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Author Topic: Ashdown Peacemaker 20 with suspected blown output transformer  (Read 12218 times)

txflood

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Ashdown Peacemaker 20 with suspected blown output transformer
« on: December 08, 2014, 10:57:13 PM »
I was looking for advice on where one could find a suitable output transformer for an Ashdown Peacemaker 20 amp. It has EL-84 output tubes and the schematic can be found online here if needed:

http://music-electronics-forum.com/t34072/

I was wondering how cost effective this repair would be for this particular amp.

Thanks

phatt

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Re: Ashdown Peacemaker 20 with suspected blown output transformer
« Reply #1 on: December 09, 2014, 04:44:15 AM »
What makes you suspect the OT?
There are many other things that will cause no output and are a lot cheaper than a transformer to fix.
Phil.

txflood

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Re: Ashdown Peacemaker 20 with suspected blown output transformer
« Reply #2 on: December 18, 2014, 01:42:23 PM »
This was a buddy's amp that he had a tech look at a few years ago and was told the "output" transformer needed to be replaced. I now have the amp and took it apart.

The tech had left the wires from the power supply transformer disconnected from the PCB so I started there to see where things really stand with this amp.
The AC inline fuse was blown and I found scorched marks around R76 in the power supply section on inspecting the PC board in detail. There is a wire that runs from the power supply at R76 to the center tap of the primary side of the output transformer. The connector is attached where the board silkscreen reads "FROM OUTPUT TX" next to R76 and the plastic insulator on the connector appears melted. R76 sits between the 2 filter caps and it does measure to be 470 ohms.

I also removed the PCB from the chassis and took a look at the area under R76. It looked like the solder had been worked on that resistor so I am guessing that the original part has been replaced. Close inspection showed that the current resistor has a 6W power rating while the schematic only calls out for 2.5W.

I disconnected all the leads from the mains transformer to the PCB and then took no load AC measurements on the secondary winding wires coming directly from the transformer. I only applied power for short intervals. The transformer buzzed fairly loudly and got warm fairly quickly even with these short tests, so I knew that wasn't a good sign already.

On the high voltage pair I only measured around 10VAC. The yellow pair only came in at 6.6VAC and the red pair at 4.8VAC. I would say this confirmed my worst fears on the power transformer.

So the question remains as to why that resistor failed and if it took the power supply transformer out with it or if that occurred later.

I contacted Ashdown and they don't have replacement power transformers but they did give me the specs on the three secondaries:

270V @ 0.15A
6.3V @ 1.6A
13V - 0 - 13V @ 0.5A

I couldn't find an off-the-shelf part to match all 3, but I found a few candidates to match the first 2:

Hammond 270FX, Weber W025130INT, or maybe Weber WPTGP (that one would be 280V)

My thought is to get one of these transformers and then add another small transformer to the chassis to take care of the 13-0-13V.

I also tried to test the filter caps to make sure there wasn't any obvious problem (visual inspection, low impedance) but could not find anything. The PCB indicates the amp was assembled in 2002 time frame so I'm thinking it would be a good idea to replace them anyway.

I'm still concerned that perhaps there is still a problem that caused the power supply failures in the first place that hasn't been addressed so I was going to proceed carefully when trying to re-energize the circuit after getting replacement parts.

Since I've never actually heard one of these amplifiers, I am curious as to what it really sounds like and how they are regarded in general. I haven't found much info on them at all.

Roly

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Re: Ashdown Peacemaker 20 with suspected blown output transformer
« Reply #3 on: December 19, 2014, 01:11:30 AM »
There seems to be some confusion here between the power transformer (PT), mains to rectifier, and the output transformer (OPT), output valves to speaker.

Quote from: txflood
The transformer buzzed fairly loudly and got warm fairly quickly even with these short tests, so I knew that wasn't a good sign already.

On the high voltage pair I only measured around 10VAC.

Those are pretty fair indications that the PT high voltage winding has a shorted turn, however it doesn't follow that this was caused by the OPT failing, in fact that would be an unusual result of a typical OPT failure.

