BA-5000 Rework

[HDJR808]

Moving along...

The rectifiers are in:



I'm sure I checked at least 10 times to make sure that all of the polarities are correct.

Next came the smoothing caps on the rectifiers, some nice blue C0G ceramics:



Followed by some initial wiring:



I had many concerns about heat damage and corrosion with the original wiring in this amp, so I have continued with the
22759 mil-spec etfe as hookup wire, as well as some 14awg high temp braided wire for the link from the filter caps to the negative speaker terminal.

As I brought up in a separate thread, it has been somewhat of a headache deciphering the wire gauges used here. Almost all of the wiring used, unless I am being completely oblivious, has no indication of AWG. There are numbers that correspond to US standards for appliance wire, but nothing seems to correlate to wire size. The solution has been to use the strand count and individual strand diameters to cross reference to an appropriate AWG.

All for now..

 

[HDJR808]

Before doing much more wiring, I wanted to get the speaker terminals and boucherot/zoebel assemblies installed.

Mechanically, there were some speed bumps due to the fact that the WBT binding posts use some uncommon thread sizes. The poles themselves mount with an M9- .75 nut, which I did have the originals of, but the original nuts are particularly deep and if used would interfere with the holes tapped for set screws that I planned on using. Turns out that the input level pots use the same threads, so those narrow jamb nuts got pressed into service and I ordered some new M9 nuts off the bay to use for the pots.

The set screws were even more of a challenge, being an M4- .5 thread, which appears to be virtually non-existent. A vendor at work was heading to Germany and was kind enough to check around there. Turns out they are a non-standard item there as well, but he was able to have the machinist at the factory there cut several of them for me. The original WBT screws are available for $2-3 apiece, but for my approach I needed a screw with a head not a regular grub screw.

For the boucherot assembly I used a polypropylene cap and some interesting Ohmite TA series non-inductive thick film reistors:



Here with leads and solder tags attached:



And mounted on the binding posts and the studs that will park the unused leads from the transformers:



And finally, the panels installed and one of the transformers back in with some temporary wire placement:

 

[Hipocrates]

Inspiring work.

 

[HDJR808]

Inspiring work.

Thanks. Got a little more done today...

Second output transformer in:



This thing's starting to get heavy!

With the covers attached:

Even being able to choose between the unit being rebuilt and the parts unit, the covers are pretty mangy- dings, scratches, and corrosion. But, this amp was never going to get any points for aesthetics so that's just the way it goes...

Binding posts are complete. The WBT0700 posts are incredibly overbuilt, which seems to be appropriate for the BA5K:



All for now-

 

[The Fuxtor]

Inspiring work.....

 

[HDJR808]

A few more items addressed yesterday, starting with the relay for the soft-start circuit:



The 3.9 ohm resistors mounted under the relay socket were both spot-on, so they will stay. The coil on the relay itself reads to spec, but leaving it out for now and will test its operation later.

The mains terminal block is in, with some of the connections temporarily made, mostly as a placeholder. I figure that some of this will need to get moved around later for access when the heatsink is installed, but for now I wanted to go ahead and keep the wiring tidy and out of harms way.



And finally the mono/2-channel switch with new wiring, new connector housing and new terminals:





Hopefully today I can get on to the input board and signal leads...

 

[HDJR808]

Didn't get too far last weekend due to a little cross-threading mishap...

The input board was cleaned and sporting new coax and terminals-



But unfortunately, when installing the board one of the screws twisted off in the mounting post. Definitely operator error, but it certainly seemed to shear off very easily, especially threading into aluminum.

Took it to work to see if one of the guys could extract it easier and cleaner than I could. Better than that, he machined me an entirely new post:



The post on the left is the new one, on the right is the original which he cut off and then machined a spacer in case I wanted to use that combo. Sure wish I had the skills and the tools to do work like this...

Next came the 2519 power supply board. The refreshed board:



and the board installed. In the background is the input board and shield:



The 2521 power supply and ground harness went in next, and some of the wiring starts to fill in...





At this point I will probably have to get a handle on matching the output transistors so that the heatsink can be installed along with the driver boards. I will save the buffer board and meters for last, so pretty much any other wiring I can do now is just going to be dangling.

 

[JOMARK911]

Excellent work . I do believe you're gonna love this amp , after restoration. You are very thorough on your work. Congrats.

 

[Hyperion]

The neatness and care shown here is inspirational.

 

[ghamilton]

 

[Overundr1]

Keep the photo's coming sir, even us grizzly old techs learn new tricks at every update.
-Lee

 

[Cosmin2000]

Congrats of your beautiful work on this Japanese masterpiece! I am watching you.

