Author Topic: Replacement toroidal transformer for EAR834 phone amp (Read 4186 times)

solarbot · « **on:** December 26, 2020, 06:26:00 pm »

Hi, I have an EAR834 phono amplifier which has failed, one of the 450V, 100uF caps on the AC supply side has failed which I think must have melted the toroidal transformer.... is that the likely cause? Replacing the CAP is easy enough but I'm not sure about the transformer. It is marked with I/V 0V--115V, 0V--115V and O/P 0V--250V, 0V--6.3V - see attached photo. I'm looking for a replacement on Farnell or similar but not sure of what would work as I can't find an exact match? The mains supply here is 240V AC so perhaps the transformer was wrong? Can anyone provide some guidance as I'm not sure what to look for? Many thanks.

SpecialK · « **Reply #1 on:** December 26, 2020, 09:21:18 pm »

I think they have connected the two 115V primaries in series for 230V use.

SpecialK · « **Reply #2 on:** December 26, 2020, 10:01:27 pm »

eBay seems to have EAR834 transformers.

solarbot · « **Reply #3 on:** December 27, 2020, 01:35:17 pm »

Thanks SpecialK, I've drawn out the wiring as per the attached which I guess confirms that your thought is correct in that the primary coils are wired in series? I've looked on ebay but none of the EAR834 transformers have a 6.3V output which I understand is the supply for the valve heaters?

SpecialK · « **Reply #4 on:** December 27, 2020, 04:58:31 pm »

Do you have continuity on all the windings?

SpecialK · « **Reply #5 on:** December 27, 2020, 05:11:43 pm »

https://www.ebay.ca/itm/50VA-HIFI-copper-O-type-transformer-0-250V-0-6-3V-for-tube-preamp/143370746861?hash=item21618ffbed:g:97kAAOSwMT5da5Lz

solarbot · « **Reply #6 on:** December 27, 2020, 06:30:34 pm »

Thanks for the ebay link, that looks to be an exact replacement :-)

I seem to have continuity across all 4 primary taps so I guess there is short between the 2 primary coils?

SpecialK · « **Reply #7 on:** December 27, 2020, 06:59:56 pm »

Not necessarily. The winding should have a pretty low resistance to a DC ohmmeter. It's just a long piece of wire. The turns present a very different impedance to AC.

Let's go back to the beginning of the story. We can try troubleshooting it with you.

Audiorepair · « **Reply #8 on:** December 27, 2020, 08:38:12 pm »

Quote from: solarbot on December 27, 2020, 06:30:34 pm

Thanks for the ebay link, that looks to be an exact replacement :-)

I seem to have continuity across all 4 primary taps so I guess there is short between the 2 primary coils?

There's an easy test for that.
Wire a 240v lightbulb in series with the primary, making sure all secondaries are unconnected.
If it lights continuously, the primary is toast.

It's otherwise quite difficult to tell with a multimeter whether the primary coils are shorted or not.

SpecialK · « **Reply #9 on:** December 27, 2020, 10:17:49 pm »

We need some background. There seem to be quite a few EAR834 phono preamps out there. Which version is this? Pictures would be very helpful.

I've noticed some (maybe all?) versions have a 285V regulator circuit using four zener diodes and a pass transistor. Is yours like this?

If you could reassemble I would test as such"

remove tubes and switch on
- 6.3V across pins 4 & 9 and 5 & 9 of each socket (if so transformer primary is okay)
- high voltage DC between 1 & 3 and 6 & 8 of each socket? (if not then maybe the regulator circuit has died)

other non-powered tests
- test the fuse(probably in the IEC mains connector) with a multimeter (continuity beeper)
- test the switch with a multimeter (continuity beeper)
- test the heater filaments of the tubes with a multimeter

Audiorepair · « **Reply #10 on:** December 27, 2020, 11:24:11 pm »

You haven't actually told us why you think the transformer is now faulty.

Have you tested it in any way to confirm this?

solarbot · « **Reply #11 on:** December 31, 2020, 01:00:45 pm »

Hi, Sorry for the slow reply, for some reason I didn't get the notification emails.....

The attached picture, i think, shows the secondaries are fried? As I think the 2no. 450V 100uF caps on the AC side were old stock (the manufacturers website said discontinued some 10 years ago) I wonder if the cap failure caused the melted transformer or perhaps the other way round? Only 1 of the 450v caps was bulging and bloated in the usual manner. The EAR834 board I have is this one: https://www.ebay.co.uk/itm/Finished-HiFi-ear834-MM-RIAA-Tube-Phono-Amplifier-Stereo-preamp-board-for-Audio/274075364171 but purchased as a completed unit: https://www.ebay.co.uk/itm/Hiend-MM-Riaa-12AX7-Tube-Phono-amp-base-on-Ear-834-Turntables-preamp-L12-37/113527627475

I'm hoping the whole board isn't fried by this event, its a pretty nice sounding amp.

SpecialK · « **Reply #12 on:** December 31, 2020, 04:56:55 pm »

Well the board should be easy to inspect for damaged traces.

The unit looks infinitely serviceable. None of the parts are unobtainium. The transformer being the most difficult part to obtain. Even that seems to be readily sourced.