An important early check are the main rectifier diodes, <peacemaker-40-b.pdf> D10-13.

Here's a radical idea - test the OPT.

Fully isolate it and apply around 6.3VAC to the speaker side.  It will have a turns ratio of around 20:1-30:1 so you should get something like 20 * 6.3 = 126VAC to 30 * 6.3 = 189VAC on the anode side.  The exact voltage isn't that critical, but if it's very much lower then it suggests that the OPT has a shorted turn, otherwise it suggests that it's okay.

Quote from: txflood
I also tried to test the filter caps to make sure there wasn't any obvious problem (visual inspection, low impedance) but could not find anything. The PCB indicates the amp was assembled in 2002 time frame so I'm thinking it would be a good idea to replace them anyway.

Don't complicate things.  If the caps hold charge from a Megger or similar high voltage source, leave them alone, at least until you can get some high voltage on them.

Until you are sure all is well you should only power up the amp via a Limiting Lamp.
If you say theory and practice don't agree you haven't applied enough theory.

phatt

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Re: Ashdown Peacemaker 20 with suspected blown output transformer
« Reply #4 on: December 19, 2014, 07:48:23 AM »
Go with Roly's advice,, He eats pentodes for breakfast and washes them down with a mathematical smoothie,  ;) :P
Meantime my observations;
The fact that R76 is running very hot suggests that something *After* that Resistor is pulling too much power.

I'd guess, the Screen grids are likely getting belted to death.
As you have 270 volt AC supply then your DC at CT is going to be pushing close to 380VDC yet R61 is only 50 Ohms so unless I'm sadly mistaken the power tubes are running way too hard.

Your power amp (V7,4 and 5) is a simple copy of the classic AC30 and they ran 300~320VDC and they were known to run hot. So R61 would need to be raised in value to compensate for the higher supply voltage.
You can see signs of over dissipation by looking at the plates of the power Valves, those little holes in the side of the plates. If there is any sign of black burn marks around those holes it's a fair bet they have been flogged to death.
 
Obviously the power tranz is dead so maybe look for something lower than 270 VAC as a replacement.
This issue of higher than needed HT voltage is common and from what I understand some transformers were likely meant for valve rectification where there is a lot of voltage drop across the rectifier Valve but with SS diode rectification the drop is only small so the working voltage gets bigger and without a rethink of power valve conditions and rebiasing the power tubes suffer greatly.

Valves are tough and can handle over voltage by a fair amount but NOT over current. So the higher the voltage the more critical the dissipation becomes. The screen grid is the weak link in power Valves and is a common failure mode.

Regarding a new transformer, just take the secondary voltage and multiply it by 1.414 (1.4 is close enough) that will give you a clue as to the DC result.
That voltage will drop when running so if the equation runs out at 320~330 VDC it will be close enough.
Keep at it, if you take time to work through it I see it as a worthwhile fix. :tu:
Phil.

txflood

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Re: Ashdown Peacemaker 20 with suspected blown output transformer
« Reply #5 on: December 19, 2014, 11:12:13 AM »
There seems to be some confusion here between the power transformer (PT), mains to rectifier, and the output transformer (OPT), output valves to speaker.

Right I was initially going by what I was told before ever looking at the amp.

An important early check are the main rectifier diodes, <peacemaker-40-b.pdf> D10-13.

Those tested fine using the diode check on my meter.


Here's a radical idea - test the OPT.

Fully isolate it and apply around 6.3VAC to the speaker side.  It will have a turns ratio of around 20:1-30:1 so you should get something like 20 * 6.3 = 126VAC to 30 * 6.3 = 189VAC on the anode side.  The exact voltage isn't that critical, but if it's very much lower then it suggests that the OPT has a shorted turn, otherwise it suggests that it's okay.
Thanks for this tip. I did get the specs from Ashdown now on the OPT and I'm guessing that it is bad as the drawing shows 8K ohms resistance on the primary windings. I am measuring a little less than 600 ohms.

Wouldn't this low primary resistance explain R76 burning up feeding the CT of the OPT?


