Transformer question

[Electron Tornado]

I have the chassis and board, etc of a 30W solid state amp that was described as having a bad transformer. The PCB looks OK and nothing looks fried, and I managed to find some schematics for it.

I tested the transformer per R.G. Keen's instructions:  http://www.geofex.com/ampdbug/pwrtrans.htm   and found the primary winding seems to be open (zero continuity) and the secondary seems fine (>1k ohms). 

Sooo, find a replacement transformer. The only info from the schematics is "Power Transformer 6099". Taking a look at the transformer that's installed gives:

Part # 5600-00 2135 (the "2135" is written by hand)
Pri:120VAC
Sec: 33 VCT
Date Code: written by hand is either "v414" or "0414"

The closest thing to a brand name is "Stars Tianjin"

Hmmm, neither the name nor the part numbers come up with any results from Google. Searching the usual parts sources for a transformer, I found this: http://www.alliedelec.com/search/productdetail.aspx?SKU=70181289  which is actually the only one I found with a 33VCT secondary. The power fuse on the schematic is listed as "~115V  T500mA", so I think I'll be safe with a transformer rated at 1A. Correct?

[iccaros]

what is the amp? I have one for a Line 6 30 watt I have no use for

[R.G.]

I have the chassis and board, etc of a 30W solid state amp that was described as having a bad transformer. The PCB looks OK and nothing looks fried, and I managed to find some schematics for it.

I tested the transformer per R.G. Keen's instructions:  http://www.geofex.com/ampdbug/pwrtrans.htm   and found the primary winding seems to be open (zero continuity) and the secondary seems fine (>1k ohms). 

It is likely that there is an internal temperature fuse that has opened. NO, do not go try to open it up and put in a new thermal fuse. It's not safe.

You're doing the right thing looking for a replacement. However, remember that the transformer probably did not randomly overheat. Something else failed that overheated it. Whether the board looks fried or not, assume that there is a shorted rectifier, bad filter cap, fried power transistor, something wrong internally, and act accordingly, or you'll have *two* burned out transformers.  

Searching the usual parts sources for a transformer, I found this: http://www.alliedelec.com/search/productdetail.aspx?SKU=70181289  which is actually the only one I found with a 33VCT secondary.

Notice that it may be that a 30Vct, 28Vct, 32Vct, etc. might work fine.

The power fuse on the schematic is listed as "~115V  T500mA", so I think I'll be safe with a transformer rated at 1A. Correct?

Hmmm. Maybe.

If the AC power line fuse is rated at 0.5A, then the input power is intended to be less than 500ma long term. That's 115V*0.5A = 57.5W. The output power it *could* put out is then 57.5W (if it was 100% efficient) or 57.5/33 = 1.74A. This is a foggy guess at things. In reality, it would use less than that, so it may be rated at something like 33Vct at 1.5A or so.

I would feel a little uncomfortable with a transformer rated at 33Vct at 1A. I think that is asking for a second transformer to fail. I'd pick one with output current of 1.5 to 2.0A rating.

And do go figure out what caused to fail.

Also look up "light bulb limiter" and worry about whether you can do the AC power wiring in the limiter and amplifier safely.

[Electron Tornado]

what is the amp? I have one for a Line 6 30 watt I have no use for

It's a Laney HCM-30. Laney is understandably tight-lipped when it comes to providing technical help via email.

It is likely that there is an internal temperature fuse that has opened. NO, do not go try to open it up and put in a new thermal fuse. It's not safe.

You're doing the right thing looking for a replacement. However, remember that the transformer probably did not randomly overheat. Something else failed that overheated it. Whether the board looks fried or not, assume that there is a shorted rectifier, bad filter cap, fried power transistor, something wrong internally, and act accordingly, or you'll have *two* burned out transformers.   

Searching the usual parts sources for a transformer, I found this: http://www.alliedelec.com/search/productdetail.aspx?SKU=70181289  which is actually the only one I found with a 33VCT secondary.

Notice that it may be that a 30Vct, 28Vct, 32Vct, etc. might work fine.

The power fuse on the schematic is listed as "~115V  T500mA", so I think I'll be safe with a transformer rated at 1A. Correct?

Hmmm. Maybe.

If the AC power line fuse is rated at 0.5A, then the input power is intended to be less than 500ma long term. That's 115V*0.5A = 57.5W. The output power it *could* put out is then 57.5W (if it was 100% efficient) or 57.5/33 = 1.74A. This is a foggy guess at things. In reality, it would use less than that, so it may be rated at something like 33Vct at 1.5A or so.

I would feel a little uncomfortable with a transformer rated at 33Vct at 1A. I think that is asking for a second transformer to fail. I'd pick one with output current of 1.5 to 2.0A rating.

And do go figure out what caused to fail.

