HF-85 Recap question

Hi folks,

I recently obtained an Eico HF-85 and plan on re-capping with k40y-9's to replace the electrolytics, but I wondered what you folks might recommend to replace all of the little ceramic disc caps that are in here. Also, I went through and checked the resistors and they are all within spec, but would you replace them anyway, or just leave them alone?

The guy who gave this piece to me brought it up on a variac before he brought it over. I hooked it up as a phono stage and found it's top end to be very nice and warm, but a little shy in the bass. My NAD PP-! is the exact opposite. I'm hoping that a cap job might flesh out a little more bass, as well as cure a hum that developed after running the unit for awhile.

Any suggestions are welcome and thanks in advance!!

any polyprop cap will do just fine. If Rs are in spec leave em. Im working on a friends 85 but just the hi level output stages, caps but no Rs, and power supply caps. Has a bad transformer so have to replace it.

The phono preamps for the HF85 (and HF-81 as well) are somewhat unique, in that they use active NFB EQ to obtain the RIAA response curve, but ONLY around the second stage of the preamp design. This has design advantages and disadvantages, but suffice to say that because of this feature, and because the filaments operate on AC in this unit, it makes the condition of the cathode bypass caps and the interstage coupling cap in the preamp very important. If the cathode coupling caps go south (open), gain drops which destroys the low end response, and hum increases because the cathodes are no longer held at AC ground level. If the interstage coupling cap breaks down, the opposite happens because the gain of the second stage is increased, which again affects EQ.

In the line stage, there are two mistakes that should be corrected as a matter of good design. The tape out signal (via .02 mfd caps) is taken directly from the plate of the last line stage which is inside the NFB loop for this section. As a result, because of the smallish .1 mfd cap used to feed the FB loop and the following tone stage, the signal at the output of the tape out jacks is actually bass boosted (besides being variable). Solution? move the .02 tape out cap to source it's signal AFTER the .1 mfd cap feeding the FB loop and tone stage. Also, there should be inserted a .22 mfd cap between the same .1 mfd cap mentioned above, and the input to the tone stage. This is to prevent dc at the cathode of the first line stage amp from appearing on the tone controls, which can make them noisy. Therefore, after the .1 coupling cap from the final line stage amp, it should go to the .02 tape out cap, the 33K FB resistor, and the new .22 mfd feeding the tone amp. This will correct all the major issues with the line stage, except for the fact that it has (IMO) an excessive amount of gain to it. But that is a whole different issue.

Finally, the power supply B+ filtering is very marginal as well, with only two filter cap sections in place before feeding the high level stages. As a result, 120 hum is evident with sensitive systems, even with good filter caps in place. The solution is to replace the 10K B+ dropping resistor with two 4.7K resistors, with an additional 40 mfd cap installed at their junction. This will remove all B+ supply hum.

Good luck with it!

Dave

Ok, so, it looks like I'm going to have to print off the responses here. I've ordered some caps from Russia, so I'll come back to this when they arrive. Thanks very much for the info!!

Ok, so I have followed dcgillespie's advice exactly as described. I have not swapped any of the ceramics yet, but I have moved the ones suggested. I have a weird problem though. When I connect the tape outputs to my amps, I get a horrible hum and this is without even turning the power on to the 85. With the pre-outs connected, my GFI pops when I plug it into the ac outlet? ity: What could be going on? It worked and played before I changed the electrolytics, although it did develop a hum before I took it out of the chain. It was not as bad as it is now though. I removed the extra cap at the junction of the 2 new 4.7k resistors and no difference. Now I'm scratching my head. Any help would be appreciated!

This may be a crazy question, i may be reading post #1 incorrectly. Did you replace the electrolytics with the Russian caps ???

Yes, I replaced them with k40y-9's in original values, but higher voltage rating. I think I found the cause of the latest hum issue. There was a blob of solder that I missed when I was cleaning up. It was shorting one of the caps on the pre-outs to the chassis. I fixed that and listened to several records with no trouble.

I have one other issue. As I said, everything is fine when listening through the phono stage, but when I switch to another source, I lose the left channel. I moved he tubes around with no change. I also made sure that it was not an input issue. The right channel is fine, but the left is barely audible. Any ideas?

bad switch contacts? only phono goes thru ph stage. all else goes thru switching system to the output stages, the tubes. so if all is well thru phono, only switches are between in and output tubes. yu follow me in this?

