They don't burn up or look any different [you wouldn't know unless you measured them] but when they go open circuit you can hear the *pop* as it happens. AC present at J851 is 47v-0v-47v. I've removed & tested each rectifier diode & they are all fine.
I've only just remembered but when it came to me a previous owner had "piggybacked" every dead 1R resistor on the board with 0R resistors... I dismissed it at the time as a lazy previous repairman but could that explain what's going on ? Maybe it caused some permanent damage.
I've only just remembered but when it came to me a previous owner had "piggybacked" every dead 1R resistor on the board with 0R resistors... I dismissed it at the time as a lazy previous repairman but could that explain what's going on ? Maybe it caused some permanent damage.
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Hmm, something of a breakthrough ! I noticed one of R851's legs felt "wobbly" and sure enough the solder joint had failed causing an intermittent connection - I just reflowed the joint & +/-57v is working again under load... Strange as it was a completely unrelated resistor yet had a massive effect on the stability of the whole board.
Isn't R851 one of the ones that failed originally ?
It just sounds to me like there is something really intermittent going on. A favourite trick of many techs (sort of a last resort 😀) would be to link all the 1 ohms out and literally blow the faulty up... in other words see what goes pop. Its not as crazy as it sounds actually, as long as you realise what could happen such as a cap blowing its top etc. Its a technique that works though... I speak from many years as a repair tech.
Or... you could try the bulb tester and see if that shows anything. The circuit might latch in a stable but faulty state drawing enough current to keep the fault alive but letting you measure and see whats dragging things down. If there is doubt over the tranny then that might be the safest option if it comes down to the wire.
It just sounds to me like there is something really intermittent going on. A favourite trick of many techs (sort of a last resort 😀) would be to link all the 1 ohms out and literally blow the faulty up... in other words see what goes pop. Its not as crazy as it sounds actually, as long as you realise what could happen such as a cap blowing its top etc. Its a technique that works though... I speak from many years as a repair tech.
Or... you could try the bulb tester and see if that shows anything. The circuit might latch in a stable but faulty state drawing enough current to keep the fault alive but letting you measure and see whats dragging things down. If there is doubt over the tranny then that might be the safest option if it comes down to the wire.
Intermittent ? I couldn't agree more 😀
... Its doing it again & I'm sure I saw a spark from D866. It was working perfectly for 5 minutes 🙄
... Its doing it again & I'm sure I saw a spark from D866. It was working perfectly for 5 minutes 🙄
It has to be worth changing them if in doubt.
Don't know why they use series pairs, you could just replace them all with four 1N4004 or 4007's. I'm doubtful tbh though that one diode is the culprit. Even if it failed short the other in the pair would work, and if it went open it wouldn't cause the 1 ohms to fail.
Worth changing though 🙂
Don't know why they use series pairs, you could just replace them all with four 1N4004 or 4007's. I'm doubtful tbh though that one diode is the culprit. Even if it failed short the other in the pair would work, and if it went open it wouldn't cause the 1 ohms to fail.
Worth changing though 🙂
I liked the sound of that so just tried it ! I "linked out" both R859 & R860 and the 57v returned... Nothing gets hot, nothing smells & no nasty noises but both R851 & R852 are now open circuit 😱 Its like it will only allow either the 18v or 57v circuit to operate... try both & its goodbye 1R resistors ! If I were to "link out" all four aforementioned resistors it would be back to how I received it but with new capacitors.
*Rectifiers now replaced with 1N4004 btw & still the same.
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Its a technique to find the fault, not intended to leave permanently like that.
Could the resistors you are fitting be underated in some way. Its not really normal practice to use standard resistors for locations like that. Usually a "fusible" metal oxide type resistor is specified which when they fail do so quietly and with no smoke.... smoke makes customers uneasy, even if it is only a tiny resistor doing it 😀
Don't rush things, make sure it stays working first with no more dramas.
Could the resistors you are fitting be underated in some way. Its not really normal practice to use standard resistors for locations like that. Usually a "fusible" metal oxide type resistor is specified which when they fail do so quietly and with no smoke.... smoke makes customers uneasy, even if it is only a tiny resistor doing it 😀
Don't rush things, make sure it stays working first with no more dramas.
Yes, that would be a bit naughty... The resistors are 1R 0.25w Carbon Film according to the label which is what was originally fitted & what the service manual specifies unless I've missed something.