Until you are sure all is well you should only power up the amp via a Limiting Lamp.
I was already thinking that I should have one of these when I attempt to replace the power transformer and energize the circuit.

Thanks for the info!

txflood

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Re: Ashdown Peacemaker 20 with suspected blown output transformer
« Reply #6 on: December 19, 2014, 11:43:17 AM »
I'd guess, the Screen grids are likely getting belted to death.
As you have 270 volt AC supply then your DC at CT is going to be pushing close to 380VDC yet R61 is only 50 Ohms so unless I'm sadly mistaken the power tubes are running way too hard.

Your power amp (V7,4 and 5) is a simple copy of the classic AC30 and they ran 300~320VDC and they were known to run hot. So R61 would need to be raised in value to compensate for the higher supply voltage.
You can see signs of over dissipation by looking at the plates of the power Valves, those little holes in the side of the plates. If there is any sign of black burn marks around those holes it's a fair bet they have been flogged to death.

The amp I have is the 20W version so R61 is 100 Ohms  and V8 and V2 are not fitted if that makes much of a difference.

I took a look at the output tubes and didn't see any black burn marks but maybe I don't know what I am looking at. The guy I got the amp from claimed all the tubes had low hours on them as well.

Obviously the power tranz is dead so maybe look for something lower than 270 VAC as a replacement.
This issue of higher than needed HT voltage is common and from what I understand some transformers were likely meant for valve rectification where there is a lot of voltage drop across the rectifier Valve but with SS diode rectification the drop is only small so the working voltage gets bigger and without a rethink of power valve conditions and rebiasing the power tubes suffer greatly.

Valves are tough and can handle over voltage by a fair amount but NOT over current. So the higher the voltage the more critical the dissipation becomes. The screen grid is the weak link in power Valves and is a common failure mode.
OK thanks for that info. What I am taking away from this is just how well thought out is this amp design and does it need changes to actually make it reliable. I don't know enough about valve amp design to know what corrections are needed here but I am willing to learn.

Regarding a new transformer, just take the secondary voltage and multiply it by 1.414 (1.4 is close enough) that will give you a clue as to the DC result.
That voltage will drop when running so if the equation runs out at 320~330 VDC it will be close enough.
Keep at it, if you take time to work through it I see it as a worthwhile fix. :tu:
Phil.


So you would recommend a replacement power transformer that is more around 220V (if I did my math right) as a good design point?

Well at this point I am guessing that both transformers are going to have to be replaced as my initial cost factor, but this looks like it might be a good learning experience and hopefully I end up with an amp I actually might want to use.  :)

g1

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Re: Ashdown Peacemaker 20 with suspected blown output transformer
« Reply #7 on: December 19, 2014, 05:08:21 PM »
Thanks for this tip. I did get the specs from Ashdown now on the OPT and I'm guessing that it is bad as the drawing shows 8K ohms resistance on the primary windings. I am measuring a little less than 600 ohms.

Wouldn't this low primary resistance explain R76 burning up feeding the CT of the OPT?
The 8K figure is impedance, not resistance.  Impedance is an AC only spec. and we can't measure it with a normal type meter.  We can measure DC resistance but it will give a much lower measurement.  Your 600ohm resistance reading does not seem low at all, some amps OT primaries measure even lower.
R76 does not feed the CT.  Rather it feeds everything after the CT and the fact it is burning up implies the problem could be anything other than the output transformer.




phatt

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Re: Ashdown Peacemaker 20 with suspected blown output transformer
« Reply #8 on: December 21, 2014, 07:48:35 AM »
Yes as G1 noted Impedance and Resistance can be confusing.

Impedance is AC so it's in the frequency realm. Example, 8 Ohm speaker may read 6 Ohms DC resistance but the Z (impedance) at 10,000 CPS might be closer to 30 Ohms. :o
A Microphone label might say 600 Ohms Z @ 1,000CPS but a multimeter will read the DCR down around ~150 Ohms.  DCR is nearly always lower than the Z Ohms.