Also look up "light bulb limiter" and worry about whether you can do the AC power wiring in the limiter and amplifier safely.

From reading your posts in another power supply thread, I was kind of afraid 1A might not be enough, which is why I asked. It seems odd that I only found the one with a 33VCT secondary. The amp isn't old and it doesn't use any exotic parts, so I'm surprised that that the transformer isn't more common. Of course, there's nothing that says that the transformer in there now wasn't an inadequate replacement for the original (possible, but doubtful).

From what I can see on the schematic, Vcc looks like it's only 22V, so a transformer with a different secondary does sound like a good option.

There are no glaring problems with any of the components on the pcb, but I've only looked at the component side so far. I can check for continuity in the paths for supply voltage and audio signal. I'll look up light bulb limiter as well.

[Electron Tornado]

R.G., is this the light bulb limiter you're talking about?

http://www.thegearpage.net/board/showpost.php?s=ef91e4a49b6ca3a68935290d7bccfd56&p=6843418&postcount=4

[Rob Strand]

From what I can see on the schematic, Vcc looks like it's only 22V, so a transformer with a different secondary does sound like a good option.

The original  33VAC ct  (= 16.5VAC-0V-16.5VAC) transformer will produce a dual supply of +/-23VDC under load.  That's the voltage you need.    That should give you about 30W into 8 ohm.  The voltages and powers agree with the amp spec and schematic.

The VA rating for the replacement should be about 65VA (say up to 80VA).    That's something like a 33V ct at 2A.  You might find a 32VAC ct (= 16VAC-0V-16VAC) off the shelf.  A 30VAC  (= 15VAC-0V-15VAC) is more common but you could lose some output power, you can compensate for this by using a larger VA transformer.

If you use a replacement you have to make sure the thing will fit  in the space available.  (You might not be-able to use the mount screws and the wires might be short too -you  can of course work around this.)

Why not get a price on a proper replacement?

[petemoore]

  Possibility of rain on a parade warning..sometimes it's easier to find something like that working than it is to find suitable replacements to make 'it' work, possibly needing more than anticipated. Already working carries a low possibility of never working, has a known cost, possibly even lower cost than replacement parts.

[Electron Tornado]

what is the amp? I have one for a Line 6 30 watt I have no use for

What are the specs on your transformer?

The original  33VAC ct  (= 16.5VAC-0V-16.5VAC) transformer will produce a dual supply of +/-23VDC under load.  That's the voltage you need.    That should give you about 30W into 8 ohm.  The voltages and powers agree with the amp spec and schematic.

The VA rating for the replacement should be about 65VA (say up to 80VA).    That's something like a 33V ct at 2A.  You might find a 32VAC ct (= 16VAC-0V-16VAC) off the shelf.  A 30VAC  (= 15VAC-0V-15VAC) is more common but you could lose some output power, you can compensate for this by using a larger VA transformer.

If you use a replacement you have to make sure the thing will fit  in the space available.  (You might not be-able to use the mount screws and the wires might be short too -you  can of course work around this.)

Why not get a price on a proper replacement?

Thanks for the info Rob. None of the usual parts suppliers seems to have the right voltage with a good enough current rating. I spoke with a U.S. distributor for Laney and they seem to think a direct replacement should be available, however it would have to come from the UK. So, I'm waiting to hear back from them, but I have a feeling it could very well be price prohibitive.

Per R.G.'s suggestion, that a lower voltage might be useable, I found one which is 28VCT, 3.6A: http://www.alliedelec.com/search/productdetail.aspx?SKU=70218302

The power section of the amplifier supplies not only 22V, but 15V and 5V as well. So, if the 28VCT was my only option, further mods to the power supply would probably be needed, but I doubt that would be an impossible task.

[Electron Tornado]

None of the usual parts suppliers seems to have the right voltage with a good enough current rating.

Well, my abilities using parts supplier's search functions must leave something to be desired.    I decided to search the Hammond web site, found a decent item and searched for the manufacturer and part # through Google. I got hits at three of the suppliers I had already searched. 

How does this transformer look:  http://www.mouser.com/ProductDetail/Hammond-Manufacturing/165P30/?qs=sGAEpiMZZMtz8P%2FeuiupSVU%252bYNFJcCD9QUugwbbK9zc%3D

Data Sheet: http://www.hammondmfg.com/pdf/5c0015.pdf

[PRR]

> secondary seems fine (>1k ohms).

Better be a LOT less than 1K. More like _1_ Ohm. (Some meters can't tell one ohm from dead short, but it should be way down there.)

> The power fuse on the schematic is listed as "~115V  T500mA", so I think I'll be safe with a transformer rated at 1A. Correct?

No. That is 0.5 Amps on the 120V side. 120V*0.5A is 60 Watts, which is not unlikely for a "30W" amp.

