HH Scott 340 Receiver Rebuild

SCompRacer

A project for a good friend. HH Scott 340 receiver. I agreed to refurb it before realizing how much effort would be required. But it's all good otherwise I'd have nothing to do. I've been bothering Ken about some details and mhardy might be next.

The multi-value can capacitors will be replaced first. There can be up to four values of same or different value capacitor in each can. Can is marked for values with symbols that match symbols on bottom wafer.

There are multiple ways to replace these can style electrolytic capacitors. CE Manufacturing® builds new multi-section electrolytic "can" capacitors on the original vintage Mallory machinery. They have a video page with three videos showing the process. They are priced around $40 - $50 each but don't offer custom builds that I am aware of. The Scott 340 needs four cans. One 75uf @ 450v x 4, one 20uf @ 450v x 2 with 25uf @ 25v x 2, one 30uf @ 500v x 2 and one 20uf @ 450v x 4.

https://www.cemfg.com/can-capacitors

Hayseed Hamfest is another source for can capacitors. They stuff new cans with modern caps. Brands used are Nichicon, United Chemi-Con, Panasonic, Illinois Capacitor, Rubycon, Roderstein, etc.. They offer can kits for some gear like Fisher, HH Scott, McIntosh. They also offer a custom quote form for values not listed. The kit for a HH Scott 299D is close to what I need for $167. They are in the UK so unsure of what shipping might cost.

https://hayseedhamfest.com/

Another way, if you have the room, is install modern caps on terminal strips in the chassis. While modern caps are smaller, there is not enough space for the fourteen values this 340 needs, so I chose the disassemble, clean and restuff the can method. It's labor intensive but lowest cost. I chose Nichicon UCY 12,000 hour rated caps. IIRC cost was 50 bucks for all values needed.

I file the rolled edge off can and careful remove the wafers. After that, some large screws, some heat and brute force to pull it out. I did it in garage as it stunk when opened. Some folks cut the can. Some use side cutters and roll the edge back. The procedure I chose, the can will slide onto the round wafers. There is a stack of three wafers. By removing one layer, I can reroll the edge over the ring with tabs. You need those tabs to attach it to wafer riveted to chassis.

I'll do the 75uf/75uf/75uf/75uf @ 450v/450v/450v/450v common positive, yes, positive ground, can first. Once the new caps arrive, I'll see if I can fit the rest in the chassis. The other three cans are common negative ground.

Adapt a cap makes a small circuit board that fits modern caps and rivets/nut-bolts to chassis. You have to figure out how to attach the can to cover them. A guy on ebay supplies a similar can kit with round wafer that can fits on. I don't want any exposed caps above the chassis and want to keep the original appearance.

xschop

Will be following this thread. Love HH Scott receivers.

pitdogg2

Wow that's ambitious!

mhardy6647

It should be worth it.
Plus, those Scott receivers were (are) absolutely gorgeous.

SCompRacer

The busy work. The insides of switches can look like this selector. While some say the corrosion is conductive, I find not cleaning them can result in scratchy sound.

I'm using DeoxIT with makeup Q tips as they are more tightly wound than regular cotton swabs. Saturating the wafers with DeoxIT can cause issues without completely removing corrosion so I soak the Q tip with DeoxIT and lightly rub the metal without bending the contacts. I fold paper wipes and saturate to clean between contacts. I've read some guys use a dollar bill.

Still have a little more to do. The forward section of selector is much harder to get at.

SeleniumFalcon

Those switches are probably silver plated contacts and, fortunately, silver oxide is conductive. It looks grim but not so bad electrically. The Revox A77 has a sliding switch bar that makes dozens of contacts when the speed is changed or the deck is turned on and off. It uses silver contacts and it is hard to disassemble, clean and put back together. But satisfying when done.

xschop

Looking at your pics, there's no brittle PCBs to worry about

SCompRacer

Parts starting to arrive! My eyes are crossed from all the researching about recapping the 340.

I stuffed and reassembled the 75uf @ 75v x 4 common positive can. I used Nichicon UCY 82uF 200v. It was the closest value to 75uf I could find. Hayseed Hamfest replaces this can with 100uf 100v. Negative leads are attached to the four terminals. I wound the leads around hole in terminal and tinned them. The positive leads connect to eyelet.

Cans are typically common negative ground, which means the can is tied to the negative terminal(s) of capacitor(s) in can. I've seen some gear where the cans are attached directly to chassis. On the Scott 340, the cans are attached to phenolic insulating wafers. A ground wire is then soldered to the can ground lugs.

The 18 Ohm one-watt 10% carbon resistors on the 75 x 4 were over 10% out of spec so I replaced them. One connects from the Selenium rectifier to first 75 Ohm, then three daisy chain along the remaining three 75 Ohm tabs.