If the Valves look ok then do not worry as the issue might have little to do with High voltage section,, remember you have 3 supplies so the blown primary winding may have been caused by the other secondaries.

15/0/15VAC drives the 12VDC SS supply section through Reg chips (IC 4&5)
That also powers the filaments (Heaters) for the AX7's

And one more AC winding powers the Filaments for the power Valves.
So you have 3 possible failure points to check.

Check the bridge diodes on the 12/0/12 supply and don't forget the fuses,, make sure they are the right value. Some clown may have put in a 10 Amp fuse rather than fix the underlying issue, meanwhile it's melted the primary. I've seen that one a few times ::)

Don't forget Roly's OTr test and build the lamp tester. :tu:

Re the transformer over voltage, anything lower than 270VAC will be a good but if 270 is all you can find then just remember to check the bias when it's up and running again but that is the least of your worries at the mo,,, need to find the fault first.
Phil.

txflood

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Re: Ashdown Peacemaker 20 with suspected blown output transformer
« Reply #9 on: December 21, 2014, 09:58:10 PM »
OK, I made a bad assumption that it was DC resistance so that sounds like potentially good news on the OT.

Thanks for the tips and it will probably be after the holidays before I will have a chance to proceed any further on the amp.

Roly

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Re: Ashdown Peacemaker 20 with suspected blown output transformer
« Reply #10 on: December 22, 2014, 01:37:11 AM »
The first thing you should do is the OPT test I suggested - if you get 100+ volts between the primary ends (anode connections) you can forget about the OPT being faulty.

The impedance (Z) of a transformer winding is its DC resistance (R) plus its AC reactance (Z = R + XL, where XL = 2 Pi f L) so the impedance will always be higher than the DC resistance, often very much higher.  This is also true for loudspeakers.
If you say theory and practice don't agree you haven't applied enough theory.

txflood

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Re: Ashdown Peacemaker 20 with suspected blown output transformer
« Reply #11 on: January 22, 2015, 09:22:07 PM »
OK so I finally had a chance to get back to the amp. I've built a cool current limiter with a pair of 250W bulbs and tried it on my fried power transformer as a test. It lit up beautifully and the transformer never buzzed or heated up at all, so I would say I now have a good limiter for future repair work.

The only AC source that I had handy tonight output 9.9VAC according to my mulitmeter. Since the transformer is supposed to have 8000 ohms primary for the EL84s to see and the speaker is 16 ohms, I believe that gives us an impedance ratio of 500 and a winding ratio of roughly 22.4.

I applied the 9.9VAC to the primary (with the source plugged into my handy dandy new limiter of course) and expected to get about 0.4V on the secondary. I measured 0.3V and I know my meter isn't the best in the world precision wise so I'm thinking that it's in the ballpark.

You guys were suggesting that I also test it the other way by applying the AC to the secondary, but I'm wondering is that the normal practice for testing an output transformer? Or do you only worry about applying voltage to a secondary if you are dealing with an unknown transformer? I've gathered that it can be frowned on as an unsafe practice or am I misinterpreting things?


Roly

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Re: Ashdown Peacemaker 20 with suspected blown output transformer
« Reply #12 on: January 23, 2015, 08:55:00 AM »
Quote from: txflood
a pair of 250W bulbs
{twice in one week?  :o }

I hope this is a typo and you mean 25 watts - the idea is to limit the power into the amp to a safe level, and two-hundred-and-fifty watts is not a safe level.


Transformers are (approximately) bi-lateral devices.  If you apply 6.3VAC to the speaker winding the step-up ratio should be, as you correctly calculated, 22.4:1, giving around 22.4 * 6.3 = 141.1VAC (or 22.4 * 9.9 = 221.8VAC) on the valve side.  These voltages are within the range of normal operation ('tho the actual voltage may be a bit less due to transformer losses).

It's also generally easy to isolate the valve side by pulling the OP valve(s), then picking up 6.3VAC from the heater line with a clip lead or two.

You also need to remember that the transformer losses will slightly load your driving transformer, so you really should measure the actual applied voltage for the most accurate results.

