New to me TMF-25, Brown PCB

A friend of mine gave me a TMF-25. He told me that one of the channels was weak when it got warm.

Check out the PCB:








The amp has worked fine for several hours for me. I've seen the TMF-25-2 Service Bulletin ("To prevent components from discoloring the PCB") and have been contemplating swapping the resistors. Anyone know if not changing these is going to cause the amp to fail? Also some of the resistors in brown areas aren't mentioned in the service bulletin.

I need to ask my friend if he was using it to power subs or something, I'm only using it for inwalls with 8"s and the amp does not get warm. According to the meters it barely puts out 5 watts at normal listening and sometimes hits 30 watts when the volume is up.

I'll post some random pics since this is my first post. The setup its in:



Yeah, the Panasonic doesn't have front pre outs. I dragged the preamp out of the closet to test the TMF-25 out.

My computer audio setup:

Caps look good, but the areas I circled look pretty toasty. I think I'd perform the service bulletin. Jazzman just redid his TFM-25, maybe he'll chime in.

TFM-25.jpg (136.09 KiB) Viewed 432 times

Yep!!! Brown is not the preferred color for the boards!!!

I have one discolored like that. No problems with sound or operation. I run it about three times a week.

Well, you know they are generating HEAT to discolor the board that way, and heat is death on components. It is basically the only way for a semiconductor to die other than bad manufacturing. I'd certainly try to mitigate the problem by implementing the tech bulletin.

The previous owner must have pushed it way harder than I do though. The heatsinks do not even get warm when I'm using it.

I'm going to do the service anyway. I'll probably put together a parts list this weekend, some of those burned spots are not covered by the service bulletin.

Caps apparently need to be replaced too. The top of the larger one on the right is energized.

I did the bias adjustment yesterday, amazing how much better it sounds. The right channel was at 3 mV and the left was at 0.5 mV.

I had the very same issues with my TFM-25, including more toasted areas on the board than are shown in the Service Bulletin. The parts were cheap so, rather than just hoping nothing would fail, I replaced every part over every toasted area on the board and elevated the replacement parts 3/8" above the board to better dissipate heat. Mauser and Digikey didn't stock those 1/4 watt flame proof resistors called out in the Service Bulletin so I substituted 1/2 watt standard resistors for those. While I was at it, I replaced all of the electrolytic caps. The link at the end of this post is a write-up I did on another web site covering the recap and Service Bulletin repair. I bet your idle current alignment is way off on that weak channel (mine was).

The parts lists in my linked writeup show only the caps and Service Bulletin replacement parts, not the other toasted parts that I replaced. So, below the link is the actual parts list I used, which includes all of the resistors and transistors I replaced (probably the same parts that could be compromised in your amp too).
All parts purchased online from Digikey except for the transistors, which were purchased from online B & D Electronics. The total parts cost was about $67 including all new caps.

Anyway, this was my very first electronics repair and it turned out OK in the end. The amp now has stable rail and bias voltages and it sounds wonderful. The new caps really improve the dynamics too. Hope this helps and good luck with your amp.

Link to writeup with photos:
http://thecarversite.com/yetanotherforu ... sts&t=1683


Toasted parts replaced:
Item Carver PN Qty B & D PART# Description
Q201, 2SA1370-E,F 4 2SA1370
Q202,
Q209,
Q210
Q203, 2SC3467-E,F 4 2SC3467
Q204,
Q207,
Q208
Q451 2SD2012 1 2SD2012
Q452 2SC1815-Y,GR 1 2SC1815

