Replacement toroidal transformer for EAR834 phone amp - Page 1
Forum section: electronics / repair

author: solarbot
author_rank: Regular Contributor
timestamp: December 26, 2020, 06:26:00 pm
content: 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.

author: SpecialK
author_rank: Regular Contributor
timestamp: December 26, 2020, 09:21:18 pm
content: I think they have connected the two 115V primaries in series for 230V use.

author: SpecialK
author_rank: Regular Contributor
timestamp: December 26, 2020, 10:01:27 pm
content: eBay seems to have EAR834 transformers.

author: solarbot
author_rank: Regular Contributor
timestamp: December 27, 2020, 01:35:17 pm
content: 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?

author: SpecialK
author_rank: Regular Contributor
timestamp: December 27, 2020, 04:58:31 pm
content: Do you have continuity on all the windings?

author: SpecialK
author_rank: Regular Contributor
timestamp: December 27, 2020, 05:11:43 pm
content: https://www.ebay.ca/itm/50VA-HIFI-copper-O-type-transformer-0-250V-0-6-3V-for-tube-preamp/143370746861?hash=item21618ffbed:g:97kAAOSwMT5da5Lz

author: solarbot
author_rank: Regular Contributor
timestamp: December 27, 2020, 06:30:34 pm
content: 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?

author: SpecialK
author_rank: Regular Contributor
timestamp: December 27, 2020, 06:59:56 pm
content: 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.

author: Audiorepair
author_rank: Frequent Contributor
timestamp: December 27, 2020, 08:38:12 pm
content: Quote from: solarbot on December 27, 2020, 06:30:34 pmThere'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.

author: SpecialK
author_rank: Regular Contributor
timestamp: December 27, 2020, 10:17:49 pm
content: 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

author: Audiorepair
author_rank: Frequent Contributor
timestamp: December 27, 2020, 11:24:11 pm
content: You haven't actually told us why you think the transformer is now faulty.Have you tested it in any way to confirm this?

author: solarbot
author_rank: Regular Contributor
timestamp: December 31, 2020, 01:00:45 pm
content: 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/274075364171but purchased as a completed unit:https://www.ebay.co.uk/itm/Hiend-MM-Riaa-12AX7-Tube-Phono-amp-base-on-Ear-834-Turntables-preamp-L12-37/113527627475I'm hoping the whole board isn't fried by this event, its a pretty nice sounding amp.

author: SpecialK
author_rank: Regular Contributor
timestamp: December 31, 2020, 04:56:55 pm
content: 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.

author: innkeeper
author_rank: Unknown
timestamp: January 01, 2021, 05:13:00 pm
content: this may work for youhttps://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

author: solarbot
author_rank: Regular Contributor
timestamp: January 02, 2021, 08:43:46 pm
content: 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.

author: solarbot
author_rank: Regular Contributor
timestamp: January 02, 2021, 09:03:10 pm
content: 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-12ax7-7025Thanks

author: solarbot
author_rank: Regular Contributor
timestamp: January 02, 2021, 09:24:16 pm
content: 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?

author: Audiorepair
author_rank: Frequent Contributor
timestamp: January 02, 2021, 09:25:58 pm
content: 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.

author: solarbot
author_rank: Regular Contributor
timestamp: January 02, 2021, 09:48:32 pm
content: 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 :-)

author: Audiorepair
author_rank: Frequent Contributor
timestamp: January 02, 2021, 10:38:19 pm
content: I think you need to stop jumping to conclusions.Logic and proof works better.

author: SpecialK
author_rank: Regular Contributor
timestamp: January 02, 2021, 11:51:51 pm
content: Quote from: solarbot on January 02, 2021, 09:48:32 pmSo 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.

author: solarbot
author_rank: Regular Contributor
timestamp: January 03, 2021, 09:42:06 pm
content: 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

author: Audiorepair
author_rank: Frequent Contributor
timestamp: January 04, 2021, 07:09:34 pm
content: 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.

author: solarbot
author_rank: Regular Contributor
timestamp: January 05, 2021, 05:02:05 pm
content: 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 ohms2 tubes: 1.54 ohms1 tube: 3.04 ohms0 tubes: no continuityI 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.

author: solarbot
author_rank: Regular Contributor
timestamp: January 05, 2021, 05:12:18 pm
content: I've just watch this:which confirms what you say :-)

author: Audiorepair
author_rank: Frequent Contributor
timestamp: January 05, 2021, 07:57:25 pm
content: Quote from: solarbot on January 05, 2021, 05:02:05 pmThat all looks perfectly normal to me, there is no short.

author: solarbot
author_rank: Regular Contributor
timestamp: January 18, 2021, 05:22:36 pm
content: Many thanks, I guess I will replace the transformer and hope that the bad capacitor was the cause as I can't find any other issue..... do you think thats a foolish move?

