Offset problem with TFM-55?

How stupid am I???

R286 not R226!

It was so burned that I misread it. This is a 100ohm.

I was looking at Q236 and it didn't make sense as one of the legs connects to the burned resistor but it wasn't R226.

now if the negative 98V cable connecting to D504 had become lose would this cause a big problem?

Yes

......Also Q236 readings are much lower than the left channel (same polarities in circuit), but reading only 12/13 compared with 38 on the left channel transistor.

I'm confused, since we're only looking for shorts. This may be significant later, but for now we're only looking for blown transistors. 12 vs 38 on what range?

......Q214 (right)

E/B: 1992 (this should be zero reading like the left channal) other way : 556

[/quote]I don't understand this at all, a reading of zero is a short, nothing should be reading zero

How stupid am I???

R286 not R226!

It was so burned that I misread it. This is a 100ohm.

I was looking at Q236 and it didn't make sense as one of the legs connects to the burned resistor but it wasn't R226.

Where is this resistor in the schematic?

R286 is at the bottom right of the right channel page. just on Q236. I'm assuming it's R286, difficult to read. But this connects straight to one leg of Q236.

......Also Q236 readings are much lower than the left channel (same polarities in circuit), but reading only 12/13 compared with 38 on the left channel transistor.
I'm confused, since we're only looking for shorts. This may be significant later, but for now we're only looking for blown transistors. 12 vs 38 on what range?

This is on the diode testing mode.

......Q214 (right)

E/B: 1992 (this should be zero reading like the left channal) other way : 556

I don't understand this at all, a reading of zero is a short, nothing should be reading zero[/quote]

This is in circuit testing with the diode function.
Yes very strange, seems to be reading something (1992). My meter shows 1 if no reading. Let me have a look at the manual to see what it says.

ALSO how should Q236 read out of circuit?

UPDATE: EVERYTHING BACK TO NORMAL

I put the 100 ohm back in and everything seems to power up ok. Will do some testing tomorrow (got to get myself ready to go out now). I think I had a short when I messed around with those coupling caps and the 100 ohm saved my ass!

OK ONCE IT'S UP AND RUNNING TOMORROW NO MORE MESSING AROUND I PROMISE TO MYSELF. JUST THOSE TRANSISTORS YOUR SENDING ME.

UPDATE: EVERYTHING BACK TO NORMAL

I put the 100 ohm back in and everything seems to power up ok. Will do some testing tomorrow (got to get myself ready to go out now). I think I had a short when I messed around with those coupling caps and the 100 ohm saved my ass!

OK ONCE IT'S UP AND RUNNING TOMORROW NO MORE MESSING AROUND I PROMISE TO MYSELF. JUST THOSE TRANSISTORS YOUR SENDING ME.

What a strange road this is turning into. If it were me, I'd get those PIO caps out of there and put the right ones back in; theyre too big, they're PIO so they won't last long, and I've only seen PIO on passive crossovers.

Too much chance of the metal can shorting things out.

If it were me, I'd get those PIO caps out of there and put the right ones back in; theyre too big, they're PIO so they won't last long, and I've only seen PIO on passive crossovers.

I took those out straight away. Metal cans and all. (keep them for the valve project).

The little polyester and polypropylene are back in it's rightful place.

This was like a little twist to the near end.

Back to the problem at first.

I put my amp back in the rack after testing voltages on the workbench, nothing switched on I connected the right speaker and bam the R286 resistor blew. This was with the amp switched off!!!!WTF.

Update: I think the transistor Q232 was shorting with the metal cover for the rca board. It was brushing the metal part of this transistor!!!! When I took off the metal cover the resistor popped again
Let me check this out.


I think this was the initial problem all along. the metal plate was shorting with the edge of Q232!!!!!!!!!!!!!!!!

music is coming out of the boxes now

8 hours of good burn in time yesterday and everything so sweet and juicy like these nutz..

Heatsinks were just warm, bias set at about 2.3 mV (which took bloody ages to get right cuz those stock pots are junk), didn't want to overdo things.

The 100V,10000uF Panasonic caps were warm to the touch. Burning in nicely.

Just looking forward to getting the new transistors (kindly sent by Rich) to fix the 30mV of dc offset I'm getting. Also the new FR603 diodes and new bias pots.

Hey Brit....

Great job Bro! For a guy that didn't even own a multimeter you really kicked ass following the input from this board. Heck, I own a multimeter (actually two) and generally know how to use them (automotive electronics, fuel management, ect.), but I must say this thread really had me concentrating...

HRDROKN,

For a guy that didn't even own a multimeter you really kicked ass

Cheers dude. Yep last time I touched electronics was at high school some time ago.
You could say my personality is a bit passionate bordering on the obsessive side, so when I focus on something my determination is second to none. Also I didn't trust any cowboy "technician" on my carver.
Have to pay some attention to my better half now
She said "Me or Carver, you choose"

.....
She said "Me or Carver, you choose"

I'm there as well, and it's the wife's day off so I have to take a time out from all things Carver or the cup won't runneth over.......

Interesting observation:

I was experimenting with 1% metal film resistors and 5% carbon ones on the RCA board.

I wanted to put metal film resistors at the signal path.

The 1% resistors were increasing the amount of dc offset. I'm thinking that the 5% tolerance is important for the dif. transistors here for them to be a little flexible in the push/pull arrangement.
As I changed pair by pair back to carbon 5% the dc offset slowly decreased.

It's balanced just below 20 mV on each channel now. Still 2 pairs of 1%'s I need to change back and also on the main board.

I guess I will need 5% metal film if I am to use this kind. Probably not worth the effort now.

20mV is in a safe range I believe, but I'd like to get it below 10 mV.

well I've got all the new transistors in place, even changing all dif.transistors on the rca board, and the new pots, including the new FR603 diodes.
I found some C2229 transistors on the main board instead of 2SC2240's . I changed them for new C2240's.

The pots are much better. Thks Rich.

When I start up the amp after a few mins I get about 10/11 mV dc offset balanced on both channels. After about an hour of heating up it rises to about 17 mV max.

I guess without matching transistors this is ok now. When I power up I get no bump through the speakers anymore. Dead quiet.

What level of dc offset are other people getting on their tfm series?? Or on your MkII amps Rich?

Haven't been around for a while.

Just popping in to say Hi to all the Carver members here.

My Carver is still going strong. In fact I just changed the inout caps on the RCA board for some little red WIMA's this weekend.

Got my Carver running well with an Aikido Valve pre-amp that rocks!!

But I do get an occasional small crack through the right channel. Still got some pending component changes though.

Been very busy with a new job as systems analyst and new hours so don't have much spare time.


Also got a new addition: An immaculate Rotel RCA 965 BX CDP. Beautiful

See you all around.