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Old 22nd August 2009, 11:55 PM   #1
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Default help w/ Eico hf-81

I'm a beginner but have built a Fender Deluxe amp, including debugging it..

I bought a hf-81, cleaned it up, replaced the power cable with a grounded one, and replaced the power capacitors.
Then powered it on for less than one min. and identified the following issues:
1) the power on light doesn't come on. The bulb appears ok - measured it's resistance. There was no DC current at the bulb, but now that I'm writing this I realize I should have measured AC..
2) the power transformer makes a buzzing noise - not terribly loud, but loud enough to be distracting and not something you want to keep.
3) one channel power tubes do not appear to behave. After about 15 seconds a high-to-low sound is produced and shortly or around that time a flash or some flashing occurs in the one channel tubes. I immediately turned the amp off and switched the tubes with the other channel. The same channel displayed the same behavior, so I'm concluding the issue is not the tubes. Then I removed the bad channel power tubes and turned on the amp for about 2 min. - no problems (other than the transformer buzzing).

It's late here, so tomorrow I'll measure the AC voltage at the power on light.
For the strange tube behavior in the bad channel I'm guessing the stage coupling caps may need replacing and are doing some wacky voltage stuff?? or maybe something along those lines??

Thanks in advance for your help. Given my Fender amp.. I do realize I'm in for a little adventure, but really look forward hearing this amp sing.

Paul
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Old 23rd August 2009, 09:35 PM   #2
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Per the power on light - the bulb is bad. And I measured the voltage at 6.3V for it - perfect.

The biggest issue is the flashing which is occurring in one channel power tubes. Like I said, even if I change out those tubes, the flashing still occurs in that channel. A strange noise occurs too, about 15 seconds after the amp is turned on.

Appreciate any advice.

Will measure voltages soon.. but I'm not sure what to look for per this issue..
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Old 24th August 2009, 02:27 AM   #3
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The voltages on the schematics should be helpful. Good luck!
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Old 29th August 2009, 01:57 PM   #4
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Got a hold of the eico manual - excellent - and measured resistances and voltages..
Power supply is at 116 V AC - I would think this good, not too high.
Filament voltage is right at 6.3 V.

Per the manual, measured all resistances from the pins, and they all check out ok, more or less within 5-10%.

Voltages check out pretty much too.
The only outliers are:
- V1/V2 grids low. V1(1) at 9 V but should be 80 V. V1 (6) at 40 V, but should be 60 V. V2(1) at 17 V but should be 80 V, and V2(6) 43 V but should be 60 V. (I believe V1/V2 are for phono only..)
- power tubes V7/V8/V9/V10 a little high. On average they measured ~370 where Eico states ~330 V. (Based on my limited Fender guitar amp experience, that should be ok.)
Pre-amp tubes V3/V4/V5/V6 measured reasonably close to Eico's spec. Also rectifiers (V11/V12) are at 300 VAC, eico wants 290 V, and 380 VDC, eico 340 VDC.

Note that these measurements were done with V9/V10 power tubes out of the amp.
Since they flash and I believe the amp goes into oscillation with them in, I left them out for these measurements.

I did try switching around tubes, to no affect..

The bad channel output transform stays completely cold.
When I hook up a (cheap) speaker to the bad channel, a high pitch sound comes on. Equals oscillation? This happens about 15 - 30 seconds after turning on the amp. Thereafter the power tubes start flashing. I always turn the amp off quickly.
Could it be I have a bad output transformer? How can I test for that?
If I get (oscillation) noise out to the speaker, doesn't that mean the transformer is ok?..

The bad channel power tubes don't seem to light up correctly when the amp is turned on. They flash white briefly, then there's no light at all. They seem almost not to be on, but then when the oscillation occurs strange flashing occurs inside them.
I double-checked the filament voltage to them but it's ok.

Thanks for any advice for next steps,

Paul
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