Eico HF-81 Help?

[wncwalker]

I'm working on restoring an Eico HF-81. When I got it it was really dirty but working. After cleaning it up with deoxit it's sounding pretty good with all original capacitors and resistors only two of the el84s are not original tubes. There is almost no hum but I get some random snap crackle pop out of the tweeters. Also when I turn it up with the bass up the amp itself resonates with the bass making almost a clicking noise from one of the two smaller transformers closest to the tubes that were replaced. I can't get the volume level turned up over about 4 without noticing some significant distortion. I have already replaced the biggest capacitors with 30 uf 500v sprague atoms as suggested on one website I saw but no real change in sound. It actually sounds pretty good but I know It can be much better. Any ideas on what I sould do next. I am on a very limited budget so I can't send it off to Craig at NOS valves like I want to.:sigh:

 

[Patrice B]

Better stop to use it and do the full recap, checking each tube and resistor (particularly the bias resistors), replacing power supply caps (I guess the 30uf were part of it).

The fact that one of the output transformer vibrate with the music is kind of worrying...

 

[mhardy6647]

The clicking is instability called motorboating; the amplifier is on the verge of oscillation. Check the original assembly instructions and make sure the polarity's right on the output transformers. Interestingly, the high-value ceramic coupling caps in the HF-81 shouldn't fail (they don't sound all that great... but they should be OK electrically speaking). Drifted resistors are always a possibility; check all the relevant voltages at the tubes. All the correct values are found in the manual, which is available from the EICO Yahoo group if memory serves.

 

[wncwalker]

Thank you. I have the manual, now I just have to learn to measure the voltages of the tubes. Can the resistors be tested without disconnecting from the circuit board?

 

[glen65]

Thank you. I have the manual, now I just have to learn to measure the voltages of the tubes. Can the resistors be tested without disconnecting from the circuit board?

You can try checking them in circuit. But if they don't check according
to their stated value you would need to disconnect at least one leg
and check the resistor again.

 

[Dmannnnn]

The majority of the carbon comp resistors on mine had drifted up considerably, some as much as 100%. The carbon comps look like little plastic cylinders with the color code bands. My amp also had some other resistor types. They had metal on the ends and seemed to be covered in a cardboard or paper material. Most of those tested within 10%.

Unless you have a capacitor analyzer you should just plan on swapping out all the coupling capacitors. The ceramic caps may still be OK, but they really do sound like ass, in my opinion. You can find decent poly caps for a buck apiece or less.

 

[gogofast]

illiois caps are less than a dollar each. doesn't make sense to leave any paper, film or electrolytic caps in there even if they test ok. leaky or bad coupling caps are most likely causing the distortion. the power transformer is a known weak link in that amp. so don't stress it too much by using it without properly updating and biasing the amp. i had restored two of these and they both sounded fantastic! one of the best of el84 amps out there.

 

[mhardy6647]

The ceramic caps may still be OK, but they really do sound like ass, in my opinion.

mine, too.

 

[GordonW]

A bad cap can definitely cause snap crackle and pop sounds. I had this happen on my Heathkit AA100. It turned out to be an arcing power supply cap! It wouldn't have to be a power supply cap- any cap that momentarily shorts could cause this.

Bass oscillation is, as mentioned, a sign of instability. When you replaced caps, did you use values close to the originals, or did you change them more than a factor of two in value from what was there? Too big of coupling caps can cause this problem, in very rare cases. Also, weak power supply caps can cause this too... it starts to act as a "relaxation oscillator" sometimes, if a power supply cap has too high of internal impedance...

Regards,
Gordon.

 

[dcgillespie]

Since the OPT noise starts with advanced setting of the bass and level controls, I suspect that it is the power supply caps that decouple the line and/or low level preamps sections as Gordon said. These can be replaced rather inexpensively, and should be done anyway to ensure any kind of long term reliability.

Dave

 

[wncwalker]

Thanks for your replies. I'm not sure which caps are the power supply caps. The manual tells me the value but not their function in layman's terms. I plan to replace parts with their original values because I figure the original designers knew what they were doing.

 

[mhardy6647]

It's a little complicated in the HF-81; the two electrolytics under the deck are part of the P/S filter, as (if memory serves) are some sections of the multi-section "can" electrolytic. The two downstairs are the ones that, if original, are likely to be the most in need of replacement.

They're the two white cylinders on the far left (middle and bottom) in this "as received" photo of mine. The multisection can's on the right - maybe it's not in the HV supply; check the schematic (the parts are all numbered and should be pretty easy to identify).

 

[wncwalker]

Thank you. The two white cylinders are the ones I replaced so far (spague atom 30uf 500v). I was thinking about replacing the multisection can next with JJ 40-20-20-20 connecting two of the 20uf caps matching the original part which calls for 40-40-20. Would this be a reasonable part to replace next or would there be better course of action ?

 

[Dmannnnn]

Thank you. The two white cylinders are the ones I replaced so far (spague atom 30uf 500v). I was thinking about replacing the multisection can next with JJ 40-20-20-20 connecting two of the 20uf caps matching the original part which calls for 40-40-20. Would this be a reasonable part to replace next or would there be better course of action ?

That is a good idea. The four coupling caps shown at the bottom of the picture could also be pretty leaky so it is a good idea to change those while you are in there. You also might want to change the bias caps. These are the smaller electrolytics in parallel with the large 160ohm resistors. You can see them at the top of mhardy's picture attached to the uppermost output tube sockets.

In the picture below you can see how I used a dremel tool to enlarge the opening for the JJ can capacitor which is fatter than the stock cap.

 

[Dmannnnn]

Oh, and in case you haven't done it yet, the first thing you should probably do is replace the stock power cord with a 3 conductor grounded cord. Make sure the chassis is well grounded. Then you can remove the 'death' cap that is wedged in next to the power transformer.

I use standard computer power cords (3A rated or more) and cut off the IEC female connector. You can find them by the boxfull at most pawnshops.

 

[wncwalker]

Dmannnnn, when you say you can remove the 'death' cap do you mean it can be taken out and not replaced after the power cord is upgraded? Also can anyone recommend type and brands to replace the capacitors. I have found out they vary from one dollar to forty dollars. Do resistors brands effect the sound of the amps?

 

[glen65]

Dmannnnn, when you say you can remove the 'death' cap do you mean it can be taken out and not replaced after the power cord is upgraded?

Yes, if you ground the chassis you wont need it.

Also can anyone recommend type and brands to replace the capacitors. I have found out they vary from one dollar to forty dollars.

There's been lots of debate concerning capacitors. I'll simply say that
you can get very good caps without spending a lot of money.

Do resistors brands effect the sound of the amps?

Brands aren't as important with resistors as they are capacitors.

 

[mhardy6647]

There's been lots of debate concerning capacitors. I'll simply say that
you can get very good caps without spending a lot of money.

some Orange and some Yellow ones come to mind... ;-)

 

[wncwalker]

Thanks. I will replace the power cord tonight. I guess you are talking about orange drops and auricaps? Auricaps are a little too expensive for me at about $15 each any specific recommendations would be appreciated.

 

[glen65]

Thanks. I will replace the power cord tonight. I guess you are talking about orange drops and auricaps? Auricaps are a little too expensive for me at about $15 each any specific recommendations would be appreciated.

He's talking about SoZo Mustard Caps or Mallory. They're good caps and not very expensive.