Actually, the selector switch was the next thing I was going to check. I just checked all of the resistors on the tubes again and they are fine. Going to check the switch now. Thanks!

Well, I've gone back through and checked all of the resistors, cleaned the entire thing again with contact cleaner, especially the switch, tightened the pins a little bit and cleaned them. I fired it up and checked the voltages against the specs I have for it and they are a little high across the board, but I get 122-125VAC at the outlet. What am I missing?

I should say that both channels play when the mode switch is in mono. Check amp mode 1 sounds fine. Check amp mode 2 is barely a whisper but does play something. Normal plays channel 1 and reverse plays channel 2. Tuner/aux mode and mono play through both outputs. I thought maybe something might be askew in the tone circuit, but as I said earlier, phono is perfectly fine in normal stereo mode and it goes through the tone too, so...:scratch2:

I wondered if one of the little ceramics on the switch could have somthing to do with this problem? I hecked the one resistor I saw in there and it's fine. Although difficult to see, it looks like all the contacts are clean and visibly move when connection is made. Maybe a short in the wire that feeds the left channel?

Thanks to Dave for his excellent advise. I'm going to spend a day in the future updating mine with his mods. In the interim here is a photo of my rebuild that addresses the posters original question about the 0.025 ceramic caps. On this rebuild I used those Russian glass bodied 0.025 200v PIO's..these are right at the voltage limits but I haven't had any issues as of yet (I wanted to use an exact value with a PIO is the reason I went for these)
..on other Eico rebuilds I've used small yellow metalized polyester "tubular axials in the same position to good effect and on my future HF-81 rebuild I'm going to up these value's to 0.033 uf per Craig of NOS Valves and use Russian K-40's....now that is a plan!
Cheers,
Kirk

I'm sorry to have gone so far off topic with my own thread. My head started spinning a little.:nono:

I boosted the values on the phono section to .33uf @400vdc and the linestage and tone circuit use stock capacitance with higher voltage rating. I've got some Solen .022uf and .033's to replace the discs with. I also put a 40UF @ 500vdc cap at the junction Dave specified, but my mentor says this may be a bit too much for the rectifier to handle? I have not added any additional capacitance to the other 2 sections, but have considered replacing the can cap. My mentor thinks I should stay around 40UF on the first section and bump the other 2 up to 100 uf or so.

I have to say that after following Dave's advice and then spinning my wheels over some stupid issues, I've actually ended up with a very nice sounding HF-85 that is also very quiet. It stands head and shoulders above the ASL Wave Line II and NAD PP-1 Phono pre that it has replaced and I'm not even finished yet! I appreciate the help folks!

BTW, we went through this thing with a scope last night and found the lost channel in the switch. I want to apologize for not taking the original advice I was given. You guys are awesome!!

xlr8 -- glad you've got it going now! As for the cap values in the power supply, in this case, there is no issue with the new 40 mfd cap, or upping the original caps to 100 mfd each, as the 2200 ohm resistor coming from the rectifier in conjunction with the winding resistance of the power transformer keeps any surge currents well in line for a 6X4 rectifier tube.

As for the phono stage, in stock form, the RIAA response of the original design is really quite accurate -- with my unit measuring +/-.25 db from 40 Hz to 20 kHz, and within -1 db down to 30 Hz. Below that, it drops off a cliff which is a very good thing to minimize rumble. Therefore, there is little to gain in increasing the value of either coupling cap in this circuit -- the first cap's time constant being so long in the first place, and the second cap being within the FB loop. In any event, the values you installed should perform well.

Good luck with it!

Dave

Well, I have finished the re-cap job, including all of the discs. I'm having a hard time believing that something this old sounds so good! It's pushing 2 Dynaco st-70's into a pair of dipole line arrays that I designed and built. It sounded pretty good before the cap job, but now, my wife agrees that there is a night and day difference. It has more of a full bodied, smooth and warm sound to it. Bass is MUCH better. It seems to fill the room rather than just rattle stuff. My ASL sounds almost tinny and stupid by comparison.:yes: I played the Eico all day yesterday and am back at it this morning, no issues.

The guy who gave this unit to me loaded it up with 5 12ax7's labeled "Hammond, Made in Great Britain" and a JAN 6x4. I thought that was pretty darn nice of him!:thmbsp:

Thanks for the help! Looks like I'll be enjoying it for quite some time!