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Working on a 2500, ideas and questions for transformer protection

I recently inherited a 2500 and have started working on bringing this unit back to life.
Have not even tried powering it up yet, but the info I have is that it would sometimes
blow the main fuse. So I want to check everything out before attempting power up.
So far have checked resistance on the primary/secondary windings and all seem
okay. Primary seems a bit low at .9 ohms but from what I've read this is normal.

I would definitely be interested in adding additional protection for the transformer
so I'll be following this thread closely. The idea of a board to hold replacement
main caps and the additional protection fuses etc sounds like a great idea.
 
It's nice to have ya on board mikey99!

I now recall playing with the idea of an add-in board and it was going to be a tight fit and possibly not enough room. On the main filter caps, I had to stuff four 1800uF's times two in each can to get the 2 x 7200uF - per a handy tip from member 'skibjr'. The 2600 has either more capacitance, a higher voltage or both and that info is at home and I'm at work. I swear we need a grant so we can quit work and do this stuff full time!!

I believe the double spike is due to the relay kicking on and shorting out the 10 ohm limiting resistor and therefore a jump in current sourcing. It wasn't finished charging the big filters yet is the idea. This is why I was thinking about slowing down the relay engaging. This would create more heat on the 10 ohm limiter resistor so the stock 10 watt cement resistor would likely not cut it. On the 2500 & 2600 I've done I installed 25 watt versions which may be enough.

Tx, that's an interesting part. I wonder what voltage drop is incurred at idle and at some power as well. You're brave to give it a shot!

I would love to hear from some other AK "titans of tech" on this whole matter!
 
rBuckner said:
It's nice to have ya on board mikey99!

I now recall playing with the idea of an add-in board and it was going to be a tight fit and possibly not enough room. On the main filter caps, I had to stuff four 1800uF's times two in each can to get the 2 x 7200uF - per a handy tip from member 'skibjr'. The 2600 has either more capacitance, a higher voltage or both and that info is at home and I'm at work. I swear we need a grant so we can quit work and do this stuff full time!!

I believe the double spike is due to the relay kicking on and shorting out the 10 ohm limiting resistor and therefore a jump in current sourcing. It wasn't finished charging the big filters yet is the idea. This is why I was thinking about slowing down the relay engaging. This would create more heat on the 10 ohm limiter resistor so the stock 10 watt cement resistor would likely not cut it. On the 2500 & 2600 I've done I installed 25 watt versions which may be enough.

Tx, that's an interesting part. I wonder what voltage drop is incurred at idle and at some power as well. You're brave to give it a shot!

I would love to hear from some other AK "titans of tech" on this whole matter!
Click to expand...

I ordered a couple of the parts to test but I am a bit skeptical. The resistance of the part really needs to be very low after it warms up or a big chunk of voltage is going to get dumped across it.

I think that your idea of changing the timing and resistance value for the start up circuit may be a fine solution. Also it may be that the 17A peak is already good enough. I really think part of the transformer that needs to be protected is the AMP rails. I am going to install the fuses and see how they preform.
 
Well I recieved the parts today and the initial results are very interesting.

At the moment I have the NTC Thermistor wired up inline with the incoming power at the plug. Actually I have it integrated into an outlet that's connected up in a rig with the power quality analyzer so I can easily move it around. Way easier to move the test gear than the receivers!

This is the before shot from the other day of the 2600 at start up... Pay no attention to the values in the boxes, look at the peak in the graph. If I move my cursor to the peak it measures 17.1A. I tried this a couple times and sometimes it's a bit lower, depending on what part of the AC cycle the unit is energized.


28887801983_e5b809129e_c.jpg



This is a shot from this evening with the NTC Thermistor installed...
In this shot I moved the cursor so you can see what the peak current is. In several attempts at a cold start I never got above 6A...


