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PostPosted: 28 Feb 2021, 10:05 
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Joined: 24 Oct 2010, 07:05
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Hello Jeff,
I think you have a short voltage sage at the moment the relay is triggered. It is unlikely that this has anything to do with the rise in the potential of the heater circuit. Do you use diodes as indicated in Bruce's diagram (reverse diodes to suppress the induction of the relay coil and a diode to suppress the reverse pulse to pin number 3)? If you use the LM555, I recommend connecting pin 4 to the positive power line, and pin 5 through a small capacitor (~0.01 UF) to the negative line.


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PostPosted: 28 Feb 2021, 16:04 
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Joined: 13 Apr 2020, 19:31
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Hi Poty,

Thanks for the reply. I do have the diodes as shown in Bruce's diagram. I have added the modifications you recommended to the 555 and the relay works intermittently, but mainly it still has the same issue. You mentioned a voltage sag, my bridge rectifier is only rated for 6A maybe that's my issue?


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PostPosted: 01 Mar 2021, 14:58 
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Joined: 24 Oct 2010, 07:05
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No, the rating of the bridge rectifier is no suspect.
The quality of heaters voltage can be controlled by any multimeter. But I have another idea: put an electrolytic capacitor (1000u or more) across power lines near the 555 just for experiment. If it helps the problem is in power, if not there may be either cold joint somewhere near 555 or broken 555.


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PostPosted: 04 Mar 2021, 22:39 
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Joined: 13 Apr 2020, 19:31
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Tried the cap and I still have the same issue. I've swapped the 555 to my working unit and it works fine. I've checked for cold solder joints and everything looks clean. As I mentioned previously, if I disconnect the 360 Vac, the relay pulls in every time without issue. I am still thinking something on the high voltage side is causing my issue, just can't seem to find it.


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PostPosted: 05 Mar 2021, 11:38 
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High voltage side? Mmm... Have you tried to use tubes from the fully working unit in the problematic one? I mean there may be some cathode-heaters leakage that works that way.


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PostPosted: 05 Mar 2021, 12:38 
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Joined: 04 Jun 2008, 20:59
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Location: Arizona, USA
Hi, You have an advantage. One amp works. So it is a matter of finding out what is different between them. There are several possibilities. I figure the simplest is to look carefully at the layout. We will get to actual troubleshooting later. It is possible that when the relay activates it is causing a spike in a circuit. It will of course cause a huge initial flow of current into the filters. This will last for only a short time but might be enough to induce a spike somewhere into the timer circuit. This could result in a timer reset. A B+ wire or component too close to a portion of the heater circuit might do this. What happens if you wait for a second timing delay? Does it reset again? What happens if you jumper the terminals on the relay and apply the B+ at power on and ignore the relay action? Does the amp work ok?

To troubleshoot the amp (really anything) there are two ways I like. The tubes need to be in place for all the following procedures. First with all power off and disconnected and any capacitors discharged use an ohm meter. Measure the resistance to signal ground of each amp terminal by terminal looking for differences. Be aware that circuits with capacitors will take time to charge up and may even not measure the same. When you find a difference look for what is causing it. You can use the same type of procedure by measuring DC voltages with the amps powered up (assuming the second one will power up if you jumper the relay). Look for differences. Some will naturally be there. The midpoint in the driver anode to cathode connection will often be off. It ought to be about ½ the B+ applied to the upper anode though. I have seen +/-20% and it is fine. Zero or nearly full B+ is not. BEWARE that the voltages are potentially lethal and use precautions when measuring the amps under power.

Good listening
Bruce

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PostPosted: 07 Mar 2021, 08:51 
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Hi Bruce,

The layout on both are exactly the same. I used circuit boards in my design (I have attached a photo for reference). One concern I have is the fact that some of the heater traces run close to the traces for B+, but this is the case for both the working and non working unit. Before adding the Cap on the 555 from Pin5 to Gnd, if I check voltage from ground to Pin5 the relay would engage and the amp worked perfectly fine. I did clean up some of the wiring underneath the board, it was a bit messy compared to the working unit. Another concern I have is the fact that I used an IC socket for the 555 timer; I am considering just soldering it in directly.

For troubleshooting I have checked every single point possible to ground on both units and I am getting the same values everywhere. Next step will be to measure voltages with both units powered up. Just working on a way to bypass the relay. I'll let you know what I find.

Fingers crossed I don't have to redesign the circuit boards, de-solder and rebuild...... :eek:

Thanks again Bruce and Poty for all your help, it is greatly appreciated!

Jeff


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PostPosted: 07 Mar 2021, 21:19 
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Hi, I don't seen anything startling in the photo. I would probably move the AC mains wires away from the IC and relay. I like to run them close to the chassis to minimize hum but usually in the power amps it doesn't make a lot of difference as long as the signal input wires are not nearby. BTW I use IC sockets all the time. No problems. Does the IC reset the relay if you wait through a second timing cycle or is the output still energizing the relay and the relay not allowing power through? Knowing which one is the problem should make it easier to track down the problem. BTW ...you didn't use the CMOS version of the 555 as they would probably misbehave, only the standard ones should be used.

Good listening
Bruce

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PostPosted: 09 Mar 2021, 21:44 
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Hi Bruce, The delay times out and the relay "clicks", but does not seem to allow power through and then the timing starts over again. This will happen over and over again with the relay clicking every 60 secs if I leave the power applied. The voltage on pin 3 never seems to drop, but that might be hard to see with a multimeter. Voltage on pin 6/7 from the RC timer drops and then starts building up again. Used a standard LM555 Timer.

Thanks again,
Jeff


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PostPosted: 10 Mar 2021, 09:36 
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Hi, ignoring the possibility of a bad 555......the circuit is doing a set/reset or if you wish a long term oscillator. So first check for short circuits or solder bridges around the IC . Next verify that the diodes at the relay are in the correct polarity and do not have bad solder joints. If you don't find any problems then hook pin 4 to the +12 volt source (at the socket is fine) and put a 0.1uf (any voltage over 16 V) from pin 5 to the - 12 volt side (at the socket is fine). The first prevents unwanted triggering and the second eliminates the possiblity of noise triggering the IC. If none of this makes a difference substitute a different 555 in the circuit. Let us know what happens.

Good listening
Bruce

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