Hello all,
sorry for my first post immediately asking for help! Just this has been driving me absolutely nuts and I am in desperate need of finding out what's wrong. I'm kicking myself over this. I'm sure someone here will be able to help me.
I've had this Hammond BC organ for nearly 2 months now, but haven't been able to play it properly at all! When I first got it, there was a loud hum coming through my external speaker. This was a bad RCA cable, so replacing this got rid of the hum. All set then! Except when I put it all back together, the volume was extremely soft. I was very lazy when I went in the back...
The pre-amp has screw terminals, so no soldering required. I thought I'd be lazy and mess with the wires while it was still switched on as to not have to go through the start-up process many times. "What's the harm, they're only audio cables" I thought. What a mistake! I'll attach a schematic below, but basically the audio wire, still attached to the Right "G" terminal, came into contact with a bare section of the B+ voltage wire which was attached to the B+ terminal. A big bluey-green spark happened for a split second, these were only in contact for the slightest fraction of a second. I immediately turned it off and tightened all wires firmly. The volume was back at full, with no hum, but now this crackling/popping.
First thought was Output Transformer, always thinking the worst. All the transformers run extremely cool, they don't even heat up a fraction. So I'm pretty sure they're fine. My engineer came round and said the transformers should be fine. He thought it was the 57 valve. I've tried 3 replacements, as well as 2x replacement 56's, but no change happens. We thought this because giving the 57 a bit of a knock stops the noise. Even cleaned the sockets, but no use. Wobbling the 57 still does stop the noise eventually though.
Any help much appreciated. Could it be a capacitor or resistor? Any way I can tell? Preferably with it turned off, I'm still a bit skeptical of going in and doing the 'tap' test. I only have a multimeter, no oscilliscope or other tools. Any ways I can definitely eliminate the output transformer or not? The noise stops completely with the 56 removed.
Schematic: http://www.captain-foldback.com/Hammond_sub/schematics/a_preamp_later.gif
YouTube video of noise: https://www.youtube.com/watch?v=-GRjJaFSzbo
Any help is really appreciated. My engineer will be taking a look very soon, but I thought I may as well see if I can eliminate anything 100%. Not like I can make it any worse, I've learnt not to be an idiot now, the hard way...
sorry for my first post immediately asking for help! Just this has been driving me absolutely nuts and I am in desperate need of finding out what's wrong. I'm kicking myself over this. I'm sure someone here will be able to help me.
I've had this Hammond BC organ for nearly 2 months now, but haven't been able to play it properly at all! When I first got it, there was a loud hum coming through my external speaker. This was a bad RCA cable, so replacing this got rid of the hum. All set then! Except when I put it all back together, the volume was extremely soft. I was very lazy when I went in the back...
The pre-amp has screw terminals, so no soldering required. I thought I'd be lazy and mess with the wires while it was still switched on as to not have to go through the start-up process many times. "What's the harm, they're only audio cables" I thought. What a mistake! I'll attach a schematic below, but basically the audio wire, still attached to the Right "G" terminal, came into contact with a bare section of the B+ voltage wire which was attached to the B+ terminal. A big bluey-green spark happened for a split second, these were only in contact for the slightest fraction of a second. I immediately turned it off and tightened all wires firmly. The volume was back at full, with no hum, but now this crackling/popping.
First thought was Output Transformer, always thinking the worst. All the transformers run extremely cool, they don't even heat up a fraction. So I'm pretty sure they're fine. My engineer came round and said the transformers should be fine. He thought it was the 57 valve. I've tried 3 replacements, as well as 2x replacement 56's, but no change happens. We thought this because giving the 57 a bit of a knock stops the noise. Even cleaned the sockets, but no use. Wobbling the 57 still does stop the noise eventually though.
Any help much appreciated. Could it be a capacitor or resistor? Any way I can tell? Preferably with it turned off, I'm still a bit skeptical of going in and doing the 'tap' test. I only have a multimeter, no oscilliscope or other tools. Any ways I can definitely eliminate the output transformer or not? The noise stops completely with the 56 removed.
