Audio amp power transformer question

Thread Starter

spuddo

Joined May 2, 2013
76
OK, but that meter reading is "open", far from zero ohms. CONNECT THE TWO PROBES TO SEE A ZERO READING.
And the transformer certainly looks like the paper can be opened up to find that internal fuse. Or is that a copper band around the windings? And while the amp does look fairly well made, that fuse buried inside the transformer places a serious doubt about the quality. I have only seen that done in lower quality items, where the transformer quality, or capabilities are marginal. And you would never ever see that in equipment designed to be durable and rugged.
One momentary line voltage spike will pop the fuse and the show is over.

So now it is time to either replace the transformer or remove it and carefully investigate. My money is on that fuse being open.
Now I question the validity of the meter readings , because an open circuit is not likely to have any inductance.

My bad yes it is open not zero.
Standard practice to connect the two probes when using this meter on 200 Ohm range. it usually settles at about 400 t0 500 milli Ohms.
Yes it is a copper band.
I agree , the the readings are strange. As you said " an open circuit is not likely to have any inductance"
My suggestion is maybe micro Ohms-----i'll need a kelvin bridge for that.
As for the momentary line voltage spike , i have no way of knowing what the owner did prior to receiving this blessing.
The transformer is no longer available according to Pioneer spare parts------it is 30+ years old.
What do you think of my micro Ohms theory?
Thanks for your inputs.
 

MisterBill2

Joined Jan 23, 2018
18,167
Given that the transformer resistance is above the range of your meter, it would certainly be above any micro ohm reading. and given that the indication is that the primary is open, there is no reason to do any more measuring.
The next step will be to either find an equivalent transformer or to repair this one.

An interesting possibility is that both terminals of that internal fuse can be seen, possibly even accessable, at the edge of the windings. Can you post an additional photo of the transformer looking down at the side of the windings with the primary connections? And, looking at the circuit diagram, let us know the secondary winding voltage? I see DC voltages of 39 and -39, as well as some higher voltages. But it would be very helpful to know the AC voltages.
And also let us know the rated watts of the amplifier, as well as the mains amps input from the label.
Because the transformer only has one secondary winding, there should be a substitute available someplace..
 

Thread Starter

spuddo

Joined May 2, 2013
76
Given that the transformer resistance is above the range of your meter, it would certainly be above any micro ohm reading. and given that the indication is that the primary is open, there is no reason to do any more measuring.
The next step will be to either find an equivalent transformer or to repair this one.

An interesting possibility is that both terminals of that internal fuse can be seen, possibly even accessable, at the edge of the windings. Can you post an additional photo of the transformer looking down at the side of the windings with the primary connections? And, looking at the circuit diagram, let us know the secondary winding voltage? I see DC voltages of 39 and -39, as well as some higher voltages. But it would be very helpful to know the AC voltages.
And also let us know the rated watts of the amplifier, as well as the mains amps input from the label.
Because the transformer only has one secondary winding, there should be a substitute available someplace..
Hi
Pioneer A-656 is rated at 70 Watts per channel.
Label on back shows 650 Watts , 2.7 Amps @ 240Volts.
The transformer is wrapped in a copper band then a sheet of hard black plastic film covering what appears to be dipped(enveloped) in epoxy or lacquer.
Not interested in touching the beast's insides , that usually ends in tears and then the owner blaming you for ruining their precious relic.
All tips welcome
Thanks Bob
 

Thread Starter

spuddo

Joined May 2, 2013
76
Hi
Pioneer A-656 is rated at 70 Watts per channel.
Label on back shows 650 Watts , 2.7 Amps @ 240Volts.
The transformer is wrapped in a copper band then a sheet of hard black plastic film covering what appears to be dipped(enveloped) in epoxy or lacquer.
Not interested in touching the beast's insides , that usually ends in tears and then the owner blaming you for ruining their precious relic.
All tips welcome
Thanks Bob
Hi.
Back again , forgot to ask in response to your quote .
"One momentary line voltage spike will pop the fuse and the show is over."
Could one momentary arc in the mains on / off switch do it ? , that's something anyone has access to.
Thanks
 

MisterBill2

Joined Jan 23, 2018
18,167
Probably NOT. But there are assoorted spikes that appear on the mains from events outside the localarea. A lightning strike in the next block can send a spike many hudreds of feet along the power lines. And that is all it would take to pop an internal fuse. THE FUSE INSIDE THE WINDING IS A DESIGN CHEAT! It is a cheap alternative to a better transformer construction and better insulation with better heat resistance.

So now the search will be for another transformer that will provide the required DC output at enough current. I see in that screen shot voltages of 39 and 49 volts, which is quite a spread. And if the power out is really 70 watts per channel, 140 total, then the transformer will need to be about 24 volts each side of the center tap and good for perhaps 4 amps.

