So what’s the result so far.
Not only the voltage on the emitter resistors but also over the two 0.1 resististors.
Do these results match, i.e. adding your 5 measured Remitter voltages divided by 0.22 equals the measured voltage voltage over the two 0.1R divide by 0.2.
When not exactly the same, there is something else to investigate.
Hans
So far the amp has been on for about 24 hours, with the left channel open, no lid. The bias has been pretty stable so far, had gone to 65mV this morning, rising slowly since yesterday, but I think when I set it yesterday the amp was not fully warmed up so not completely stable yet. This morning I turned it down to 55-57mV and it's been relatively stable since (I saw it move to 53 mV minimum to 60mV maximum. The individual emitter voltages seem consistent with the total bias, ranging around 9-11mV. I notice the transistors that are further away from the Vcc rails have the lowest emitter voltages, but overall within 2mV more or less, so I guess sufficiently matched. The voltages over the two 0,1ohm resistors match as well, about 26 and 27mV respectively.
The right channel is closed but I checked the bias as well and it's been stable too so far.
I'm not sure whether I should keep it on for much longer, the problem is that here in Rome now is pretty hot now, and if I turn off the AC, the ambient temperature gets to about 32-33C degrees. The open channel doesn't benefit from air ventilation so it gets pretty hot, I'm not sure about the accuracy of my meter but I measured 82C degrees earlier, so I turned on the AC, now it's about 60C degrees, obviously not the 43C degrees expected, since it is not in vertical position.
Overall, it seems that there is a trend toward a very slow rise of the bias, probably 1mV every couple of hours. I'm not sure however, maybe I should observe for longer without readjusting bias if it gets to high.
Given these findings I have the doubt that what happened the other day was due to too hot ambient temperature: the amp was closed with the lid on, AC turned off, and the amp was close to a window which in the morning receive some direct sunlight (although with a thin curtain), could it have been too hot for the amp?
Also, when I mounted back the transistors on the heatsink during the repair last autumn, I noticed that after the first days of use the heatsink became very greasy, from the silicon based thermal compound somewhat dripping. I hope it's not fault of the paste, either poor quality or too old, that it melted and dripped away too much. What do you think? I could try removing one of the transistors that can be removed without disassembling the amp and check if there still is adequate quantity of thermal compound. It was the first time I used this thermal compound but it looked ok to me and I was careful to apply it correctly, just a thin layer, but the very oily/greasy heatsinks made me think...
The right channel is closed but I checked the bias as well and it's been stable too so far.
I'm not sure whether I should keep it on for much longer, the problem is that here in Rome now is pretty hot now, and if I turn off the AC, the ambient temperature gets to about 32-33C degrees. The open channel doesn't benefit from air ventilation so it gets pretty hot, I'm not sure about the accuracy of my meter but I measured 82C degrees earlier, so I turned on the AC, now it's about 60C degrees, obviously not the 43C degrees expected, since it is not in vertical position.
Overall, it seems that there is a trend toward a very slow rise of the bias, probably 1mV every couple of hours. I'm not sure however, maybe I should observe for longer without readjusting bias if it gets to high.
Given these findings I have the doubt that what happened the other day was due to too hot ambient temperature: the amp was closed with the lid on, AC turned off, and the amp was close to a window which in the morning receive some direct sunlight (although with a thin curtain), could it have been too hot for the amp?
Also, when I mounted back the transistors on the heatsink during the repair last autumn, I noticed that after the first days of use the heatsink became very greasy, from the silicon based thermal compound somewhat dripping. I hope it's not fault of the paste, either poor quality or too old, that it melted and dripped away too much. What do you think? I could try removing one of the transistors that can be removed without disassembling the amp and check if there still is adequate quantity of thermal compound. It was the first time I used this thermal compound but it looked ok to me and I was careful to apply it correctly, just a thin layer, but the very oily/greasy heatsinks made me think...
Sorry I didn't calculate voltages as you asked. I calculate from the 6 emitter resistor voltages divided by 0,22 about 268mA. Whereas by dividing the voltage across the two 0,1ohm resistors by 0,2 I get about 315mA (since the room is now much cooler the bias has risen to 63mV). I'm not sure if this is normal discrepancy given the tolerance of the emitter resistors (5%).
By the way, I have now the speakers connected (6ohm load), since they were connected the other day when the bias was 90-100mV. DC offset is 0V.
p.s. actually I noticed if I measure exactly over the two 0,1ohm resistors (not over the emitter resistors), I get about 2mV lower voltages, even though their all soldered to the same trace there's 2mV potential difference between the 0,1ohm lead and some of the emitter resistors leads, so the calculated current over the two 0,1ohm resistors is more 300mA than 315mA (is the resistance of the meter leads and the contacts of the clips causing this?).