Schematic: http://www.captain-foldback.com/Hammond_sub/schematics/a_preamp_later.gif
YouTube video of noise: https://www.youtube.com/watch?v=-GRjJaFSzbo
Any help is really appreciated. My engineer will be taking a look very soon, but I thought I may as well see if I can eliminate anything 100%. Not like I can make it any worse, I've learnt not to be an idiot now, the hard way...
Can you be sure that this problem hasn't actually been there all along ?
The big give away here is that you say its "physical" and responds to prodding the board which could be anything from a dry joint to a faulty part.
Am I mistaken, or is the crackle only present when there is audio coming through... as if the crackle is introduced earlier on.
The big give away here is that you say its "physical" and responds to prodding the board which could be anything from a dry joint to a faulty part.
Am I mistaken, or is the crackle only present when there is audio coming through... as if the crackle is introduced earlier on.
No this crackle definitely was not there before the accident. I did have this working 100% fully. Then when I assembled and moved the organ back the volume was soft. Still, no crackle. Only after this accident was there crackle.
The crackle is present as soon as the valves warm up. It happens all the time after the valves have warmed up, whether audio is coming through or not.
At first it was very intermittent and at worst still very soft. Over time it's becoming more constant and louder - it's getting worse as time progresses. It doesn't seem to be getting worse with use, but just time. I can leave it sitting for a week without use, but the problem is then worse than the week before. It hasn't gotten too much worse recently though, but I would still say it gets worse with time.
The crackle is present as soon as the valves warm up. It happens all the time after the valves have warmed up, whether audio is coming through or not.
At first it was very intermittent and at worst still very soft. Over time it's becoming more constant and louder - it's getting worse as time progresses. It doesn't seem to be getting worse with use, but just time. I can leave it sitting for a week without use, but the problem is then worse than the week before. It hasn't gotten too much worse recently though, but I would still say it gets worse with time.
No problem
The wiper is the middle terminal of the tone control that goes to R99. If that point is connected to ground (where C8 goes) then the amp should be silent. Its possible that C8 could be the culprit. If you lifted (to isolate) one end of either the cap or R12 then it would remove it from the circuit. No crackle... faulty cap. Its a long shot but worth a try. There are only those few components that could have suffered directly with the high voltage.
Thanks Mooly for all the help so far! My schematic reading skills aren't too bad, but still definitely a newbie here, so a complete noob! Everyone has to start out somewhere though and I'm definitely interested in learning as much as possible, even when all my gear is working! Definitely in this for the long haul, hopefully I'll get the hang of it soon enough! Telling me what components to check is a brilliant help, I was having a bit of a hard time identifying the possible culprits from the schematic.
I'm very fortunate this is a BC (one of the very first 1930s Hammonds). The later 1950s and 1960s models get much more complicated! I have one of those too, however.... :-D
Do I need a soldering iron to do any of these tests? I chucked mine out a while ago... Sorry for all the questions, but as you can see, due to my very limited understanding I need things explained to me in quite a bit of detail...
I'm very fortunate this is a BC (one of the very first 1930s Hammonds). The later 1950s and 1960s models get much more complicated! I have one of those too, however.... :-D
Do I need a soldering iron to do any of these tests? I chucked mine out a while ago... Sorry for all the questions, but as you can see, due to my very limited understanding I need things explained to me in quite a bit of detail...
Well I have a confession too I don't normally work on valve gear at all but hey... they use the same bits and bobs as solid state
Yes, you will need a soldering iron at some point to replace or undo components.
Any of those components around the input (the grid) of the first valve could have suffered with the overvolts. I'm still dubious that the fault seems "physical" though. Have you tried tapping those components with an insulated tool of some sort. C37 is another possible too.
I don't normally work on valve gear at all but hey... they use the same bits and bobs as solid state Yes, you will need a soldering iron at some point to replace or undo components.
Any of those components around the input (the grid) of the first valve could have suffered with the overvolts. I'm still dubious that the fault seems "physical" though. Have you tried tapping those components with an insulated tool of some sort. C37 is another possible too.
Well, electronics is electronics I guess!
OK, that's definitely on my list. Hey, it also figures that my soldering skills aren't great either! I'll definitely keep out of this beauty with a soldering iron, but I will practise my soldering skills again soon. One bad accident (followed some bad advice off someone else) gave me a bit of a scare so I chucked anything electronics away!
