What toroidal transformer should i order to replace this? - Page 1
Forum section: electronics / beginners

author: jaunty
author_rank: Regular Contributor
timestamp: October 29, 2020, 03:30:29 am
content: I have an old (early 80s) vintage digital delay (music gear) and am finding a dead short on one of the secondaries (measuring 0.1 ohms or less between each end of winding and centre tap on LOWER secondary indicated on schematic. It looks to me as though each of two primaries has it's own secondary and i'm wondering if i can even replace this with a toroid (or would i need two?) - i just like toroids because they are quieter and seem less 'cranky' than E-frames.Now given the following schematic what specs would you look for?I considered repair but thought it might get pretty difficult assuming laminations are glued together.or should i find someone to rebuild this for me? (replacements are definitely unavailable from the OEM)

author: bob91343
author_rank: Super Contributor
timestamp: October 29, 2020, 06:10:56 am
content: Looks like you need two 30V CT windings at whatever current is being drawn.I wouldn't think the laminations are glued together. It wouldn't hurt to get a bit rough and try to take it apart.However, most faults in transformers occur near the terminations. If you have a shorted winding that should show up in an inductance measurement. Maybe it's just the diodes that are shorted. To find a short on both halves of a winding is exceedingly rare.

author: eblc1388
author_rank: Frequent Contributor
timestamp: October 29, 2020, 06:17:25 am
content: Quote from: jaunty on October 29, 2020, 03:30:29 amThe winding you have referred to is a 7Vx2 C.T., 6.3A winding, and will definitely have a very low winding resistance. It is not easily measured accurately using a common DVM. I also doubt that the short will be to both windings and the center tap. Dead short on the secondary winding of a transformer are rare because of the low voltage and method of placing the turns on the coil former. You problem might be in the circuit after the transformer.I suggest you powered up the transformer alone to find out if there is any problem with the windings. With the possibility of a faulty transformer, I would first put in series a high power appliances, such as either a 100W filament lamp, a toaster or an iron with the mains wire before going to the transformer. With this setup, a short of the winding will provide an immediate indication on the lamp, toaster or iron.

author: jaunty
author_rank: Regular Contributor
timestamp: October 29, 2020, 07:27:33 am
content: well i had the xformer disconnected totally ... measuring between the two ends and centre tap of the winding i get 0.1 ohms or so on the DMM - i know DMM resistance measurements aren't too conclusive - but compared to the other secondary which puts out similar loads - that measures 2.8 ohms ... so i figured it would be in the ballpark if there weren't an issue with it (?) even though transformers are voodoo (at least to me!)i guess it certainly wouldn't hurt to try to take the thing apart.

author: jaunty
author_rank: Regular Contributor
timestamp: October 29, 2020, 07:31:57 am
content: already did that before posting. it's the first thing i do. it was an older and abused looking piece of gear sold to me 'for parts' - so the first thing i do is a dim bulb test THEN sort out the PS - i found no issues in any other part of the PS - bridge diodes fine - put in all new caps etc - still lit up the bulb. lights up a 60W bulb 'orangish' but not full-on bright. the only thing left really was the xformer. Even a 7V rail supply should STILL measure at least 1 ohm plus, no? can i ask how you came up with the 7V figure? esp when this supplies multiple rails of 15, 12 and 8v?

author: eblc1388
author_rank: Frequent Contributor
timestamp: October 29, 2020, 04:28:03 pm
content: Quote from: jaunty on October 29, 2020, 07:31:57 amIf there is a short on the secondary winding, the bulb should light up with nearly full on brightness.Quote from: jaunty on October 29, 2020, 07:31:57 amA 7V 6.3A winding has very low resistance, I would say probably well below 1 ohm. Let's say it is 1 ohm. Then the loss with 6.3A passing through it will be I2R= 6.3x6.3x1 = 39.7W so it is highly unlikely that the resistance is so high. More like 0.1 ohm or below to minimize the loss.The voltage test is one good method to determine if a transformer has shorted turns. You would need to isolate the transformer from the remaining circuit, power up the primary via a dim bulb and measure voltages on respective secondary windings to be certain. If the voltages are balanced then I don't think there is a shorted turn in it.I guess that the winding is 7v x2 or 7.5V x2 because it was marked on the schematic "+8V UNREG" at the output of one of the windings after rectification. See capacitor C5, input to the LM317K and the grey wire P1-1.

