Tascam 38 Modification

Jim Williams Mod

I notice there has been some discussion about this mod on tapeop questioning which values are correct.
The values on my graphical schematic are the right ones- try calculating the gain stages if you are curious why.
I could see why this might be a little confusing from what Jim says in the section I’ve published here.

The Tascam 38 tape reel-to-reel was introduced as a prosumer tape machine, occupying the niche between hi-fi and professional grade audio equipment.  Its got a nice sound, but a good share of weaknesses. When overdriven it is prone to flab, the inputs prefer -10 to +4 levels, and the control relays tend to stick eventually. As it turns out, with a few simple tweaks it can really be overhauled into a high quality machine.

Tascam 38 Mods“Unscrew that bar that goes across those 8 pcb’s, those are your record/play cards, one for each channel. I think channel 1 is towards the front, but it’s been many years so I could be opposite of the order. Use 2 hands, gently pull straight up on the card and it will come out. Use care in reseating them so pins align. If you want to get into it, you can replace the opamps with National LME49720′s or AD8599′s and then pull out the cheesy step up transformer used to get cheap gain off the heads. Bridge it and the remove the resistor from pin 2 to ground on the opamp. Replace it with a 91 ohm or so, low enough to make up the lost gain to unity. That will open up the playback quality to no end. They can then sound like MCI -JH-24 or Studer A-800. It’s also possible to replace a couple of parts in the bias circuit so you get more, enough for +6, maybe even enough for +9 levels with GP-9 type tape. The output trims can be elevated to get +4 unbalanced outputs and reducing the record trim allows +4 db inputs as well. Lots can be done to tweeze these machines, I made a few very good sounding records with one back in the early 80′s on a heavily modified machine. Doug Sax at Mastering Lab was my mastering guy then and he always commented on the excellent sound I brought and was very suprised to hear they were recorded on a modified 38 and a custom console.”

Jim Williams of Audio Upgrades

I followed up his post with some emails to clarify a few things, and this is the modification I arrived at.  I think the most important thing to do would be removing the step up transformer and upping the gain in the recovery circuit. If you want to take the mod a step furthur I would also recommend recapping the signal path with some nice new electrolytic capacitors. I like Panasonic FM caps. I used Nichicon Muse for my CS-60 refurb, but its hard to say if the brand is worth it or not. I’ve heard the argument that pretty much 105C caps are created equally.
As far as op-amp choices go, from what I understand the national LME part is very natural, good low end- not great but acceptable. Supposedly the AD8599 is darker, but clear. Obviously take descriptions like these with a fat grain of salt. Both are very good ICs compared to what you’re going to find in most prosumer stuff.

Useful links:

Original TapeOp Thread
“My Adventure Modding a Tascam 38″