Super Stock Eico HF-85

[Sam Cogley]

My HF-85 has been stuck in rebuild limbo while I work on some other projects. Started thinking about it again tonight, and managed to spend a whole lot more time than I expected re-writing the schematic based on rebuild/upgrade ideas I've found in various threads. A lot of these ideas came from dcgillespie, especially in these threads:
http://www.audiokarma.org/forums/showthread.php?t=434119
http://www.audiokarma.org/forums/showthread.php?t=368270
http://www.audiokarma.org/forums/showthread.php?t=309259

The fact that Eico included the internal diagrams for those stupid PEC chips on the HF-81 schematic allowed me to add that info to the HF-85.

I've considered using jaymanaa's DC heater modification, but will not be doing so just yet. However, I am going to be changing the pilot light to an LED pair to reduce current draw on the transformer. This is not yet reflected in the schematic.

This schematic is for an HF-85 (or HF-81) with a modification I came up with a few months back - using parts from two selector switches to build one with the pair of seven-input wafers. As designed, the HF-85 has 6.5 inputs.

The tape head input is no longer a tape head input, so I repurposed that switch to play with variable phono cartridge loading. Right now the schematic shows 47k and 100k as the options, I'll might change that to 47k and 68k. I also might try to come up with some sort of socket arrangement so that various resistances can be tested with minimal fuss and no soldering.

One schematic has one phono input and six line-level inputs. The second version wires the tape head input into the phono input, allowing for two turntables and five line-level sources. I haven't decided which way I'm going to build it, yet, but I'm rather leaning towards the two-phono version. Five line-level inputs seems sufficient.

I'm leaning towards using close-tolerance metal film resistors throughout the unit. That might be overkill, I've also thought about using carbon films everywhere but the power supply and phono section.

The loudness gain correction has been implemented.

It's not yet reflected in the schematic, but the .03uF cap on the power input will be changed to a .1uF modern X/Y safety cap.

Per one of the above-mentioned threads, the 10ohm resistor stage in the HV supply has been split into two 4.7ohm resistors with an extra 40uF cap. I also upgraded the first cap from 20uF to 40uF. Considering the ridiculously loose tolerances of those old can caps, I don't think this will be a problem. However, I might change the first cap back to 20uF if there is a chance the power supply as designed will over-stress the transformer.

From reading dcgillespie's restoration and upgrade notes, I think the .25uF output coupling cap can be eliminated with the addition of the .22uF cap before the tone stage. The schematic will be updated pending verification.

If you see any mistakes or have any suggestions, please feel free to comment!

Edit: Photobucket reduced the resolution on me, I'm uploading them as attachments.

Edit 2: That's even worse! Back to the Photobucket links, if anyone can't read these and wants to have a look, send me an email through the AK system.

 

[Sam Cogley]

I wonder if the 500V, 40/20/20/20uF can cap made by JJ would provide sufficient filtration? Any thoughts?

 

[Sam Cogley]

Schematics updated to remove the final .25uF coupling cap, changed phono loading to 47k/68k, and ground cap upgraded to modern .1uF X/Y. A fuse will also be added to the design.

 

[jessdw]

It's not yet reflected in the schematic, but the .03uF cap on the power input will be changed to a .1uF modern X/Y safety cap.

Thanks for sharing your schematics! They are beautiful. I'm doing a few things to my HF-85 right now as well. One thing I'm curious about is the value of the safety/death cap. Shouldn't it be 0.01uf? I only ask because that was the advice someone gave me when I was working on my AF-4. Also someone had recapped my HF-85 before I bought it and used a 0.1uf Orange Drop there and there is definitely AC on the chassis.

 

[jaymanaa]

Sam, you should see the selector on the 81 I just finished. It is nearly nekkid, just a few wires on the bottom. I'm only using the two center discs, one side for inputs to amp, the other side for grounding unused inputs. I have two aux inputs, one stereo tuner, and one phono. Definitely rebuild the pecs with top quality components. IMO the active tone circuits are what makes the 81 / 85 as good as they are. After a PECS re-do I think they are the best tone circuits I've heard. I didn't use the power switch on the treble control of the 81 due to the fact that it's nearly impossible to hit the same exact spot every time you power up. I didn't like the AC up in front of the amp anyhow, so I mounted a nice little rocker back where the tape RCAs were. My 85 is in my "never sell" stack.

