TFM-45 Bias,offset,damping adjustment procedure

Hello,
Does anyone know the procedure for adjusting bias,ect. on a TFM-45?The amp works well but has developed a slight muffle in the mids/lower highs that seems to disapear at higher volumes.At lower volume(just off zero)it is very noticeable.Only on the left channel.I've done all the trouble shooting to say that it is in fact in the amplifier.Dif. speakers,Preamps,amp ect..I'd like to make sure the amp is operating per specs before I look any further.Beings it seems to go away at higher output I suspect it may be a bad solder joint on the left PCB or related curcuit.I'll be Reflowing the solder joints regardless. Anyone had any experience with this problem?It doesn't seem to need a major repair as it has very stout output and doesn't fatigue in the least through amazing silvers at ungodly volumes.(Ribbons are perfect,it is NOT in the Amazings.problem reverses sides when outputs are reversed,and non existant with another amp)If anyone has a service manual for this that'd be awesome.I'd ask the powers that be but being a newbie to the forum,I'm not allowed yet,and I don't think I can wait that long to get this running 100%.I hope I'm not out of line asking this way either.I'd like to listen at a little lower volume before my nieghbors call the cops(again).but this little sizzle is gotta go.
Thanks!
Peace,Robert

The top rail should be ±118V (adjust RP1 on the power supply board). Bias should be 2.5mV across both 0.05Ω 5W emitter resistors R94 and R95 on each amp board (adjust RP1).

I'd say about 99% certain that you need to do the 'damping pot' adjustment: apply a 2k Hz signal (can't remember the mV level of this AC signal) to the amp and adjust these pots until you get 1 V (I believe) out at the speaker terminals with an 8 ohm dummy load. VERY sensitive adjustment, so just tweak the pots very small amounts! There is a thread here with all the details (including how to download and set up Virtual Testbench to provide the input signal). This adjustment "drifts" and there is a Service Bulletin to correct this on all 45's that were made before a certain serial number; but even after the "fix" I'd still check this once in a while. It is really "shocking" how much better and more musical and "perfect" sounding this amp is AFTER this adjustment is made. It will knock your socks off when you hear the difference! If you cannot do it yourself, bring in the manual that details how to do this to a good repair shop and also have them do the SB update for this adjustment.....you will be so glad you did (assuming yours has not had this done all ready). - Ed

add: yes, the fact that it is in one channel only is also a very strong indication that it is the damping adjustment problem. Go to the "other" Carver site (PM me and I'll give you the address if you not aware of this site) to get a service manual....

Thanks Bill and Ed for the response.I registered and tried to get the service manual from the other site but it appears that the Manuals link doesn't go to manuals for some reason.I'm kind of curious as to what point serial number-wise was the cut off point for TFM-45'S that need/don't need the fix.I know my amplifier was manufactured in 1990 according to the markings on the transformer.I'm away from home now so I do not have the serial# handy.As far as doing the fix myself I'm capable.I don't do a tremendous amount of work on S.S. gear,I do what I need to as we have no decent repair shop within 200 mi.But I have been restoring/building and rebuilding my own tube amplifiers/preamps as a hobby (I'm not a school trained tech)for many years.From what I understand the fix involves changing the trimmer pot and associated resistor to dif. values,and severing a trace on the board.sounds fairly straightforward but I'd like to find out if mine is one that is or is not in need of the fix.
The virtual bench would be great for the signal generator feature.In the past I used test tapes I had for Reel to reel alignment to set levels, and while they work for most things I've used them for I don't think they have the proper freq. for this application,and the minor fluctuations inherent to tape would probably hinder an accurate reading if they did.
Thanks to all for the help,And seems by the time I'm done asking questions I might have the required posts for the service manual download after all!
Thanks again,
Robert

I'll post the serial number cutoff when I get home....not sure why the manuals link no work: perhaps the site was being "worked on".

If I can, I'll also try and post link to the big thread here that goes into all the details for this adjustment...look for all this ~8:00 pm PDT

Ed

I registered and tried to get the service manual from the other site but it appears that the Manuals link doesn't go to manuals for some reason.

The link at the top of the main page didn't work for me either.
I sent you a PM with info that will help.

