Eico hf-85 volume pott
- Thread starter shelby1420
- Start date
The real problem with the HF-85's touchy gain (and it is surely touchy as designed), is -- once again -- the controversial loudness switch design. When you look at the gain through out the line/tone stage circuits, they are all really pretty normal. The gain of the tone stage itself is <1 at a flat setting, which is true of almost all feedback type tone circuits. In this case, it is more precisely about .90. The line stage has a typical gain of about 14.5, which together produces an overall gain of about 13. This is very similar to that of say a Dynaco Pas preamp, that has an overall line/tone stage gain of 10. The big difference between the two however, is that when the loudness switch in the Pas preamp is turned off, the volume control loudness tap is shunted to ground through it's 4.7K loudness switch resistor, where as in the Eico, that control tap is left open circuit when the loudness switch is off. This has a huge effect on how the control operates through out the first half of it's rotation. This is also why there is such a big drop in gain in the Eico when the loudness switch is engaged.
The easiest solution is to use the same loudness components that Eico used, but connect them as Dyna did in their preamp. That is in essence, that one end of the 10K resistor should be grounded, the other end should go to one end of the .025 cap, whose other end goes to the loudness tap. Then, the loudness switch should act to short out the cap when turned "off", and allow it to be in circuit when turned on. That means that only two of the switch terminals will be used for each channel. If you do this, you will find that all the touchiness of the control is gone, and the loudness switch will operate "normally", without the large change in gain associated with the original design.
Dave
The easiest solution is to use the same loudness components that Eico used, but connect them as Dyna did in their preamp. That is in essence, that one end of the 10K resistor should be grounded, the other end should go to one end of the .025 cap, whose other end goes to the loudness tap. Then, the loudness switch should act to short out the cap when turned "off", and allow it to be in circuit when turned on. That means that only two of the switch terminals will be used for each channel. If you do this, you will find that all the touchiness of the control is gone, and the loudness switch will operate "normally", without the large change in gain associated with the original design.
Dave
shelby1420
Enjoying the music.....
Interesting...... what if I just bypass the loudness switch by snipping the two wires on the pott it self, will that work:scratch2
no great urge to have the loudness switch anyways........)
Interesting...... what if I just bypass the loudness switch by snipping the two wires on the pott it self, will that work:scratch2
No. Just swap the resistor and capacitor on each half of the loudness switch. Disconnect them completely and you'll just have too much gain at low volume...
shelby1420
Enjoying the music.....
Hey Tom, not following you re swapping, what exactly should I be doing?? Sorry if this sounds like a dumb ass question, really like the sonic of this unit tho!!!
The capacitor should be where the resistor is and vice-versa - and this error is present in most EICO equipment with a "Loudness" switch. I just looked inside an ST-84 that I'm looking at for a friend - the caps will fit fine where the resistors are - long leads - but it'll need new resistors to reach the terminals.
jaymanaa
RIP 1961-2018
Wow, I would not have noticed that in a million years.:thmbsp:
shelby1420
Enjoying the music.....
very cool Tom, gonna have to give that a try!!!!:thmbsp:
I would emphasize that to implement this mod to the loudness control switch correctly, simply inter-changing the cap and resistor is only part of the required change. You also need to move the lead coming from the level control tap to the middle switch terminal as well -- meaning that now only two switch terminals are used as I stated, where as originally, all three were used. If you don't move this lead, simply swapping the cap and resistor will produce no change at all in the switch's operation.
It's interesting to note that Eico didn't consider the original design of their loudness switch circuits an "error" at all. Since the original idea of the loudness function was that it was only supposed to be used at low (i.e. background music levels), they felt that their's was the best execution of the idea: It boosted the bass frequencies, and lowered the gain all in one easy step, which they felt you would be doing anyway in that situation. The problem is, at the time, almost all other manufacturers used configurations that provided the boost without the drastic change in gain. That format is what caught on and became the norm, even though there was no "norm" established for the switch's function to begin with. But Eico staunchly held to their approach clear up until the bean counters finally removed the switch altogether in their final "Classic" series of vacuum tube units. At any rate, it was the rather unique thinking at Eico once upon a time that caused their units with a loudness switch function to seem so out out of step with what most of us are accustom to today.
Dave
It's interesting to note that Eico didn't consider the original design of their loudness switch circuits an "error" at all. Since the original idea of the loudness function was that it was only supposed to be used at low (i.e. background music levels), they felt that their's was the best execution of the idea: It boosted the bass frequencies, and lowered the gain all in one easy step, which they felt you would be doing anyway in that situation. The problem is, at the time, almost all other manufacturers used configurations that provided the boost without the drastic change in gain. That format is what caught on and became the norm, even though there was no "norm" established for the switch's function to begin with. But Eico staunchly held to their approach clear up until the bean counters finally removed the switch altogether in their final "Classic" series of vacuum tube units. At any rate, it was the rather unique thinking at Eico once upon a time that caused their units with a loudness switch function to seem so out out of step with what most of us are accustom to today.