On the 33VCT side this would be 60W/33V= 1.8 amps. 2A would be fine.

> the only one I found with a 33VCT secondary.

It isn't fussy, except higher might be real bad and much-lower might suck. Look for 28VCT 2A, a more common spec than the wacko 33VCT. So it becomes a 25W amp, that's still better than the zero-Watt amp it is now.

> it doesn't use any exotic parts

When you order ten thousand transformers you can get ANY voltage you want.

> Vcc looks like it's only 22V

Read that again and understand what you are working on.

33V AC CT in a sand-amp is typically a bi-polar power supply. There's three power wires. One is connong ground zero-volts. One is plus 23V. One is minus 23V.

> not only 22V, but 15V and 5V as well

Surely the 15V is two, plus/minus? And it is gotten from the 23V rails with regulators?

That's why you _could_ get to trouble with much-lower voltage. Not enough headroom for the regulators. 24VCT (a super-common part) is probably too low.

Don't be looking at 150VA or 3.6A irons. While ideally the amp only takes what it needs and the rest of the capacity loafs, in practice an oversized PT and typical stage hook-ups may smoke much faster than the original design. And you got it because there was a problem, and you are not yet sure you found the main problem or just a sub-problem.

Here. Go to RadioShack and get:

25.2V CT 450mA Std Chassis-Mount Xformer Model: 273-1366 $7.39
10 Ohm 10W 5% Wirewound Resistor (2-Pack) Model: 271-132 $2.19
8-Ohm Non-Inductive Resistor Model: 271-120 $2.69

The black leads go to wall-power via whatever switch and fuse Laney used. LETHAL voltages, fire hazzard!

The PT CT goes where the old one was.

In each remaining lead, splice a 10 ohm 10W resistor and go where the old PT's side leads went, to the bridge rectifier. These resistors must be on a fire-resistant (but non-short) surface. Wood is fine as long as you keep your nose open.

Re-check all connections.

Step back. Power on.

If the resistors get HOT, shut down! You have a short. Probably a blown rectifier but maybe a bad power cap (the BIG ones). It is possible to quickly voltage-probe and guess which it is. Dead-zero on the AC side of the FWB, replace bridge. 0.6V AC into bridge, replace power caps. (Maybe only one is bad today, but do them both.)

Ideally the resistors stay cool and the power cap terminals come up to 18V DC in both polarities.

If so, check DC Volts at speaker connection. If it is stuck almost to an 18V power rail, the amp is sick/shorted and will burn your speaker, eventually the PT if the speaker is connected. If well under 1V DC, you may be lucky. Connect a sturdy speaker in series with that 8 ohm 20W resistor (or another 10 ohm 10W), then to the amp. Be ready to un-plug if it is ugly!

At this point you should, with ear in speaker, hear hiss and hum. Touching small parts on the PCB may give a big buzz, showing it will amplify body-hum. It _may_ even "play", though very-lame because of the 10 ohm power protection resistors. If they stay cool at idle, take them out and go direct. The amp should now "play better", though with the low voltage and under-size transformer it will be lame and not ready for heavy work.

If it gets this far then you can hunt that 30V-34V CT 2A PT.

The 24VCT transformer is handy for many 12V 15V or +/-15V regulated chores.

[Rob Strand]

I had a quick look in the US for transformer and things look pretty thin on the ground.   The prices seem ridiculously high as well.  I'm quite surprised.    Maybe someone else can recommend a local supplier.

Maybe get a quote from Laney.  A distributor should get regular shipments and the shipping costs *should be* minimal.   Based on the US transformer prices I saw, could even be a cheaper option!

[petemoore]

  Shop used...get an amp to hold you over.
  Whole PS recto/filterboard + transformers are available in old junk radios, I've wired up a number of 'honker-irons' for amplifier supplies..look for a big one then use a dinky 2vac WW [so as to better not kill yourself when measuring the secondary to get the transformer voltage ratio] and figure out what the conversion is.
  A small pile of such irons is sitting around..I think I've built enough amps to carry me and my clones over until I blow up a few...lol.
  If you were here I'd just hand you one of them..

[Electron Tornado]

> secondary seems fine (>1k ohms).

Better be a LOT less than 1K. More like _1_ Ohm. (Some meters can't tell one ohm from dead short, but it should be way down there.)

Yes, that was a typo. It should have read <1k ohms.

Also, yes, the power is +/- 22V and +/- 15 volts which comes from the 22V rails. There is also a +5V line for one of the chips.

I'm fine with using a 28V transformer. A 25W amp instead of 30W isn't a big deal.

If I understand correctly, tests with either the light bulb limiter mentioned by RG, or using the resistors mentioned by Paul R, is a means to test whether there is a problem downrange from the transformer that would damage the transformer or the speaker. I have an 18V 1A transformer on hand. Would that be OK just for that kind of testing?

