CT F2121 Recording Current

[GaryJarcia]

Hello AudioKarma! Long time lurker, first time poster.

I'm in the process of servicing one of my decks, a CT F2121, beautiful deck. Got it for a great deal on Ebay and got it functioning great mechanically minus some low RW torque that I still need to address.

I've gone through a bit of the electrical adjustments section already, but i'm finding myself stuck on measuring recording current. The recording current adjustment states:

"This adjustment is performed with the tape bias current cut off. By opening the solder connection in the copper foil (shown * in fig 21), power is not supplied to the oscillator circuit, thus interrupting the bias circuit."

My issue is, I cannot locate this anywhere on Fig 21! I may just be missing it and I know the image is blurry, but this has absolutely stumped me. Any help by someone who knows what I need to disconnect would be much appreciated, thank you!

 

[GaryJarcia]

I'm beginning to think they never marked the spot at all! I went in with photoshop and compared the enlarged version provided on page 35 of the service manual. I made them the same size, put the enlarged version in front, and slowly removed sections looking for differences. Besides the enlarged version adding voltage measurements, they were identical, one just worse quality.

The schematic is pretty low quality too, I will keep searching and report back.

 

[chazix]

solder connection in the copper foil

Have you tried a visual inspection of the solder side of the board? If it is really present, I would think this would be pretty easy to spot - a blob of solder without any component leads sticking into it.

Armchair quarterbacking, I know...

chazix

 

[GaryJarcia]

Have you tried a visual inspection of the solder side of the board? If it is really present, I would think this would be pretty easy to spot - a blob of solder without any component leads sticking into it.

Armchair quarterbacking, I know...

chazix

Thats what I thought too! A high chance I could just be overlooking it, but i've stared at that board vs schematic for awhile with no luck. The schematic for the entire thing is very grainy so it's hard for me to see where the power for the oscillator unit is coming from. Any ideas by chance?

 

[GaryJarcia]

I was under the impression it was connection 10, but when I tested with connection 10 removed, no dice. It appears it could be 7, but 7 looks like the oscillator output. I'm definitely missing something.

 

[chazix]

I was under the impression it was connection 10, but when I tested with connection 10 removed, no dice. It appears it could be 7, but 7 looks like the oscillator output. I'm definitely missing something.

My read of the schematic would be that oscillator power supply is indeed pin 10. Are you saying that it still oscillates when 10 is disconnected? My only guess at an explanation for that would be that the schematic is in error. (Wouldn't be the first time. On the other hand, the PC trace drawing also shows the power input to be 10, so at least there is consistency.)

I don't have a guess at the function of pin 7. It is some sort of input, but not a power input. I think a DC input of a volt or two applied to pin 7 would quench oscillation, but I don't think that's something that you want to exploit without knowing what's connected to the pin.

If someone better-clued than me doesn't come along here, one thing you might consider is asking a moderator to move the thread to the "Tape" forum. Folks there very likely know their way around bias oscillators better than I do.

 

[spicer]

Looking at the overall schematic, pin 10 is the supply and 7 is the mute. Output is 14 for the erase head and 12 and 13 are for the record head, left and right. The schematic is on the hi fi engine site with excellent resolution.

 

[markthefixer]

Look at the block diagram, the output to the heads is 12 & 13 to the record heads, and 14 to the erase head.

Pin 10 is the power to the oscillator, 11v for standard, 18v for CrO2 tapes.

 

[GaryJarcia]

Doh. Thanks everyone! It was pin 10 after all. I was driving myself crazy trying to figure this out. I must have mis-measured.

I think part of my problem is the trimpots on top of the rec/play board are in pretty terrible shape. The resistance jumps around like crazy even after spending 15+ minutes working thoroughly with deoxit. Planning to replace them with equivalents and re-doing the adjustments from the beginning. Thanks again!

 

[GaryJarcia]

Thank you all again for your help. The service was completed per the manual's specifications and wow, what a little beast! Now to fix the idler on my F9191....for another thread.