There is a significant difference between a good transformer operating unloaded with about 5-10% losses, and a dud transformer with a shorted turn(s) with about 90-95% losses.

It's "unsafe" in the sense that 150 volts will give you a shock, but you are careful not to touch it while the soup is boiling, right?  The reason for doing it step-up rather than step-down is that the results are rather more definitive, say 5 or 10VAC rather than the expected 150-160VAC.  As you found you need to apply a fair bit of voltage to the valve side to get anything measurable on the speaker side.

Whatever, it looks like your OPT is healthy.
If you say theory and practice don't agree you haven't applied enough theory.

txflood

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Re: Ashdown Peacemaker 20 with suspected blown output transformer
« Reply #13 on: January 23, 2015, 10:25:00 AM »

I hope this is a typo and you mean 25 watts - the idea is to limit the power into the amp to a safe level, and two-hundred-and-fifty watts is not a safe level.


No actually I meant a 250W bulb. I thought the idea of the device is that if your equipment under test has a short in it, it will still be protected as the light bulb will then turn on and the power will be flowing to it. The large wattage bulbs are required as they have the low resistance needed for this to work in practice.

https://www.youtube.com/watch?v=wRFRwOnLsZI

The best part is about the 7 minute mark when he shorts the circuit with a screwdriver.

 :o

He has a link to his suggested 250W bulb being sold on Amazon.

Roly

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Re: Ashdown Peacemaker 20 with suspected blown output transformer
« Reply #14 on: January 23, 2015, 12:00:48 PM »
{There had to be a reason for this "300 watt" co-incidence}


Well this is a video that needs to be resounding and loudly bagged.  "The larger the better" is just plain WRONG, so I'm not going to do this guy the honor of watching more than 60 seconds of his misinformation.


The Limiting Lamp represents a non-linear resistor in series with the Device Under Test (DUT).  When it is cold its resistance is low, but as it heats up its resistance rises rapidly, by a factor of around x10-20.  In this way it acts rather like a self-resetting fuse, and like a fuse it has to have a suitable rating for the load, not just "bigger is better".

In our case the DUT is a guitar amplifier, either valve or solid state, and typically with an output power rating somewhere in the range of 10 to 100 watts.  Since they are almost always Class-AB or Class-B the input power will be of the order of twice the output power, say 20 to 200 watts.

In a valve amp there is heater power to consider and this may be up to about 30-odd watts in a larger amp, while in a s.s. amp we will be looking at an output stage idle current of around 50mA across, say, +/-35V supplies;

P = E * I

2 * 35 * 0.05 = 3.5 watts


The object of a Limiting Lamp is to allow you to liven up a circuit which has a fault so you can do some faultfinding, without doing any more damage.  This will mean a lamp with a starting value of around 25 watts for a s.s. amp, and perhaps 40 watts for a valve amp.  As a rule of thumb you should use a lamp rated at no more than the amp nominal output power.

Another particular need for a Limiting Lamp is where you have repaired a s.s. amp and need to bring it up gently to confirm correct operation, and not blow up all your new transistors if you have missed a fault.  Again you would start with a 25 watt lamp and work up to say 100 watts if all seems okay.

A "limiting lamp" of 300 or more watts simply won't provide a safe power limit into your amp, it will pass an excessive amount of power, and you can expect smoke before it is repaired, and to replace all your expensive new transistors after you have repaired it and missed something - it is simply useless for the task.  {a similar consideration applies to using a Variac which provides a fairly "stiff" supply and no non-linear limiting like a lamp, and requires greater care in use}

For electrical stuff, large motors, heaters, etc, there may be some call for such a high power limit, but using a lamp of 100 watts or higher initially with electronic equipment such as a guitar amp (and particularly lower power ones) is simply asking for serious trouble.

This video is going to cause a lot of heartache to novice amp repairers and it should be taken down.

 :grr
« Last Edit: January 23, 2015, 06:05:42 PM by Roly »
If you say theory and practice don't agree you haven't applied enough theory.