Dolby A301A repair

[JohnRoberts]

Thanks!

I experimented with selectively taking out modules. It appears one amplifier module is to blame, the unit works fine with it not fitted. It uses only easily attainable transistors, so this will probably be an easy fix.

That PS is a bit of a trapeze act with transformer windings relative to each other so loading down the main could disrupt the other voltages.

It would be an educational discrete design 101 exercise to figure out what was going on. Perhaps some current limiting. But the easy fix is also a good thing.

JR

 

[living sounds]

I replaced all the transistors on the defective modules with modern substitutes:

BC108C, BC109C  ->  BC550

2N3702 -> BC557B (had to bend legs and insert it backwards, but it's only one)

Works, and the noisefloor is noticeably lower than on the other module.

But one problem remains: The inputs of both channels get summed for some reason. No idea if this could be wired for M-S or someone tampered with it. Does anybody have a wiring diagram for A301? Unfortunately the pdf I was able to grab off the net only has a few schematics in it.

Is Amplifier -> Control  -> Compressor -> Compressor (followed by the same for channel 2) even the correct order?

Thanks!

 

[living sounds]

The cards were inserted in the correct order, but this is what the internal wiring looks like:

The record/play switches are rendered useless.

Looks like I really need that wiring diagram... it would be great if someone could upload it.

Thanks!

 

[barbaroja]

Hi. Did you fix it?

 

[soapy_goat]

Looks like I really need that wiring diagram... it would be great if someone could upload it.

6 years later... did you figure it out? I hope this helps future spelunkers.

 

[living sounds]

Thanks, I still need to fix it. There was a member here who had made extender cards, but he stopped replying at some point.

 

[Tubetec]

I didnt know about this project ,
I took the opp to snatch up a couple of the later 361's a few years back ,
even with NR disengaged these things add a hell of a polish to any sound that runs through them ,gain is variable on ten turn pots behind the front panel ,so its easy to establish a reference level
I think its the Cat 22 that turns the Dolby unit into a 4 band single ended comp/expand noise reduction system intended for film soundtrack , Ive a feeling it would work rather well to a pristine digital tape path .

 

[soapy_goat]

Thanks, I still need to fix it. There was a member here who had made extender cards, but he stopped replying at some point.

you are probably thinking of Tubular Audio? I think he is very busy. Here is the schematic for the alignment card if you want to cut your own.

 

[Tubetec]

I see a lot of epoxy blob tantalums in my Dolby units to save space ,a cold check for shorts is a good idea ,but no guarantee they dont breakdown after voltage is applied and you'll smell the acrid toxic stench as the series resistance goes up in smoke .

 

[soapy_goat]

I see a lot of epoxy blob tantalums in my Dolby units to save space

yes the 36X series cards are tantalum cities!