 

[cubby01]

Out of curiosity what would it cost in parts to build a DIY kit like this today? Anyone care to through a swag parts cost out there for a basic (ie not boutique) BOM?

 

[jaymanaa]

I wonder if the 500V, 40/20/20/20uF can cap made by JJ would provide sufficient filtration? Any thoughts?

Marginal at best IMO, any chance of replacing one or both of those dropping Rs (4.7K) in the filter string with some 1hy chokes?:scratch2: Just thinking out loud.

 

[jaymanaa]

Maybe the 155H http://www.mouser.com/ds/2/177/5c0032-57977.pdf

 

[Sam Cogley]

Marginal at best IMO, any chance of replacing one or both of those dropping Rs (4.7K) in the filter string with some 1hy chokes?:scratch2: Just thinking out loud.

If I really was the Doctor, I could make the HF-85 bigger on the inside. As it is, that little thing is jammed full of bits.

I'll probably just go with 40/40/40/40 or 20/40/40/40. I can whip something up with some Nichicons that should fit.

 

[jaymanaa]

If I really was the Doctor, I could make the HF-85 bigger on the inside. As it is, that little thing is jammed full of bits.

I'll probably just go with 40/40/40/40 or 20/40/40/40. I can whip something up with some Nichicons that should fit.

Eico built the most shallow chassis didn't they. This 81 I just finished is 10 pounds of stuff in a 5 pound can.:drool:

 

[s-petersen]

I think the AF4 is even more crowded

 

[ferninando]

Jay, I think we'd all like to see some pics of your 85. I know I would. Do you have any?

 

[jaymanaa]

Jay, I think we'd all like to see some pics of your 85. I know I would. Do you have any?

I will snap a couple.

 

[jaymanaa]

Here she is, with a picture of the builder (in his younger days) setting on top. I undressed her for the perves among us.

(I still need to rebuild the PECs)

 

[Sam Cogley]

Modded the 2 phono schematic a little bit.

 

[jaymanaa]

This is the 81 that got a little cramped. Too many mods to remember.

 

[jaymanaa]

Hey Sam, what if you used a FET or something and make one of the phono stages MC. That would be slick.

 

[Sam Cogley]

That would be pretty cool, but there is only one phono stage. I just has two different inputs. Basically a fancier version of hooking two turntables to one input with a pair of RCA Y cables.

Now that I look at the switch and schematic again, I've figured out how to completely separate the two phono inputs, except for the cartridge loading. As a solution, I think one will be permanently set at 47k and the other will have adjustable loading. Should I go with 47k and 68k there? Or should I make it 68k and something else? I don't have any Grado carts or I'd set it at 18k.

If I felt like futzing around enough to make a new faceplate, it might be fun to use a stepped attenuator with precision resistors for variable cartridge loading in the hole currently used by the somewhat Rube Goldberg late 50s stereo/mono switch (Eico was far from the only offender on that front) and move the stereo/mono switch to the tape EQ position. As I don't, and I somewhat appreciate the versatility of ol' Rube's contribution, I'm going to leave it alone.

A note for all those playing along at home: you'll never see the wafer sides in the lower positions on the schematic look like that unless you stole Superman's eyes. They were consistent in depicting all of the switch wafers as they're viewed from the front of the preamp - which means the back side of all four wafers is mirror-imaged. They also didn't depict it in the installed orientation, but turned up on its side. It's not as bad as that insane Fisher R-20 schematic, but it's close.

 

[s-petersen]

I think people built them using the pictorial,and not the schematic,as an AF4 I repaired had all of the terminals off by 1 so it worked strangely. Paint by numbers

 

[Sam Cogley]

Two Phono version now updated with two completely separate RIAA circuits! If anyone wants to follow Jay's suggestion and make one of the phono circuits MC-capable, please share your ideas.

Also, it's worth noting that you don't need to use my kludged-up 7-input selector switch to implement the "Tape Head" and "Mic" input modifications. I chose "Tape Head" to convert to line-level since it already says "Tape," even if it's no longer a tape head input. Good spot to plug in the Tandberg.

The only thing making it difficult to convert all three low-level inputs to independent RIAA equalization is the lack of a third non-contact tie point on the switch wafer.