I'll keep to tests which don't require soldering for now anyway. I know these will be limited though.
I see that if any overvolts went down the G terminal line, the Output Transformer would've gotten damaged. Is that correct? As I've said, transformers are fine after hours of use, so I doubt there's any arcing etc there. Just wanted to check my schematic skills out there a bit. I wouldn't have guessed anything by the tone control would be a problem though, so I have some lessons to do I see! I was thinking it could've gone straight down to R51 or R58, but this was just a guess, I still have a lot to learn. I'm just wondering what would make this problem seem to be around the area of the 57, apart from of course the 57 being noisy, or is that why you're suspecting the area around the 57? I know things can come across differently on the internet, just to be clear, I'm not doubting you, just curious and am tyring to learn a bit in the process!
Well there's a problem with that. The 57 is very high gain, so it has this huge metal shield around it. It's also one of those with a little metal cap on top to connect it to the (input?) grid. Removing that metal can makes it a bit noisy anyway, but touching it makes a noise like a mircophonic valve would make. This is supposed to be normal though due to it being high gain. I've tried 3 NOS 57s, so it can't be the valve. The valve socket/base isn't loose at all either. I don't know why it's showing physical signs to the 57 valve, but I've tried everything I (and my engineer) cna think of there with no success. We really did think it was the 57. We're thinking resistors or capacitors now, really it surely has to be now, but any testing I can do would be great for my own curiosity.
I also forgot to mention - there's a B+ power supply in the bottom of the organ. This is a small amplifier, it has a power transformer, two electrolytic can capacitors and a rectifer valve (5U4 if I recall correctly). Is it possible that this could be causing the problem? Maybe any way to isolate that which I'm not thinking of? To me though, the preamp does seem more likely, but this is just a thought.
OK, that's definitely on my list. Hey, it also figures that my soldering skills aren't great either! I'll definitely keep out of this beauty with a soldering iron, but I will practise my soldering skills again soon. One bad accident (followed some bad advice off someone else) gave me a bit of a scare so I chucked anything electronics away!
I'll keep to tests which don't require soldering for now anyway. I know these will be limited though.
I see that if any overvolts went down the G terminal line, the Output Transformer would've gotten damaged. Is that correct? As I've said, transformers are fine after hours of use, so I doubt there's any arcing etc there. Just wanted to check my schematic skills out there a bit. I wouldn't have guessed anything by the tone control would be a problem though, so I have some lessons to do I see! I was thinking it could've gone straight down to R51 or R58, but this was just a guess, I still have a lot to learn. I'm just wondering what would make this problem seem to be around the area of the 57, apart from of course the 57 being noisy, or is that why you're suspecting the area around the 57? I know things can come across differently on the internet, just to be clear, I'm not doubting you, just curious and am tyring to learn a bit in the process!
Well there's a problem with that. The 57 is very high gain, so it has this huge metal shield around it. It's also one of those with a little metal cap on top to connect it to the (input?) grid. Removing that metal can makes it a bit noisy anyway, but touching it makes a noise like a mircophonic valve would make. This is supposed to be normal though due to it being high gain. I've tried 3 NOS 57s, so it can't be the valve. The valve socket/base isn't loose at all either. I don't know why it's showing physical signs to the 57 valve, but I've tried everything I (and my engineer) cna think of there with no success. We really did think it was the 57. We're thinking resistors or capacitors now, really it surely has to be now, but any testing I can do would be great for my own curiosity.
I also forgot to mention - there's a B+ power supply in the bottom of the organ. This is a small amplifier, it has a power transformer, two electrolytic can capacitors and a rectifer valve (5U4 if I recall correctly). Is it possible that this could be causing the problem? Maybe any way to isolate that which I'm not thinking of? To me though, the preamp does seem more likely, but this is just a thought.
My reasons for "suspecting the area around the 57" are mainly based on you saying that you can wiggle that particular valve and get the noise to stop. If you remove the 56 valve it stops too. So that all points to the problem being around that area. If the problem were in the output stage which is after this point then it would still make the noise.
