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250M Won't Come Out Of Protection After Complete Rebuild of Both Amps.

Davidmac5

Davidmac5

AK Subscriber
Subscriber
Hi all,

I just completed rebuilding both the left and right amps on a 250M, I replaced everything, pre drivers, drivers, outputs all with matched HFE, I replaced the differential pair, both filter caps, etc. Brought her up using my DBT and all is fine except she will not come out of protection. I'm able to set the bias and DC offset no problem, I get 54.60 volts at the + and - posts on both amps. If I disconnect both amps from the filter caps the protection relay clicks, if I reconnect them the relay does nothing, if I connect one amp and not the other I get nothing, if I switch sides I get nothing, disconnect again relay clicks reconnect zip zero nada. What am I missing here? This amp should be singing like a Canary but it's not. Any help figuring this one out would be greatly appreciated.
 
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You didn't say if it was working before the rebuild so I'm assuming it was. Since it's something you did to make this happen you're going to have to go back over all of your work.

Check to be sure parts are in the correct direction, that they're the right parts. Check all of your solder joints, check for solder bridges. Etc., etc. etc.
 
You may need to rebuild the protection circuit, but in the mean time check all the voltage inputs to the protection circuit. Either you have DC on the outputs or something on the amp boards is dragging down the voltages the protection circuit "senses".
 
I failed to mention in my first post that this amplifier did not work at all when I received it. The right channel outputs were all shorted, there were a couple of burn spots on the circuit board where those resistors had burned up and the pre-drivers had been replaced as well as the burnt resistors, someone prior to my acquiring it did some replacement of those defective components. The right channel board did not have any issues that I could find but it didn't work either so I went ahead and rebuilt it also.

Just checked the right channel output and I have 53 volts on it, so the problem is there, left channel seems to be fine. Just need to figure out where that 53 volts is coming from in right channel.
 
Davidmac5 said:
Just checked the right channel output and I have 53 volts on it, so the problem is there, left channel seems to be fine. Just need to figure out where that 53 volts is coming from in right channel.
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you will need to check resistors. I've had a few resistors drift and cause a similar problem.

I'd only power the left channel with a DBT (150w bulb). remove the B+ and B- on the left channel at the PS caps. no sense blowing up your good channel by mistake.

From memory, measure the voltage at the yellow/green/blue wires to the MJE182 temp compensation transistor (mounted on the heat sink). If they don't match the schematic, you will need to determine why.

I'd compare voltages on the schematic with your measurements to help narrow down the problem. If all the transistors are good and not blown, the resistors are your next thing to check.

if you get stuck, post your findings and I'll try to help.
 
I checked the yellow/green/blue wires going to MJE180 on the heat sink and I'm getting 50 to 51 volts on each wire and that is with the wires disconnected from MJE180, the schematic calls for 0V/-0.7/+0.7 transistors Q507 and Q508 have been replaced, MJE180 tests good, so not sure where to look next.
 
As mentioned above check the resistors I also had some that were highly drifted you can compare both channels to see if you find a difference, there’s also a few zener diodes on those boards which I always change as a precaution also worthwhile rebuilding the protection board as mentioned it’s easy and caused issues on all of my 250m over the years the sps transistors on them are usually suspect as are the capacitors goodluck with it
 
Will any zener diode work? The service manual just says zener diode it does not give a specific description or number.
 
Focus on the resistors. I've done many 250/240/1200 amps and haven't seen a zener failure. Check the 39ohm ones.

The protection board is an easy rebuild too. The relay should not close if you are getting b+ on the output. I'm not at my desk but I'm pretty sure I posted transistor subs on one of my posts.

I've increased the time before it engages too by increasing the 220uF cap on the board (I hope that is the one).
 
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I just looked up a service tag on one. Blown channel. Q801-804 (outputs) and q505 and q507.

Since your transistors are good (and installed correctly), resistors are next.
 
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This 250M is kicking my tail, I have checked every transistor, transistor pin orientation, numerous resistors, solder joints, possible solder bridges, I disconnected the output wire to the relay board and I still have 53 volts at output pin 19, I replaced Q507, Q508, Q505, Q506, Q501, Q502, R529, R530, R527 and R528, I have double checked everything and my problem remains, I'm missing something but I will be darned if I know what it is.
 
Well I checked every component on the right channel and compared it to every component on the left channel which is working fine and the only discrepancy I could find are the two zener diodes on the right channel have continuity in both directions and the working left channel zeners only have continuity in one direction. I'm going to replace the zeners on the right channel but the parts manual doesn't tell me which one, it just says zener, does anyone know if there a specific one I should use with the 250M? Thanks to everyone for the help.
 
Davidmac5 said:
Well I checked every component on the right channel and compared it to every component on the left channel which is working fine and the only discrepancy I could find are the two zener diodes on the right channel have continuity in both directions and the working left channel zeners only have continuity in one direction. I'm going to replace the zeners on the right channel but the parts manual doesn't tell me which one, it just says zener, does anyone know if there a specific one I should use with the 250M? Thanks to everyone for the help.
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They are 17v zener diodes, the cross reference list I've seen calls for 1N5247. You would probably know if they weren't working since you'd be missing your + and - 17.7 or so V used in the input side of the amplifier causing most if not all of the voltages on the left-hand side of the 250M< schematic to be wrong
 
Swimmer brought up something. You should make sure your power supplies are working. +-17 on the front end and b+- on the outputs.

Feel free to post pics of the board. You didn't say but someone might see something wrong.
These are very difficult to get working. If you plan to fix this yourself, you will need to check voltages, resistor values and even caps. Even a cap installed backwards will cause problems.

Since one channel works (I think), you can compare sides. I always disconnect the working side so any mistakes won't blow it up but I don't know your skill level.
 
Davidmac5 said:
This 250M is kicking my tail, I have checked every transistor, transistor pin orientation, numerous resistors, solder joints, possible solder bridges, I disconnected the output wire to the relay board and I still have 53 volts at output pin 19, I replaced Q507, Q508, Q505, Q506, Q501, Q502, R529, R530, R527 and R528, I have double checked everything and my problem remains, I'm missing something but I will be darned if I know what it is.
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It'll be the last thing you check.....
 
Well I finally got the right channel sorted out and I have a clean sign wave signal at term 19 output, but I still have no signal to the right channel speaker so moving to the protection circuit board I discovered that the right channel contact in the relay is fried. What to do now? Anyone have a good used 250M protection relay? lol. Any thoughts on how I solve this problem? Thanks for your input.
 
250M working again. After rebuilding both amps I replaced the fried protection relay which was the last remaining issue with a Omron relay I rewired and the amp is working perfect both channels. This one was kicking my tail but I finally beat it. Thanks to everyone for your valuable and appreciated input.
 
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