Help identifying components in HF-81 front end

mike_p

Downhome Upstate
Am I correct that the 2-100K resistors, each in parallel with a 47 pf cap in the HF-81 front end, are separate from the phono stage? If I'm swapping-in a 2P6T rotary switch for the nightmare of an original 4 wafer selector switch, and using a stereo phono input and 5 line level (Aux) inputs, can I ditch those components?

I was really confused by the depiction of the amp's front end and first (phono) stage in the Sam's Photofact schematic, which seems to transpose one of the 2.2 Meg resistors (part of the RIAA network) with one of the 100K resistors (apparently not part of the RIAA network), in distinction to how that is shown in the factory schematic included in the owner's assembly manual.

What was the function of the 100k/47pf pairs? EQ for the other low-level inputs?
 
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You are correct about Sams' HF-81 schematic transposing the phono EQ resistors at the Channel 2 area. Sams often made "clerical errors" so our vigilance is often needed. Channel 1 shows the original phono EQ : 2.2 Meg//2700 pf + 90K//850 pf.

My EQ deciphering indicates Eico chose a bit too much high freq. cutoff with the 90K//850 pf. More precise RIAA occurs with 91K//825pf. In my upgraded HF-81, I chose 2.4 Meg//2730pf + 91K//815pf for the Phono EQ switch position. I set my HF-81 up to accommodate three phono inputs. The Mic and Tape "Low Level" inputs were "converted" to accurate phono inputs. In fact, the Tape (Tape Head) input in my unit also has two switched high freq EQ "choices" via the Tape EQ slide-switch. The "3 3/4" position now provides the NAB LP/Columbia LP EQ, while the "7 1/2" position provides proper AES EQ. Conversion using the stock 4 wafer Function Selector switch and Tape EQ slide switch required significant "studying" of the available schematics alongside the actual layout in my amp. The results were very worthy and I also incorporated my upgrades in my beloved HF-85.

Now, concerning the Mic input, if you check the schemes again, you will notice the 100K//47pf are actually Eico's chosen feedback EQ network for the Mic input. Also note the .025 uf (25000 mmf) ceramic disc cap from the Mic input jacks to the first stage. That cap resides on the wafer for each channel's EQ circuitry. Yes, the schematic can easily mislead. That is why studying the actual layout and switch progression is required. If you were using the original selector switch, bypassing that input cap, installing a 50K input resistor at the Mic jacks, plus converting the 100K//47pf to a more proper phono EQ network, could change the Mic input into another phono input.

The Tape Head EQ arrangement is even more involving. The original feedback EQ network is 600 pf in series with either 68K or 330K, the chosen resistor switched in via the Tape EQ slide switch. Conversion to phono EQ does require some wiring plans and knowledge of the mathematics concerning phono EQ networks. Using tried and true circuitry "can work," but, testing and truly verifying the results will require appropriate test equipment.

Eico's HF-81 and HF-85 are great platforms for "retro mod" restorations. In my HF-85 preamp, I alternate the use of PIO and polypropylene caps in every other stage, thus attempting to not duplicate/amplify any distortion products inherent with any type of cap. With my HF-81, I chose vintage, mil-spec, paper and PIO in alternate stages, to retain the classic's heritage. Other than the HF-81 having a Balance control and stereo Level control, as well as lacking the Loudness switch control, while the HF-85 uses dual level controls (concentric and clutch capable) and that (almost useless original) Loudness switch, the 81 and 85 preamp stages are essentially almost identical.
 
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