Dave
Kegger -- I would not abandon your effort with this, as while my suggested loudness switch mod does correct the switch's function and operation of the level control, it does not correct the rather significant noise that comes from the unit's design when used with sensitive power amps (i.e. HF-60, 89, etc.). The feedback level around the line amp stage is rather insane anyway, indicating that a lower mu tube should have been used to begin with. Even if the the same line stage gain is maintained, the HF-85 would benefit from a 12AY7 based line stage, as then the feedback resistor would not present such a heavy load to the second stage of the line amp.
On a separate note for the 85's line amp, one absolute error that Eico did make with it's design is the signal presented to the tape out jack. People often prefer the signal there to drive their power amps as it is tapped into the circuit at the output of the line amp, but before the tone amp stage. Therefore, it's a cheap and easy way to "bypass" the tone circuits. However, the signal there is inaccurate (presenting a bass boosted signal), because the signal is taken from within the NFB loop of the line stage. Therefore, that signal is subjected to the frequency errors contributed by the combination of the line stage output cap (.1 mfd) and the feedback resistor (33K) of the original design. To correct this error, simply move the connection of the .02 tape output blocking cap from before the line out coupling cap (.1 mfd), to AFTER it. Doing so will then present a dead flat output signal at the tape output jacks for an accurate bypass of the tone control circuits.
Dave
A bit of squinting at the fuzzy ST-84 schematic proves you correct... I think values of .02 (or .022) and a resistor of 10K would be closer to common practice, and those are the values I'll try. I'll post a before/after schematic.
BrokenHill
Well-Known Member
dcgillespie & Tom Bavis, Thanks from me too.
shelby1420
Enjoying the music.....
Great stuff guys, gonna try this tonight!!!
jaymanaa
RIP 1961-2018
Hey Rick, this Alps has 4 connections.:scratch2: They are calling the extra one a tap for bass boost. I wonder how close to the original Eico it would be?:scratch2:
http://cgi.ebay.com/ws/eBayISAPI.dll?ViewItem&item=300445052057
http://cgi.ebay.com/ws/eBayISAPI.dll?ViewItem&item=300445052057
shelby1420
Enjoying the music.....
Very interesting Jay, sure sounds like it could be real close!!! For the 13.00 I think its worth it to try it out, I have done the loudness switch mod and hot digidy dog it worked!!! Now I just need to figure out how to get them ( volume left and right) equal all the time......... also gotta figure out whats up with the friggin phono....re soldered every spot, no help........
Hi Rick --
Hopefully from your last post, it sounds like the loudness switch mod is working for you now. If not, PM me (again) and I'll be happy to help. Otherwise, I hope the gain control operates more smoothly for you now.
Regarding your phono noise, here are a couple of suggestions to try:
1. Have you tried adjusting the hum balance control? It is located on top of the chassis behind the treble control, and will have the most effect when the phono input is selected. With no phono leads connected and the gain advanced, you should be able to find a definite minimum hum point with adjustment of the control.
2. Check the wiring neatness -- particularly of the heater wiring. This will be the twisted brown and yellow leads that run through out the unit. Try to keep them away from any wiring connected to pins #2&7 of the phono preamp tubes (two tubes nearest the selector switch). If you do move any wiring, readjust the hum balance control, as it's minimum hum point will likely have changed.
3. If your AC power wire is original, try reversing the plug for minimum hum. Converting to a 3-wire system with a green wire ground is great for safety, but often causes ground loops with other 3-wire devices. If your 85 has a three prong plug now, temporarily try it with a cheater adapter to lift the ground, and see if that makes any difference.
4. If the cathode bypass caps in the phono preamp section are original, they likely need to be replaced. Due to the particular design of the HF-85's phono circuits, these caps will cause the preamps to hum if they are bad. Bad caps here will also destroy the accuracy of the RIAA curve the circuits are trying to create. These are the caps connected from pins 3&8 to ground on each preamp tube.
5. Try different 12AX7s in the phono preamp stages. Because these tubes are operated with AC on their heaters, some tubes will have much more hum than others.
6. Finally, a simple test will help determine the origin of the hum. With your power amp plugged into a full time power source, and the 85 set to phono with the gain advanced and the hum present, unplug the 85 and listen. If the hum disappears immediately when the 85 is unplugged, then the problem is due to AC wiring infecting the audio circuits within the 85's chassis. However, if the hum just fads away with the rest of the noise as the 85 cools down, the origin is outside the 85. This is a handy tip I've found to help isolate hum problems.
I hope these suggestions give you some new things to try, or at least help isolate the problem for you.
Dave
shelby1420
Enjoying the music.....
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:
