So it powers on and you hear the click to indicate the power amp has also just switched on but then......
The speaker cone shoots forward and there is a "zoooogt" noise with each forward excursion. It does not fire the cones backward what so ever. It ran great for over a decade for me up until maybe 4 years ago.
I have no respectable experience with power amp repair, design or modification. I am 6 months into my electronic engineering degree and do not start learning about power amps for a few weeks. I have a fair amount of experience in custom preamp design but again, no familiarity with standard industry circuits. I've always taken the you design it, you build it, you blame nor thank nobody but yourself for the outcome.
My father gave me this amp nearly 20 years ago so I would really like to salvage it. I won't make any theoretical guesses about what's wrong because I am truly ignorant about these symptoms.
I will however degrime it until maybe we can get it running again!
It's unbelievable how good all the small caps look.
The speaker cone shoots forward and there is a "zoooogt" noise with each forward excursion. It does not fire the cones backward what so ever. It ran great for over a decade for me up until maybe 4 years ago.
I have no respectable experience with power amp repair, design or modification. I am 6 months into my electronic engineering degree and do not start learning about power amps for a few weeks. I have a fair amount of experience in custom preamp design but again, no familiarity with standard industry circuits. I've always taken the you design it, you build it, you blame nor thank nobody but yourself for the outcome.
My father gave me this amp nearly 20 years ago so I would really like to salvage it. I won't make any theoretical guesses about what's wrong because I am truly ignorant about these symptoms.
I will however degrime it until maybe we can get it running again!
It's unbelievable how good all the small caps look.
Ah yes, very good observation!
The single direction throw would be a sign of DC.
I will dummy load test them and post my findings.
Thanks man!
Yes, that's a possibility, especially since the electrolytic caps in this amp are most likely in need of replacement. The speakers are DC-coupled to the output stage, so any DC-offset that gets introduced anywhere in the amp finds its way to the speakers. The "protection block" (F-2041 in schematics) should, however, detect this and disengage the speaker relay RL901.
Another possibility is a failed component (other than already mentioned elcos).
Please measure the DC-voltage on the speaker terminals of both channels. According to page 19 it must be 0 V with +/- 30 mV tolerance.
Edit: it seems that pre-amp and power amp can be disconnected from eachother by removing the links on the rear panel, see page 6 (PRE-OUTPUT and MAIN-INPUT).
If you do measure DC-offset on the speaker terminals, then remove the links and measure again. If the DC-offset disappears, the fault is not in the power-amp but in the pre-amp.
Another possibility is a failed component (other than already mentioned elcos).
Please measure the DC-voltage on the speaker terminals of both channels. According to page 19 it must be 0 V with +/- 30 mV tolerance.
Edit: it seems that pre-amp and power amp can be disconnected from eachother by removing the links on the rear panel, see page 6 (PRE-OUTPUT and MAIN-INPUT).
If you do measure DC-offset on the speaker terminals, then remove the links and measure again. If the DC-offset disappears, the fault is not in the power-amp but in the pre-amp.
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Thanks for the patience and help guys.
I am getting DC!
First, I connected an 8 ohm dummy load and sure enough, DC every time the relay switched on. I'm assuming the relay is part of the protection circuit?
I see it arch and turn on then off. With each on click, 30 volts DC.
Then, I connected it without the dummy load.
The amp switches on and I get a constant 30 volts DC. No relay switching the circuit in and out.
This is is occurring on the Left and Right channels of both amps A and B.
I will disconnect the preamp today or tomorrow and we will then see if I am still getting DC. If the relay switches in and out with the dummy load on, would this indicate that it is a DC blocking cap on the power amp that has gone bad?
I am curious to know if this is a common indication or if it still involves further testing to specify where the faulty capacitor is.
I must also confess.
I have had a small handful of audio electronics die always knowing it was likely due to a part costing either a few cents or maybe a couple dollars and was ignorant so rarely did I mange to repair the item.
Now however it's just so very exciting to know I will be able to repair just about anything that gets ill, especially once I've completed my degree and have a bench full of test equipment!
I am getting DC!
First, I connected an 8 ohm dummy load and sure enough, DC every time the relay switched on. I'm assuming the relay is part of the protection circuit?
I see it arch and turn on then off. With each on click, 30 volts DC.
Then, I connected it without the dummy load.
The amp switches on and I get a constant 30 volts DC. No relay switching the circuit in and out.
This is is occurring on the Left and Right channels of both amps A and B.
I will disconnect the preamp today or tomorrow and we will then see if I am still getting DC. If the relay switches in and out with the dummy load on, would this indicate that it is a DC blocking cap on the power amp that has gone bad?
I am curious to know if this is a common indication or if it still involves further testing to specify where the faulty capacitor is.
I must also confess.
I have had a small handful of audio electronics die always knowing it was likely due to a part costing either a few cents or maybe a couple dollars and was ignorant so rarely did I mange to repair the item.
Now however it's just so very exciting to know I will be able to repair just about anything that gets ill, especially once I've completed my degree and have a bench full of test equipment!
I sometimes use an incandescent light bulb of some great voltage than the supply rails (Ej, a 48V lamp for +-40V amplifiers), so you may work without your hand at the VOM. The current in the lamp and then its power, must be in the margin of the SOA of the amplifier, the the lowest wattage of it, the best. Obviously, as the guy said, disconnect the speaker(s).
Yes, it is.
Better not do that too many times. The arching wears out the contacts.
Hmm, the protection circuit should switch off the relay wether or not the dummy load is connected. The arching may have welded the contacts on the relay shut.
See if turning the speaker selection knob to and fro between SPKR OFF and A + B makes the relay click. If it clicks, measure if in SPKR OFF position, you still have DC.
