Audionics of Oregon BT-2 Preamp restoration

[Organgrinder]

Hey all!

Thought I'd share my little project for the time being, cause I know some of you love seeing the guts of these old workhorses.
Been working on this old Audionics of Oregon BT-2 preamp the last couple days. For the last few months, its been doing daily duty as my phono preamp for my Pioneer PL-71 that I recently restored. The sound is beautiful, except the BT-2 has some room for improvement.

There are two things I'd like to fix, or at least improve:
1. There is a slight noise in the right channel, which only exists on the phono input. It doesn't seem to be related to any controls; I've cleaned them all thoroughly with Deoxit and they are all good solid connections. The pots all work smoothly and make no noises when turned, especially after being cleaned. It is an intermittent sound, dull bursts of white noise that are just noticeable when the preamp is idle and no record is playing. As said, it only happens when the phono input is selected.
2. The phono input has hiss/noise when the volume is cranked past 50% which grows to be fairly loud beyond that. The other inputs are dead silent by comparison.

(First pic has the boards unsoldered already, I got ahead of myself before taking photos!)
The photos show the phono preamp board, removed from its shielded housing. I have a whole new set of Panasonic RC caps on the way to replace all the electrolytics, and I will see how that sounds before I do anything more to this board. I suspect the TL072 opamp is the cause of the hiss at high volume, and possibly the noise I am hearing in the right channel... Anyone have a good idea what to do about that? I'm toying around with ordering a different one (OPA2134 perhaps? ). I have little experience with swapping and playing with these guys, and would love some input on how to improve this stage!

The other board is what I presume the other inputs and the main output stage. It is potted, and a look underneath doesn't really reveal much, other than no more chips are involved. Those 16uf caps gotta get swapped out too....

The power supply is beefy, and again, new caps on the way. Comparing it to other BT-2's, mine looks more crude and homebrew... wonder if it is an older version? All voltages measure in spec, but it'll be peace-of-mind getting all that freshened-up.

I'd love any input on what can be done to help this out, especially those who have experience with this preamp or ones like it!

Thanks!

 

[ryuuoh]

From reading around, yours appears to be an early unit. I wonder why they potted that one circuit?

 

[Organgrinder]

New caps are in, everything except the 16uf pair on the potted board. I'll update tonight on if there are any improvements. Hoping this cures some of the noise, if not, that op-amp is next on the list to swap out. I have a 8 pin socket and OPA2134 in the mail which I'm gonna pop in just to see.

 

[Organgrinder]

Well took longer than expected! Finding precious time to work on things is getting rare, but I've managed to replace all of the electrolytic caps. I'm happy to report, this cleaned up absolutely all of the right channel noises, and seemed to take down the overall hiss I was getting a slight amount.

I did a test on my PL-71 using ELP's Trilogy album, and as a whole, everything sounds more crisp and punchy. I like it! I think there is still room for improvement, but if this was the be-all-end-all, I would not complain.

I do plan on swapping out the TL072 op-amp when the OPA2134 comes in, just to see what difference that makes. Other than that, there isn't much I can do about whatever lay beneath the potted board, so hopefully nothing there is going to be an issue. I'll update when I get the new chip in, but for now I think this is where I will leave it.

 

[Organgrinder]

Popped in the OPA2134 today! Took about 10 minutes to do (I'm getting really fast at tearing apart this preamp!)
The noise is much lower and the sound is very good! I thought it sounded great before, and I don't think there is a massive change in character, but I do feel it was worth doing to bring the noise floor down.

Before and after photos:

 

[ryuuoh]

Nice.

 

[Organgrinder]

Update:

So that dark, windy sounding noise is back in the right channel. I notice this after the preamp is on overnight. It still only occurs in the preamp channel, so i am going to suspect that one of the transistors on the preamp board, or less likely, a resistor. Might have to pull the oscilloscope out for this one, but I'll save that for another day. The noise isn't offensive enough for me to not use the preamp, in fact, I am enjoying The Nice live at the Fillmore East 1969 right now! But every time I lift the arm I hear a light thunderstorm in my right ear!

I'll post more when I have a solution!

 

[Billy Dio]

Did you ever get that noise eliminated? I've looked carefully at PC power supply boards and they like to rubber shrink wrap resistors and exposed metal legs (like the split you did vs. running radial caps), and they also shrink wrap entire ceramic caps. Seems to all be an effort to reduce or eliminate unwanted noise.

BTW, if you are looking to clean up the accuracy of the sound a bit, replace those green (polyester?) film caps with something like metalized polypropylene caps instead. They typically clean up and improve accuracy if in the audio path from my personal experience. I've run Vishay Roderstein which are roughly 1/4" x 1/4" x 1/4" square (very small foot print). On my "low-end" MCS 3233 which received a full recap + the Vishay, with a couple higher-value orange drops in the pre-amp section, while listening to FM through the headphones, you wouldn't believe how detailed it is.

Also run those film caps in other units with great results.

 

[pwauk12]

I know this is an old thread, but how did you get the main potted section out of the case?

I have been trying ever more forceful measures, but do not want to break anything

thanks

 

[Splatter Pak]

The OP is lucky he got an early unit. Mine had every board coated with some kind of epoxy, making any upgrades or repair almost impossible. In time, I let it go.

 

[pwauk12]

yes I am beginning to wonder that myself, though the "solvent" has been mentioned, so I may investigate a little further- of course if I have success I will report back

 

[70smidfiguy]

So here's a BT-2 i'm working on, one channel not working.

The main module with the phono section and the "epoxy" board required brute strength to get it out of its rusty container. (In for a penny, in for a pound)

After inspection, as you can see, some resistors were blown, the million dollar question is, what is the status of the components inside the epoxy?

Hopefully I can find a schematic and try and replace what I can.

I wonder if perhaps some of the Apple Engineers were involved in the design of this thing when they were younger!

 

[HornyTubes]

I know this is an old thread but a few additions for those researching. The silicon potted board (left side and directly connected to the phono inputs) shown above is the phono board, not the preamp board. In the photo in the post directly above the potted phone board is accessed by removing the connections on both sides, the 3 screws holding it to the bottom and then sliding the board out the front side. There are a few beads of silicon in front of the board to stabilize it that will need to be removed. The is no need to open the riveted can the board is in. I started down that path before sliding it out.