Eico hf-85 volume pott

[shelby1420]

Hi Dave, yup the other mod worked wonderfully, thank you!!!!

I tried to do the adjustment as stipulated in the manual, however the hum is so bad I cannot get the volume past 11 on the dial without having to leave the room, it is that bad..........
I will check the wiring, brown and yellow tonight
I have tried the plug turning to no avail
swapped out 3 sets of 12ax7 and two sets of 5751
I will try the test you have suggested in point 6 in a bit, thanks a ton for all this information Dave!!!!:thmbsp:

Ok checked the wiring, on this unit two tubes closest to the selector switch there are no wires coming from pins 2 or 7 ( there resistors from both and a jumper wire from pin 2 to a tag board, see pic....)

If I am seeing things correctly the little black cap and the shinny blue ones are the caps you mention in point 4, yes?

I did the test and when I turned off the hf 85 the hum went away immediately!!!

 

[dcgillespie]

The wiring in fact looks pretty neat in your unit, so it's nice you came upon a nicely built unit. The blue and black caps you mention are in fact the cathode bypass caps for the phono section. They appear to be original, and should be replaced, if only due to age, let alone being bad -- which I bet they are. It will cause a significant amount of hum when they are bad -- which will go away immediately when turned off, exactly as your unit did.

I would also recommend that all the bigger black caps and the wax caps be replaced as well. These type caps were notorious for leaking DC on through to the next stage over time, which can cause all sorts of problems.

The HF-85 is a great little preamp. Yeah, the level control needs the loudness switch mod done to it, and there are some things you can do to the power supply to help it out as well. But overall, it's a great unit -- and a great unit to learn and work on, too. Good luck!

Dave

 

[shelby1420]

The wiring in fact looks pretty neat in your unit, so it's nice you came upon a nicely built unit. The blue and black caps you mention are in fact the cathode bypass caps for the phono section. They appear to be original, and should be replaced, if only due to age, let alone being bad -- which I bet they are. It will cause a significant amount of hum when they are bad -- which will go away immediately when turned off, exactly as your unit did.

I would also recommend that all the bigger black caps and the wax caps be replaced as well. These type caps were notorious for leaking DC on through to the next stage over time, which can cause all sorts of problems.

The HF-85 is a great little preamp. Yeah, the level control needs the loudness switch mod done to it, and there are some things you can do to the power supply to help it out as well. But overall, it's a great unit -- and a great unit to learn and work on, too. Good luck!

Dave

Ok, thanks a ton for your advice/expertise/and help!!!!!!:thmbsp:

 

[BrokenHill]

Has anyone tried replacing the carbon composition resistor's with metal film in the HF-85 ?

 

[shacky]

I believe HF-85 is same preamp as HF-81. Dan Nicotera who leads the Yahoo Eico Tech group just recapped my HF-81 and did a great mod on my phono section. It's amazing now. Can turn volume up 3/4 with no noise.

Check it out:

http://tech.groups.yahoo.com/group/eico/message/12687

You may have to sign up first.

 

[BrokenHill]

Sweet! I have heard that too about the HF-81 & HF-85, I just joined the
Yahoo Eico Tech group and just got my HF-85 working a few nights ago,
I will be doing a lot of modifying and will probably need some tips from that
Eico Tech group.
I know that Ronnie Earl sounds awesome on that clean HF-81 you have.:thmbsp:

 

[seanrr]

I know this is an old thread, but... I have a question related to the topic of discussion. I just got a nice HF-85, but I share the same complaints about the volume dial(s). Here is my question - I have a homemade passive preamp that is really nothing but a volume control. Should it work to hook my passive preamp to the tape out or preamp of the HF-85 and then go from the passive to the amp? The idea would be to get the settings right on the HF-85 and then just control the volume through my passive preamp...which would be easier and smoother. Presumably adding this additional component complicates my system a bit and theoretically degrades the sound some, but I am thinking it would give me the best of both worlds - the HF-85 sounds better than the passive, but the passive has a nice smooth, easy to use volume control. Opinions?

 

[dcgillespie]

Another option is to turn on the loudness switch and see if you like the way the level control reacts with the loudness turned on. The bass will be boosted, but try to disregard that and simply judge if the level control action is acceptable for you in this condition. If it is, then a simple rewiring of the loudness switch will allow the level control to always act as if the loudness switch is turned on volume wise (whether it is or not), while actually turning the switch on would then simply apply the bass boost function -- but not affect the level so significantly like the stock arrangement does.

The problem with placing an attenuator between the output of the preamp and the input of the power amp is that is raises the effective output impedance of the preamp allowing the effects of the interconnects to have more influence on the sound, and also creates a condition where the input of the preamp could be overloaded if the attenuator is set too low.

Good luck with your preamp!

Dave

 

[seanrr]

I realize this thread is over a year old, but...I wanted to add a question. I too have an Eico HF-85, and I love the sound, but I hate the volume pot. I want one control for both sides that is balanced. otherwise, I keep trying to get the sides balanced.

So - is it a completely crazy idea to use the tape out of the Eico into a passive preamp and then out to the amp? Does this add too much complexity to my system, and will I lose the dynamics of the HF-85? Or is this a reasonable solution to the volume pot question (which this thread did not seem to solve)? I will try it myself, but as I do I wanted to hear people's thoughts on this.

