hopeless problem with a transformer (not Farnell) - Page 1
Forum section: electronics / general

author: Grapsus
author_rank: Regular Contributor
timestamp: December 05, 2014, 07:02:15 pm
content: Hello,Last week I ordered a mains transformer from Farnell (France) in order to repair a guitar amplifier. I was sure that the problem was the mains transformer, the secondary had huge resistance and the amp worked fine when I powered the circuit directly by a lab supply. I found the schematics online : the mains transformer has 230 Vac input and 32.8 Vac output with a center tap (2 x 16.4 Vac). It didn't look like a common value and I didn't find an exact replacement, so I decided to go with a 2 x 15 Vac transformer to stay on the safe side. I found a nice toroidal transformer on Farnell which could even handle more power than the original transformer (50VA vs 38).I carefully wired the new device in place and double checked against the datasheet that the two windings were correctly joined to form a center tap. When I powered the amp, boom ! many active devices (mainly op amps and a power audio IC) released the magic black smoke. I measured the transformer output again and again with all of my DMMs : each winding was producing 18 Vac instead of 15. The transformer had a sticker which clearly said "Primary : 0-230V, Secondary: 0-15V" and my mains is 230V so WTF ?!I wrote a mail to the Farnell support complaining about the situation and saying that I didn't even care for a replacement since the board I intended to repair was now too damaged. I just wanted them to control their supplier and give me an explanation. The guy cynically directed me to a replacement form online. I filled in the form but I replied to the guy that I didn't care about a replacement. Today a UPS guy came in without any warning, delivered a new transformer and took the old one. Just for fun I measured the new transformer : 18 Vac !!! On the sticker it says that it was produced on the same day at the factory and it has the same "QC 22" sticker so I think they come from the same batch (I wonder what their QC procedure is if they don't even check the output voltage).The situation is just hopeless, what do you think I can do so that Farnell actually understand the problem, check their supplier etc. instead of hiding behind their return procedure ? Did you encounter similar problems ?I agree that I might have checked the transformer before using it, but Farnell is a well-known supplier, it's not like I ordered a low cost piece of junk from eBay. If we take this direction then we should check all the components we use so it's not really viable.Here is the transformer I ordered :http://fr.farnell.com/jsp/search/productdetail.jsp?CMP=i-ddd7-00001003&sku=1675058Edit:I was being a fool for complaining to Farnell, everybody agree that 18 Vac is within spec. Now I'd like to understand what went wrong.Here is the schematic :http://support.fender.com/schematics/guitar_amplifiers/Frontman_15G_15R_schematic.pdf

author: gibbled
author_rank: Regular Contributor
timestamp: December 05, 2014, 07:08:38 pm
content: 15vac is the output voltage under load. Sounds like the measured 18vac was with no load on the transformers output.

author: amiq
author_rank: Regular Contributor
timestamp: December 05, 2014, 07:08:45 pm
content: Well, it does say the typical regulation is 14% - you should have checked what it actually was.

author: nctnico
author_rank: Super Contributor
timestamp: December 05, 2014, 07:10:29 pm
content: An unloaded mains transformer always produces a higher voltage than it's rated voltage.There is probably a problem in the power supply section of the amplifier. Perhaps a shorted regulator? A few volt difference shouldn't be a problem because the mains voltage can vary a lot (officially between 216 and 253V) so the original transformer could also output 18V in a worst case condition. Besides that the AC value is RMS. The peak voltage is 21V. If a regulator has failed I can imagine this is too much for some circuits.

author: lewis
author_rank: Frequent Contributor
timestamp: December 05, 2014, 07:13:24 pm
content: Nothing wrong with the transformer. Transformer voltages are quoted under full load, and the output voltage will be higher at open-circuit or partial load. This is known as 'regulation' and is quoted in the datasheet for the transformer. It is the percentage change in output voltage from no-load to full-load. Your particular transformer has a regulation of 16% which means that you can expect the output voltage to rise by 16% at open circuit. So that's just over 17V already. Factor in the mains voltage variance and your 15V transformer can easily output 18-20V.The most likely cause of the 'boom' was the fact that the amp was still faulty. Something had to cause the transformer to die in the first place. Powering the unit off a bench supply is a good start, but you need to take into account the transformer regulation and rectification and smoothing process to provide an accurate DC voltage. If the transformer is 15V, you can have upto 18V AC. Rectify and smooth that and you get 25V DC. Powering off 15V DC might not tease out a fault that 25V would.Edit - empty thread when I replied, then BAM! Three replies as I was typing.

author: mikeselectricstuff
author_rank: Super Contributor
timestamp: December 05, 2014, 07:18:49 pm
content: Quote from: amiq on December 05, 2014, 07:08:45 pmNot to mention that mains voltage can vary by at least 10%

author: Grapsus
author_rank: Regular Contributor
timestamp: December 05, 2014, 07:31:47 pm
content: Thank you for your replies ! I'm absolutely not an expert in power electronics. My mains is 234V (so around 2% higher) and I just tried to load the transformer up to 10W with big resistors, the output moved from 18.1 Vac to 17.8 Vac. With 14% regulation, the worst case seems to be 15 * 1.14 * 1.02 (mains difference) = 17.4 Vac. What is the general practice regarding the voltage indication for transformers ? maximum voltage under load ? maximum without load ? or minimum under load ? Is it still possible that they put the wrong label on this transformer ?