author: jaunty
author_rank: Regular Contributor
timestamp: November 03, 2020, 08:24:54 am
content: I guess this is what drives me a bit bonkers about dim bulb testers ... lovely idea though they are - i guess i had the impression it was a bit of an 'all or nothing' proposition - that if there was ANYTHING beyond the FAINTEST glow there was a fault in the connected circuitry or power supply.is there any sort of dictum - like the amount of glow being proportional to the current draw in a given device??I've also lost HOURS and HOURS trying to troubleshoot circuits attached to dim bulb testers, thinking there were fundamental problems with the power supply when in fact the supply just wasnt' getting full load.does anyone know of a great FAQ with all the ins and outs of dim bulb testers? I guess one should never try to troubleshoot with the tester connected?

author: mikerj
author_rank: Super Contributor
timestamp: November 06, 2020, 08:37:24 am
content: Quote from: jaunty on October 29, 2020, 07:31:57 amWhat voltage do you measure on the output of the secondary you suspect is faulty?

author: Doctorandus_P
author_rank: Super Contributor
timestamp: November 06, 2020, 04:13:27 pm
content: Testing power supplies with light bulbs is a very simplistic test, but can be useful none the less.The trouble you have is completely caused by your false assumptions.Just stop making those assumptions, and start making real measurements.Have you for example measured the (cold & hot) resistance of the bulb? Have you measured the primary voltage on the transformer with the bulb in series?Also, a 60W light bulb is not the best. If the device you test with it also uses 60W, then you will have a significant voltage drop over the light bulb. If you use a 200W light bulb the voltage drop over it will be much less. The higher the wattage of the bulb, the lower it's resistance, and the lower the influence on the device you're testing.A secondary transformer winding wich is designed for 7V 6.4A will have a much lower DC resistance than the 2.8 Ohm you think it has. The true DC resistance is very likely to be below what you can reliably measure with a normal DMM. 0.1Ohm sounds about right for a 1V sag under full load.A much better way is to simply connect it to mains voltage, and then wait some time. If the transformer gets very hot (when nothing is connected to the secondary windings) there is something wrong with it.

author: mkiijam
author_rank: Regular Contributor
timestamp: November 06, 2020, 09:17:20 pm
content: Steps to AC bliss.1. Buy a cheap Variac clone.2. Turn the AC voltage down all the way.3. Hook up everything including your favorite Fluke to measure AC Volts on the secondary.4. Turn Variac up slowly.5. Watch Fluke to see if volts is going up.If you got fancy and hooked up your back-up meter as a current monitor, then you could see if the current is going up as you turn the Variac up. Some of them have little current gauges on the front, some AC volt gauges

author: jaunty
author_rank: Regular Contributor
timestamp: November 10, 2020, 09:56:07 am
content: yes perhaps i was putting too much stock in the dim bulb tester - i'd seen someone comment (perhaps in this forum) that if there was ANY result at ALL (even the faintest light from the dim bulb tester) then there was a ground fault present. Certainly I'd tried dim bulb testing a number of pieces of equipment and found the result null - but perhaps not. I was just being overly cautious. There do not seem to be issues at the moment and i have been working with this piece of equipment without the bulb for at least a week now - troubleshooting other areas. though I am not at all uncertain that there aren't problems SOMEWHERE (perhaps a shorted chip somewhere- we will see what pops up)

author: srb1954
author_rank: Super Contributor
timestamp: November 10, 2020, 12:30:30 pm
content: Quote from: jaunty on October 29, 2020, 07:27:33 amIf you have an LCR meter another way to test a transformer is to measure the inductance of all the windings. The inductance ratio of any two windings should be related by the square of the turns ratio between those windings. The winding voltages aren't specified on your diagram so it isn't possible to determine the exact turns ratio. However, by taking an educated guess at the voltages, and hence the turns ratio, I would expect one of primary windings to be about 70 times the inductance of the lower voltage secondary winding or about 15 times the inductance of the higher voltage secondary winding.Another check is to measure the change in inductance of the primary winding when one of the secondary windings is externally shorted. For good transformer there should be a significant reduction in measured inductance when one of the other windings is externally shorted.If you have a major internal short in any winding you will tend to measure lower than expected inductances on all windings and the measured inductance will not be greatly affected by externally shorting other windings.