Here is the "big thread" that details this adjustment process:
http://www.carveraudio.com/phpBB3/viewtopic.php?f=8&t=5300&hilit=TFM

serial numbers of 42/45's that need the damping pot drift SB fix:

TFM-42: below 01200
TFM-45: below 02760


Ed

Thanks again everybody for the help.I appreciate it.I did finally get the manual,and all related service fixes.the only thing I'm a bit cloudy on is the test point for the 118v rail voltage.I also have to download the signal generator.
I did open it up tonite and it already has the damper adjustment drift fix installed per the factory instructions so thats good news.The serial # on my amp is 1042.below the cut off for not needing the fix- b.t.w.Do these still drift a bit even after the fix?
I'll set it up torrorow and report back the results.I hope it cures the problem,and with the same noticable results you had Ed.Although other than the low volume fuzzyness thing Its one hell of a good sounding powerful amplifier as is.
Thanks Again,
Robert

I'd look on pages 36 or 37 for someplace convenient to the adjustment trimmer on the power supply board to test.

"The serial # on my amp is 1042.below the cut off for not needing the fix- b.t.w.Do these still drift a bit even after the fix?"

Hey, Gunner: you mis-read the post. Serial numbers below 2760 need the fix. Your's may have had the SB fix done later, but it did not come that way from the factory if its serial is 1042....I'd check the max value on the pots to make sure they are the "non-drift" size (500 ohm, I believe). If the fix has been done, it much less likely to "off" enough to cause problems, but I'd definitely do the check anyway and at least one of them is likely to be off a bit in any case...if fix not done, then it an almost certainity that one of them will be way off.

No,I understood what you said about the cutoff point for the drift fix,mine being 1042 it would need the fix done,if it wasn't already.(which it is)2 500k trimmers,2 750k resistors,and a trace from trimmer pot to 3rd term. of u2 severed.It appears to NOT be done at the factory, but what is done is done to the factory description in the service updates in the back of the service manual.(which also has the serial# cut off of units needing the fix)It doesn't look like a hack job,in fact it looks pretty good.I'll check the settings anyway,who knows when the fix was installed .15-20yrs. is more than enough time for things to vary from the original settings regardless.I did download the virtual testbench,And I'm putting together the dummy load now.So we'll see if it makes a difference.

I can't thank you all enough for the advice,someday I hope to be able to return the favor,
Robert

Maybe I'm just having a brain fart here but where exactly is the bias tested from?across r94 and r95 is what it says in the manual,but it seems I'm getting no reading from my iterpitation of the instructions.moving the pots does nothing where I'm trying to test at on either board.The amp works well on both channels (with the exception of the initial fuzz at low vol. issue that started this procedure)so I do know that no reading here can't be possible,It has to be my dumb ass.So to clear up any misconceptions I may be having ,where EXACTLY do I put the leads of my m.m. to test for bias?I'm more than slightly embarrased that I'm having difficulty,so please be gentle!I did set the rail voltage to 118v,it was at just under 120v.My tube amps are much more dumb-ass friendly,I've recapped,rebuilt and done about all a hobbiest can do to them,(Fishers,and Dynacos mostly)but this simple setting on this Carver for some reason is giving me trouble.(No I'm not an Idiot)...........really!
Thanks for all the Help,
Robert

I just saw a picture of a TFM-45, and it looks like the trimmer is right between a pair of big white block resistors. At the top of each leg I believe is where you put your meter (in the millivolt range). You'll get negative millivolts one way and positive the other. The trimmer in the middle adjust the value. I could be wrong about this because I've never been in one of these before, but that's just what it LOOKS like.

You'll have one lead on R94 and the other lead on R95

Thanks Billd.I Do know where the pots are,and the 2 resistors R94 and R95,that the manual tells to check across.I've tried the upper leads of the resistors, + from my meter on one,R94.and -on the other,R95.And the same correleation on the lower leads of R94,R95.Not much success,moving the pots does nothing.( yes set to MV)I let the amp idle for about 20min. before attempting to get a reading.Are shorting plugs in the inputs required for this measurement? 2.5 mv across the resistors is what I'm looking for,Am I not measuring across the resistors?Or am I missing something?If I truly had zeros across the board like I have at this point(If I'm Correct on the test point)I'd think I'd have a non functioning amp on my hands,which I don't.Wow, feeling dumber by the minute.Again thanks for the help,
Robert

I can only go with what the manual says because I've never done a TFM-45 before, but it looks to me that where the thin arrows point, you should put your two leads (nothing to ground) to measure and then adjust with the pot. Of course, the amp should have been on and warmed up for about 5 minutes before you perform this.

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