 

[a.wayne]

Any updates

 

[HDJR808]

even us grizzly old techs learn new tricks at every update.

Not having any training in electronics, I certainly wouldn't expect that I would be the source of new tricks for real techs, but very much appreciate the thought and everyone's posts of encouragement. What understanding I do have of this kind of work is almost entirely due to the knowledge shared on this forum. It is this knowledge that has allowed me to navigate through several other refurbishment projects and now the BA5K. In particular the previous work of Echowars and ScottRT has been extremely helpful on this one, and also, in addition to taking a ton of pictures, I am fortunate that I do have a working example I can use as a reference. But I do find myself wishing that I had the time to learn more of the electronics theory that underlies all of this, especially at the point where I am now where I need to match all the output transistors. Which leads me to today's update...

Following the the diagram from Trnsfmr in this thread: http://audiokarma.org/forums/index.php?threads/output-transistor-matching-in-powerful-amps.730458/

I am putting together a rig to match the outputs. Using the damaged heat sink from the parts unit, today's project will hopefully be to cobble together a tidy setup that I can use in the future. Hopefully more photos will follow later...

Again, many thanks for the responses and the years of shared knowledge.

 

[HDJR808]

Made a little dent in matching the outputs today, resulting in some numbers that I can crunch, some head scratching, and a request for advice...

As noted before, my plan was to build a test rig based on the diagram posted by Trnsfmr. One of the spare TO-3 sockets and wiring on the heat sink:


And a piece perf board for the 10k pot and 10ohm resistor:



and mounted with dedicated leads for power supply and metering:



The head scratching began when I did a dry run with an MJ21193, figuring I didn't want to subject any precious 2sa908's to experimentation. As per Trnsfmr's diagram, and some quick math, I should have been seeing 1.5A of collector current at the stated 15V across the 10ohm resistor. Well, I'm guessing I have a wire crossed somewhere, since my little rig would start to pull too many amps from the power supply and voltage across the resistor would only make it to 8 or 9 volts before the power supply would start to bog down. So, it seemed that the better part of valor was to get a direct reading on the collector current and I just patched in an ammeter to set the current. I also chickened out a little bit and decided that I would take my measurements at 1A of collector current instead of 1.5- might not be optimum, but hopefully it will give a valid comparison and I won't take the chance of frying anything.

I started with the PNP devices, of which I have a mixture of 8 2sa908's and 8 2sa909's that were in various combinations on the parts unit and the project unit. This brings me to a request for advice. The data sheet indicates that aside from the 909 being a higher voltage device, the specs are the same for the 907, 908, and 909. Any opinions on whether it would be safe to mix and match the 908's and 909's? If not mixed on the same channel, maybe 908's on one side and 909's on the other? I understand that common wisdom here is that it is fine to just replace all of the Sanken outputs with 1193's and 94's, but since I theoretically have enough of the Sankens to get good matching numbers I am hoping that I can consider the 908 and 909 to be interchangeable.

As far as readings, the 908's ranged from .6765Vbe to .7873Vbe, and the 909's ranged from .6498 to .7567Vbe. A quick check of hfe on my meter showed the 908's (all "o" gain range) ranging from 30-50, and the 909's (6 "o" and 2 "r") ranging from 12 to 42, with 2 suspect devices with readings of 4 and 6. Now comes the math for the percentage of Vbe variation, and I will also run them all on the curve tracer to see how they compare.

 

[JOMARK911]

Don't MIX and match 908 and 909. If you try you'll find 909's,1586's on the net. Not counterfeits.

 

[HDJR808]

Don't MIX and match 908 and 909

Thanks very much for the feedback. Just to help my understanding though, I'm curious what would be the factors that would make it not a good idea since the specs are identical other than maximum voltage?

 

[JOMARK911]

Other than the max voltage , the hfe. It might be very difficult to match hfe. Ideally hfe should be within 2 units.

 

[Overundr1]

Mixing the two within one channel will create potential load imbalance across them, mixing them channel to channel could affect left-right balance as well as potentially marginally upsetting the power supply sag under high draw conditions. I doubt that factory workers gain matched the outputs during initial assembly years ago however today we have the technology, the time and the ability to get it just right
As a side note, and I think I mentioned this a while back in another thread I ask during restoration if the client would prefer gain matched new outputs, saving and returning the originals or clean and re-install the factory pieces. Most prefer new ones for longevity and save the n.o.s. ones in case the owner decides to sell the unit later on. Often those originals have hfe value swings of 10% or more if the unit was run hard.
-Lee

 

[JOMARK911]

Oh they were matching them all right. Service manual states that a specific batch of transistors be used, o or y. Also it states that if one found to be at fault , all be replaced. Same batch.