28929478734_47b09e1e9e_c.jpg


I repeated this test warm and the current rises a bit but was much lower from the previous day tests.
I also measured the voltage drop across the part at start up and as the part warmed up and got stable. At start up it was around 10V but dropped very quickly and after about one minute it settled in at 1.75 VAC at idle. If the receiver is cranked up and drawing power the part should heat up further and the voltage drop should go even lower. It's late and the wife is watching the tube so I can't let it rip... When I checked the resistance of the part immediately after shutting the unit off it was about 1.5 ohms. There may be a bit of error in the measurement due to other parts of the circuit. The other interesting thing is that the lights in the room don't seem to dim when I hit the power now.

The part gets quite warm in operation. It would need to be located in an area where a bit of latent heat is not a concern. Around the location of the soft start resistor might be good. As I think about it you could just get a metal J box from homer d's and mount the part in line and put an outlet and cord on it, exactly like the test rig.

All in all I am quite pleased with this result. I am definitely going to install it on the 2500 and 2600.

Next thing will be the amp rail fuses. I think what I will do is measure the current draw at a known power and calculate the current at max power and size the fuses appropriately. Maybe with a lower inrush fast acting fuses can be used. I think I can test the fuses using the dummy load at a lower power and fuse rating than max and see how it works. My concern is that one of the rails dropping out could cause high DC at the speaker outputs. I suspect that the DC protection circuit would open the speaker relay. I just want to make sure that I don't get nuisance trips.
 
As a technically inept owner of a 2500, I'm thankful to those technically gifted members who are applying their knowledge and experience towards reliability improvements and solutions for this classic receiver.
:beerchug:
 
[QUOTE="txturbo, post: 9912699, member: 213089"
Next thing will be the amp rail fuses. I think what I will do is measure the current draw at a known power and calculate the current at max power and size the fuses appropriately. Maybe with a lower inrush fast acting fuses can be used. I think I can test the fuses using the dummy load at a lower power and fuse rating than max and see how it works. My concern is that one of the rails dropping out could cause high DC at the speaker outputs. I suspect that the DC protection circuit would open the speaker relay. I just want to make sure that I don't get nuisance trips.[/QUOTE]


We're you able to measure the amp rails? Any definitive conclusions to prevent current spikes?
 
Thermistors are a bad idea for Class aB amplifiers, from a performance perspective. Line current varies too much, so the source impedance is constantly changing. All that work the designers did to provide that awesome power supply for that awesome high-end piece is out the window.

The 2500/2600 already have a soft-start circuit. You could bump up the resistance on the soft-start resistor, or see if you can lengthen the delay of the relay, but the only way I'd use a thermistor with a Class aB is if the thermistor took the place of the soft-start resistor so that it got bypassed when the soft-start relay engaged. Such an arrangement also means you don't have to worry about the heating effects of the thermistor as well, since they DO get smokin' hot at large currents.
 
EchoWars said:
Thermistors are a bad idea for Class aB amplifiers, from a performance perspective. Line current varies too much, so the source impedance is constantly changing. All that work the designers did to provide that awesome power supply for that awesome high-end piece is out the window.

The 2500/2600 already have a soft-start circuit. You could bump up the resistance on the soft-start resistor, or see if you can lengthen the delay of the relay, but the only way I'd use a thermistor with a Class aB is if the thermistor took the place of the soft-start resistor so that it got bypassed when the soft-start relay engaged. Such an arrangement also means you don't have to worry about the heating effects of the thermistor as well, since they DO get smokin' hot at large currents.
Click to expand...

I am not sure about the sonic aspects of the NTC thermistor. It's certainly not going to improve it. I could do some distortion measurements and see what effect it has. What I really do not like is the fact that the part gets hot.. Like really hot. It's like putting a little heating element inside the receiver.

I think the idea of looking at the soft start resistor is a good one. Maybe there is no issue with the inrush current as it is, it's going to have some value... In any case I will buy a couple different resistors and try them out. The nice thing is that I can easily compare the results...

I think the bigger issue is the protection of the secondaries and how it relates to inrush. If fuses are used they may fry on power up unless a more aggressive approach to inrush is used.

