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Old 22nd February 2019, 07:08 PM   #1
phyciocc is offline phyciocc  United States
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Unhappy Problem (my fault!) with Elekit tu-8500

Last year I put together a tu-8500 preamp kit to mate with my tu-8200 (non-dx) kit. A couple of days ago I was playing aroundwith another amp and was swapping power cords, speaker cables etc.



The power cord of the TU-8500 (and of the TU-8200) are not three-prong but two-prong with the understanding (I think) that ground needs to be connected at the plug separately.



Long story short (no pun intended), my tu-8500 grounded itself to the case of the other amp through (you guesses) the ground lug in the back of the pre-amp. Big spark.


Needlessly to say, now the pre-amp does not work well anymore.

I looked for obvious burned components (besided the ground lug) but no luck. It still comes on, goes through the countdown and then power comes on. The sound is masked by a strong buzzing sound. The switch in the back to give 1x and 3x amplification is still effective. It points to a broken electrolytics in the power supply (I hope...), but is there anything else I should check first?
I really liked the duo together!
Thanks!!

Marco
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Old 22nd February 2019, 09:39 PM   #2
Hmavv is offline Hmavv  United States
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Its going to be a pain and some work..

Your going to have to test every component to make sure it still meets the specifications.. resistors, and caps. Your also going to have to replace all the IC packages with new ones, unless you can figure a way to test them properly.
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Old 8th March 2019, 09:12 PM   #3
phyciocc is offline phyciocc  United States
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It is alive!
I replaced a few things based on my meager understanding.

The DC voltages were good (the B+ at 140V and the filament at 12), but they looked a bit "noisy" on the oscilloscope. So I replaced a couple of caps in the B+ side, 3 of the diode bridge rectifiers and two voltage regulators, and a couple of Zeners (the caps were an overkill, as the original tested fine).
The one replacement that that had the most immediate effect was one of the voltage regulators. The quality of the BUZZZ changed immediately after that one.

And I could hear music below the buzzz.

Now the pre-amp is playing nicely, as far as I can hear.

But I do believe a problem persists.
The power cord that comes with the Elekits (both the Tu-8200 and the Tu-8500) are non-polarized with the ground connection at the power end (to attach to the socket screw, I think).

Well, if I use the original power cord with the TU-8500 the BUZZZZZZ returns. With a regular three-prong polarized ground plug, it works fine.

Any idea?

Marco
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Old 9th March 2019, 05:35 AM   #4
Hmavv is offline Hmavv  United States
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Which cord were you using when you shorted???
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Old 9th March 2019, 06:11 AM   #5
jean-paul is offline jean-paul  Netherlands
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It is very hard to understand what happened with such a vague description. There should be no difference when using power cords with 2 or 3 wires. That is when things are done according electrical rules/regulations. Devices that have a 3 prong IEC inlet need a 3 wire power cord for PE and should never be used with 2 prong cables. In fact 2 prong IEC connectors do not even fit physically in 3 prong IEC inlets to avoid using non-PE cabling with PE devices. Your TU-8500 with 3 prong (so it needs PE) IEC inlet grounded itself to the case of the other amp through the ground lug in the back of the pre-amp (how can that happen?) which seems to indicate a very serious safety issue. If one is so clever to hamper/modify 2 prong/2 wire connectors for use with a device that has a 3 prong IEC inlet with PE then one is doing dangerous stuff.

Simply said: a device that has a PE pin or a IEC inlet with PE needs that PE pin to be connected to the mains PE. This ensures the case/chassis to be at PE potential preventing electrocution when something is wrong in the device. A loose mains wire that touches the casing will then cause a short to PE and either a fuse or a breaker will switch off mains voltage. No electrical shock, no pain, no frustration.

When native English speakers ask something there very often is a need to ask several times what exactly the issue is. I would like to advise to use a better problem description assisted with pictures/drawings if one wants effective help with problems.
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Last edited by jean-paul; 9th March 2019 at 06:34 AM.
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Old 9th March 2019, 04:29 PM   #6
phyciocc is offline phyciocc  United States
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Hello:
This is what the original power cord of both my Elekits looks like. The receptacle is standard IEc 320 receptacle. The Power cord is three-wire but the plug has two prongs that are not polarized (they will go in the power socket in the wall in any orientation) with earth ground separate, to be attached to a screw in the wall socket if needed).

With this cord, Buzzzz.

With a standard US power cord (three prongs plug, polarized and earth ground connection (IEC 320 to Nema 5-15P), the buzz goes away.
As I said, something is still not quite right. But the preamp works.
Marco
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File Type: jpg Powercord.jpg (15.6 KB, 44 views)
File Type: jpg power1.jpg (21.9 KB, 46 views)
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Old 10th March 2019, 11:16 PM   #7
Hmavv is offline Hmavv  United States
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When you are using the original cord with the grounding wire, you are grounding it? And if so how?

I can tell you from my experience that this pre-amp needs a really good ground, it picked up a ground loop issue I wasn't aware of with my last pre amp.

The screws on my plate did not have good contact with ground so I'm using a blank plug with a ground prong.
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Old 10th March 2019, 11:25 PM   #8
phyciocc is offline phyciocc  United States
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With the original cord (and before the spark) it did not needed to be ground at the external contact. It does now. I am not sure I completely understand why it needs the extra ground now, but I may have an idea. I'll follow through and report.
Marco
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Old 11th March 2019, 04:25 AM   #9
Hmavv is offline Hmavv  United States
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Please do! I'm really interested in what you find!
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