One possibility is that that internal fuse connection comes to one of those terminals that we see on the primary side, where the white and yellow wires are connected. That black plastic is probably just a cover, because a molded transformer would cost more than just a plastic cover to look pretty. So the next step will be to ose the ohm meter to see if there is a connection to ay of the vacant terminals on that side.

So there seem to be two options at this time. Get another transformer with similar ratings, and replace this failed one, or else do some surgery and see if that internal fuse can be bypassed. An external fuse would be required but that would be a very simple addition.
How able is the TS? Are they able to purchase a replacement transformer and install it? Or NoT???
 
Last edited:

Thread Starter

spuddo

Joined May 2, 2013
76
On the schematic the internal fuse is shown between pin 2 and pin 4 of the transformer. TS seams unable or unwilling to measure the fuse.


Probably NOT. But there are assoorted spikes that appear on the mains from events outside the localarea. A lightning strike in the next block can send a spike many hudreds of feet along the power lines. And that is all it would take to pop an internal fuse. THE FUSE INSIDE THE WINDING IS A DESIGN CHEAT! It is a cheap alternative to a better transformer construction and better insulation with better heat resistance.

So now the search will be for another transformer that will provide the required DC output at enough current. I see in that screen shot voltages of 39 and 49 volts, which is quite a spread. And if the power out is really 70 watts per channel, 140 total, then the transformer will need to be about 24 volts each side of the center tap and good for perhaps 4 amps.

One possibility is that that internal fuse connection comes to one of those terminals that we see on the primary side, where the white and yellow wires are connected. That black plastic is probably just a cover, because a molded transformer would cost more than just a plastic cover to look pretty. So the next step will be to ose the ohm meter to see if there is a connection to ay of the vacant terminals on that side.

So there seem to be two options at this time. Get another transformer with similar ratings, and replace this failed one, or else do some surgery and see if that internal fuse can be bypassed. An external fuse would be required but that would be a very simple addition.
How able is the TS? Are they able to purchase a replacement transformer and install it? Or NoT???
Hi.
Please explain what TS means---don't forget i'm an old fart from Australia.
Regards
 

MisterBill2

Joined Jan 23, 2018
18,167
On the schematic the internal fuse is shown between pin 2 and pin 4 of the transformer. TS seams unable or unwilling to measure the fuse.
OK, I missed that part of the drawing. If that is the case, and the device is indeed found to be open, then it will be a simple matter to connect an external fuse of a suitable size across the terminals and see if the ampplifier works, or if it has an actual problem.
But if there is an actual problem it does not seem like the TS will be able to find and repair it, based on the discussion thus far. There are two basic ways of diagnosis and repair, the first being to check every part and replace every part that is defective. The other method is to understand the circuit and the symptoms and then check the parts that would cause the symptom. This is often much faster to do, even if it does require making some measurements.
 

Thread Starter

spuddo

Joined May 2, 2013
76
OK, I missed that part of the drawing. If that is the case, and the device is indeed found to be open, then it will be a simple matter to connect an external fuse of a suitable size across the terminals and see if the ampplifier works, or if it has an actual problem.
But if there is an actual problem it does not seem like the TS will be able to find and repair it, based on the discussion thus far. There are two basic ways of diagnosis and repair, the first being to check every part and replace every part that is defective. The other method is to understand the circuit and the symptoms and then check the parts that would cause the symptom. This is often much faster to do, even if it does require making some measurements.
Hi.

When i first tested for dc resistance i had put a jumper wire (fuse ) across pins 2 and 4 , still showed open circuit.

The attached photos should explain it all.

Thanks to all for your interest.

Any further comments welcome.
 

MisterBill2

Joined Jan 23, 2018
18,167
OK, this next request only requires an examination of whatever information you have about that amplifier. To locate an alternative transformer we must know the AC output voltage that the failed transformer supplied. Given that voltage and the stated wattage, we can calculate the current fairly closely. At that point we can know the specifications for the replacement transformer.

As I examine the photos of the test arrangement I see no part of it done as I would do it. Those clip leads use some of the thinnest wire commonly available and my experience is that in addition they are of very poor quality. Without exception, as I have acquired such leads I have replaced the wires with much better quality wire. So the clip leads may have sabotaged your test results. Next, there was no need to include a series fuse for a short duration test . If the fuse were failed then there would be no current .
Something does not seem correct here. Please do a resistance measurement between terminals 2 and 3, and between terminals 3 and 5, and also between terminals 2 and 5.
 