By the way, I have now the speakers connected (6ohm load), since they were connected the other day when the bias was 90-100mV. DC offset is 0V.
p.s. actually I noticed if I measure exactly over the two 0,1ohm resistors (not over the emitter resistors), I get about 2mV lower voltages, even though their all soldered to the same trace there's 2mV potential difference between the 0,1ohm lead and some of the emitter resistors leads, so the calculated current over the two 0,1ohm resistors is more 300mA than 315mA (is the resistance of the meter leads and the contacts of the clips causing this?).
Last edited:
Apart from your thermal conducting grease, everything else seems O.K.
Thermal grease should be able to withstand high temp without desintegrating into an oily substance, so you may have used an inferior make.
I never noticed this happening.
But I realise that correcting this involves quite some labour, so I hope it is still doing its job properly in conducting heat from the TO3’s to the heatsink.
If not, your transistors will become too hot and thermal runaway is on its way.
If possible try to measure the temp of your TO3 transistors and compare that to the temp of your heatsink. Differnce should only be a few degrees.
Hans
A quick measurement on the right channel shows about 3C degree difference, max 4 (46-7C vs. 49-50C degrees). Of course I will try to make more accurate measurements on all transistors. If this is the cause of the runaway, do you think it needs fixing? The issue happened only once so far, the other day because it was really hot and the amp was in front of the window. Also, I realize that I never used the amp during the summer season, because I got it in September 2020, and last summer I didn't have speakers and didn't use the amp. So who knows if the problem could have happened anyway.
In case, do you have any suggestions for thermal compound that I can find online?
Thanks
p.s. I really hope it's not necessary, changing the thermal paste would really be hard work...silly mistake from me that I didn't use a known reputable make at the time..
In case, do you have any suggestions for thermal compound that I can find online?
Thanks
p.s. I really hope it's not necessary, changing the thermal paste would really be hard work...silly mistake from me that I didn't use a known reputable make at the time..
Last edited:
Dow Corning 340 is the standard heat sink compound.
I don't know the oil matters much. The oil is there to evenly distribute the solid particles across the sink. After 20 years the oil evaporates, the transistor is left with only the particles transferring the heat.
Don't understand the dividing by 6 in post 244. 60 mv / .22 ohm = 272 ma. That is too much idle current. You want 40 ma or less, ideally 20 ma. Each transistor stands or overheats alone.
I don't know the oil matters much. The oil is there to evenly distribute the solid particles across the sink. After 20 years the oil evaporates, the transistor is left with only the particles transferring the heat.
Don't understand the dividing by 6 in post 244. 60 mv / .22 ohm = 272 ma. That is too much idle current. You want 40 ma or less, ideally 20 ma. Each transistor stands or overheats alone.
The 60mV are the sum of all 6 pairs of output transistors, so 272mA of idle current sounds ok to me. As far as I know, idle current per each transistor should be 45mA, not 20mA. In my experience the amp sounds bad with such low idle current.
Anyway, I found the thermal compound I used: https://shopware.donau-elektronik.d...eitpaste/3860/wp235-waermeleitpaste-35-g-tube and emailed the manufacturer to ask if it is adequate for use in my application/working temperature. And I also mentioned the oily substance that dripped after first application.
p.s. I learn from here that it is actually NOT silicon based, but made of a synthetic grease, and that it's made for applications up to 150C degrees. Still, I'm not sure whether the oily substance is to be expected...
Last edited:
Sometimes the oil & particles separate in the tube due to too long sitting on the shelf. You have to mix it by squeezing the tube up & back. To repeat, the oil does nothing to conduct heat.
Multiplying idle current may help figure out your transformer & fan requirement, but doesn't do ***** for figuring out whether your transistors are going to withstand use. 12 mv per transistor doesn't add up to 60 mv in a parallel circuit. Voltages add in series circuits, currents add in a parallel circuit. Get your Kirchoff's laws right. If you have 60 mv on any individual OT emitter resistor, the idle current is way out of norms.
I'm totally suspect of amps with .22 emitter resistors, anyway. Designed to die, IMHO. My 650 w/ch amp with 5 pairs TO3 MJ15024/25 per channel has .5 ohm emitter resistors. Two 5 watt 1 ohm resistors parallel for each transistor. BTW my amp brand was known in the 1980's 90's for 24/7 watt ratings suitable for beach bars & dance halls.
Multiplying idle current may help figure out your transformer & fan requirement, but doesn't do ***** for figuring out whether your transistors are going to withstand use. 12 mv per transistor doesn't add up to 60 mv in a parallel circuit. Voltages add in series circuits, currents add in a parallel circuit. Get your Kirchoff's laws right. If you have 60 mv on any individual OT emitter resistor, the idle current is way out of norms.
