Eico HF-81 is blowing Fuse

Discussion in 'Eico Place' started by vaathi, Jun 6, 2017.

  1. vaathi

    vaathi baski Subscriber

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    Hi all..
    I picked up Eico HF-81 which has work done by NosValves decade back.. I did post about that earlier in one of the thread. I powered it up in variac at 40 volts and every 20 minutes stepped it up. I did not see the pilot light on even when i turned it on at 40. I slowly brought it up to 117 thinking the light will come up as voltage increases. So I checked the usual suspect and it had a blown 2 AMP Fuse. I changed it to 3 AMP fuse as suggested in service manual and powered it in variac. Again this one blew up too. I will upload the pics from the underside tomorrow. I did check the underside of the amp and I dont see any burnt or swollen resistors or caps.
    As a last effort I want to bring it up in a DBT to check for the sake of it. Also Can I use a slow blow fuse instead of the Fastblow.
    What should I do next to trouble shoot the issue.
    [​IMG]

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  2. vaathi

    vaathi baski Subscriber

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    I removed all the tubes and still it is blowing fuse. What next when trouble shooting this amp which was fully reconditioned a decade ago.
     
  3. RWood

    RWood The future is not what it used to be Subscriber

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    Sounds like a short on one of the secondaries. You said the pilot light didn't come one -- is it part on f the filament circuit? Maybe a component lead or terminal is touching something it shouldn't.
    I'd go with very close visual inspection before you fire it up again.

    If you can't see anything, try measuring the AC voltage of each secondary, at a low enough voltage on the variac to keep the fuse from blowing. Disconnect the secondaries if you have to, if you otherwise can't find anything, until you find the problem.

    I would not go to a 3 amp fuse, or a slow blow. The blown fuses have been saving your amp!
     
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  4. vaathi

    vaathi baski Subscriber

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    Thanks Rwood. :trebon: I checked visually and I did not see any suspect wiring. Also I checked the Primary transformer T1s Resistance for internal and external and everything checked out. I then checked the Red Wires for short and I did not see any.

    Also checked the 6.3V output which connects to all filaments in parallel and to the pilot lamp. There was an old light bulb holder which was not used but left attached to the chassis. New LED buld with new holder was paralleled from the old bulb connection. I checked for short in the old bulb holder and it is shorted. I hope that is the short that is causing the fuse to blow. I will completely remove the old bulb holder and solder the new one properly and check it once again. I set the variac voltage to 40 and it blew the fuse.
     
  5. RWood

    RWood The future is not what it used to be Subscriber

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    I think you are on the right track.

    There are three secondaries on this amp: the yellow 6.3V for the two rectifier filaments, the red HT also going to the rectifiers, and their center tap, and the green 6.3V for the rest of the tubes' filaments. The yellow and red secondaries "dead end" at the rectifiers when those tubes are removed so they are the less likely suspects. The green, however, travels all throughout the amp and should contain voltage even with the tubes pulled. That you've already spotted one issue there is also incriminating. Continue your focus there.

    What I'd do next is disconnect the green secondary at its first stop, which should be the hum balance pot. Clip your AC voltmeter leads to these two green wires and start your variac at 0, observe what results you get as you gradually raise the supply voltage. At 40 volts from the variac I would think you'd have around 2 vac on the green wires, and hopefully no fuse blows. If all is well slowly raise the variac voltage and with fingers crossed, you get to 120V from the wall and 6 - 7 V on the unloaded filament line. That would indicate a good PT, but a short somewhere down the line from the hum pot, as it goes to the tube sockets and lamp holders. With the secondary disconnected you can then use your ohmmeter on the rest, to help you track the problem down.

    There is one other possibility I noticed on the schematic. On the PRIMARY side there was originally a line-to-ground capacitor which, if it is there and failed short, would give you the same results. Look on the primary right after the fuse, and see if it is still there. That cap should be a Safety-Rated cap (designed to fail OPEN if it ever fails). I did not see that cap listed among the replacements on the bill of sale that you posted. Take a look and let us know.
     
  6. vaathi

    vaathi baski Subscriber

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    Hi Rwood. Thanks for your patience in explaining on what to do next. I also reached out Craig @NOSValves and he was detailed in his steps to debug the issue. Let just say customer service at its best even though he worked on the unit decade back. He asked me check the hum pot for short. I have already disconnected the new led bulb and craig didn't install the new LED bulb. I will disconnect the old bulb holder and check for short without the circuit and then go back to Hum pot as suggested.
    Also as you mentioned I dont see the c48 which is supposed to be 0.3 600V capacitor. In one of the schematics I am referring to, they modded it to connect after the fuse as shown in the following schematics.
    http://home.earthlink.net/~eico_hf81/schematic/eico_hf81_schematic_full.gif
    I will check with craig why that was not installed and also reason to leave that out.
     
  7. vaathi

    vaathi baski Subscriber

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    I removed the old socket holder. When I test it out of circuit I do not see any short. Only other connections to the green leads are Hum pot. I will remove the hum pot and try to check for shorts in the circuit.
     