I replaced the Callins 10uf 25v bypass with a Sprague Atom 10µF / 150V. I learned you can't always find exact uf or voltage replacements for vintage gear. I learned it's OK to use slightly higher uf and voltage caps.

The Scott 340 has two Callins electrolytic caps. The other, a 25uf 25v, was in tuner section buried under some Micas. I could see a crack in it. While researching I found a video on youtube showing one that cracked in pieces. I removed it and it was open inside. I read one should get rid of the Callins as they will crack and leak. I also learned Fred Callins co-designed the first viable miniature electrolytic capacitors for use in palm sized transistor radios.

For the 30uf @ 500v x 2 can next to it, I just did a terminal strip with two 33uf 500v over it. Once I turn it right side up, they will be under it.

I removed the third can and will disassemble, clean and stuff it. It's a 20uf @ 450v x 4. The fourth can I'll do a terminal strip in chassis.

Yes, I number wires so I don't get lost in reassembly.

And yes, I caught and rectified not soldering the resistor to the rectifier terminal and snipped the ground.

pitdogg2

Impressive!

xschop

Are you renewing the wiring as you go or is the insulation still good after all these years?

SCompRacer

Thanks Ivan!

xschop wrote: »

Are you renewing the wiring as you go or is the insulation still good after all these years?

So far, I have not encountered any burnt or brittle wiring so I'm not replacing wiring. It's like 22-gauge solid tinned wire. I've had to snip ends off some desoldered wiring and remove a little insulation and it's still pliable, no insulation cracking off. I'm able to loop wire around terminals and squeeze tight with needle nose before soldering.

I watched a video of a Scott 399 restoration, and that unit had some shorting that damaged some wiring.

SCompRacer

All done with the four old electrolytic multivalve cans. Stuffed the first, installed two 33uf @ 500v under chassis for can two (30uf 500v original value). Stuffed can three with four 22uf @ 450v caps (20uf @ 450v original value). Fourth can under chassis with two 22uf 450v and two 27uf 160v (original value 20uf @ 450 and 25uf @ 25v). I had laid out all fourteen caps under there and I felt it was a little too tight to do them all in chassis.

I had two used, known good Sovtek 5AR4/GZ34. Stuck them in (no other tubes) and powered on through dim bulb tester with 75 watt bulb (I don't have a variac). No smoke, nothing getting hot.

Tested voltage at the 33uf 500v cap which gets fed from the two GZ34/5AR4, 357.9vdc. Since all looked well I removed the dim bulb tester and plugged it direct into wall and got 505.9vdc. Schematic shows 470v there but that's with all tubes in it. Mains voltage is higher now and can affect voltages and ability to adjust tube bias. I need to do more research on that.

Next up, a trip to Aaron's and test the tubes to see what's good and what's not. One 7951 is visibly bad, white inside and glass is loose. We have a mix of HH Scott (which could be original), Telefunken, Sylvania, RCA and some rubbed clean.

I'm going to try operation with all tubes before recapping the rest of unit. The carbon resistors on can three and four measured good so I reused them.



Voltage with dim bulb tester (mains voltage goes through bulb then to gear plugged in).



Voltage with dim bulb tester removed, plugged direct into wall. You can see I wrote symbols and values on chassis so I wouldn't screw up with the 20uf 450v and 25uf 25v values.



Multivalue cans have symbols next to values on outside of can that correspond with symbols on bottom of can. If you are stuffing one with different values, you have to ensure the value matches the symbol.

agfrost

Very cool, Rich.

tonyb

Very nice, and nice of you Rich. Looking at that puppy naked , lots of work. Must be a real good friend.. Looking forward to thoughts after all is done on the sound as I'm a big Scott fan myself.

SCompRacer

Greetings Tony! Maybe it's a test of friendship...

SCompRacer

The volume pot was dripping DeoxIT. It's a great product but I think some folks get carried away with it. The pot felt very rough and when I disconnected wires and Ohm'd it with an analog meter, needle movement was not smooth as pot was turned.

Tough spot but you can remove a couple screws each side and tip front panel forward. Too many wires to disconnect and remove that front plate.

I watched a video where a guy successfully resorted a soaked pot. It was suggested to soak the wafer in rubbing alcohol to dry it out. Clean the element surface with straight DeoxIT D100L, dry. I tapped on the rivets to tighten connections without breaking the wafer. The center contact ring was loose on both wafers.

Fader grease is applied sparingly on metal-to-metal surfaces and on raised ring where it contacts back of plastic wiper. Feels like a new pot and both measure well.

It's busy with stuff connected to it. You want to get it back together right. A couple of resistors connect from below.

EndersShadow

Your wizardry knows no bounds Rich!