I should have the receiver assembled in the next few days and can do some experiments.

I am curious... Does anyone have experience with a blown transformer in a 2500 or 2600? Was the transformer blown in isolation or did the transformer fail along with some other part(s).
 
txturbo said:
What I really do not like is the fact that the part gets hot.. Like really hot. It's like putting a little heating element inside the receiver.
Click to expand...
Bugs me too. But if you use a thermistor in place of the soft-start resistor so it gets bypassed...

txturbo said:
Does anyone have experience with a blown transformer in a 2500 or 2600?
Click to expand...
Nope. I've had three 2500/2600 units here through the years and all were described as having blown transformers. In reality, none did (all had blown outputs and an open soft-start resistor). So, I can't help there.
 
Discussions now seem to be focused on right where I'm at on my 2500, rail fuses would have saved me some grief ..... :)

Finally got around to powering up the 2500, (plugged into a dim bulb tester of course) and found that the soft start 10 ohm resistor
was open. Traced this to a potential short on one of the main amp boards....its the outer board on the right side of the receiver.
I knew I should have disconnected those rails before powering on the first time :(

I then disconnected the rail voltage from both channels amp boards, replaced the soft start resistor with a 12 ohm
10 W.... had a few 10 ohm 20 W but not a good idea to up the wattage here. Powered on again and now the soft start circuit
works correctly, fan runs fast at first then drops way down in speed. I get panel lights and the function selector works but nothing
on the scope.

Didn't think to measure the rail voltages at the time .....but I will do that quickly and then recap the P800 board, then track down the short
in the main amp board. Hopefully being the outer board might help access if its a shorted output transistor.
 
Maybe not the best thread to ask this question , but has anyone found a spreadsheet or Mouser/Digikey importable
parts list for the 2500 ? Mainly interested in the capacitors for now.....

I have a copy of the service manual but its time consuming going through the manual, verifying the actual part on the board, etc.
 
"but I will do that quickly and then recap the P800 board, then track down the short
in the main amp board..."

If you find the short do take a mug shot of the offending cap.;)
 
mikey99 said:
Discussions now seem to be focused on right where I'm at on my 2500, rail fuses would have saved me some grief ..... :)

Finally got around to powering up the 2500, (plugged into a dim bulb tester of course) and found that the soft start 10 ohm resistor
was open. Traced this to a potential short on one of the main amp boards....its the outer board on the right side of the receiver.
I knew I should have disconnected those rails before powering on the first time :(

I then disconnected the rail voltage from both channels amp boards, replaced the soft start resistor with a 12 ohm
10 W.... had a few 10 ohm 20 W but not a good idea to up the wattage here. Powered on again and now the soft start circuit
works correctly, fan runs fast at first then drops way down in speed. I get panel lights and the function selector works but nothing
on the scope.

Didn't think to measure the rail voltages at the time .....but I will do that quickly and then recap the P800 board, then track down the short
in the main amp board. Hopefully being the outer board might help access if its a shorted output transistor.
Click to expand...


I think what your describing is why the rails need some sort of protection.

OWNER:
1. AMP blows an output or develops a short somehow
2. Receiver draws enough current to blow the main fuse
3. Main fuse is replaced, system is powered on and now draws high current through the soft start till it opens the resistor.
4. Receiver sent for service.

Technician:
1. Finds blown resistor.
2. Resistor replaced and powered up, maybe uses current limiting device on power up. If not blows resistor again.
3. Finds bad amp section

If the technician is not familiar with the receiver maybe things don't go so well and some of the steps are repeated.

The moral to this is that the transformer is potentially subjected to several over current situations, with some percentage failing.
 
I'm at the point of replacing shorted output transistors in one channel of the 2500..... Before I put this back
together and press the power switch it might be a good time to add some type of fuse protection on the amp
rails. Not hard to do but the trick is figuring out what size fuse to add :-) And as was mentioned
previously, adding a fuse too small, and one channel dropping out suddenly when the fuse blows , might
potentially damage the speaker or other components.
 
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