Last edited:

Thread Starter

spuddo

Joined May 2, 2013
76
OK, this next request only requires an examination of whatever information you have about that amplifier. To locate an alternative transformer we must know the AC output voltage that the failed transformer supplied. Given that voltage and the stated wattage, we can calculate the current fairly closely. At that point we can know the specifications for the replacement transformer.

As I examine the photos of the test arrangement I see no part of it done as I would do it. Those clip leads use some of the thinnest wire commonly available and my experience is that in addition they are of very poor quality. Without exception, as I have acquired such leads I have replaced the wires with much better quality wire. So the clip leads may have sabotaged your test results. Next, there was no need to include a series fuse for a short duration test . If the fuse were failed then there would be no current .
Something does not seem correct here. Please do a resistance measurement between terminals 2 and 3, and between terminals 3 and 5, and also between terminals 2 and 5.
Hi
The images are in the following order:
1------calibration @ 200 Ohm setting . Test leads add 110 milli Ohm.
2------Pin 2-3
3------Pin 3-5
4------Pin 2-5
5------Pin 2-4

Power supply part number is ATS1287 , no longer available from Pioneer.
Total power 650 watts , the amp is 70 watts per channel.
The power supply schematic differs from the actual circuitry. Only 4 diodes not 8 as shown. Have not been able to find the the matching schematics.
It shows what i consider to be a full wave rectifier on each rail.
i guess with +/- 49.5 (no signal input) , taking in to account volt drops across diodes , 50+ volts ac required per rail in output.
So maybe we need 240 in 100+ out , 650 watt + capacity.
With a closer look at the transformer schematic , it seems it should show a clear separation between pin3/5 and pins 2/4.
These pins have never been connected together , each one is a separate voltage setting.
Thanks for listening.
 
Last edited:

MisterBill2

Joined Jan 23, 2018
18,167
OK, and thanks for the details. So it appears that possibly the schematic does not exactly reflect the specific model that you have, and certainly yhe name plate, with all of the voltage choices listed, does not relate to tha transformer in the circuit or the transformer in the amplifier.Themains terminals should be more than 2.2 ohms, I believe.
If you have a low voltage transformer available, possibly six or twelve volts, it can be used to apply that voltage between pins 9 and 11, and then check for voltages between pins 3 and 5.

If nothng is available then itis time to carefully disconnect the wires from the transformer and begin exploratory surgery, with the goal being to bypass that internal fuse, OR discover thhat thetransformer is indeed burned and the only hope is replacing it. The replacement will be a 48 volt center tapped transformer. That voltage will allow the amplifier to put out most of therated power, and live a much longer life.
 

MisterBill2

Joined Jan 23, 2018
18,167
For this application physical size matters a great deal. Given that the goal s torepair the amplifier and have the repair last, It makes sense to go a bit lower in the voltage. In addition, I think that I saw on that same drawing a tag of 39 volts, probably at full output. THAT tells me that the big transformer was a bit undersized for current capability, and that the 70 watts power may have been an IHF power rating.
AND we know that the amplifier is only two channels. That is what there was in that era among this class of system.
 
https://www.radiomuseum.org/r/pioneer_stereo_ampliifer_a_656.html

It looks like there is a little more to the specifications than meets the eye:
Specification
Continuous power output
(both channels driven at 20Hz-20kHz)
T.H.D. 0.005%, 8Ω
T.H.D. 0.008%, 4Ω
70W + 70W
95W + 95W
Power bandwidth5Hz-80kHz (T.H.D. 0.05%, 8Ω)
Damping factor (1kHz/20Hz-20kHz) 8Ω200/70
Dynamic power output
(on EIA dynamic test signal)
150W/190W (4Ω/2Ω)
Intermodulation distortion (at rated output)0.005%

It does support a 4 ohm load continuous and a 2 ohm load briefly.
For 4 ohms P = I^2 / R; I = sqrt(95/4) = 4.9A

Hammond has this http://www.hammondmfg.com/pdf/5c007.pdf design note which states that I D.C.= 0.62 X Sec.I A.C
so the secondary I rating for one channel should be. 4.9 A /0.62 or 7.9 Amps.

When you read the specs:

Continuous power output
(both channels driven at 20Hz-20kHz)

It basically leave that specification out. There is nothing in that column. There is just distortion ratings and peak ratings.
So, the specifications are iffy.

I went through this with my amp and you basically have to look at all of the limits. The load, the rails, the loaded rails. then you have to make the current rating of the secondary 40% higher.

in the initial design of the power supply for my amp, I used a 18 Amp constant voltage transformer and it sounded really nice, but the hum was intollerable. The Leach Amp in Audio magazine specified a 70 VCT transformer at 3 Amps for both channels. They mentioned electrostatic speakers which don't need the current. Search online for the Leach Amp. The amps output devices are rated for 30 Amps. The speaker fuses are 3A AGX fuses. My speakers are 100 W units, but the dome tweeters are only rated for 35 W.