You need to isolate the stages to try and pin this down. Does it make the noise if you remove the first stage 57 valve while leaving the 56 valve in place ?
The transformer T4 looks like an inter stage phase splitter that will feed the main output amplifier.
In your first post I was thinking too, that you had applied the B+ voltage to the input (with talking of RCA cables and so on) but I see you mean you applied it to the secondary of T4. That could be a problem because it will have created a tremendous voltage spike on the primary winding as well as on the secondary due to the inductance of the unit, and its possible therefore that the insulation has been compromised. But again, that doesn't fit with the fact that you can wiggle V57 and get it to stop.
You need to isolate the stages to try and pin this down. Does it make the noise if you remove the first stage 57 valve while leaving the 56 valve in place ?
The transformer T4 looks like an inter stage phase splitter that will feed the main output amplifier.
In your first post I was thinking too, that you had applied the B+ voltage to the input (with talking of RCA cables and so on) but I see you mean you applied it to the secondary of T4. That could be a problem because it will have created a tremendous voltage spike on the primary winding as well as on the secondary due to the inductance of the unit, and its possible therefore that the insulation has been compromised. But again, that doesn't fit with the fact that you can wiggle V57 and get it to stop.
Yes basically B+ voltage (about 200V DC) got applied to the secondary of T4. The wires came into contact with each other, so I would assume this is what happened. Only for a fraction of a second, but I know in electronics that can be more than long enough to cause damage!
T4 was my first thought and worry. But I was told by someone on a Hammond mailing list that if the noise stops with the 56 removed, it shouldn't be T4. My logic here was that with the 56 removed, T4 has no input, but is still making an output, hence if the noise was originating in T4 it could and would still be output. Is this making any sense, or am I talking rubbish?
Well I thought if the insulation was damaged that the transformer would start smoking, or even set on fire? At least it should overheat or have a bad smell, but after an hour of use again today, nothing. No heat, no smell, thankfully definitely no smoke or fire either! After being on for about 6 or 7 hours in total after the accident, all transformers are still cold to the touch and no bad smells or smoke.
I'll remove the 57 tomorrow and report back with what happens. Only so much moving of 25-odd stone organs I can do in a week, it's quite a performance, especially in my small room!
T4 was my first thought and worry. But I was told by someone on a Hammond mailing list that if the noise stops with the 56 removed, it shouldn't be T4. My logic here was that with the 56 removed, T4 has no input, but is still making an output, hence if the noise was originating in T4 it could and would still be output. Is this making any sense, or am I talking rubbish?
Well I thought if the insulation was damaged that the transformer would start smoking, or even set on fire? At least it should overheat or have a bad smell, but after an hour of use again today, nothing. No heat, no smell, thankfully definitely no smoke or fire either! After being on for about 6 or 7 hours in total after the accident, all transformers are still cold to the touch and no bad smells or smoke.
I'll remove the 57 tomorrow and report back with what happens. Only so much moving of 25-odd stone organs I can do in a week, it's quite a performance, especially in my small room!
Thats making perfect sense Fortunately valves tolerate overloads far better than semiconductors, a spike lasting a microsecond can kill a semi, with a valve its a case of an overload either flashing over internally in a valve and damaging it or heat buildup causing the anode to start to glow and overheat.
Passives such as resistors and caps could suffer with a few seconds overload and transformers are tough and stand a lot of abuse.
So because the noise stops with 56 removed, then thats a pretty good assumption the tranny is OK but its not conclusive proof. Under correct operating conditions there is a current flow in the primary and consequently a volt drop across the primary. Under those conditions an interwinding short could show. That doesn't happen with 56 removed. So we have to test now with 56 in place but with no input connected to it, and the easiest way to do that is to pull valve 57. Thats a rough and ready way to isolate the stages but it still leaves the anode load and coupling cap in the mix at this point.
So depending on the outcome of the test with 57 removed, that determines where we look next. If its silent then the problem would appear to be around 57. If its not silent then we look at isolating and checking the parts around 56, in particular the caps. Only after all that can we say that the problem might be the tranny.
Fortunately valves tolerate overloads far better than semiconductors, a spike lasting a microsecond can kill a semi, with a valve its a case of an overload either flashing over internally in a valve and damaging it or heat buildup causing the anode to start to glow and overheat. Passives such as resistors and caps could suffer with a few seconds overload and transformers are tough and stand a lot of abuse.