author: SpecialK
author_rank: Regular Contributor
timestamp: January 18, 2021, 09:23:00 pm
content: People who reapir this stuff more regularly would probably use a variac to bring the voltage up slowly and watch for excessive current draw. Or even use a "dim bulb tester".Since that's probably not very practical, I suggest buying an energy meter or wattmeter. They are cheap and have other practical uses areound the house. Plug the "repaired" preamp into one of thes and observe wattage on power up. At any time if the consumption goes much over 50W unplug to avoid damage to the transformer and then continue to troubleshoot.https://www.amazon.co.uk/Lowenergie-Monitor-Electricity-Electric-Monitoring/dp/B00G955V0E/ref=sr_1_4?dchild=1&keywords=watt+meter&qid=1611004622&refinements=p_76%3A419159031&rnid=419157031&rps=1&sr=8-4

author: Audiorepair
author_rank: Frequent Contributor
timestamp: January 19, 2021, 08:09:38 am
content: I would suggest you do use the mains lightbulb in series, and run the pre-amp without any valves in to test.Measure the HT and heater voltages at the valve sockets, and leave it for a while.Without the valves there will be very little current draw, so the bulb probably won't even glow, and the HT voltages will be close, if not higher, than those expected with the valves in place, so a good burn in test perhaps.Then try it with the valves in place, and again leave it running a while, though the HT and heater voltages will probably now be slightly lower than normal, as the bulb will probably be soaking some power and glowing slightly due to the heater current draw, so the mains input voltage to the pre-amp will be reduced.The bulb should prevent any damage to the transformer should anything on the PCB short out, and you can safely leave it, just keeping an eye on the glow from afar.And then you will have a "dim bulb test" for any future projects.Quote from: solarbot on January 18, 2021, 05:22:36 pm

author: solarbot
author_rank: Regular Contributor
timestamp: January 25, 2021, 06:01:04 pm
content: Thanks for the advice folks, much appreciated - the transformer has arrived so I will proceed following your valuable guidance :-)

author: SpecialK
author_rank: Regular Contributor
timestamp: January 26, 2021, 04:25:56 am
content: I think you are going to do alright. Still would like to hear your result.

author: solarbot
author_rank: Regular Contributor
timestamp: January 29, 2021, 02:28:13 pm
content: Progress is being made :-) I have it all back together with the new transformer installed, a bulb on the live supply along with a clamp meter. The bulb lights up breifly which is I guess the caps charging, the clamp meter measures about 0.1A and there is no smoke or heat :-)Volatges at the valve sockets are:Between pins 4&9 and 5&9 = 8.4V, all the same at each socket.Between pins 1&3 and 6&8 = 303V, 278V and 285V, all slightly different at each socket?I'm guessing these will be closer to the expected values under load, just building up the courage to insert the valves!

author: fcb
author_rank: Super Contributor
timestamp: January 29, 2021, 04:06:31 pm
content: Good luck with the repair. Sadly Tim de Paravinci died last December.The 834 was cloned ALOT by people, so there should be ample help out there to bring it up.

author: Audiorepair
author_rank: Frequent Contributor
timestamp: January 29, 2021, 06:26:25 pm
content: Quote from: solarbot on January 29, 2021, 02:28:13 pmOnce the valves are in place you will read very different HT readings at the valve base, as pins 1 & 6 are connected to the HT by resistors.Without the valve in place you will read the power supply voltages before these resistors.Once the valves are in place there will be current drawn through these resistors and thus voltage lost across them, so the actual voltage at the valve pin will be lower, maybe much lower, so don't panic!There is a diagram here that shows expected voltages on this circuit, I don't know how reliable this is, but it just goes to illustrate my point.Look at the top HT line from right to left.B+ goes to 2K and 100uF to ground. Then 20k and 100uF to ground. Then 200k and 100uF to ground.This gives 3 different HT voltages to the 3 valves, but ONLY when the valves are in circuit and drawing current.Readings without the valves in place will put these 3 voltages much closer together than the real world, and can be very misleading/confusing.

author: solarbot
author_rank: Regular Contributor
timestamp: February 01, 2021, 03:01:51 pm
content: So, last night I got to listen to OMD Live at the Hammersmith Apollo for the fitst time thanks to all your help and patience - many, many thanks - I've learned quite a bit and repaired my amp so couldn't ask for more than that.In summary, the 415V caps used in this build must have been old stock and dried out after just a year. Only one of the caps was bulging but both were replaced, all the other components remained unchanged. The failure of this capacitor caused one of the secondaries to melt which of course needed replacing - so 2 new caps and a new transformer fixed the amp nicely and I can now remember just how sweet it sounds :-)Thanks again folks