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PostPosted: September 18th, 2019, 10:19 pm 
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Based on the photos there does not appear to be any observable issues with passive parts. Time now to test tubes. But -- now that I think about it -- the Music Angel I had in the past was a PP KT-88 based amp. It had four KT-88s, two 5814s, and two 6201s. The recommended fuse was rated at 6-amps. I would have to wonder why the fuse in your amp. is only 2-amps. ??? I would bet that the fuse that ended up in your amp. is not the size needed. Hell -- my pre-amp uses a 2-amp. fuse.

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PostPosted: September 19th, 2019, 5:32 am 
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This is an AE amp kit from Cary, correct?

1. Never turn on a tube amp without any tubes (no load) when it is not on a varaic. The reason for this is that with no current, the voltage will peak up beyond the rated voltage of the capacitors, possibly damaging them.
2. A 2 Amp fast blow seems a little low to me for a stereo KT88 amp. That being said, the fact it blows does indicate you have a problem. FInd the Cary manual with respect to their recommendation.
3. I think the problem is pretty well isolated to the power supply. It can be:
a. A shorted winding in the power transformer
b. A shorted power supply capacitor
c. One of the heater winding is grounded to the chassis through poor construction (check all tube sockets), or one of the taped off unused winding are in contact with the chassis.
d. The plate supply is grounded to the chassis through poor construction (again, check all tube sockets).

YOu will need to test, UNPOWERED, each of these. You will need to isolate the power transformer windings (including B+ centertap to ground) from amp to check these. Remove connections (i.e. de-solder) to the rectifiers or connections to the first PS capacitor. The B+ winding (red) should show a resistance in the 10s of ohms between the leads. Heater windings should show a few tenths of a ohm. It should be an open circuit between the each set of windings. If you read a short or open circuit in any winding, or a short between windings, then you have a blown transformer.

If these check out OK, then inspect your power supply capacitors for any swelling or leakage of electrolyte that indicate damage. IF you seen any evidence of stress, replace.

You just need to systematically go through the amp.

David


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PostPosted: September 19th, 2019, 5:42 am 
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I just checked a product manual for a similar stereo KT88 amp, and it lists a 2A SLOW BLOW fuse.

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PostPosted: September 19th, 2019, 7:29 am 
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SoundMods and David, fuse sizes can be all over the charts. The Citation II (PP KT-88) used an 8 ampere fuse, forget if it was slow-blow though.

All good advice in posts above.

Variac and series incandescent bulb testing are essential, as has been pointed out.

If you are not familiar with the incandescent bulb deal, it's pretty simple.

Assuming the unit uses an IEC power cord, take a disposable computer monitor type el cheapo power cord. Razor knife open it near the middle for, maybe, 6". Buy a pigtail lamp socket, a 100 watt or so incandescent lamp (not LED or CFL) and a pair of Wirenuts at your favorite hardware store. Cut the "hot wire" in the power cord; the one connected to 120 volts, not ground (use your voltmeter to test). Connect the black lead of the pigtail socket to the "hot" wire coming from the wall-end of the cable. Connect the white wire of the pigtail socket to the other end of the "hot" conductor, going to the amp. you can connect the amp to 120 volts with a bulb inserted and do your testing. Keep in mind that this is deadly stuff! Don't work in a situation where you are on a concrete floor, or otherwise possibly grounded. Only one hand in the amp at a time. Wear insulated gloves if possible. If you are using multimeter probes, ensure that only the very short tip of the probe is exposed; tape the shaft if necessary.

As you identify and/or clear "faults" the pigtail light will dim or go completely out.


