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 Post subject: Allied 369 - Dead Output Channel
PostPosted: May Tue 30, 2017 6:41 pm 
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Joined: Dec Thu 08, 2011 4:03 am
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Location: Orland Park, IL
So here's the story. I have a 2 channel (stereo) Allied 369 amplifier with only one working channel output. The schematic below is for one channel. I drew it rather quickly, so expect there to be a couple mistakes. I discovered that R2 went up in smoke and Q1 had a dead short. Q2 was also questionable. I ended up replacing both Q1 and Q2 transistors with TIP120's and Q3 with an NTE159M just to be on the safe side. Plugged it in and no luck. I tried replacing the other three transistors with 2N2222's. After that, both R1 and R3 started smoking. Trying to undo whatever mistake I made, I changed back the three transistors leaving only the three I replaced initially. After plugging it in, R2 and R4 started to smoke. This failure closely resembled the problem the circuit had to begin with.

Before this thing starts on fire, I decided to post here. If somebody could pinpoint where I went wrong that would be great. Otherwise, I am going to try hunting down the original transistors and replace them all at once.

By the way, I checked caps for shorts and checked the resistors in circuit.
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 Post subject: Re: Allied 369 - Dead Output Channel
PostPosted: Jun Mon 05, 2017 5:38 pm 
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Joined: Dec Thu 08, 2011 4:03 am
Posts: 1464
Location: Orland Park, IL
HELP!!

I just replaced all six transistors with NTE equivalents and no luck. There is about 4V across R2 while the working channel has almost no voltage across. I believe R1, R2, R3, and R4 are all getting hot but I'm afraid to plug this thing in for too long to find out.

One thing I forgot to draw on the schematic is the capacitor value after the fuse and speaker output. Value of the cap is 1000uF at 25V and the output is at the negative side of that capacitor.

I am powering up the bad channel with no input or output loads connected, but I don't think that should cause smoke.


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 Post subject: Re: Allied 369 - Dead Output Channel
PostPosted: Jun Mon 05, 2017 5:45 pm 
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Joined: Jan Thu 01, 1970 1:00 am
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Location: Albuquerque, NM 87123
Replaced with NTE equivalents? I think not. A TO220 is NOT a replacement for a TO3, and using all 2222 isn't likely to help.


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 Post subject: Re: Allied 369 - Dead Output Channel
PostPosted: Jun Mon 05, 2017 5:47 pm 
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Joined: Dec Thu 08, 2011 4:03 am
Posts: 1464
Location: Orland Park, IL
The NTE equivalent for a 2SD91 is a TO220 package. Look it up in the NTE site.

I took out all of the 2N2222's a while ago and started over. That was a bad idea.


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 Post subject: Re: Allied 369 - Dead Output Channel
PostPosted: Oct Fri 20, 2017 5:42 pm 
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Joined: Dec Thu 08, 2011 4:03 am
Posts: 1464
Location: Orland Park, IL
I'm back to fighting with this thing. The root cause was the 560 ohm pot, which opened up. I replaced that, along with all of the bad transistors. Now, it doesn't smoke, but Q4 keeps failing. I replaced every electrolytic capacitor with exact values, checked the tolerances of every resistor again, and replaced Q4 for the third time. At this point, everything was working fine, and the channel sounded great. The success was short lived though. After a few minutes, the channel failed. I haven't checked Q4 since, but I assume it failed again, since the biasing is all messed up again as it was before.

I am replacing Q4 with NTE382. NTE doesn't give an exact cross reference for that transistor. Does someone have a datasheet for C904?


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 Post subject: Re: Allied 369 - Dead Output Channel
PostPosted: Oct Fri 20, 2017 7:18 pm 
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The originals where to-66's which a to -220 can replace .

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 Post subject: Re: Allied 369 - Dead Output Channel
PostPosted: Oct Fri 20, 2017 8:11 pm 
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Location: Albuquerque, NM 87123
What were the numbers on the original transistors? I suggest trying to get those. Also, operating an amplifier with incorrect bias is just asking for burnt up transistors.

NTE often lists replacements that "probably" will work. A transistor with a totally different package is a sign they don't have the correct replacement.


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 Post subject: Re: Allied 369 - Dead Output Channel
PostPosted: Oct Fri 20, 2017 11:39 pm 
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http://www.datasheetarchive.com/pdf/dow ... uery=2SD91

A TIP 41C should work fine for a 2SD91 , TIP 41 C's are dirt cheap .

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 Post subject: Re: Allied 369 - Dead Output Channel
PostPosted: Oct Sat 21, 2017 10:32 pm 
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Joined: Feb Sat 12, 2011 2:29 pm
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Location: Fayette County, Pa
With all the smoked transistors / resistors this has endured there is a god chance some parts not evidenced of burning mah have been stressed / changed values. Unless these are also replaced you will continue to go through this.

