Milestone 1 passed and beyond!

Recapping preamps, done!
Balanced inputs on channel 7 and 8, done!
Recapping PSU done!

This wasn't as much work as I anticipated. I definitely thought that the piggy board mod would be more hassle. But sometimes you are just lucky I guess. Karma I hope. There is two variants on the 7 and 8 channels and hopefully I will get to test it out this weekend.

When I ordered PSU caps I couldn't resist order OPA2134 and OP275 for one channel so now is channel 1 modded as Ampex 354 explained in the gearsutz thread, with new amps and coupling caps removed. I didn't do the EQ mod or gain reduction mod. I don't remember if he did so, but I added 100nF decoupling on the pins for the new amps and upped the supply rail bulk caps C16, C17 from 10uF to 47uF. I also added an extra ground cable from the far end of the board back to the ground connector hoping to reduce ground path resistance.
I cannot say anything about sound quality improvement yet but the noise floor on channel 1 was significantly reduced by this mod.

Decoupling caps on the solder side.


Little dark picture but this is the board with the extra ground cable. I really want to try stargrounding one board and break the busground, adding a cable from the preamp board straight to the PSU ground and see what impact that would do on the noise level.

PSU upgrade done! Wow

Yesterdays project was to do the PSU and I must say this is the most rewarding step I've done so far. The noise in the machine is drastically reduced by swapping/upgrading the bulk electrolytes. If it's one thing you want to do to this puppy it is this. I upgraded from 2200uF stock to 2x4700uF low impedance caps. The new caps were 18x35mm and the clearance to the bottom lid is about 36 mm so I recommend to you not to use no more than 30mm height. There is also a 6V rail that has 1000uF stock that i upped to 2200uF. Stock dimensions on the PSU caps are 2x 16x30mm and 16x20mm.

I removed the PSU board from the underside by pulling it to the side, under all the cabling, after removing and carefully mark all connectors. After dropping a screw in there i found a much better way. The cabling to the back panel are long enough so if you remove 4 screws on the back you can fold it right out and remove the PSU board that way. Much easier. You still need to remove the bottom cover to unscrew the board and remove the connectors though.

The board inside the chassis. It was really tricky pulling it to the side under the cabling. Don't do it. Pull it out from the back instead.

The board

The caps

My way of doing a little dual cap assembly


The assemblies mounted on the boards. There was, of course, less space than I estimated when i ordered the caps and 4700uF was the biggest caps my convenient supplier had in stock. I recommend getting larger capacitance in one capsule if you are not used to work on electronics. It is very easy to do this if you use stock dimension caps. I recommend this upgrade even for non experienced people.

I couldn't resist adding some extra caps in there. These are 47uF on the secondary side of the regulators in the holes for the cables. there are 22uF mounted on the secondary, I feel this is way to little and they are mounted too far from the regulators. Notice the marking on the connector, can't stress this enough, you will forget were they were when you reassemble.

Conclusion, as stated in the beginning, if you would like to do just one thing on your machine, upgrade the PSU and enjoy the loss of ssssssssssssssssssssssss. I wouldn't say its dead silent but its way better than before.

Recapping, done (sorta)

I am finished with recapping the channel boards, caps for the PSU and some extra goodies are coming in in a day or two.

Channel 7 and 8 are now balanced input

I have installed and tested the INA217 boards on channel 7 and 8 and it seems to work just fine. Channel 8 is just the preamp board with unity gain installed were the transformer is on the stock boards. On channel 7, after reading the schematic, I have done some further modifications.

Removed R4 and shorted R3, taking S1-1 out of the picture.
Removed the voltage divider formed by R7 and R8 by removing R8 and short R7.
Added a 10K gain set resistor on the INA board, getting 6dB gain in this stage.
Replaced gain set resistors on U1-1 that are switched by S1-2
R10: 10K 19.4dB
R9: 2K7 10.24 dB
R11(stock): 540R 3.23 dB

Replaced first coupling cap with 47uF elyt + 1uF poly caps paralleled.Added ceramic decoupling on the rails at U1.

As this is just testing right now I just lift the component lead that are in the signal path for easy reassemble if the mod is not beneficial. Here R4 lifted out of circuit.


R8, eerie close to R12 and the jumper

R3 and R7 shorted on solder side. If you do permanent mods, replace the resistors with proper jumpers. That's what I am going to do when I am satisfied.

Since I am doing this on spare time when geek junior is asleep I have only done some very basic testing. Comparing the noise and estimating the gain by tapping a very crappy dynamic mic. The mod works, no doubt about that, channel 8 is a bit less sensitive than stock and channel 7 is some dB less sensitive than channel 8. Since I am mainly intending to record slamming drums on it I suspect that less sensitive inputs might be beneficial. A real situation test will clear that out.

My intention with spreading the gain over two chips was to reduce noise floor and Transient intermodulation distortion. However I estimate the noise to be about the same, taking into account the different gain of the channels.

Preamp board schematic, thanks to fellow gearslut

Ampex 354 at the gearslutz forum was kind enough to take snapshots of his schematic, thank you very much. I hope it is OK to post it here and that TEAC will be good sports about it. I hope it can stay up so that I have something to refer to.

Project thread on Gearslutz.com

I started a thread on Gearslutz.com regarding this project HERE:
Good comments and suggestions from other posters. Thank you =)

Replace phono jacks with XLR

The phono jacks on channel 7 and 8 are mounted on washers that cover holes that fits XLR jacks so no modification on the back panel have to be made. I used Neutrik jacks that were available in a store nearby and they fit in the original JAE holes if you tilt them a bit.