Another problem....(solved)

[kidjan]

I came home the other day, and although my amp was turned on, the green light in the switch wasn't illuminated. A quick check turned up a blown fuse.

After replacing the fuse and flipping the switch, the result was another blown fuse.



SO, I started troubleshooting. I suspect something in the heater circuit, so I removed J8 and J9 to isolate the transformer from the rest of the circuit, and tested from there. When I tested the resistance across J8 and J9 without the power tranny hooked up, I get about 200 ohms (which falls in line with R40 and R41 being hooked up in serial--this is without tubes in). When I test between green and green-white on the power transformer (PA774) I get a very, very small resistance--almost nothing--which leads me to believe that I have a short in the power tranny, although I don't know if I should be getting a resistance here. I also get the same resistance going between the brown and brown-white lines.

Anyone have any ideas? Every time I flip the heater switch, the fuse blows instantaniously....I've also thought perhaps there's something wrong with the IRCL, or a short somewhere (couldn't find ANYTHING), since clearly I'm getting way too much current....

[Shannon Parks]

When I tested the resistance across J8 and J9 without the power tranny hooked up, I get about 200 ohms (which falls in line with R40 and R41 being hooked up in serial--this is without tubes in).

Sounds good...

When I test between green and green-white on the power transformer (PA774) I get a very, very small resistance--almost nothing--which leads me to believe that I have a short in the power tranny, although I don't know if I should be getting a resistance here. I also get the same resistance going between the brown and brown-white lines.

No, this is normal. I just check the resistance of these two windings and they are indeed very low resistance (my DMM says 0.1 ohm).

Anyone have any ideas? Every time I flip the heater switch, the fuse blows instantaniously....I've also thought perhaps there's something wrong with the IRCL, or a short somewhere (couldn't find ANYTHING), since clearly I'm getting way too much current....

1) First, disconnect all the power tranny taps (you may still have the filament taps disconnected).
2) Check the impedance of J6-10 and J6-11 to ground, to make sure there are no shorts there.
3) If an IRCL ever failed, it would most likely fail open, which would prevent the unit from working, but also prevent blown fuses.
4) Check the impedance of the on/off switch pin 1and pin 2 to ground (should be high impedance).
5) If you flip it on at this point and blow a fuse, you either have a wiring issue (odd, since it was working - a very tricky short maybe?) or maybe your tranny shorted out. Maybe bypass the IRCL & neon switch at this point to verify if you do have an internal short. If your tranny checks out by itself with no hookups (other than the primary AC of course), report back and we'll take it from there. Good luck!

Shannon

[kidjan]

I finally got the time to take a look at the amp, and worked through the circuits, and determined that the only thing that could be causing it was my power tranny (PA774 replica). I also blew out the IRCL while fooling around--board was a bit scorched where the IRCL was, but for the most part it took the vast majority of the damage (saw some nice fireworks :D )


Anyways, I ordered a new IRCL and tranny, dropped them in, and it now works. I guess my question is why the tranny failed in the first place, since it worked fine for about a month. Also, how hot should the OPT be running? Mine always ran smokin' hot--not sure why.





Lastly, Shannon--any idea when the eiclones are going to be out? I'm dying to build a pair. :D Muchos gracias-jer

[Shannon Parks]

Anyways, I ordered a new IRCL and tranny, dropped them in, and it now works. I guess my question is why the tranny failed in the first place, since it worked fine for about a month. Also, how hot should the OPT be running? Mine always ran smokin' hot--not sure why.

Thanks for the update, Kidjan - I was just thinking about your amp this past week. Sorry to hear the bad news.

I'm not too familiar with the repo PA774 (though I've tested one), but the original does get very warm/hot. Assuming that this is Ned's, if you felt that the transformer was defective I'm sure he would replace or refund it. You can always double-check the HV current by looking at the voltage drop of R43.

Lastly, Shannon--any idea when the eiclones are going to be out? I'm dying to build a pair. :D Muchos gracias-jer

Haha! You all don't click on my Ebay link very often, do you? I've been trickling a few pairs onto Ebay over the last several weeks. I greatly appreciate these early adopters - they end up helping me tweak the product even more. Official Eiclone launch should be soon - I'll put the manual online for perusal.

