Offset problem with TFM-55?

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brit01
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Re: Offset problem with TFM-55?

Post by brit01 » Sat Oct 04, 2008 4:50 pm

IIRC anti-clockwise will reduce it; just a little bump should do it
Clockwise did the trick (maybe because I'm in the Southern Hemisphere :lol: :lol: ).
got it down to 98.1 Volts :D :D :D

Also the humming from the tranny is a little less I'm sure.

Great. Thanks a lot Rich.

Regarding the right bias, I don't think I can get into R248. An old looking green resistor pointing vertically. It's base on one side does not show any metal contact to touch with the probe. Left side yes, end that points vertically.
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Re: Offset problem with TFM-55?

Post by RichP714 » Sat Oct 04, 2008 5:06 pm

brit01 wrote:
IIRC anti-clockwise will reduce it; just a little bump should do it
Clockwise did the trick (maybe because I'm in the Southern Hemisphere :lol: :lol: ).
got it down to 98.1 Volts :D :D :D

Also the humming from the tranny is a little less I'm sure.

Great. Thanks a lot Rich.

Regarding the right bias, I don't think I can get into R248. An old looking green resistor pointing vertically. It's base on one side does not show any metal contact to touch with the probe. Left side yes, end that points vertically.
OK, the PS voltage is fine now. Those two emitter resistors should be quite large, white boxes though, not green vertical cylinders.

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Re: Offset problem with TFM-55?

Post by BillD » Sat Oct 04, 2008 7:07 pm

BTW, if it won't go down — well, I've been there.
It should sound like it isn't there!
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Re: Offset problem with TFM-55?

Post by brit01 » Sat Oct 04, 2008 7:42 pm

Just concerned about the left and right idle adjustments.

Left idle does not change at all when I turn the adjustment. I haven't found the right channel testing points yet.

If I can't change the bias (currently 1.85 mV), what would be a good course of action? Leave it and hope nothing fails :( or get a technician to overhaul the bias circuit?
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Re: Offset problem with TFM-55?

Post by RichP714 » Sat Oct 04, 2008 7:50 pm

brit01 wrote:Just concerned about the left and right idle adjustments.

Left idle does not change at all when I turn the adjustment. I haven't found the right channel testing points yet.

If I can't change the bias (currently 1.85 mV), what would be a good course of action? Leave it and hope nothing fails :( or get a technician to overhaul the bias circuit?
Currently the bias is low (providing you're on the right points and cranking it doesn't make the transformer buzz louder) so there's a low chance of blowing anything up.

Also, being low, her voice is not as clear as it should be, and being unable to adjust the bias is a sign that there's trouble somewhere in the circuit, and it should be fixed.

That mV reading you're getting at the output is upsetting too; not knowing whether it's AC or DC it could be a DC offset or a low level oscillation; either is bad

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Re: Offset problem with TFM-55?

Post by brit01 » Sat Oct 04, 2008 7:57 pm

That mV reading you're getting at the output is upsetting too; not knowing whether it's AC or DC it could be a DC offset or a low level oscillation; either is bad
Not good news for me. I was recommended a tech by my vintage hi-fi dealer. Kind of pricey but I heard he is good. he has fixed bias problems for other clients.

I'll check the AC/DC reading tomorrow. Got guests coming round now. \:D/ \:D/

thanks Rich for all the tips here
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Re: Offset problem with TFM-55?

Post by RichP714 » Sat Oct 04, 2008 7:58 pm

brit01 wrote:...... or get a technician to overhaul the bias circuit?
If you bring it to someone that doesn't have experience repairing Carver magnetic field power supplies, make sure they leave the PS alone, especially the triac and triac control circuit; they can permanently destroy the amp by messing about with that, and your problem has nothing to do with that part of the circuit.

After the power supply, the TFM-55 is a rail switching amplifier, and some techs have trouble making sense of the schematics. The design is basically a 50 watt amplifier with two more 'gears'. Again, this shouldn't be the issue with your bias problem, so if they ignore the commutators and other elements of the rail switching and focus on the bias network they should be fine.

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Re: Offset problem with TFM-55?

Post by brit01 » Sat Oct 04, 2008 8:12 pm

yes I've heard this before. I will make sure he is aware.
He has serviced some before. I will check how many.

I know he serviced an old Linear Phase 400 model a month ago and know he has been sent 'the cube' power amp to fix.
I will speak to my hi-fi dealer to get more info about him.
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Re: Offset problem with TFM-55?

