Offset problem with TFM-55?

......Question: Can you clarify where I place the negative probe of the multimeter? Also just to note that there are 2 diodes D503 soldered in parallel (on top of each other).

Those two (D503 and D503a) are the right ones. The negative probe will be on chassis (any unpainted metal) and the positive lead on D503 (or D503a) cathode.

......2 problems:
I can't seem to find the centre rail adjust! See photo.

centre idle.jpg

There is no centre rail adjustment, only a top rail adjustment

......Also TP202 and TP204 are missing!!
I'm thinking the some mods were done on the right channel these were not replaced.
Is there a way around this?

On the M-1.0 and TFM-35 these test points are small diameter metal posts rising up from the circuit board about an inch or so. I've never seen the inside of a TFM-55 and the illustrated breakdown doesn't point them out very well. I'll look for another point on the schematic that would be equivalent

Also to add to the above I'm getting only about 1.85 mv on the left channel idle.

sorry, that should be 2.5mV +/- 0.5 so it was probably good. Unfortunately those pots are not very smooth in their adjustment reaction, so it's probably out now. Add to that the fact that it didn't change and hmmmmmm.....

.....Can I adjust this to 2.5 mV before knowing the right idle? Well I tried a quarter both ways and no effect on the mV setting

1/4 turn in either direction is substantial. if it made no difference, either you're on the wrong test point or the bias circuit is fubar. To check:
1. make note of the pot position
2. for the left channel IIRC counterclockwise is less and clockwise is more (reversed for right channel)
3. BRIEFLY, and with your ear to the transformer (uh,,,,,magnetic field coil) crank the pot to max and back.
4. if the mag coil starts humming you're on the wrong test point and it IS increasing the bias.
5. if the mag coil doesn't get louder the bias circuit has a problem

......Also I'm getting about 310 mV from the left speaker terminals! nothing from the right terminals (the right is dead silent when I power up, the left just a little bump)

is that AC or DC? either way it shouldn't be there. It's not destructively bad, but it's also not right.

There is no centre rail adjustment, only a top rail adjustment

Hi Rich,

Only 2 bias adjustments left and right. i can't find any top rail adjustment

On the M-1.0 and TFM-35 these test points are small diameter metal posts rising up from the circuit board about an inch or so

Yep found the metal posts on the left but they are missing on the right. Maybe removed when the mods were done

I will have another tweak tomorrow and listen to a difference in hum.
Could the bias controls be dirty? contact cleaner will help? Or maybe the whole circuit is just real messed up due to the mods that were done with a previous owner
I love this amp and don't want it to die on me.
What effect could it have if i keep the bias to the min on both rails to avoid high bias problems as there seems to be something wrong here?

Cheers

There is no centre rail adjustment, only a top rail adjustment

The adjustment pot is R25; it's usually in the centre of the amp, should be close to U1 or U2 chips.

Yep found the metal posts on the left but they are missing on the right. Maybe removed when the mods were done

What Mods were done? the posts may have been broken off accidentally, on the right channel they go across the top of R248 and bottom of R250 (the white rectangles) each resistor is 0.02 ohms, so 0.04 from test point to test point (same on the other side)

I will have another tweak tomorrow and listen to a difference in hum.
Could the bias controls be dirty? contact cleaner will help? Or maybe the whole circuit is just real messed up due to the mods that were done with a previous owner

The pots could be corroded or tarnished, but that wouldn't make them not change anything at all.

I love this amp and don't want it to die on me.
What effect could it have if i keep the bias to the min on both rails to avoid high bias problems as there seems to be something wrong here?

Cheers

I've been playing around with bias setting a bit lately, trying to find the sweet and juicy like these nutz. spot with minimum crossover distortion without a setting high enough to let the amp go into thermal runaway at high volume. Interestingly, that point is usually more or less at the point the service manual specifies (Guess they know what they're doing). Too low and the amp sounds grainy, too high and it runs hot, way too high and it'll destroy itself.

......Also I'm getting about 310 mV from the left speaker terminals! nothing from the right terminals (the right is dead silent when I power up, the left just a little bump)

My output terminals show (with no input hooked up) 10 mVAC and 08 mVDC

What Mods were done? the posts may have been broken off accidentally, on the right channel they go across the top of R248 and bottom of R250 (the white rectangles) each resistor is 0.02 ohms, so 0.04 from test point to test point (same on the other side)

All power transistors on right channel were changed at some point and power caps I believe with previous owner.

Can I test these points directly across the resistors you mention as the posts are missing?

I'll have another look for the top rail adjustment later.

What Mods were done? the posts may have been broken off accidentally, on the right channel they go across the top of R248 and bottom of R250 (the white rectangles) each resistor is 0.02 ohms, so 0.04 from test point to test point (same on the other side)

All power transistors on right channel were changed at some point and power caps I believe with previous owner.

Can I test these points directly across the resistors you mention as the posts are missing?

I'll have another look for the top rail adjustment later.

Yes, although you might want to use clip leads to prevent slipping and shorting anything, the resistors make a ladder, the test points are the top rung of one and the bottom rung of the other

The adjustment pot is R25; it's usually in the centre of the amp, should be close to U1 or U2 chips.

Found the R25 on the power board to the left of the tranny. Maybe because this is a 230 V version.
And I assume the idle current will be different to your 110V version.

The adjustment pot is R25; it's usually in the centre of the amp, should be close to U1 or U2 chips.


Found the R25 on the power board to the left of the tranny. Maybe because this is a 230 V version.
And I assume the idle current will be different to your 110V version.

I'm getting about 103 Volt reading for the top rail. Does this indicate a problem? I should lower it to 98 volts right?

The adjustment pot is R25; it's usually in the centre of the amp, should be close to U1 or U2 chips.

Found the R25 on the power board to the left of the tranny. Maybe because this is a 230 V version.
And I assume the idle current will be different to your 110V version.

No, idle current should be the same

May I make what I think is a constructive suggestion here? Why don't you and RichP set yourselves up on some sort of IM session so that you can interact in real time? Would make things a whole lot faster and easier for the both of you. And, if either of you have a Webcam setup, would maket things even better! Skype, MSN, Yahoo Messenger, etc ...

Although I do enjoy reading about all of these technical details, I do find that the progress is frustratingly slow. Why spend weeks goign back and forth if it is something that could be done in an hour or so?

The adjustment pot is R25; it's usually in the centre of the amp, should be close to U1 or U2 chips.


Found the R25 on the power board to the left of the tranny. Maybe because this is a 230 V version.
And I assume the idle current will be different to your 110V version.

I'm getting about 103 Volt reading for the top rail. Does this indicate a problem? I should lower it to 98 volts right?

103 isn't horribly high; should be 98 but that's close enough to not overload the power supply caps or gain stage transistors.

103 isn't horribly high; should be 98 but that's close enough to not overload the power supply caps or gain stage transistors.

It's more like 104.5 - tested it again.

clockwise or anticlockwise to reduce it?

......Skype, MSN, Yahoo Messenger, etc ...Why spend weeks goign back and forth if it is something that could be done in an hour or so?

I don't have any IM software installed, and won't; that's for my 'reckless' wife and kids computers, and part of the reason why we each have separate systems and I've had no viruses in X yrs.

Other than that, and finding a part of the day each would have available, it would work fine.

103 isn't horribly high; should be 98 but that's close enough to not overload the power supply caps or gain stage transistors.

It's more like 104.5 - tested it again.

clockwise or anticlockwise to reduce it?

IIRC anti-clockwise will reduce it; just a little bump should do it