Shannon

[kidjan]

Thanks for the update, Kidjan - I was just thinking about your amp this past week. Sorry to hear the bad news.

It happens. No biggie.

I'm not too familiar with the repo PA774 (though I've tested one), but the original does get very warm/hot. Assuming that this is Ned's, if you felt that the transformer was defective I'm sure he would replace or refund it. You can always double-check the HV current by looking at the voltage drop of R43.

What is the "HV current?" You'll have to excuse me--my electronic skills are pretty well non-existant.


Glad to hear about the eiclones--I'll be ordering a pair as soon as they're out. I think a few other people who've seen my amp are pretty interested in them too, so....I can't wait!

[EWBrown]

The HV (high voltage) current can be measured through the 50 ohm (or the paralleled 100 ohm 3 watt resistors in the Rev C) by *carefully* measuring the voltage drop across this resistor, the total DC current should be around 150 to 160 mA (35 for each of the four EL84s, if biased properly) and 10-20 for the 12AX7 and 12AU7 stages) this would appear as 7.5 to 8 VDC drop across the 50 ohm resistor, given it as being 50 ohms, each volt drop represents 1/50 amp, or 20 mA. Just remember, this resistor is at the high voltage potential of 360 to 380 volts DC above the ground/chassis, which can deliver a nasty if not possibly lethal shock - the best method is to use clip leads and connect the meter across the 50 ohm resistor, then power it on and monitor the voltage , another slightly safer method is to measure the voltage at each end of the resistor, referenced to ground, the same voltage drop will be apparent, either way will work fine. As the old time :backinmyday: electricians would say, just use one hand when measureing HV, and keep the other hand in a pocket, this prevents getting an across-the-heart shock, which is the most dangerous.
:shockingzap: :eeeeek: :scrambleup:

/ed brown in NH

[kidjan]

Okay, I didn't understand most of that. :oops: 'Scuse me while I try and milk you for more information.

The HV (high voltage) current can be measured through the 50 ohm (or the paralleled 100 ohm 3 watt resistors in the Rev C) by *carefully* measuring the voltage drop across this resistor,

When you say fifty ohm, which resistor is this (like, R42 or R14 or whatever)?

the total DC current should be around 150 to 160 mA (35 for each of the four EL84s, if biased properly) and 10-20 for the 12AX7 and 12AU7 stages) this would appear as 7.5 to 8 VDC drop across the 50 ohm resistor, given it as being 50 ohms, each volt drop represents 1/50 amp, or 20 mA.

The numbers add up, but why does a volt drop represent 1/50 amp (guess: Ohm's law?)

Just remember, this resistor is at the high voltage potential of 360 to 380 volts DC above the ground/chassis, which can deliver a nasty if not possibly lethal shock - the best method is to use clip leads and connect the meter across the 50 ohm resistor, then power it on and monitor the voltage , another slightly safer method is to measure the voltage at each end of the resistor, referenced to ground, the same voltage drop will be apparent, either way will work fine. As the old time :backinmyday: electricians would say, just use one hand when measureing HV, and keep the other hand in a pocket, this prevents getting an across-the-heart shock, which is the most dangerous.
:shockingzap: :eeeeek: :scrambleup:

/ed brown in NH

Sound advice (no pun intended). Dying isn't high on my list of things to do at the moment.






A few things I've been wondering:
-could my selection of transformers have anything to do with the power tranny running hot? For reference, the PA774 gets hot to the point where I can't put my fingers on it for longer than about half a second. Could it be possible that it doesn't play nice with the 1650F's?

-With this irregular mix of trannies, what should I be biasing the tubes to? I've been biasing the EL84's to about 315 mA just to be one the safe side...

-Should I hook up both the 4 and 8 ohm taps for the opt's, or do I only hook up one? Currently I have the 4 AND 8 ohm taps wired up, and some of the stuff I've read here seems to indicate that you only hook up one? (or perhaps I'm going nuts)

-Could not having the trannies grounded properly contribute to this?


Thanks in advance...
jer

[Shannon Parks]

A few things I've been wondering:
-could my selection of transformers have anything to do with the power tranny running hot? For reference, the PA774 gets hot to the point where I can't put my fingers on it for longer than about half a second. Could it be possible that it doesn't play nice with the 1650F's?