R201 KA25ST911J 3 910Ω, 1/4W, 5%, carbon
R202
R204
R205 KA25ST102J 4 1kΩ, 1/4W, 5%, carbon
R206
R207
R208
R209, REPLACE WITH 62W, 1/4W, carbon, FP
R210,
R211,
R212
R218 KA25ST122J 2.2kW, 1/4W, 5%, carbon
R219
R220
R229, REPLACE WITH 1.5kW, 1/4W, carbon
R230
R243, REPLACE WITH 22W, 1/4W, carbon, FP
R244
R245, REPLACE WITH 27W, 1/4W, carbon, FP
R246
R281, KA25ST222J 2.2kW, 1/4W, 5%, carbon
R282
R283, KA25ST562J 5.6kW, 1/4W, 5%, carbon
R284
R451 KA16ST471J 470W, 1/6W, 5%, carbon
R452 KA16ST102J 1kW, 1/6W, 5%, carbon

Note:
Must verify R235-238 are 1/2W (some amps came with ¼ W)

Note the following corrections to my parts list shown in previous thread.

Corrections for parts list-- C307-C308, C415-C415 should be:

NS-25TW 470M 8 47µF 25v EEU-EB1E470 493-1547-ND 2 C307-C308, C415-C416 $0.21 $1.68


Also forgot to mention the 1/2 watt standard resistors shown in my parts list are substitutions for the 1/4 watt flameproofs called out in the TFM-25-2 Service Bulletin-- because I couldn't find the flameproofs.

Also, the P10038-ND (10,000uF) caps are taller/thinner that the original caps and only clear the top cover by about 3/16"--- not a problem as long as you don't drop a brick on top of the amp but if you prefer shorter/fatter caps, use P10040-ND's.

I have a TFM-55 with the same brown spots on the PCB. I ran into some problems while recapping, I think the soldering iron was too hot, and burned a few solder pads off. So while dealing with this, the PCB cracked on the bottom right side just under the power supply components there. I think I fixed that problem, then somehow caused a spark to fly on the left amp side. Now I can't get a reading to set the bias pots for either amp. When I check the very top transistors on the heatsink on the right, I seem to get a short. That heasink gets very, very hot. The relay no longer kicks in, but the amp does power up.
I only noticed these brown spots when I discovered the transistors in the burnt areas had become loose from their solder pads. Plus I found a burnt resistor-R218. I can replace all the problem brown area components, but I am at a loss as to what is the real culprit behind these problems. I don't know where to look. Any help would be appreciated. Please don't suggest sending it to someone, I am broke.

Hello Backsigns, I would start by looking for a dead short somewhere. Then figure out what you fried. A magnifying glass works wonders.

Thanks for your reply. I have been looking for the fried part on the left amp since it happened. Can't seem to find it.
Could all this have been caused by all the raised leads on resistors touching each other on the boards?

Yes. When they touch, you have just changed the circuitry. That may be where your short happened. Now, a raised trace wouldn't hurt a thing [so long as it didn't touch anything and wasn't creating an open].

Please explain in detail what you mean to help us to help you.

It's been awhile since I did this, but I believe I powered up the amp, when the resistors on the input board were all mostly leaning into each other. I can't tell you the timeline of the events that occurred, since it's been awhile. The first problems occurred-the relay switching on and off when I created some bridges while trying to repair the burnt solder pads. But I noticed the resistors had all bent a little bit on the input board after this. I was fixing a bad solder joint on the left amp when I touched on of the solder pads with a tweezer a little lower on the board. After the PCB cracked I tried to fix the broken traces. There were no components on this area, but did affect the amp by not turning on the relays. I could still set the rail adjustment. But I have never seen any readings for either side for bias adjustments. At one point I could trip it on by touching the probe to the IC (which is where most of the solder pad problems were) but not now. Those transistors on the darkened parts of the board had all been blown off 2 or all their solder pads. I had not noticed these burnt areas before, so I don't know when this happened. After all this, I was checking out the transistors on the heatsinks from ground to each leg, and only one leg of each transistor on the right, very top 2, (The hot, hot heatsink) measured nothing. All other transistors had readings on each leg from ground.
I'm not sure I know what you are asking me. I hope this helps.

I guess I don't understand what you are asking.