Offset problem with TFM-55?

I just had a chat with an acquaintance who deals with vintage hi-fi about my Carver set up.

I mentioned that when I switch on the power amp there is a little bump through the speakers and also importantly during several recordings the cones move in and out very slowly about 0.5-1.0 cm.

He said that it has an offset problem and something about the bias needs adjusting. I've heard about this before but do not know any more than that.

Can anyone confirm if my TFM-55 needs servicing?

Thks

If you have a multimeter you can do the alignment yourself.
There are three adjustment trim pots, roughly in the circled areas:

Capture.JPG (156.46 KiB) Viewed 1561 times

The center adjust is for rail voltage; with nothing hooked up, and the unit turned on, adjust that pot for 98 volts DC from chassis (negative probe) to diode D503's cathode (the side with the stripe).

Then, from a cold start, let it warm up for about 4 minutes.

Left channel idling bias:
Put the probes across TP201 positive and TP203 negative and adjust VR201 for 2.5 millivolts

Right channel idling bias:
TP202 and TP204 VR202

That's it.

.....He said that it has an offset problem and something about the bias needs adjusting. I've heard about this before but do not know any more than that.

Can anyone confirm if my TFM-55 needs servicing?

Thks

If you look at the output of the speaker terminals with a voltmeter are you getting any reading (either in AC or DC.

A slow pumping in and out of the cones implies a very slow AC signal, which isn't good. A DC offset should get 'caught' by the offset protection circuit (you wouldn't see the cones moving anyway).

Either way, it's most likely a situation that's not going to go away by doing the alignment, but it's free to start there.

The moving in and out very slowly is almost certainly from sub-sonic frequencies. Power draws that do nothing for you. A 20 hz filter will remove that.

Great stuff guys.

Have to get myself a multimeter. What kind/range should I look for?

The moving in and out very slowly is almost certainly from sub-sonic frequencies. Power draws that do nothing for you. A 20 hz filter will remove that.

I heard this before. Mentioned this to the guy in the store and he didn't agree. There was a nice Kenwood pre-amp with a sub-sonic filter and asked what the cut-off point was. 20Hz he said. That's when I brought up the discussion.

implies a very slow AC signal

Can you tell me a bit more about this? What causes the slow AC signal and how can it be improved?

My girl will be in Miami in 2 weeks. Can I order one of the filters on line? Any suggestions?

Tomorrow I will have a further look at the schematics you posted and look under the tfm hood.

Cheers.

Brit, this kind of filter is usually built into the preamp. I don't have a manual for your CT-3 but check to see if it has a Low or Subsonic filter either on the front or back of the unit.

......Have to get myself a multimeter. What kind/range should I look for?

Anything that'll have a 100V range and a millivolt range; maybe even autoranging so you don't have to change ranges; should be pretty cheap.

......Can you tell me a bit more about this? What causes the slow AC signal and how can it be improved?

Unfortunately, if it were a 'normal' rumble from a turntable for example, 20 or even 10 cycles per second is pretty fast to the eye, and a subsonic or rumble filter would clear it; this sounds like a different fish. A slow pumping suggests a problem with the amp (assuming it does this with NOTHING plugged into it as a source)

A slow pumping suggests a problem with the amp (assuming it does this with NOTHING plugged into it as a source)

It just occurs when I'm playing music.
It's quiet and still when nothing is played.

Brit, this kind of filter is usually built into the preamp

No filter on the CT-3.

Perhaps one of these:
http://www.hlabs.com/products/crossover/

Cheers

This one would work:

HP - SUB ( Sub-Sonic ) # 900111
12 db / Octave NEW!!!
20, 25, 30, 35 Hz
Straight through jacks ( Bypass )

http://store.hlabs.com/pk4/store.pl?view_product=13

Hi Gary,

Not sure which one you were referring to sorry.

This one I have listed above: I could place this between the pre and power amp? I imagine that would be a good idea so no sub sonic frequencies hit the power amp.

Cheers

This one would work:

HP - SUB ( Sub-Sonic ) # 900111
12 db / Octave NEW!!!
20, 25, 30, 35 Hz
Straight through jacks ( Bypass )

That's the model number from the page you linked.

That's the model number from the page you linked.

Getting old. Can't see the model number on my screen. Symptoms of coming in at 5 am every day as we both know.

I think I should go for the 30Hz one as I have smallish speakers anyway.

Or maybe even this one that is adjustable and has an extra output!
http://store.hlabs.com/pk4/store.pl?view_product=12

I have Klipsch F-1 speakers. Which crossover should I go for? 20/30 or 50 Hz or even higher? Really not sure.

What d'ya think?

Just bought the sub-sonic 30Hz high pass filter RCA level from partsexpress. 25 bucks.

Lets see if this makes any difference. Got to wait until my other half gets back from Miami.

Thks Gary and co

There are three adjustment trim pots, roughly in the circled areas:

djust that pot for 98 volts DC from chassis (negative probe) to diode D503's cathode (the side with the stripe).

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).

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

centre idle.jpg (21.06 KiB) Viewed 1429 times

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?

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

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

If you look at the output of the speaker terminals with a voltmeter are you getting any reading (either in AC or DC.

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)