Thanks everyone for the helpful replies so far.


Almost forgot to add a question fundamental to this discussion - what things will cause a power transformer to fail? I think Paul R. mentioned a couple of things along with how to test for them. Are there any others?

[R.G.]

Almost forgot to add a question fundamental to this discussion - what things will cause a power transformer to fail? I think Paul R. mentioned a couple of things along with how to test for them. Are there any others?

There is really only one thing which causes power transformer failure. That is insulation failure, which allows wire shorting and runaway overheating.

How the insulation fails can be subdivided into three main classes:
(1) overheating
(2) voltage punchthrough
(3) environmental

Environmental is things like mechanical abrasion of the insulation, wear-through caused by vibration, water seepage, fungal degradation of the insulation, chemical attack and so on. Military transformers are potted in cans because of these issues.

Voltage punchthrough is why transformers are high-pot(ential) tested. The AC power line has all kinds of spikes and trash on it. Modern practice is to test them to 4000V between primary and secondary.

Overheating is the big one. Overheating happens from
- too much current drawn from the secondary; the ohmic heating in the wire resistance causes heat, and a hot spot can have insulation failure
- too much voltage or too low a frequency (or both) on the primary; this runs the core inductance into saturation and it can no longer limit the AC power line current, causing runaway ohmic heating, and insulation failure from heat. Note that you **cannot** saturate a transformer from the secondary, only the primary. However, the end result of secondary overloading and primary saturation is very hard to distinguish. The smoke smells the same.
- preventing cooling from happening; blocking airflow or conduction of heat to a chassis can do this
- kind of environmental; ambient temperature too high to get the internal heat out.

What you're probably most concerned with is secondary overloading. The light bulb limiter and the 10 ohm "chokeoff" resistors do the same thing - they prevent a secondary load from causing overheating or fuse popping until you can see what's happening.

By the way, for line-frequency transformers, power handling is pretty much proportional to mass. Transformers which run on the same AC power line and have the same weight (well, OK, and style; compare E-I to E-I and not E-I to toroidal and vice-versa) will have quite similar power output V-A.

[PRR]

> typo. It should have read <1k ohms.

I say again: this size secondary should be near one ohm (not K ohm).

However the open primary makes this moot.

> an 18V 1A transformer on hand. Would that be OK

Is it CT? While you can fake-it for little pedal amps, in a Power amp a fake ground will make more confusion than clarity.

18VCT will give DC voltages so low that I would not be sure the amp would bias-up even approximately. I used to do this for a living so I could try-and-see, with some (smokey) experience of mis-powered amps. But it would have flustered me my first few dozen attempts. I think 24VCT is a better first try.

> what things will cause a power transformer to fail?

R.G. covered it fantastic.

Your specific symptom, an open primary without clear sign of fire, strongly suggests the Thermal Cutout he mentioned earlier. This was a reaction to the real danger of transformer fires, which at best stink enough to start panic, and sometimes set fire to drapes, sets and from there sometimes the building.

The cutout will open if the transformer hots-up significantly above the insulation's long-term rating.

In a power amp, this could mean the transformer first developed an internal short. Defect in the insulation, lightning spike.

It can also mean the _amplifier_ turned into a short which of course cooks the transformer. Which is why you want to power-up gently with some power limiting (resistor or lamp). Abused transistors melt to a short. Small transistors on large power then burn-open, but beefy output transistors on a right-size power transformer will often hang-in until the transformer gives-up.

I have also seen these thermal cutouts fail "for no reason". The cutouts are cheap. A bad weld, dirty muntz-metal, bad seal, stuff happens. However digging it out is usually impractical and always unsafe.

And yes, transformer is like bologna: you want a same-pounds (+/-40%) replacement.

And Ohms Law applies. If you find a 25W-30W _8 ohm_ one channel amp carcass with a good PT, that's an excellent bet.

[Electron Tornado]

> typo. It should have read <1k ohms.

I say again: this size secondary should be near one ohm (not K ohm).

However the open primary makes this moot.

Specifically, in the secondary, I have 1 ohm between the center tap and each end, and 2 ohms between the ends.

> an 18V 1A transformer on hand. Would that be OK

Is it CT? While you can fake-it for little pedal amps, in a Power amp a fake ground will make more confusion than clarity.

18VCT will give DC voltages so low that I would not be sure the amp would bias-up even approximately. I used to do this for a living so I could try-and-see, with some (smokey) experience of mis-powered amps. But it would have flustered me my first few dozen attempts. I think 24VCT is a better first try.

No, it wasn't CT. I asked before actually checking it since I recalled having one around. The better question perhaps is, when testing for a short downrange of the transformer, how critical is the secondary voltage on the transformer being used for the test?

Thanks to you and R.G. again for your help.