The PA774s do get very hot. I wouldn't worry about it. Ned's are rated for 180mA service in the HV secondary. Your AC mains could be a little higher than most, but I imagine that the transformer manufacturer has that figured into the equation.

-With this irregular mix of trannies, what should I be biasing the tubes to? I've been biasing the EL84's to about 315 mA just to be one the safe side...

I think you are conservative with this setting (315mV, right?) and should be fine.

-Should I hook up both the 4 and 8 ohm taps for the opt's, or do I only hook up one? Currently I have the 4 AND 8 ohm taps wired up, and some of the stuff I've read here seems to indicate that you only hook up one? (or perhaps I'm going nuts)

Black to black yellow = ground. Green to green/yellow = 4 ohm. Yellow = 8 ohm. You don't have to connect 4 or 8 ohm taps to terminals, but green to green/yellow must always be jumpered (unless you are doing the 16 ohm config and I forget how that one is).

-Could not having the trannies grounded properly contribute to this?

Grounding them is for safety and no current <should> be passing through them.

Thanks in advance...
jer

No prob.

Shannon

[EWBrown]

I've found that the original and the replacement PA-774s do run hot to the touch, (I haven't taken an actual temperature measurement) and even the Hammond PTs get quite warm. The OPTs should not get too warm, and most of that will be from radiated heat from the PT and the nearby EL84 tubes.

Ohm's law: E(voltage) = I*R (current (amps) * resistance (ohms) )

So, assuming 160 mA (.16 amp) across 50 ohms resistance that should
yield 8 VDC drop across the resistor(s). The power dissipated in the resistor would be P = E^2 / R = 64/50 or 1.28 watts.

One thought I've had, but not yet tried, is if the Hammond 272FX/HX power trannie is used, and the 5 volt filament winding is not being used,
perhaps that could be wired in series and in opposite phase with the primary, which would "buck" the line voltage and deliver something closer to 120 VAC to the primary. Typical line voltages I've encountered range between 123 and 127 VAC. Of course, this does not apply to the PA-774.

The ICL-90 gives a few VAC drop as well, so that should help ease the prinary voltage down to a more reasonable level.

/ed brown in NH

[Shannon Parks]

Keep forgeting to mention this, but maybe leaving the filaments on isn't the best idea. I don't think you'll gain much leaving the amp in standby mode for long periods. Just my 2 cents.

Shannon

[EWBrown]

I just finished Rev C #2, using an "Uncle Ned" PA774 replacement and Hammond 1620 OPTs. The chassis is a 12X8X2 painted steel hammond.
There is a noticeable 60 Hz mechanical "hum" coming from the 774, I haven't yet determined if it is the tranny just vibrating, or if the residual magnetic field is getting picked up by the steel chassis and making it vibrate. THis mechanical "hum" is quite faint, not really loud enough to be annoying, but it can be felt by touching the amp chassis anywhere.
Prior to this I have been using aluminum chassis and never had this situation. There is no hum in the speakers.

I'll try putting a thin rubber piece under the PA774 and see if this goes away. BTW, the Dynaco ST-35 and SCA-35 also have a similar mechanical "hum" they both have steel chassis, and the cage top comes quite close to the top of the transformers.

One more thing (learned by experience), if yu plan to use one of the Hammond 12X8 top cages, a minimum clearance of 3/8 inch is required front and back, as the chassis has an inside "lip" along the front and back, for mechanical strength.

/ed brown in NH

[kidjan]

Bah. It happened again.





So, back to square one: what can I do to troubleshoot this bad-boy? I won't have time to mess with it till this weekend, but any sort of advice is much appreciated. I have a feeling I've done something very, very wrong with the board and it's causing me to fry either the IRCL or the transformer (I don't know which is bad, but I suspect it's one of those).


Thanks for everyone's input up to now--I'm sure if I beat my head on it for a while, I'll get it working right. It's a shame too, because going back to my old rotel is somewhat of a chore now....

[Shannon Parks]

Bah. It happened again.

Could you mail the whole thing (minus tubes) to me? Bad tranny and all? Thanks,

Shannon