TFM-24 problem

Well after getting the C-11 up and running I now have a new problem. My TFM-24 will not play very loud and both channels are distorted. I swapped preamps and speakers as well as using different cabling and inputs (tuner and CD). I opened the case and blew it out (20 years of dust). I see no obvious problems other than some discoloration of the PCB around the area of R202 and R205 (prehaps normal due to age and heat). I have obtained a service manual for the TFM-25 which is supposed to be identical to the TFM-24 except is have meters instead of LEDs. In the schematic it shows IC101 as being common to both channels after that the circuit appears to seperate completely. So is IC 101 the drive amp listed in the block diagram and is it possible this is what has failed. As I stated the amp plays but the output is weak and distorted and the power output LEDs never budge. Any help would be greatly appreciated, I am going out of town, but will check the forum regularly for any suggestion I might try when I get back.

I'm guessing that IC101 is probably a dual (or quad) opamp and has separate amplifiers for the left and right in the same DIP package. I'm guessing that the caps can't keep up the charge, or the optocoupler is failing to keep up the rail voltages. Maybe RichP or some of the other techies have a better idea of what is going on.

Does it say on the schematic what IC101 is?

I looked up the chip and it is a dual op amp. It drives the current on two output transisters that in turn supply the line input to the amp stages.

Well I have returned from vacation and hopefully with help can track this problem down. I have removed the bottom cover and inspected the unit. On the PCB I have found what appear to be cold solder joints in the area of R201, R203, R208 and R209. There appeared to be flashing in these areas as well, I cleaned the PCB and the tracing appears to be in good shape. Does anyone know how I can find what has failed on this unit, using only a fluke 87. I have a fair amount of electronics experence but am new to audio circuits. Since the output protection is not tripping and the problem affects both channels I am leaning towards the input circuits on the MI-801 board. I have purchased my own copy of the service manual for reference. Any help will be greatly appriecated.

I tru to make sure none of the output transistors are shorted first. Then I adjust the idle voltage on the power supply. If that fails (can't get it up or down to the point it needs to be, you've got a power supply problem. If that's OK, adjust the bias on the amp boards. At least that's a start, and then you can start isolating the problem.

I will take your advice, I bring my Fluke home tommorrow and check the output transistors and go from there.

As they say - If it works, it's a Fluke!

Otis, before you do anything DISCHARGE caps C501 and C502 (the 2 big ones) with a 120v lamp or power resistor (~100 ohms or so), otherwise you can further damage the amp if you short the wrong PCB traces with the test probes. If you have a lot of discoloration around Q201,202,204,204 then check back to one of my earlier posts (maybe 2-3 wks ago) about replacing those transistors and changing a few resistor values. I know you have the same problem in BOTH channels but I've seen these transistors fail in both channels at nearly the same time when the amp had a ventilation problem. While you could have an issue with IC101 its also possible the above transistors can cause a similar problem.

Current Limited, yes I have heavy discoloration around these transistors. I read your post concerning the service mod. But I have a question, you mention that the transistors should be changed with a TO-92M, I googled for them and found several TO-92M's listed. The carver manual listed them as 2SA1370 E,F and 2SC3467 E,F but digi-key does not show them. Is there a readily avalable substitute for these transistors, the resistors are easy to find. Also I have included several pictures, the large caps have some type of residue on them. Do you think they are leaking and should be replaced as well? You help is greatly appreciated.

I'd check those caps against the specs (the black ones). All the caps look OK. I was trying to make out the values, and couldn't. But it does look like those caps might be newer.

The transistors are available through http://www.bdent.com they are the exact same ones - just a new set of replacements. You should have the giant TO92 (TO92M) in there already, they just get heat damaged and the new replacement resistor values will reduce the current through them and increase the lifetime of the new transistors. The capacitors should be ok (2 X 6800uf/35v , 2 X 3300uf/63v and 2 X 10,000/100v) sometimes the bonding adhesive gets discolored from the heat and people mistake that for leakage, I can't really see in your example if that is the case. The 10,000uf caps should keep their charge for days or weeks, so be aware of that when servicing these amps. That was a great idea Carver had, it reduces the startup surge and prolongs the cap life, it also keeps the caps formed - unless the amp sits for a very long time between each use.

Current Limited, I saw those you mention at Bdent but the lack of the E,F at the end of the transistor listing threw me off. Does the suffix have any real meaning? Thanks for your expertise it has been very helpfull and greatly appreciated.

The "E" is the DC current gain h(sub)fe of transistor, the last batch I ordered from BDent came as the E version which is correct. E is the only version I ever saw in the TFM25 amps but the service manual lists E or F, they are both the "same" transistor but the F has more gain. Don't expect your TFM to behave differently by going to the F grade, the circuit gain is usually (but not always, so no flames here) set by the resistors and capacitors around the transistor.

Current Limited, Thanks for the invaluable info, I get those parts coming and do the service mod, then let you know how it went.

Here's my trusty capacitor discharger.

Cap Discharger.JPG (331.74 KiB) Viewed 765 times