've worked on reach bias within 3-4 minutes (even though it's still recommended to wait generally ~30 min. So - wondering if this is normal, related to the noise if not?


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 Post subject: Re: Fisher 800-T and Thermistors...
PostPosted: Oct Sun 28, 2018 10:34 am 
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Greetings to mbear2k and the Forum:

I'm curious... was the CL40 thermistor there from the factory or was that something you added? If the latter, try bridging it and see if the start-up is normal. Some amplifiers do strange things when the supply voltage is far off the design value.

Regards,

_________________
Jim T.
KB6GM


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 Post subject: Re: Fisher 800-T and Thermistors...
PostPosted: Oct Sun 28, 2018 1:56 pm 
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Good thought - I added the thermistor, but will try jumping out.


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 Post subject: Re: Fisher 800-T and Thermistors...
PostPosted: Oct Wed 31, 2018 1:21 am 
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I jumped the CL-40 and no change. I'll keep digging!


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