Carver TFM-25 Repair Questions

[Tweekster]

I’m slowly working on a Carver TFM-25 with a blown right channel, working through the schematic and trying to educate myself on the 3-rail system, and have a few questions. Repairing this amp will be mostly an educational experience, and only used for a bedroom stereo. Sonic perfection would be nice to have, but it’s not fully required.

I have built amplifiers off and on since the days of the SWTP Plastic Tiger and Tiger .01, even recently repaired a couple of Adcom 555’s, but I lack the component level skills of knowing what is a good or bad transistor substitution.

I have found one of 2SA1302’s to be open, but due to the number of counterfeits on the market, I would rather replace the PNP/NPN pair with the Hitachi 2SA1943/2SC5200 from Digikey instead. Will this work okay? Should I just get a few of each, match them, and replace them all?

My father bought this amp brand new in 1991 and it worked well until a few of years ago, when he lost the right channel. A thorough cleaning and visual inspection shows the amp in excellent shape, no brown areas on the PCB, no obvious blown or burnt components, and only a few suspect solder joints, which I touched up. The funny thing, is the left heatsink has 4-2SA1302, 1-2SA1301, 4-2SC3281, 1-2SC3280, the right heatsink has 5-2SA1302, 4-2SC3281 and 1-2SC3280. Per the schematic, the left side should be the same as the right side (the correct layout) so it looks like a factory error. From the specs, and to my untrained eye, it looks like the 2SC3281 is just a more robust version of the 2SC3280, but I’m not sure of what the sonic difference should be, or if any of the resistors should be changed for the difference in current between the 2. My question is, can I replace the 2SC1301/2SA3280 pair with the Hitachi 2SA1943/2SC5200 pair? Do I need to changes any resistors? Try to dig up matched 2SA2SC1301/2SA3280 pairs? Try a better substitution?

Last question, I usually make it a habit to change the drivers when I change any outputs, because they are cheap, and it’s cheap insurance. This amp uses 4 pairs of 2SA1370/2SC3467 as drivers and pre-drivers. Digikey doesn’t stock these, but places like B&D, Newark, MCM, etc do. I’m not sure which distributor to trust, or if I should try a possible better substitution from another reliable source.

Sorry for the long winded post, and thanks for your time!

Win

[toddyaudio]

I've used the SA1943/SC5200 (Toshiba) in dozens of Carver repairs and have never had any problems, some people have even noted an improvement with the new transistors (but I have not heard any). I've repaired many Carvers where different transistors were used on the outputs/commutators (I'm talking about the TFM 25/35/45, M1.0t here), but MOST of the time the same transistors are used in these areas. So your case of different transistors is not uncommon. If you use the SA1943 in one side, make sure its compliment is the SC5200 - not the older transistors.

[anatech]

Hi Win,
toddyaudio is correct here. You can use the MJL type parts from On Semi also. They are excellent.

Now, check the areas to the rear just to the left and right of center line. There are some TO-92M type transistors that run hot. The board may be a bit brown here. I have seen those transistors go open or leaky, so just change them.

I believe there was a modification to reduce the standing current in Q202 and 204 (repeat for the other channel). Look for over heated resistors and overheated capacitors as well (small ones in that area). I think it's important to find out what the mods were and execute them. If memory serves, there was also another mod for the bias transistor near the outputs (there are two per channel, one adjustable and the other fixed).

-Chris

[Tweekster]

Thanks for the responses guys!

Toddyaudio,

I'll get a few pairs of 1943/5200 to replace the blown outputs, and currently unmatched 1302/1380 pair.

Chris,

Yes, I have a copy of Service Bulletin #TFM-25-2, dated 4/15/92, which details the modifications, "To prevent components from discoloring the PCB and to improve circuit reliabilty."

It shows to replace the 4 pairs of 2SA1370/2SC3467 predrivers/drivers spaced 3/8" to 1/2" above the PCB, and replaces a few resistors with higher values. It also replaces a few resistors with the same value, but in 'Flame Proof'.

Two of the 1370/3467 pairs are the TO-92M, Q202, Q204 you're thinking about. After all my research I expected to see the PCB very brown in that area, but it only has the slightest discoloration. I'll do a further check on the other components in those areas. This amp really is in much nicer shape than I expected, but I'll still replace them and do all the mods.

I don't have a copy of the bias transistor mod, so I'll need to research that a little further.

I guess reading all the counterfeit posts, buying components has me a bit gun shy, besides places like Digikey, Newark, etc, where's a good, reliable source for the 1370/3467 pairs and the 1/4W flame proof resistors?

Thanks again for the help, it's greatly appreciated!

[anatech]

Hi Win,
Those vendors are fine. Good products. Use "Metal Oxide" resistors.

I believe the bias mod is part of that one you have there. If you could scan it and send it to me, I'll both check it out for you - and have a copy of my missing paperwork.

-Chris

[inttech1]

Does anyone know the values of R505 and R506 resistors?
They are near the regulator transistors on the right side looking down with the top off.

I am currently repairing one, and the color bands are burnt right off so I don't know the value.

Any help would be greatly appreciated

[anatech]

Hi inttech1,
Nice! The values are not on the schematic. Priceless. I don't have the parts list for that one either.

Measure both of them. They act as a fuse between the 30 VDC rails and 15 VDC regulators. I don't know if they are also using them to split the heat power between the actual regulator pass element or not.

It is possible those resistors are simply overheated. They may function fine, you'll have to measure them to be sure.

-Chris

[inttech1]

Hi Chris,

Thanks for your reply. Amongst other issues with this amp, including the need for the factory Service Bulletin upgrade, and blown outputs on one channel, this was the last thing I was trying to get resolved.

I believe the resistors should be identical for both positions, but when you remove them they measure very differently.
From what I can make out, the bands on both are the same from what I can make out.

I really don't know which one is closer to the actual.
One measures about 22 ohms and the other is over 43.
Perhaps someone owns one and could tell me the next time they remove the cover for cleaning, what the band colors are....

I really appreciate your input!

Ron

[anatech]

Hi Ron,
I believe these are different values.

There was an upgrade to allow the pre-driver transistors to run at a lower temperature. You could also upgrade them to a different case style (TO-126 or equiv.). The changes where some resistor value increases in the emitter circuits I believe.

-Chris

[Bigred]

Yes Ron, R505 and 506 are indeed the same values (+/- 15 reg line. ) They are 56 ohm flame proof resistors.