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PostPosted: September 19th, 2019, 7:40 am 
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Shashi,

Look at this thread on Audio Asylum. I would guess your amp is an AES AE-25 Super Amp. This confirms use of a 2Amp SLOW-BLOW fuse. Also, has test voltages for the amp to check for proper operation. Trying to find the manual, but it is tough to locate.

https://www.audioasylum.com/cgi/vt.mpl?f=tubes&m=272322

Also check out this video for general orientation:

https://www.youtube.com/watch?v=vDrEr9k3234

David


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PostPosted: September 19th, 2019, 8:21 am 
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David McGown wrote:
Shashi,

Look at this thread on Audio Asylum. I would guess your amp is an AES AE-25 Super Amp. This confirms use of a 2Amp SLOW-BLOW fuse. Also, has test voltages for the amp to check for proper operation. Trying to find the manual, but it is tough to locate.

https://www.audioasylum.com/cgi/vt.mpl?f=tubes&m=272322

Also check out this video for general orientation:

https://www.youtube.com/watch?v=vDrEr9k3234

David


Thanks David. I will look at the video this evening.
My AE 25 was upgraded by Dennis Had, probably to DJH version. It was sitting in storage for about a decade. Started using it about 3 years ago without a problem.
Called Cary a couple of years ago, they could not find the manuals.


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PostPosted: September 19th, 2019, 8:36 am 
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Location: Potomac, MD
Shashi,

If you get stuck finding the short, I could probably find it quickly with the stuff I have if you wanted to bring it over some evening.

David


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PostPosted: September 19th, 2019, 8:45 am 
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More observations:

1. The B+ power supply reservoir capacitors (large electrolytic caps on either side of hte amp), appear to be series connected with balancing resistors across the terminals. If so, this is a good thing, it is extremely unlikely the power supply capacitors are overstressed and should be good.
2. The amp appears to use a fixed bias arrangement for each channel. The AES AE-25 in the video appears to be a cathode bias version. Cannot follow the wiring, but it appears there is a test jack and pot on the back panel for adjustment.
3. From a construction point of view, I don't like the right channel signal wiring running along side the heater wiring. This could use some rerouting. Also noticed that the pairs of AC wiring out of the power transformer is not twisted (Red B+ and Green Heater). It is good practice to do so for low noise. It may be nice to rework this once you get the amp back up and working again.
4. I would keep an eye on the Jensen PIO signal caps. These are prone to drying out or leaking over time. The end seals are not hermetic. They do sound nice, but there are better choices these days.

Once you get it up an running again (try Slow-blow fuse and a re-tube, that may just fix the issue), it should be a pretty solid amp.


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PostPosted: September 19th, 2019, 9:27 am 
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Variacs are $50 on eBay. A 5 amp model should be enough. You should always have one anyway. I would not do *anything* else until you get a variac, or borrow one from someone, and a digital multimeter I sounds to me like a shorted power transformer. First, remove all tubes and measure the resistance between pins 4 and 6 on the rectifier socket. If you get 100 ohms or so, then plug the amp into a variac and SLOWLY bring up the voltage. Measure the AC voltage from either pin 4 or 6 to ground. If it doesn't start to rise quickly, stop. If all goes well you should get 175-200 VAC. Then shut down and measure the resistance between pin 8 on the rectifier socket and ground. If you don't get a dead short, try adding the rectifier and power up again, DC voltage from pin 8 to ground. If all is good you'll get upwards of 400VDC. That means no shorts in the power supply chain. Power down again, add the *input tubes only, and power up again. If you can get full voltage, then the problem is with the output tubes.


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PostPosted: September 19th, 2019, 10:50 am 
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Thanks David. I will look at the video this evening.
My AE 25 was upgraded by Dennis Had, probably to DJH version. It was sitting in storage for about a decade. Started using it about 3 years ago without a problem.
Called Cary a couple of years ago, they could not find the manuals.

If you had no problems after the amp. that sat idle for ten years, then for three years no problem -- I am getting very suspicious about the power supply electrolytic capacitors.

One or all may have finally broken down.

Having said that -- I just found some additional information -- "TUBE FUSE (Bias Circuit) •Cathode current output tube fuse. Never replace with any other fuse than a ½ Amp Fast-Blow type fuse. AC POWER FUSE •Use only 3 Amp slow-blow for 115 volt units. •Use 2 Amp slow blow on 230 V export versions."

So -- will the real recommended fuse please stand up and be recognized?

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