Post by brit01 » Wed Oct 08, 2008 2:49 am

That mV reading you're getting at the output is upsetting too; not knowing whether it's AC or DC it could be a DC offset or a low level oscillation; either is bad
Should I be taking the speaker terminal output reading with or without speakers connected? Maybe a stupid question I know.
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Re: Offset problem with TFM-55?

Post by RichP714 » Wed Oct 08, 2008 10:43 am

brit01 wrote:
That mV reading you're getting at the output is upsetting too; not knowing whether it's AC or DC it could be a DC offset or a low level oscillation; either is bad
Should I be taking the speaker terminal output reading with or without speakers connected? Maybe a stupid question I know.
Without

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Re: Offset problem with TFM-55?

Post by brit01 » Wed Oct 08, 2008 5:50 pm

Left output: 0.35 mV
Right output: 0 mV


There is definitely something up with the left channel. I was advised to first check out the input differential transistors. There should be a pair and this is normally the where the first problems are spotted when bias is out or can't be adjusted.

Richard: Do you know what this is about and where I can see these on the schematics and in the amp ?

Also: The only white rectangular resistors on the right are: R254/R258 and R252/R256
Would it be one of these pairs to measure the right bias?
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Re: Offset problem with TFM-55?

Post by RichP714 » Wed Oct 08, 2008 8:33 pm

brit01 wrote:......Left output: 0.35 mV
Right output: 0 mV


There is definitely something up with the left channel. I was advised to first check out the input differential transistors. There should be a pair and this is normally the where the first problems are spotted when bias is out or can't be adjusted.

Richard: Do you know what this is about and where I can see these on the schematics and in the amp ?
Here's a snippet of the schematic after the input buffer. THe gain stage of the push-pull CAN be out-balanced by a failed component there, but I'd expect the offset to be MUCH greater then; VOLTS; enough to trip the offset protection. Wouldn't hurt to check them though.

On the left channel (top) the positive swing is Q103, the negative Q109
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brit01 wrote:......Also: The only white rectangular resistors on the right are: R254/R258 and R252/R256
Would it be one of these pairs to measure the right bias?
Those resistor number pairs are actually just above and below the test point, they should be 2 watt devices, the ones you're looking for should be the largest. In other amps I've worked on these are big, rectangular, encased in a block of ceramic. They're usaually 5 watt though, a 3 watter COULD be just a fat looking cylindrical resistor I guess.

Anyway, the resistors you're looking for are the fattest ones (R248 and R250). One leg of both of these is a short to R 236 and R238. Once you find those, the test point is across the OTHER legs.
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Re: Offset problem with TFM-55?

Post by brit01 » Thu Oct 09, 2008 3:00 am

Left output: 0.35 mV
Right output: 0 mV
Hi Rich,

Sorry I think I meant 350mV which is very very bad if I'm right
The meter was on the 20V setting so 0.35 V. !! :shock: :shock:

I will have a look at the diagrams now.
On the left channel (top) the positive swing is Q103, the negative Q109
Can't seem to find the Q109 :? I see the Q103 and Q105.

Cheers
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Re: Offset problem with TFM-55?

Post by RichP714 » Thu Oct 09, 2008 9:58 am

brit01 wrote:......Can't seem to find the Q109 :? I see the Q103 and Q105.

Cheers
My mistake, it's q105, although I SWEAR it was a NINE when I looked last night........

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Re: Offset problem with TFM-55?

Post by brit01 » Thu Oct 09, 2008 12:42 pm

brit01 wrote:
......Can't seem to find the Q109 :? I see the Q103 and Q105.

Cheers
My mistake, it's q105, although I SWEAR it was a NINE when I looked last night........
I'm inside looking at the board now and see the Q103 and Q105. How do I test these to see if they're ok or not? or would it be a simple cheap replacement?

Also Resistors R248 and R250 are most definitely not the white rectangular components. I've got the amp up on my workbench with a clear view: R248/R250 are round green resistors. old looking parts with one end up in the air.

The posts TP's can be seen from underneath.

I measured these again from underneath and above and really strangely I'm not getting any reading from either channel.

The other day when I got 1.85mV the speakers were connected I think.

I have everything disconnect now and get no reading from these posts :? :? :? :? :?
TFM-55, CT-3, Klipsch F-1, Rotel RCD 930AX

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