700B Customer Build

Hexis22

Hexis22

Chief Journeyman
Joined
Dec 28, 2022
Messages
868
Location
Wisconsin
Started a new customer 700B build.

This ones getting the full White Oak treatment, along with new chassis and faceplate.

This amp was previously "serviced":
- 4 outputs replaced on Right channel
- Black painters tape covering the lower power supply cap
- String used to tie power switch wires together

It appears the amp was dropped on the corner of the transformer resulting in a bent chassis.

After repairing the Left channel input to the control board (open solder joint), the amp successfully played music on both channels.

Initial bench testing measured Bias at 340mV on both channels and output offset was 16mV Left and 30mV Right.

Pictures of the functioning donor:

IMG_6634-4K.jpg


IMG_6642-4K.jpg


IMG_6640-4K.jpg


IMG_6650-4K.jpg


IMG_6651-4K.jpg
 
White Oak components arrived yesterday and made good progress on the control and backplane boards today.

Everything cleaned multiple times with Isopropyl Alcohol and inspected under magnification for dreaded solder balls.

Very humbling reviewing your own work under magnification, but a great way to improve.

850_2909-5K.jpg


850_2912-5K.jpg


850_2924_01-5K.jpg


850_2926-5K.jpg


850_2921_01-X2.jpg
 
I realize it's not likely, but anyone have a meter hold down bracket or power supply cap strap from a scrap 700?

The meter bracket was in place at one time, but missing as shown in picture below.

The power supply caps are held in place by ineffective double sided foam tape. I did not realize the bean counters hit the 700B, this is the first 700 I've worked on without a strap around the caps. Or possibly the previous "service" did away with the original strap.

I may just use an aluminum plate, similar to the 400 design for holding the caps and fabricate a new meter bracket.

Meter%20Bracket-M.jpg


IMG_6649-4K.jpg
 
Hexis22 said:
I realize it's not likely, but anyone have a meter hold down bracket or power supply cap strap from a scrap 700?

The meter bracket was in place at one time, but missing as shown in picture below.

The power supply caps are held in place by ineffective double sided foam tape. I did not realize the bean counters hit the 700B, this is the first 700 I've worked on without a strap around the caps. Or possibly the previous "service" did away with the original strap.

I may just use an aluminum plate, similar to the 400 design for holding the caps and fabricate a new meter bracket.

Meter%20Bracket-M.jpg


IMG_6649-4K.jpg
Click to expand...
I appear to have a meter bracket Eric. I do not have a cap hold down strap though
 
Hexis22 said:
I realize it's not likely, but anyone have a meter hold down bracket or power supply cap strap from a scrap 700?

The meter bracket was in place at one time, but missing as shown in picture below.

The power supply caps are held in place by ineffective double sided foam tape. I did not realize the bean counters hit the 700B, this is the first 700 I've worked on without a strap around the caps. Or possibly the previous "service" did away with the original strap.

I may just use an aluminum plate, similar to the 400 design for holding the caps and fabricate a new meter bracket.

Meter%20Bracket-M.jpg


IMG_6649-4K.jpg
Click to expand...
Hello Eric,
I have WOPL'd (3) 700b's. 2 had the double-sided tape holding the caps in place, and one had the strap. Since you are using a White Oak chassis, and will have the transformer out, it won't take much to fabricate a center stud truncated just below the die cast nest for the caps. I did this as opposed to using a capscrew threaded blind into the die casting. By using a stud, (simple piece of threaded rod) you can thread the die casting (using appropriate tap drill of course) and add a nut for double security on the blind side. Then the bridge rectifier can be secured with another nut and you can fabricate the flat plate that you mentioned as the 400's used. It looks very neat, is virtually foolproof and no danger of threading a capscrew into the transformer :oops:. I was shocked that the bean counters cut corners with the double-sided tape. The more you build, the more you will see this. Not too surprising when you disassemble a 40-year-old amp that has been serviced by some "fly-by-night" shop, but almost unbelievable from the factory.
Sounds like a couple guys have a meter bracket for you, but it wouldn't be to hard to fab one from the aluminum stock you use for the cap hold down plate.
Looking great Eric !!!!
 
Vintage 700b said:
Hello Eric,
I have WOPL'd (3) 700b's. 2 had the double-sided tape holding the caps in place, and one had the strap. Since you are using a White Oak chassis, and will have the transformer out, it won't take much to fabricate a center stud truncated just below the die cast nest for the caps. I did this as opposed to using a capscrew threaded blind into the die casting. By using a stud, (simple piece of threaded rod) you can thread the die casting (using appropriate tap drill of course) and add a nut for double security on the blind side. Then the bridge rectifier can be secured with another nut and you can fabricate the flat plate that you mentioned as the 400's used. It looks very neat, is virtually foolproof and no danger of threading a capscrew into the transformer :oops:. I was shocked that the bean counters cut corners with the double-sided tape. The more you build, the more you will see this. Not too surprising when you disassemble a 40-year-old amp that has been serviced by some "fly-by-night" shop, but almost unbelievable from the factory.
Sounds like a couple guys have a meter bracket for you, but it wouldn't be to hard to fab one from the aluminum stock you use for the cap hold down plate.
Looking great Eric !!!!
Click to expand...
Thanks for the feedback.

Prime path for the cap hold down is a stud from underside of the cradle, through the rectifier, nut to secure the rectifier, and a plate over the caps.

It should be robust, just need to stay clear of all the solid core wire loops.
 
I believe I used a hand reamer and opened up the hole in the bridge rectifier from .225" to .250" (body size for a 1/4-28 stud). Using a .213 (#3) drill then open up the die cast cap cradle and tap for a 1/4 -28. You could use a 1/4 - 20 (#7 Drill) but I used the 1/4 -28 for more TPI in the die casting, which of course has a nut on the blind side for a lock nut on the stud. The limiting factor was the .225" hole in the bridge rectifier, which hand reams easily to .250". That step could be eliminated, and you could use a 10-32 (5/32" tap drill) or 12-28 (3/16" tap drill). The #12 hardware is sometimes hard to find.
 
Last edited:
The 1/4-28 stud is very robust with the nut under the die cast cap cradle, and won't interfere with the solid core wire loops. Using that setup, with Joes rectifier board on the bridge rectifier held in place with another 1/4-28 nut and a flat washer is super neat looking as well.
 
I have hold downs, brackets, anythingbyou need Eric. I made quite a few cap hold downs out of 1" aluminum angle. Measure where they would sit, trace around the cap and make the cut outside and smooth em up . They look pretty cool too.
 
Updated the cap cradle to allow use of stepped washers to isolate the transformer mounting bolts.

This involved filling the back side pockets with JB Weld and drilling out the mounting holes for increased diameter. I also had to increase the diameter on the transformer chassis mounts as well.

New stud installed for mounting the updated rectifier/snubber assembly and cap hold down plate, as well as new rubber grommets.

IMG_6726-4K.jpg


IMG_6734-4K.jpg
 
You must log in or register to reply here.
Back
Top