author: mrflibble
author_rank: Super Contributor
timestamp: December 05, 2014, 07:35:57 pm
content: Quote from: Grapsus on December 05, 2014, 07:31:47 pmEverything is possible, but far more likely is that it is the correct label and you just experienced a somewhat expensive lesson.

author: wraper
author_rank: Unknown
timestamp: December 05, 2014, 07:37:48 pm
content: Quote from: Grapsus on December 05, 2014, 07:31:47 pmThere is typical regulation in the datasheet, not maximum.

author: Monkeh
author_rank: Super Contributor
timestamp: December 05, 2014, 07:38:28 pm
content: The regulation value istypical, not maximum.Welcome to reality, not paper: Everything has tolerances.It's likely that the transformer would have been perfectly suitable, and the circuit you connected it to was already damaged (and is what damaged the old transformer).

author: IanB
author_rank: Super Contributor
timestamp: December 05, 2014, 07:38:38 pm
content: Quote from: Grapsus on December 05, 2014, 07:02:15 pmTo repeat what everyone else has said, there doesn't appear to be any problem with the transformer at all. It is completely to be expected that a 15 V transformer will produce 18 V when measured without a load. All small transformers work this way.It seems your amplifier blew up because it contained an uncorrected fault, not because of any problem with the transformer.

author: IanB
author_rank: Super Contributor
timestamp: December 05, 2014, 07:43:49 pm
content: Quote from: Grapsus on December 05, 2014, 07:31:47 pmRoughly speaking, it means "this transformer will produce no less than 15 V under expected variations in load and mains supply voltage". For instance your mains could go down to 220 V and you could put the maximum load of 50 VA on the transformer and you should still get 15 V out of it.General practice is that incoming voltages are regulated at the point of use. Your amplifier would have taken the transformer voltage and rectified it, smoothed it and regulated it before feeding it to other components. If there was some fault in the regulation circuit then bad things would happen.

author: Grapsus
author_rank: Regular Contributor
timestamp: December 05, 2014, 07:44:52 pm
content: Ok, thank you for taking your time to explain this to me.Just for my curiosity : the datasheet doesn't specify a maximum value, how are you supposed to design a circuit with that ?

author: Zero999
author_rank: Super Contributor
timestamp: December 05, 2014, 07:46:22 pm
content: The transformer will give 30V out with 230V on the primary and an 18 Ohm resistor on the secondaries connected in series.The maximum value is simply the rated voltage, plus the regulation factor.

author: Grapsus
author_rank: Regular Contributor
timestamp: December 05, 2014, 07:56:16 pm
content: Ok I realize that I was a fool for complaining to Farnell. But still I wrote to their technical support, they could have said to me that the device was within spec instead of sending the exact same one...

author: Rasz
author_rank: Super Contributor
timestamp: December 05, 2014, 11:21:22 pm
content: Farnell is not geared towards selling to amateurs. They expect you to know better.

author: iampoor
author_rank: Frequent Contributor
timestamp: December 06, 2014, 06:56:14 am
content: Quote from: Grapsus on December 05, 2014, 07:56:16 pmDont take this the wrong way, but its assumed you already know that......

author: Seekonk
author_rank: Super Contributor
timestamp: December 06, 2014, 08:57:45 am
content: This is a very common in electronics, results being quite different than expected. You need to step back andevaluate what you have done. Electrons always know what they are supposed to do. We can only make a good guess. Schematicsoften have mistakes in terminology. It could be that that 16V was center tapped and that you have applied double the voltage.This seems reasonable since multiple IC's went up in magic smoke. You have the schematic, so tell us the what those partswere. It seems a possibility that the operating voltage of them was greatly exceeded. In any case it is unlikely such asmall variation of transformer voltage would cause the problem.

author: iampoor
author_rank: Frequent Contributor
timestamp: December 06, 2014, 09:21:47 am
content: Quote from: Seekonk on December 06, 2014, 08:57:45 amAnd if it did....the amp would be destined to fail!

author: Grapsus
author_rank: Regular Contributor
timestamp: December 06, 2014, 10:37:48 am
content: Quote from: Seekonk on December 06, 2014, 08:57:45 amI'll be more carefull next time I deal with a part of electronics I don't know, I usually use only low power stuff and digital components.Here is the schematic :http://support.fender.com/schematics/guitar_amplifiers/Frontman_15G_15R_schematic.pdfThe three wires comming from the transformer are in section C8, P6 is the center tap and P5 and P7 should measure 16.4 VAC at operating point.The big TDA2050 literally vaporized and many AOPs smoked too.

author: Chris Jones
author_rank: Regular Contributor
timestamp: December 06, 2014, 11:56:12 am
content: To avoid having this problem in future, you might like to invest in the purchase of a Variac. That allows you to gently turn up the mains whilst monitoring the supply rail voltages, input current, etc. as you do it. The first sign of things going to unsafe voltages or getting hot, you can turn it down again, usually with no damage.(Using a Variac doesn't necessarily help for switched mode power supplies though - those can stay off until the mains voltage is turned up quite high, and then start all of a sudden and blow up, or poorly-designed ones can even be more likely to blow up when the input is brought up gently, because they draw constant input power and therefore draw unexpectedly high currents when the mains is lower than normal.)

author: Andy Watson
author_rank: Super Contributor
timestamp: December 06, 2014, 12:31:46 pm
content: My guess would be that one of the bridge rectifier diodes failed (CR7-10) short-circuit, leading to overheating of the original transformer. A shorted diode would not have shown-up when powered by your lab supply. The new transformer had the capability to drive AC into amplifier's DC supply - releasing the magic smoke.