You have one good channel so use that as a point of reference. You will also need a variac to bring the voltages up slowly to prevent problems. There are three key points to watch as you do this. Once all parts have been checked / verified good / or replaced. Start by applying about 20 VAC to the line. The amp probably won't work at this point, but measure the voltages at the + side of the 1000 mf cap on both channels (bootstrap). They should be about the same. Alos monitor voltages across R1 and R2. They should remain under .2 Voltas at all times.

If the first measurements are similar, raise the line voltage to about 40 volts and repeat. As before voltages should be similar. Note that around 50 volts bias stability should occur and the bootstrap voltage should be about half of B+ with respect to ground.

If still within specs, raise the line to about 75 volts. By now the good channel should be working as should the repaired channel. (You may need to use a signal generator to obtain the test signal since the front end of the amplifier may not work at the reduced voltage.) In any event both channels should be the same as for voltages. Let it run like this for about 10 minutes and keep close tabs on voltages. it is possible your problem is drifting bias / thermal runaway. If anything is drifting turn off and discontinue tests until the cause is isolated / repaired. You should also adjust bias at this point to prevent possible over current. This is common at this stage.

If it is still good, raise the line voltage to about 90 volts. By now the amplifier should be working on both channels. Repeat measurements / observations as done in the previous step. Bootstrap must now be at half B+, otherwise discontinue test and troubleshoot. Let it run like this for about 10 more minutes. Recheck bias to ensure within specs.

If still good raise line voltage to 120 volts and monitor for any heat build up. Be certain R1 and R2 stay at under .2 volts, and the bootstrap remains at half of B+. I recommend a burn in time of at least 2 hours before calling it fixed. Durring this time watch for any irregularities, especially heat build up or hum in the audio which may indicate an imbalance. If any of these occur, discontinue tests and troubleshoot. Do a final bias adjustment and set to design specs

The previous series of tests are the basic process I would use on a newly built / untested amplifier to qualify it as operational. I have adapted them to your amp using your schematic. You may need to further modify them to suit your amp, but this process usually gets an amp into specs without blowing up a lot of parts.


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 Post subject: Re: Allied 369 - Dead Output Channel
PostPosted: Nov Fri 03, 2017 5:08 pm 
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Joined: Dec Thu 08, 2011 4:03 am
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Location: Orland Park, IL
Well, I ended up biting the bullet and replacing every part on the board. Since I don't have a variac, I very briefly plugged it and monitored the bootstrap. Then, plugged it in slightly longer, and finally, kept it on for over an hour and closely monitored the bootstrap. As a final test, I connected the signal and cranked the volume as high as it would go to see if it would throw the biasing off. Switching between AM and FM would give it quite a jolt, but it would quickly correct itself.

The only conclusion I could come up with is, one of the resistors may have been drifting with the heat. Then, after switching from AM to FM, it through the bias off and smoked a transistor. We'll never know for sure. Anyway, the stereo is all back together and sounding great. Thanks for the assistance.


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 Post subject: Re: Allied 369 - Dead Output Channel
PostPosted: Nov Sat 04, 2017 10:10 pm 
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Location: Vancouver Island, Canada
What I see here is a mis-diagnoses, no offense. You started replaceing transistors that probably would have tested good.
So for next time, give it some more time before you rip into it. 8)
I have done the same, so it is not like I'm pointing only at you. Congrats on getting it fixed, too.

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 Post subject: Re: Allied 369 - Dead Output Channel
PostPosted: Nov Sun 05, 2017 9:22 pm 
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Joined: Dec Thu 08, 2011 4:03 am
Posts: 1464
Location: Orland Park, IL
I actually did test each transistor and still have them, with the exception of Q4 which kept failing. I ended up replacing them all again, which was more a matter of aggravation than anything. It was faster and cheaper to do so rather than feeding this board parts one at a time and watching it all go up in smoke. Six months is enough time for me to be working on this amp. Unfortunately, as you said, I was not able to pinpoint the problem. We can conclude that neither the transistors nor the electrolytic capacitors were causing the failure of Q4. All of the resistors tested good. This leaves the following: One or more of the resistors were drifting with the heat, or one (or more) of the mica caps were leaking. I think the resistor drifting is more likely. These DC coupled amps are extremely flaky and sensitive.


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 Post subject: Re: Allied 369 - Dead Output Channel
PostPosted: Nov Sun 05, 2017 11:10 pm 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 16835
Location: Albuquerque, NM 87123
I seriously doubt that is a direct coupled amplifier. I see a transistor mount on the heat sink that has no transistors under them; that is for bias transistors--I hope those were installed. Could be for bias diodes. If those were left out then you still have a problem.


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