I'm totally suspect of amps with .22 emitter resistors, anyway. Designed to die, IMHO. My 650 w/ch amp with 5 pairs TO3 MJ15024/25 per channel has .5 ohm emitter resistors. Two 5 watt 1 ohm resistors parallel for each transistor. BTW my amp brand was known in the 1980's 90's for 24/7 watt ratings suitable for beach bars & dance halls.
Last edited:
When only 4 degrees temp difference is measured, I would leave it as it is.
Now that both channels have bias currents of ca. 275mA, exactly as it should be, what current is drawn from 220V mains ?
If still 1.2A that would be a bit worrying.
Hans
Ok I will check later when I get home. However, when I checked the mains current drawn previously, bias was also set to about 275mA. In the manual I read that power consumpion at idle should be approximately 225W, at 220V it should be 1A. Would my 1,2A still be concerning?
Regarding the thermal compound I received the documentation from the manufacturer: thermal conductivity is 0,8W/mk (not great right?), application area -30C to 150C degrees, melting point 50C degree (?). This explains the oily dripping I experienced, but since it is rated to work up to 150C degrees I guess the melted grease is not an issue.
225W is probably on the safe side, also valid for countries with 110V mains.
The reason I asked is because of your humming tranformer.
When 1,2A is drawn instead of some 0.8A, it could mean an interwinding short in the transformer, consuming 80watt, enough to let a transformer hum.
The 1.2A you are referring to, is that rms or average current, that makes a difference.
Hans
Ok, current is 1.3A now. The amp was well warmed up but I had to turnit off to connect the meter, it started at 1,4A but now I think it’s stable at 1,3A after a few minutes. I couldn’t find it on the meter’s manual but I think it’s RMS measurement since I read AC TRMS on the display. What do you think? Both transformers hum, does it mean they both have internal shorts?
See post 236. Best test for internal short of transformer is to measure maximum wattage out with a load resistor. P=(v^2)/Z where Z is load impedance. Audio power amp transformers drive 8 ohm or 4 ohm loads, see your spec sheet. Bargain amp SWTC tiger was known for buzzing transformers.
Dow corning 340 has conductivity of .0014 cal/sec-cm-degC . Calorie(g) to watt-sec conversion is 4.18
Do you mean a load on the secondary? These transformers have a lot of windings and performing such a test would be very hard work just to disassemble everything and setting a test safely. I would only do it if it’s strictly necessary.
They have 30+ years so it is possible they have been damaged at some point or some parts have become loose. As far as I know these transformers are high quality and are not supposed to hum if working properly.
Amps I've worked on there is a way to disconnect secondary from bridge rectifier, butt splice, insulate connection, and run 14 ga wires over to load resistors on their stand. The only winding that uses significant watts is the rail to the rectifier to the output transistors. That is usually the winding that would have a shorted turn. That is the one I test for volt-amps or wattage. I have 8 ohm & 5 ohm 225 watt log resistors, suitable for this test or for testing the music on speaker output after repair.
Reputation doesn't mean much on a 30 year old used amp. The transformer rivets shouldn't be loose in that price point, but you hear what you hear.
Frankly an amp with rattling transformer that I intended to use at 1/8 to 70 w in my music room, wouldn't last very long in my collection. Especially one with with history of thermal runaway. I bought a working CX302 for $115 last winter. Fan is noisy but suitable for installing behind the wall of a church to power the auditorium. For $170 I bought a working CS800s 2 years ago with a much quieter fan.
Reputation doesn't mean much on a 30 year old used amp. The transformer rivets shouldn't be loose in that price point, but you hear what you hear.
Frankly an amp with rattling transformer that I intended to use at 1/8 to 70 w in my music room, wouldn't last very long in my collection. Especially one with with history of thermal runaway. I bought a working CX302 for $115 last winter. Fan is noisy but suitable for installing behind the wall of a church to power the auditorium. For $170 I bought a working CS800s 2 years ago with a much quieter fan.
Last edited:
Yes you are right, it is not very likely that both transformers are having issues.
And with all those secondaries you can’t do a simple test.
But what could be done as a start is to measure the primary current of both individual transformers through P301 / P302 or P323 / P324.
Hans
Ok, this look easy enough to do. Will try to do it in the next days. What are we looking for? Large differences between the two channels?
You may not be able to but I could. No test, no knowledge. You start with the main rail voltage, 3 or 4 wires. I'd pull the 2 wires br303 wiggle, solder load wires to it, measure AC voltage into 8 ohm. rating 200 w/ch into 8 ohms, Two 4 ohm 225 w resistors for the test, wired series. I put those logs on metal stands to keep them off the floor or the coffee table. Then when BR303 tested, test br302 for the other transformer. You'll need some heat shrink tubing to insulate the wire splice while you are testing.
If each transformer can't put out 300 va on the +-83 winding, it is scrap. At 50 hz a cheap dual slope integration DVM would adequately measure the AC voltage.
If they can put out the right voltamps, then you just have noisy capable transformers.
Last edited: