Offset problem with TFM-55?

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BillD
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Re: Offset problem with TFM-55?

Post by BillD » Sat Oct 11, 2008 2:39 pm

Taking a long time to reach means you're charging up a cap with the battery in your meter at the same time. Once the cap is "full", you'll get the desired resistance. Do that often enough and you'll need a new battery.
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Re: Offset problem with TFM-55?

Post by brit01 » Sat Oct 11, 2008 3:15 pm

http://www.diyaudio.com/forums/showthread/t-56632.html

Interesting read about the transistors I'm looking at. this guy has a similar problem. I have 350mV on the left output and 0mV from the right.

I'm a little confused of how to measure the transistors here. Disregard this I found this great guide:
http://www.kpsec.freeuk.com/components/tran.htm

Maybe I should try swapping them to see if this changes the output?
TFM-55, CT-3, Klipsch F-1, Rotel RCD 930AX

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Re: Offset problem with TFM-55?

Post by RichP714 » Sat Oct 11, 2008 3:30 pm

brit01 wrote:http://www.diyaudio.com/forums/showthread/t-56632.html

Interesting read about the transistors I'm looking at. this guy has a similar problem. I have 350mV on the left output and 0mV from the right.

I'm a little confused of how to measure the transistors here. Disregard this I found this great guide:
http://www.kpsec.freeuk.com/components/tran.htm

Maybe I should try swapping them to see if this changes the output?
Don't have the schematics with me, but the thread was talking about 359mV on the output of a preamp I think; not on the output of an amp

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Re: Offset problem with TFM-55?

Post by RichP714 » Sat Oct 11, 2008 3:32 pm

BillD wrote:Taking a long time to reach means you're charging up a cap with the battery in your meter at the same time. Once the cap is "full", you'll get the desired resistance. Do that often enough and you'll need a new battery.
Yes, drifting up or down implies your meter battery is charging a capacitor. If you're only looking for shorts that's okay, but if you're looking for a specific reading it's better to lift one leg of the resistor you're reading

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Re: Offset problem with TFM-55?

Post by RichP714 » Sat Oct 11, 2008 3:33 pm

brit01 wrote:.....Maybe I should try swapping them to see if this changes the output?
Not a safe idea for the transistors; one of them is NPN and the other is PNP, so it's likely that both will blow up

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Re: Offset problem with TFM-55?

Post by brit01 » Sat Oct 11, 2008 3:45 pm

Not a safe idea for the transistors; one of them is NPN and the other is PNP, so it's likely that both will blow up
tested all 4.

Not sure which side the Base or Emitter were.
eg
flat part facing me
x,y and z

x-z one way reading
x-y no reading either way
y-z one way reading

Nothing failed here no?

I was thinking of swapping the NPN left for the NPN right to see if this changes the 350mV reading from the left output.


In comparison I found a trans, Q301 (C2229) that shows a reading across the base and emitter both ways!! I believe this is fubar yes?
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Re: Offset problem with TFM-55?

Post by RichP714 » Sat Oct 11, 2008 5:04 pm

brit01 wrote:.....
Not sure which side the Base or Emitter were.
eg
flat part facing me
x,y and z
most usually XY and Z are emittoer, collector, base
brit01 wrote:.....I was thinking of swapping the NPN left for the NPN right to see if this changes the 350mV reading from the left output.
I see, I thought you were going to try to swap polarities, not channels. That would be safe, but I think it more likely that something in the bias network of those transistors has drifted or is failed and causing the offset rather than the transistors themselves.
brit01 wrote:.....In comparison I found a trans, Q301 (C2229) that shows a reading across the base and emitter both ways!! I believe this is fubar yes?
Depends on the rest of the circuit. The orignal push-pull pair we were talking about, they are emitter followers, and you're right, the emitter base junction should behave like a diode. This Q301 I have no idea how it is positioned in circuit, so I can't tell you how it would respond (also, I'm 'sampling' gin gimlets right now; gotta get them right you know ;) )

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Re: Offset problem with TFM-55?

Post by brit01 » Sat Oct 11, 2008 5:40 pm

.I was thinking of swapping the NPN left for the NPN right to see if this changes the 350mV reading from the left output.
I see, I thought you were going to try to swap polarities, not channels. That would be safe, but I think it more likely that something in the bias network of those transistors has drifted or is failed and causing the offset rather than the transistors themselves.
I will check out all parts on the input board.

But I've also found several transistors with reading on both base and emitter. Mostly the ones that had been resoldered in when the mods were done. Looking at the schematics I believe these should be one way. The little arrow on one side inside the circle?
Maybe I can take one out and test it? This should read only one way right (out of circuit)?

Q216(2SC2240)
Q230(2SA970)
Q301(2SC2240)
Q302(2SA970)

Well talking about gimlets I might just and make myself a cool rum and coke :D :D :D

thks Rich
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Re: Offset problem with TFM-55?

Post by RichP714 » Sat Oct 11, 2008 6:48 pm

brit01 wrote:.....Looking at the schematics I believe these should be one way. The little arrow on one side inside the circle?
Maybe I can take one out and test it? This should read only one way right (out of circuit)?

Q216(2SC2240)
Q230(2SA970)
Q301(2SC2240)
Q302(2SA970)

Well talking about gimlets I might just and make myself a cool rum and coke :D :D :D

thks Rich
The arrow points to the N junction, so an arrow pointing 'out' is NPN, and one pointing 'in' is PNP. Out of circuit they should definitely only read one way, yes.

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Re: Offset problem with TFM-55?

Post by brit01 » Sun Oct 12, 2008 9:54 am

http://cgi.ebay.co.uk/ws/eBayISAPI.dll? ... :middle:uk

http://cpc.farnell.com/jsp/search/brows ... tid=358370


Finding it very hard to find 10000uF 100v caps in the UK.

The only ones I can find are 6800uF that are pcb mounted and the right size.
all the 10000uF are much bigger and screw type.

Now my question is would the 6800uF be ok? I never push my amp hard and have very easy speakers to drive. Probably never get out of first gear.
TFM-55, CT-3, Klipsch F-1, Rotel RCD 930AX

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Re: Offset problem with TFM-55?

Post by brit01 » Sun Oct 12, 2008 10:14 am

adding to the above:

I found in USA 8200uF Panasonic caps:
http://www.digikey.com/scripts/DkSearch ... =P10037-ND

Could be an option. :-k
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Re: Offset problem with TFM-55?

Post by RichP714 » Sun Oct 12, 2008 10:38 am

10000uf 100V cap part # on digikey:
ECET2AP103FA

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Re: Offset problem with TFM-55?

Post by brit01 » Sun Oct 12, 2008 10:42 am

10000uf 100V cap part # on digikey:
ECET2AP103FA
found them thks Rich.
Was having connectivity problems with this web site.

:D :D :D
TFM-55, CT-3, Klipsch F-1, Rotel RCD 930AX

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Re: Offset problem with TFM-55?

Post by brit01 » Sun Oct 12, 2008 2:12 pm

Also noticed one of the little 25V 47uF caps on the power supply board is all swollen :-k

I'm going to replace R230 as it is fried.
This is 1.5K but what watt value is it? 1/2 watt?
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Re: Offset problem with TFM-55?

Post by Jag_97470 » Sun Oct 12, 2008 4:06 pm

That was some very interesting reading! I admire the talent on this board!! but, ummmm, I think if I have a problem, I'll send it to Rolland! haha.

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