I then made it with two 3A secondaries and it still isn't enough current.

A friend an I compared my amp against a Macintosh tube amplifier driving Klipsch Voice of the Theater speakers with horns.
The horns need voltage. His 7W tube amp sounded louder which makes sense. We were listening to classical music. I happen to really like folk music.

I am now running a 500 W AC sine wave voltage regulator before the amplifier.

I have to agree, the transformer is undersized and the specifications are written in a deceiving manner, but you need to look at all of the limits and the SOA (Safe Operating Area) to find out the real capabilities.
 
Continuous power output is specified at 70W + 70W 8Ω and 95W + 95W 4Ω.
Technically, it does not say that.

Why?

There is nothing (It's empty) to the right of the box that states:
Continuous power output
(both channels driven at 20Hz-20kHz)


There is a 200/70 across from:
Damping factor (1kHz/20Hz-20kHz) 8Ω

so, Damping factor is stated.

So, from a technical point of view continuous power output is not rated, otherwise it would be in the right column which is empty.

It might be inferred that rated power output is 70W+70W because of:

intermodulation distortion (at rated output)0.005%

At the 0.005% distortion level, the output is 70W+70W.
 

MisterBill2

Joined Jan 23, 2018
18,167
All of the ppreviousdiscussion provides more incentive to investigate the present transformer much farther., because it might be a challenge to locate an adequate replacement.
It may be that while the manufacturer can no longer supply a replacement, they can provide the actual specifications, at least voltage and current ratings. With that information an informed search for a replacement could be done.

Really, though, opening up the enclosure should not be so difficult, those are not molded plastic pieces, they snap into place to cover the less pretty windings. Thus with a bit of examination it will be possible to gain access and see what is actually inside. And with any good fortune, to bypass that internal fuse.
 

Audioguru again

Joined Oct 21, 2019
6,672
Why question the detailed specs? It is rated at 70W continuously per channel at any audio frequency with very low distortion.
If you turn up the volume control then its output power and distortion are higher like any amplifier.
Of course the power into 4 ohms is higher, but the sound will be only slightly louder.

The "dynamic" power is more (was called "music power" a long time ago) because it is only for short durations, before the power supply filtered voltage runs down.

Its max AC power input is 650W maybe because it gets very hot with a class-A circuit and the volume control setting is turned up too high so that each channel produces 70W of sounds plus another 70W of distortion.
 

MisterBill2

Joined Jan 23, 2018
18,167
Thereason to consider the specifications s that theyreflect the efficiency of an amplifier. I can have a class "C" amplifier that is much more efficient but produces perhaps 50% or more harmonic distortion. Driving a tuned and resonant magentostrictive head it works well for welding plastic. That is one extreme, the opposite being an ultra linear class "A" amplifier that cannever reach 20% efficiency. But they could each put out 70 watts solid. And so the amplifier 70 watts out tells us not much about how much current it draws. So we will need to do some moore analysis of the circuit to see that.
Then comes the tedious part of seeking an equivalent transformer.
And we have not heard if the TS is a service tech in a repair shop, a hobby person, or an engineer asked to fix it for a friend, or some other situation.. And how badly the amps owner wants it fixed.
 

Thread Starter

spuddo

Joined May 2, 2013
76
Thereason to consider the specifications s that theyreflect the efficiency of an amplifier. I can have a class "C" amplifier that is much more efficient but produces perhaps 50% or more harmonic distortion. Driving a tuned and resonant magentostrictive head it works well for welding plastic. That is one extreme, the opposite being an ultra linear class "A" amplifier that cannever reach 20% efficiency. But they could each put out 70 watts solid. And so the amplifier 70 watts out tells us not much about how much current it draws. So we will need to do some moore analysis of the circuit to see that.
Then comes the tedious part of seeking an equivalent transformer.
And we have not heard if the TS is a service tech in a repair shop, a hobby person, or an engineer asked to fix it for a friend, or some other situation.. And how badly the amps owner wants it fixed.
MB2
I am a retired old fart , that was at one stage a radio tech in civil aviation , some 40 odd years ago.
40 years on i am a hobbiest , and the knowledge is sketchy , that's why i come to forums.
AAC has been a big help.
Yes i am repairing friends amps.
When something is for free , they say "go ahead knock yourself out , no pressure ,take you time"
When you mention this parts failed and there might be more parts kaput , and it might cost $x 's , they generally say " nah , just chuck it down the tip ,i'll by a new one".
Gotta love em.
I get all this fun (fault finding) for free , their still friends-----it's a win win.
MB2 as per your post #38 , i'll let you know the results.
Best regards.
 
Top