Which may indicate a problem in a circuit common to both channels, e.g. a power supply rail. Measure if rail voltages are present (see page 15, B1..B4) and check the fuses. Given the offset voltage of 30 V, I wouldn't be surprised if the negative rail has failed (which should measure approx. -31 V).
If the fuses are still OK (measure continuity, sometimes a fuse looks to be OK, but isn't) make sure they contact the fuseholder well (measure continuity of the fuse from outside the fuseholder). If (one of) the fuses have (has) blown, things get more complicated.
Don't make any assumptions yet and take one step at a time. Please first check the voltages and report back.
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Look for an accredited online school recognized by your department of education. I'm enrolled at CIE which is Nationally accredited and though it's a correspondence technical college, has survived 80 years with these methods. Good school. I have a few complaints, a couple big but overall, I would still rate them a solid 7.5-8 out of 10 after my 1st semester.
It's cheap, respected and they finance you. I've found a lot of students that under official documentation have gone onto get their masters at irl colleges, work for major corporations, the military, authored books, industry handbooks etc. Even if I take 2.5 or 3 years to get the 2 year Associates degree, I will have done it for about $6.5K to $8K USD.
jitter so far the relay has survived. I was concerned about the contacts as well and have avoided turning it on more than necessary but so far, no destruction or welding the contacts together.
I was going to disconnect the preamp and run the same DC tests because it's so easy to just pull the rear jumpers. I meter tested the fuses a few years ago when it died which were new and, still functional.
I had started to assume what you are saying. Since it's occurring on all channels, in all preamp input settings and with all the filters switched in or out, I'm probably looking at a faulty cap in my power supply.
I will first measure the power rails to determine if I have a ground failure. I did however also measure AC through the speaker terminals when I tested for DC and it was also present.
One of the major concerns I have seen expressed on this forum is that speaker relays may get welded shut in a major overcurrent situation, like DC-offset at power rail levels. But if they are fine, than I still find it strange that they do not switch under no load conditions. Did the manual switching off of the speaker outputs yield a different result?
Please do, it may eliminate part of the circuit to look for the fault.
Repeat anyway as I suggested. Bad contacts have a way of being intermittent.
With a short in the power supply rail as a result of (e.g.) a shorted output transistor, you would have a blown fuse. If that is indeed not the case, then the excessive AC may indicate a dried out smoothing cap in the psu. A scope would be very useful now, it would reveal excessive ripple.
But first things first: rail voltages before and after fuses.
I hate to admit I'm "that guy" but when I checked the fuses again, sure enough a quick blow for A and for B were blown. I will have replacements later today however, I am certain that when the amp went, I replaced them and I know that did not resolve the issue. I believe the reason I shelved it is because it ate through the pack of fuses I bought and in those days, I knew absolutely nothing outside of standard car stereo install circuits, passive guitar circuits and speaker crossover circuits.
However, best case scenario is most definitely that it was just a fuse issue but I won't bank on it.
However, best case scenario is most definitely that it was just a fuse issue but I won't bank on it.
the power amp stage is a fairly good example of the "Lin" topology for power amplifiers.
I would definitely suspect dried out capacitors here. Dried out C811/C812 would definitely make things behave oddly. I think at this stage you would end up chasing your tail over faults related to dried out caps, and it would be best to consider blanket renewal of all electrolytics first.
I would definitely suspect dried out capacitors here. Dried out C811/C812 would definitely make things behave oddly. I think at this stage you would end up chasing your tail over faults related to dried out caps, and it would be best to consider blanket renewal of all electrolytics first.
This is what I assumed from square one since it's such a popular fix for countless devices but, I also wanted the experience of trouble shooting like a pro and the help has certainly been appreciated.
Just for the heck of it, I took a pic and yes, it's as grungy as it looks..grungier in fact. I hit it with a lot of industrial contact cleaner and the air hose which took off absolutely nothing!
An externally hosted image should be here but it was not working when we last tested it.
You'll definitely wanna give that a good clean! You can tell here they were still building solidstate amps like valve amps of old
Shotgun replacing all capacitors is a justified action on a piece of equipment this old - it's not really considered a "non pro" tactic. You may find the fault is cured by just this alone, but you're equally as likely to find a problem still exists. The main issue is, the circuit is unlikely to be operating correctly as it is now with all those old dried out caps not doing their job - especially the power supply smoothing capacitors. For example you may well be seeing oscillation simply because the power supply has too much ripple.
To really know for sure about things like supply ripple, you would need to use an oscilloscope which would show you the amount of ripple. However, simply based on age alone, i would expect it to now be unacceptably high
Shotgun replacing all capacitors is a justified action on a piece of equipment this old - it's not really considered a "non pro" tactic. You may find the fault is cured by just this alone, but you're equally as likely to find a problem still exists. The main issue is, the circuit is unlikely to be operating correctly as it is now with all those old dried out caps not doing their job - especially the power supply smoothing capacitors. For example you may well be seeing oscillation simply because the power supply has too much ripple.
To really know for sure about things like supply ripple, you would need to use an oscilloscope which would show you the amount of ripple. However, simply based on age alone, i would expect it to now be unacceptably high
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Right. IMHO those 50/50 odds aren't good enough, hence my suggestion to take some measurements first. You need to establish if the amp is still worth the time, effort and money needed to fix it. What good is it to you if you replace all caps only to find that a lot more has failed?
First step: measure rail voltages (in front of the fuses). Measure DC as well as AC. Excessive ripple will probably show not only as high AC voltage but also as a DC voltage that's not close to the + or - 31 V it should be.
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