Eico HF85 Balance/Volume update

[JohnW]

I am working on restoring an Eico HF85 preamp and want to change out the volume control.

The current control has two separate sections hooked together, one for each channel. This serves as both balance and volume controls. Unfortunately, it does neither very well.

I want to put in an ALPS RK27 dual pot, but don’t want to give up having a balance adjustment. I don’t really need the “MODE” selection, since I am pretty consistently “Stereophonic Norm”.

So, the plan is to remove the “MODE” and convert to a standard center detent balance control and change the “LEVEL” to a modern dual pot.

Currently the “MODE” is wired directly to “LEVEL” in the schematic. The “LEVEL” pot is 500k impedance.

Two questions:

1. Should the rewired volume control sit ahead of the balance control in the circuit, or after, or does it make any difference.

2. If the current volume control is 500k, what should the impedance for the new controls be: (500k Bal -> 500k Vol), (250k Bal –> 250k Vol), or something else?

 

[Sam Cogley]

1) rewire the "loudness" functionality before you decide that the volume pot is misbehaving.

2) if you're unsure of why the "loudness" functionality is screwed up, look at some of dcgillespie's posts on the subject, or review the revised schematic in my "Super Stock" HF-85 thread.

 

[jaymanaa]

There's an error in this that makes the pot way too sensitive. IIRC a resistor and cap are wired parallel and should be serialized. Tom B knows where it is I'm sure.

 

[JohnW]

I had read these posts earlier and have already performed these mods.
Thanks for putting this together.

Even with the loudness changes my current pot doesn't track very evenly for the first quarter, and then there is a distinct step about half way that is a little different between 1 and 2. It could be better.

 

[ferninando]

I did a gnd up rebuild of hf85, shitcanned the ldns completely and replaced the vol pot with an ALPS blue dual vol pot of same value. works for me.

 

[Sam Cogley]

I had read these posts earlier and have already performed these mods.
Thanks for putting this together.

Even with the loudness changes my current pot doesn't track very evenly for the first quarter, and then there is a distinct step about half way that is a little different between 1 and 2. It could be better.

Good deal. Just wanted to make sure you were aware of the rather screwy way the volume pot was wired into the stock HF-85, as it has been blamed for plenty of volume pot problems. :thmbsp:

There's an error in this that makes the pot way too sensitive. IIRC a resistor and cap are wired parallel and should be serialized. Tom B knows where it is I'm sure.

De plane! De plane! Look up!

 

[JohnW]

I found some interesting reading on this subject here:

http://www.angelfire.com/electronic/funwithtubes/Amp-Tone-A.html

The author doesn't recommend adjacent tone and volume controls due to the possibility of high frequency loss from capacitance in potentiometers.

One option would be to wire the balance control right before output, or perhaps between the first and second gain stage.

I think I will need to give this a little more thought, maybe some experimenting.