I just "linked out" all four 1R resistors [R851, R852, R859 & R860] and everything works perfectly so your theory of underrated resistors sounds very plausible to me.
I just "linked out" all four 1R resistors [R851, R852, R859 & R860] and everything works perfectly so your theory of underrated resistors sounds very plausible to me.
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When you say works perfectly, is that just the board on its own or the whole amp ? If just the board then you need to be careful when you come to try the whole amp. I would definitely use a bulb tester then, its asking for trouble not to do so.
And if the whole amp is working... then fingers crossed.
(its very unusual as I mentioned to have normal carbon resistors as a safety feature... fuse... because they are unpredictable in how they behave. Although the average current may be low, peak charging pulses on each cycle are high. Very strange......)
And if the whole amp is working... then fingers crossed.
(its very unusual as I mentioned to have normal carbon resistors as a safety feature... fuse... because they are unpredictable in how they behave. Although the average current may be low, peak charging pulses on each cycle are high. Very strange......)
Just the board in isolation. I've left it powered up since my last message, checking voltage & temp every couple of minutes - Voltage is spot on & everything is cool... Theres a minor buzz from the transformer but that's entirely normal here, the only one's that don't buzz are potted [due to my 258v mains I think - I've seen it at 262v on the odd occasion but the leccy company aren't interested!] Lots of dead smoothing caps thanks to them 😡
I'll sort out a dim bulb tester before connecting the rest of the amp.
As ever, I'm very grateful for your help 🙂
I'll sort out a dim bulb tester before connecting the rest of the amp.
As ever, I'm very grateful for your help 🙂
There is transformer buzz in critical moment, could be transformer failure. I would strongly suggest not to power amp without 1ohm resistors.
Dont worry, I wont power the amplifier up without either higher rated 1R resistors or a dim bulb tester... So far its just been the reg board & nothing else.
Transformer buzz is entirely normal here, everything does it... 260vac into a 230vac primary doesn't go down too well !
Transformer buzz is entirely normal here, everything does it... 260vac into a 230vac primary doesn't go down too well !
The leccy company have to be interested by law. 262 is way outside the legal requirements. The buzzing is caused by harmonics on the supply (not a totally pure sinewave) or a slight overall "DC offset" (caused by devices that draw power unequally from each cycle such as half wave rectifiers, thyristor controlled power devices etc)
Yeah I think the legal max is 253v but they simply stated "no one else has complained."
I'm moving in 6 weeks so wont be my problem any more ! FYI I'm currently in the deepest, darkest depths of Cornwall which is how they can get away with it IMO.
I'm moving in 6 weeks so wont be my problem any more ! FYI I'm currently in the deepest, darkest depths of Cornwall which is how they can get away with it IMO.
The new resistors arrived this morning & I've just fitted them. They are 1w fusables.
Now 18v measures 19.5v under load & 57v measures 63v under load... Hooray ! Fingers crossed that's the end of the matter !
Now 18v measures 19.5v under load & 57v measures 63v under load... Hooray ! Fingers crossed that's the end of the matter !
If only Mooly, if only !
All of the power supplies are behaving now but the relay's wont engage yet all output transistors are okay, I'm able to set the bias etc etc - Very odd 😕
Too tired to deal with it any more tonight... I hate this amp 😀
All of the power supplies are behaving now but the relay's wont engage yet all output transistors are okay, I'm able to set the bias etc etc - Very odd 😕
Too tired to deal with it any more tonight... I hate this amp 😀
Huge DC out the outputs - 26v ! Suppose I better check the outputs & drivers again... But surely I wouldn't be able to set the bias if they were damaged ? ... I'll compare the board voltages to the schematic & see if there's any major discrepancies. Also, I was wrong before... Its not the transformer buzzing its the relay [L801] located on the main power supply board chattering so fast it sounds like a buzzing transformer.
An externally hosted image should be here but it was not working when we last tested it.
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😀 I see its "one of those" amps with opto controlled bias.
Without having the unit to work on and get a feel for whats happening I would suggest you,
1) Bulb tester.
2) Short out the vbe multiplier on both channels to force the bias to zero. That is C to E on that darlington.
3) Short the relay out and then power it up and see whats what.
Without having the unit to work on and get a feel for whats happening I would suggest you,
1) Bulb tester.
2) Short out the vbe multiplier on both channels to force the bias to zero. That is C to E on that darlington.
3) Short the relay out and then power it up and see whats what.