So your job is to find out what component is shorted that caused the transformer to get melty.

Test the valves. There's quite a large possibilty that a tube went bad, more than any other component. In particular test the heater filaments. Each tube has two filaments, so that's six opportunities for there to be a dead short across the 6.3V winding. You will want to power these individually from a bench power supply with maybe a current limit set to 0.5A. I think they should settle at around 150mA draw once warm. If you can't do this, then replacing all the tubes probably is a good idea.

Test the rectifier diodes and the bridge rectifier.

Replace the transformer. Power the unit with no tubes instaled and see if you have 6.3V across heater pins and ~280V plate to cathode pins.

innkeeper · « **Reply #13 on:** January 01, 2021, 05:13:00 pm »

this may work for you
https://www.ebay.com/itm/Toroidal-transformer-50VA-230VAC-260V-0-12A-6-3V-1A-6-3V-2A-Weight-0-8kg/333626119667?hash=item4dadaa95f3:g:FOIAAOSwB6Re5l3f

solarbot · « **Reply #14 on:** January 02, 2021, 08:43:46 pm »

Thanks again, I've tested for shorts across the 6.3V power rail and all 3 tubes cause a short so I guess they have all blown! I will look at them more closley on the bench soon as you suggest, visually they look perfect! With all 3 valves removed there is no short on the 6.3V rail.

Thanks also for the transformer link, very useful. It looks better made than the original and is less expensive. As it has a single 230V primary I guess thats no problem as I'm on 240V here. The outputs are 260V-0.12A,6.3V-1A,6.3V- 2A and the oringal is 0-250v at 0.12A and 0-6,3v at 3.17A - would I simply wire the 2 6.3V secondaries in parallel to get 3A, just 0.17A shy of the original?

Happy new year to you all.

solarbot · « **Reply #15 on:** January 02, 2021, 09:03:10 pm »

The valve pin out is below, any idea which ones are the heaters I need to check?

The product page: https://www.jj-electronic.com/en/ecc83s-12ax7-7025

Thanks

solarbot · « **Reply #16 on:** January 02, 2021, 09:24:16 pm »

So, the 6.3V rectifier diodes all appear to be OK but the bridge rectifier has a short across pins 1 and 4 which I guess is faulty and perhaps gets the finger pointed at it as the source of the problem?

Audiorepair · « **Reply #17 on:** January 02, 2021, 09:25:58 pm »

It is highly unlikely the valve heaters would have failed short.

You are probably just measuring the normal few ohms they have at cold.

The heater is between pins 4 and 5, with a centre tap at pin 9.

Edit: Yes, if the bridge rectifier is shorted between pin 1 and 4 it is definitely toast.
You would need to take it off the PCB to be 100% sure, or at least cut one of those pins instead.

solarbot · « **Reply #18 on:** January 02, 2021, 09:48:32 pm »

Thanks, I will check the valves now I have that info, I hope they are ok.

The rectifier is now off the board and tests normal so the hunt goes on :-)

Audiorepair · « **Reply #19 on:** January 02, 2021, 10:38:19 pm »

I think you need to stop jumping to conclusions.

Logic and proof works better.

SpecialK · « **Reply #20 on:** January 02, 2021, 11:51:51 pm »

Quote from: solarbot on January 02, 2021, 09:48:32 pm

Thanks, I will check the valves now I have that info, I hope they are ok.

The rectifier is now off the board and tests normal so the hunt goes on :-)

So if the recifier tests good, then there is probably a short down stream. Which power supply is this? Do they rectify the 6.3VAC for the heaters? It seems they do, as there are a lot of electrolytic capacitors in the pictures I've seen.

Anyway, let's see some more pictures of you're disassembled unit.

solarbot · « **Reply #21 on:** January 03, 2021, 09:42:06 pm »

Hi, below is what the board looks like at the moment. I have removed 3no. 2200uF electrolytic caps, the rectifier and a 10uF electrolytic cap (still in place in the photo), all from what I think is the 6.3V heater supply. I am still measuring a 1 ohm short across the positive and negative though!... not sure what to look at next?

PS the back of the PCB is flipped to make it relate to the front

Audiorepair · « **Reply #22 on:** January 04, 2021, 07:09:34 pm »

Are you testing with the 3 valves in place as in the photos?

Those 3 (six, really) heaters in parallel could well measure less than 2 ohms.

solarbot · « **Reply #23 on:** January 05, 2021, 05:02:05 pm »

I am testing with the tubes in place, here are the resitances across the 6.3V rail as I remove them:

3 tubes: 1.04 ohms
2 tubes: 1.54 ohms
1 tube: 3.04 ohms
0 tubes: no continuity

I do know that one of the 450V 100uF on the 240V secondary puffed up, could this have caused the failure of the transformer? Difficult to measure the resistance across the cap but its somewhere about 0.6M ohms directly after discharge as compared to 4M ohms of the good cap.

solarbot · « **Reply #24 on:** January 05, 2021, 05:12:18 pm »

I've just watch this: which confirms what you say :-)


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Author Topic: Replacement toroidal transformer for EAR834 phone amp (Read 4186 times)

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