  8. vaathi

    vaathi baski Subscriber

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    Hi @RWood.. I disconnected all the secondary wires from the circuit and I still see the bulb glow bright in the Dim bulb tester. According the owners manual it says it should be Fuse holder or primary or secondary shorted or J16 ( female socket).
     
  9. RWood

    RWood The future is not what it used to be Subscriber

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    What size bulb?

    Is there a capacitor connected at any point on the primary, either to ground or across the primary? If so remove it and re-test.

    Do you have a variac? If so, clip your AC voltmeter to one of the secondaries and watch your voltage rise as you increase the supply voltage from zero. Do this to each of the secondaries and either confirm that they are working or find which one(s) are not.
    It could be that there is a short within the power transformer. What you need to do is to remove all external possibilities for a short, until you either confirm that the PT is good or condemn it as bad.
     
  10. Patrice B

    Patrice B AK Subscriber Subscriber

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    And in the case the PT is dead (I hope not), Heyboer make a more robust replacement (I have one spare for mine, in case...).

    Patrice
     
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  11. vaathi

    vaathi baski Subscriber

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    Hi @RWood I used 60 W bulb. I dont want to blow too many fuses. So I used DBT. All secondaries are disconnected from the circuit. I will check the voltage. Also I want ot check the ohmage of Primary which is supposed to be 1.2 or 1.7 ( from the schematics I got). For the Red wires the voltages should be in the range of 280 VAC according to the schematics. I do have and Variac. I will measure the volts in the secondary. I will mostly run it thought DBT going to variac.

    Thanks Patrice. I am also leaning towards PT being dead and I was about to ask where to get it. Thanks a lot for letting me know.
     
  12. RWood

    RWood The future is not what it used to be Subscriber

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    Make sure this cap is removed:

    Eico HF81.JPG

    Also, I think 60 watt is kind of small for this amp, but being cautious is fine. I would do as you said, work with the variac and bring up the voltages slowly, monitoring them with your meter.
     
  13. vaathi

    vaathi baski Subscriber

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    I dont see any C40 cap in the circuit. I guess Nosvalves removed that when he restored the unit. I checked the AC line connecting it to the variac and DBT. When I remove the Fuse, it shows the correct AC voltage at the points ie each pin of J16. I made sure that J16 does not have any internal short. When I connect the fuse, Voltage read at the same points are 20 vac. I dont know what else to check other than to conclude transformer is shot. Currently all the secondaries are disconnected from circuit.
     
  14. tubeactive

    tubeactive AK Subscriber Subscriber

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    I am actually surprised this is still a mystery...Let us see if we can deduce the problem rather quickly, okay ? It really is simple to check any power tranny with an Ohmmeter. Since you have disconnected all of the secondaries, how about disconnecting the primary ? The two black primary leads should read about 1.7 Ohms to each other, only ! They should not show any continuity to the tranny iron itself, nor to the chassis.

    Likewise, the High Voltage secondary should only show continuity from each red lead to each other and that center tap red/yellow lead. While the Sams lit indicates about 70 Ohms, center tapped for the HV winding, since there were numerous suppliers, anything from 70 Ohms to about 100 Ohms, center tapped should be fine. That means 35-50 Ohms from each red lead to the red/yellow lead and 70-100 Ohms from red to red lead is fine. There should be no continuity to the iron frame or amp chassis ( unless the center tap lead is still connected to the amp chassis).

    Now, let's check the two 6.3 Volt secondaries. The yellow leads are for the 6CA4/EZ-81 only. They should read very low DCResistance to each other only. Check the sockets for the EZ81s. None of the pins should show continuity to ground/chassis or iron tranny frame. The thick green leads are for the 6.3 Volt filaments for the rest of the amp. As you have seen, the thick green leads also connect to the 47 bulb, across the winding, as well as the hum pot across that winding. The "center tap" of that humpot gets grounded to the amp chassis, when the humpot is centered.

    So, where is the short to ground ? Where is the short from one winding to another winding ? If the power tranny primary is short internally, DCResistance readings should indicate this. If it is shorted to ground/amp chassis, that would fry the fuse instantly. Perhaps, the amp switch-on/off wiring is the problem. Are any of the two fuse connections showing continuity to ground/chassis ? That would be a major problem. How about the on-off switch wiring ? One "leg" of the tranny primary winding is taken to the fuse and then to the on-off switch on the treble control. Both AC Primary lead "legs" are also connected to the AC receptacle on the rear of the amp chassis. Neither the rear AC receptacle connections or the on-off switch connections should read any continuity to ground/amp chassis. Likewise, the fuse connections should not read any continuity to the amp chassis.

    So, an instant fuse blow or very bright DBT bulb glow definitely indicate a short. If none of the above mentioned wiring is short or none of the tranny windings are short to another tranny winding, then your tranny might still be good. The last "short" tests to check are each tube socket to ground. Black tube sockets do have a carbon element, being black. They can short over time and arc. I know you have disconnected the tranny leads, but mention of shorting socket leads or wiring leads "melted" to chassis can occur. Also, the hum pot, on-off switch and fuse wiring or the fuse body are now most suspect. Yes, the fuse body, being black plasticized bakelite can arc to ground over time. Likewise, the AC receptacles can arc over time and their black bodies can hide the arc potential...