 

[jessdw]

Thanks for all the great info about the loudness correction. I'm getting ready to do this myself. I'm wondering if there is a simplified option for those of us that want to eliminate the loudness switch altogether. Could the loudness tap on the volume pot (of each channel) be connected via the 10K resistor to the grounded terminal of the volume pot? It seems like this would accomplish the same thing as leaving the modified loudness switch in the "OUT" position.

 

[dcgillespie]

Yes, that would cause the volume control to react (level wise) as if the loudness were turned on, but without the actual loudness effect taking place.

Dave

 

[TBL_HK]

The way it goes with the Loudness is not too good coz the vol pot itself just doesn't act linear to the rotation.

My Dynaco PAS3X has been given a new ALPS 250k pot with Loudness tap. I like the "feel" of the smoothness with which the ALPS switch works. If anyone agrees, one should try to fit a high grade replacement vol pot such as the ALPS I mentioned.

I guess the only drawback is there is no way to use balance function on that switch coz the two channels are actioned on one shaft at the same time.

 

[TBL_HK]

The way it goes with the Loudness is not too good coz the vol pot itself just doesn't act linear to the rotation.

My Dynaco PAS3X has been given a new ALPS 250k pot with Loudness tap. I like the "feel" of the smoothness with which the ALPS switch works. If anyone agrees, one should try to fit a high grade replacement vol pot such as the ALPS I mentioned.

I guess the only drawback is there is no way to use balance function on that switch coz the two channels are actioned on one shaft at the same time.

After some testing with the EICO HF-85's vol pot, I suspect that the reason for their ill temperament may be due to their linearity on their carbon sweep path, instead of being "logarithmic" and therefore resistance (volum change) progresses as the knob moves towards the 12 o'clock position.

Anyone knows where to get a logarithmic vol pot for HF-85 with long dual shafts?

Terence

 

[dcgillespie]

The original Eico pot on the HF-85 is in fact an audio taper device, but its ill manners can be traced to the excessive gain of the line stage, coupled with the fact that the loudness tap is not loaded when the loudness switch is turned off in the Eico design, where as it is in the PAS and other designs. As a result, there is a significant change in volume level as the loudness switch is engaged or disengaged in the Eico, where as there is very little change in the PAS.

With the loudness switch turned off in the EICO design, that plus the gain of the line stages means the level control is operating down in the weeds most of the time, where the channels usually track very poorly, and overall sound level is hard to control accurately. Eico countered this concern by installing level controls on all of their power amps, so that the gain of the 85 could be contained, forcing its level control to operate up higher on its rotation range. But used with power amps without this feature, then noise and level control action with the 85 becomes quite frustrating.

Simply replacing the pot with a quality modern pot will only help with channel tracking, but do little to solve the underlying gain problems. Reconfiguring the loudness switch so that a load is always maintained on the loudness tap will help significantly with that, and make the 85 a much more usable device.

Dave

 

[ferninando]

does anyone have a schematic of the HF85 with all the "fixes" installed? ie: loudness switch, vol pots, gain issues, etc. With abit of verbage included for each fix. Something sure would be nice to have. parts changes shown with values.
PM me if you like.

 

[TBL_HK]

I think the design of the volume pot in the HF-85 magnifies the difficulty in achieving channel balance at low levels because at min position, the two shafts do not rotate the pointers on the carbon pads at sync.

Admittedly the EICO two-stage line level design is superior to the Dynaco as the latter had passive tone controls. But in actual use, the EICO resulted in frustration EVERY time I used it. My two McIntosh C20's are far more reliable even though their pots are just as worn.

Terence

 

[dcgillespie]

TBL -- The Eico design is very frustrating as you say, but do understand that the tone controls in all of the Dynaco PAS designs are of the active feedback type, just as they are in the Eico preamplifier.

Dave

 

[shacky]

Don't want to hijack thread. But how about new balance pots? Is there a replacement option?

My balance pot needs a tweak now and then to get right channel back.

 

[theschlepp]

Is it OK if I wake up this thread again?
I recently scored one of these little beauties at a yard sale for $50. A really nice guy who used to be into audio but has moved on to other hobbies passed it on to me. Well, it is all original, except I replaced the coupling caps. I have read through Dave Gillespie's posts, (he seems to be the authority in these) and it looks like altering the RC circuit on the loudness pot and adding a cap to the power supply are the main mods to do, besides swapping out the RCA jacks. I'd like more info on changing those RCA's. DO I just buy some gold plated ones from Taiwan or China (ebay), and mount them to a piece of phenolic or PCB? That seems like a lot of drilling and such. Or does someone make a nice dual RCA mounted already to a piece of plastic? How do you handle the wiring if you start eliminating unnecessary RCA jacks? Which ones truly are unnecessary? Also, wouldn't some nice twisted shielded pair cable go a long way to quiet the HF85? If so, has anyone out there used TSP? What kind?
Sorry for all the questions and I hope you aren't bored with this unit yet?
Thanks,
Eric

 

[shacky]

Check ou the Eico Group over at Yahoo. Dan Nicotera modded my connectors and vastly improved phono stage. He may be able to guide you. Craig at NOSValves recapped mine and got it sounding silly good :yes: