Ok. I understand what we want to try. However, what I am calling the first 5227 by the two MR-500 diodes has more or less supply voltage on all three legs. It is the only 5227 that I had direct continuity with the red turn on lead and the MR-500's. It does not make sense to me. The two back to back 5227 closer to the heat sink with the 4 output transistors seem to be the ones that need jumped? The collector on those two was 0v at rest and fluctuated between 0-6v when trying to power up the amp.
However, those two 5227 do not show a direct connection to the red turn on lead or MR-500's diodes. Are they the Second 5227's? The schematic is for a single pr-250 not a pr-2100 and I think I am seeing some minor differences on top of realizing I have two of most everything. But it seems to me these 5227's are the ones to jump collector to emitter? Also, could I just jump B+ to the collector of the suspect 5227? I could make up a fused jumper?
However, those two 5227 do not show a direct connection to the red turn on lead or MR-500's diodes. Are they the Second 5227's? The schematic is for a single pr-250 not a pr-2100 and I think I am seeing some minor differences on top of realizing I have two of most everything. But it seems to me these 5227's are the ones to jump collector to emitter? Also, could I just jump B+ to the collector of the suspect 5227? I could make up a fused jumper?
No, even if it is in good working order. I think in terms of basic car circuits(relays, switches, etc.) I'm a mechanic and have been fooled by a test light glowing but not a fan motor running; no current.
I will give it a try tomorrow. The amp is at my shop and I tinker between customers.
Thanks and fingers crossed!
Oh, the output transistor is being difficult to find a data sheet on and I know I need to replace all on the affected channel. Maybe you can give me a suggestion when I reach that point?
I will give it a try tomorrow. The amp is at my shop and I tinker between customers.
Thanks and fingers crossed!
Oh, the output transistor is being difficult to find a data sheet on and I know I need to replace all on the affected channel. Maybe you can give me a suggestion when I reach that point?
Well I tried jumping the first 5227 and no reaction at all. That first 5227 has supply voltage on all three legs without activating the turn on lead. Does not make sense to me but also no wonder nothing happened when jumped. It took all I had in me to not try jumping the "second" set of 5227's close to head sink.
I don't understand these old circuits as well as I should. For some, timing was everything and the small electrolytic caps could cause all sorts of malfunctions. As an example, some of these supplies needed a pulse at some point in the circuit to start the oscillation of the supply.
The tantalum caps in these older amps were also problematic.
The tantalum caps in these older amps were also problematic.
Ok great. I went ahead and tried jumping the second 5227's. Outer legs together. Amp starts making a steady state high frequency "singing". My amp clamp showed it pulling 13amps! Nothing seemed to get hot, but I only jumped for like 10-20 seconds at a time.
I know that is more than enough time to pop something but nothing happened as far as heat/smoke! It is too awkward(and scary) to jump the 5227 and try to measure for rail voltage. Maybe with a more suitable jump wire I can free up my hands.
I did not see the pre-amp come on when I jumped the second 5227, no indicator light.
I feel I may be getting into trouble but still have not blown the two 5 amp fuses.
I know that is more than enough time to pop something but nothing happened as far as heat/smoke! It is too awkward(and scary) to jump the 5227 and try to measure for rail voltage. Maybe with a more suitable jump wire I can free up my hands.
I did not see the pre-amp come on when I jumped the second 5227, no indicator light.
I feel I may be getting into trouble but still have not blown the two 5 amp fuses.
Hi Perry, CHEM2010,
Sorry for the wait, a lot going on here.
Before starting to do anything else, we will assume that the amplifier will get fixed at the end of this, so let's get rid of a few problems first. Replace all those tantalum capacitors as they do love to short unexpectedly. I've been thinking of replacing them with the poly-aluminum capacitors to retain the good high frequency characteristics. If you would rather replace them with electrolytic capacitors, use the ones designed for switching power supplies. Use the highest voltage rating that you can get and will fit without being too big.
The one component that stands out for me is the 2.2uF, 35 V capacitor that runs from the center tap of the secondary to the voltage sense line that comes from the pair of 1N4003 rectifier diodes back to the base of the voltage adjust transistor through a 2.2K resistor and the 1K0 pot.
This amp starts oscillating because of an imperfect match on the secondary windings that sends a pulse that is fed back to the bases of the switching transistors (MJE3055T). The voltage is controlled by the amount of base drive through the MR500 diodes.
It's been decades since I have worked on this amplifier family and I'm running on hazy memories, so please bear with me in case I remember something wrong.
Also, since the filter capacitors are working at several KHz, it would be a fantastic idea to replace them with types intended for high frequency circuits. Only bother doing this if you either have a defective capacitor for sure, or the amplifier is showing signs of life.
To turn these amps on, I used a light bulb (that also indicated it was supposed to be running as well). 1A at 12 V will work fine as most head units would draw about this much if they didn't have a "high power" amplifier built in (rated at 20 watts per channel, an optimistic claim).
If you developed a DC offset, this amplifier will continue to run. It takes a short to drag the sense line down enough to shut it down normally. So your output change should have fixed it. However, you need to replace both output transistors and carefully check the 68 R resistors for an open.
I used to modify the amplifier by putting another resistor from supply to the point driven by the op amp, then move the driving point to the midpoint of the two diodes. Adding a capacitor across those two diodes (one across the pair) also helps reduce distortion a bit. Don't use a TL072 or equiv. to replace the op amp as they have very poor output current capabilities. If you get it running, you could use an OPA2134P that should be a little better than the original op amp used. But that's if you are into improving these amps. Back then, any improvements we welcomed by the owners as the only other choice would have been the MGT 2100 or 2200 (excellent amps also). I'd love to get an old MGT 2100 to rebuild as I have some ideas I want to try. I gave mine to a guy who wasn't the friend I thought he was.
Let me know how you do on this. Too bad I'm so far away, because sometimes you just have to have it in front of you on your own bench for the memories to really come back.
-Chris
Sorry for the wait, a lot going on here.
Before starting to do anything else, we will assume that the amplifier will get fixed at the end of this, so let's get rid of a few problems first. Replace all those tantalum capacitors as they do love to short unexpectedly. I've been thinking of replacing them with the poly-aluminum capacitors to retain the good high frequency characteristics. If you would rather replace them with electrolytic capacitors, use the ones designed for switching power supplies. Use the highest voltage rating that you can get and will fit without being too big.
The one component that stands out for me is the 2.2uF, 35 V capacitor that runs from the center tap of the secondary to the voltage sense line that comes from the pair of 1N4003 rectifier diodes back to the base of the voltage adjust transistor through a 2.2K resistor and the 1K0 pot.
This amp starts oscillating because of an imperfect match on the secondary windings that sends a pulse that is fed back to the bases of the switching transistors (MJE3055T). The voltage is controlled by the amount of base drive through the MR500 diodes.
It's been decades since I have worked on this amplifier family and I'm running on hazy memories, so please bear with me in case I remember something wrong.
Also, since the filter capacitors are working at several KHz, it would be a fantastic idea to replace them with types intended for high frequency circuits. Only bother doing this if you either have a defective capacitor for sure, or the amplifier is showing signs of life.
To turn these amps on, I used a light bulb (that also indicated it was supposed to be running as well). 1A at 12 V will work fine as most head units would draw about this much if they didn't have a "high power" amplifier built in (rated at 20 watts per channel, an optimistic claim).
If you developed a DC offset, this amplifier will continue to run. It takes a short to drag the sense line down enough to shut it down normally. So your output change should have fixed it. However, you need to replace both output transistors and carefully check the 68 R resistors for an open.
I used to modify the amplifier by putting another resistor from supply to the point driven by the op amp, then move the driving point to the midpoint of the two diodes. Adding a capacitor across those two diodes (one across the pair) also helps reduce distortion a bit. Don't use a TL072 or equiv. to replace the op amp as they have very poor output current capabilities. If you get it running, you could use an OPA2134P that should be a little better than the original op amp used. But that's if you are into improving these amps. Back then, any improvements we welcomed by the owners as the only other choice would have been the MGT 2100 or 2200 (excellent amps also). I'd love to get an old MGT 2100 to rebuild as I have some ideas I want to try. I gave mine to a guy who wasn't the friend I thought he was.
Let me know how you do on this. Too bad I'm so far away, because sometimes you just have to have it in front of you on your own bench for the memories to really come back.
-Chris