    I hope you find the short very soon. I have never seen a bad power tranny being so mysterious. They always indicate their short with an ohmmeter. If the tranny reads right, then wiring, fuse, power receptacle or power switch is suspect. Best of Luck with your resto...
     
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  15. vaathi

    vaathi baski Subscriber

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    @tubeactive Thanks for the detailed post to check the power Xformer. I checked for the DC resistance and both the yellow and green shows a negligible value less that a ohm. Primary shows a value of 1.7 ohms as suggested in the schematic. When I test the Red-Red the resistance is 25.6 ohms and somewhat close value to 30 ohms from either Red wire to the Center tap. I checked the fuse holder for any grounding issue and I didn't see any. So based on the DC resistance value Can I deduce/assume that Red secondary is shorted internally?
     
  16. tubeactive

    tubeactive AK Subscriber Subscriber

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    Yes ! Unfortunately, you have found the short ! A good B+ HV winding would read close to 60 Ohms red to red lead. Furthermore, this stereo amp, as well as a few others, having 50 to 60 Ohm CT (center-tapped) B+ winding is an advantage over the typical 100 to 200 Ohms CT B+ windings of usual mono amps. These lower than usual DCResistance B+ windings yield a special "sound" which can be very evident when you restore an Eico HF-81 or other desirable, classic stereo tube amps, like a Pilot SA-232.

    So, I guess you should check with Heyboer about their B+ winding DCR and current abilities. If theirs is 60 Ohm CT, that would be good. 50 or less ohms would even be better, but not likely...

    When I rebuilt my HF-81, I noticed Eico's strange wiring for their 6CA4/EZ-81 rectifiers. First, they connect the indirectly heated cathodes to the filaments of each EZ-81. That "changes" the indirect heater to a direct heater type. Thus, B+ is available sooner as the tubes warm-up. That might be okay, but they chose a "down the road" problematic arrangement whereby each rectifier tube is connected as a half-wave rectifier. Unless the two old rectifiers are a "matched pair" they will conduct each half of the HV B+ winding unevenly, eventually possibly causing harm to the power tranny !

    I had a rectifier problem occur with my HF-81 when I was using it in good, unrestored condition for a while. While listening to this sweet sounding amp, distortion presented itself. The initial check for "near original" tube amps is checking for leaking DC voltage on the final stage coupling caps to the output tubes. They were measuring fine; not even .2 V DC leaking from any of the four caps. Shutting down, then testing the rectifier tubes, this indicated one weak one and a strong one. I found a matched pair in my stash and plugged them in. Refiring, sweet music again, for a short while. I noticed the hot-running EZ-81 socket connections might be related. I tightened the sockets, cleaned tube pins and enjoyed the music for a good, long session. While conducting some "actual layout vs. schematic" studying, I saw the half wave rectifier connections. I knew from past restos, many high power mono amps also chose this "separate diode" wiring method. If changed to two full wave rectifiers, with each rectifier tube effectively in parallel with the other, if one should "weaken" or fail, the amp will still work, albeit with a possibly lower B+.

    So, I made that wiring change and replaced the one weak and one strong pair of EZ-81s in the Eico amp. Just as I predicted, on very loud passages, the amp now distorted. With two strong rectifiers, the Eico runs fine all the time. Cleaning and tightening the sockets, plus changing to the two "full wave-rectifiers per bottle" wiring arrangement is highly recommended. I have since almost fully restored my HF-81 and truly recommend everyone do the same. Maybe, twenty years ago it was cool to use a near original. Nowadays, the true sonic potential of vintage amps cannot be reached without replacing some caps and resistors; especially the caps...
     
  17. vaathi

    vaathi baski Subscriber

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    I have asked for invoice number from Heyboer for the transformers. I also asked for the Spec Sheet if one is available. In another thread jaymanaa said he will not restore any HF-81 without the Heyboer Xformer. So guess it will work out. Thanks for detailing the quirks in rectifier circuit. Since I am novice w.r.t this hobby I will defer those changes for later. :)
     
  18. vaathi

    vaathi baski Subscriber

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    I got invoice for the Heyboer HF81 transformer. I have already ordered and specs are good enuf. Except for the voltage for red is 295 instead of 280. I read somewhere it is well within tolerance level. Specs were sent in a XLS format and I am not able to upload that in this thread for some reason. Site complains that extension is not allowed... Whatever that may be..
     
  19. Shadowdog

    Shadowdog Super Member

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    So the old transformer on has bad filament sections? If so maybe still good for spare etc do to it's nice ohm specs and just add separate filament transformers.
     
  20. vaathi

    vaathi baski Subscriber

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    Filament sections are fine but the main Voltage secondary is shorted.
     

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