So because the noise stops with 56 removed, then thats a pretty good assumption the tranny is OK but its not conclusive proof. Under correct operating conditions there is a current flow in the primary and consequently a volt drop across the primary. Under those conditions an interwinding short could show. That doesn't happen with 56 removed. So we have to test now with 56 in place but with no input connected to it, and the easiest way to do that is to pull valve 57. Thats a rough and ready way to isolate the stages but it still leaves the anode load and coupling cap in the mix at this point.
So depending on the outcome of the test with 57 removed, that determines where we look next. If its silent then the problem would appear to be around 57. If its not silent then we look at isolating and checking the parts around 56, in particular the caps. Only after all that can we say that the problem might be the tranny.
Done the testing today as agreed.
With the 57 valve removed, the crackling and popping is still present. With the 56 removed however, the noise goes completely. With the 56 removed, the volume of the organ output is incredibly soft (I assume this is perfectly normal), but I could actually play the organ and hear it playing without any crackling or popping - wow, that was a nice change! Like I said, extremely soft volume, but at least I know the organ is capable of giving out a sound without the crackling.
Interesting observations: With the 56 valve inserted, but the power of the organ switched off and even the mains cable completely disconnected (so it has no connection to the mains at all), if I tap on the 56, every time I do there's a small pop, almost like tapping on a microphone just a bit quieter. Even with the organ switched off for 5 minutes, so there's no heat left in the valve, and the mains cable fully disconnected, still get the pop! If I tap the top with the organ switched on, then there's a buzz/hum, kind of like the sound you'd get from a microphonic valve I guess? But it does this with all three 56s I have.
I hope this helps a bit and maybe makes it a bit clearer what could be suspect and what to test next.
With the 57 valve removed, the crackling and popping is still present. With the 56 removed however, the noise goes completely. With the 56 removed, the volume of the organ output is incredibly soft (I assume this is perfectly normal), but I could actually play the organ and hear it playing without any crackling or popping - wow, that was a nice change! Like I said, extremely soft volume, but at least I know the organ is capable of giving out a sound without the crackling.
Interesting observations: With the 56 valve inserted, but the power of the organ switched off and even the mains cable completely disconnected (so it has no connection to the mains at all), if I tap on the 56, every time I do there's a small pop, almost like tapping on a microphone just a bit quieter. Even with the organ switched off for 5 minutes, so there's no heat left in the valve, and the mains cable fully disconnected, still get the pop! If I tap the top with the organ switched on, then there's a buzz/hum, kind of like the sound you'd get from a microphonic valve I guess? But it does this with all three 56s I have.
I hope this helps a bit and maybe makes it a bit clearer what could be suspect and what to test next.
OK, that seems to show the 57 stage isn't the culprit. I'm slightly puzzled that you can hear it well enough to play with 56 removed. Puzzled as in "how is the audio getting through". There is the interwinding capacitance that will couple audio but enough to be usable ? Hmmm.... You see one end of the primary is open with 56 removed. Could there be some kind of internal short I wonder.
What happens if you tap the transformer rather than the valve ? Perhaps try that by switching off, then quickly pulling the valve and try tapping ?
I'll think some more over this...
What happens if you tap the transformer rather than the valve ? Perhaps try that by switching off, then quickly pulling the valve and try tapping ?
I'll think some more over this...
Well it isn't really usable at all, it's incredibly soft and half the tones of the organ aren't even audible whatsoever, only the upper quarter tones are audible and even then I'm picking them out. Definitely isn't usable, so maybe this is just normal due to the interwinding capacitance?
I've tapped the transformer with it switched on. The thing literally doesn't heat up the slightest, none of the transformers do actually. Hammonds are incredibly light on their valves (hence why most still have the original 80 year old valves) and transformers. If I tap on it with it switched on, nothing happens at all. No sound, no change. There's no sound from tapping the transformer with the 56 either in or out.
Incidentally, the transformers in the older models such as the BC are particularly robust, much more so than any other Hammond or nearly any other amplifier made. As my engineer said: short of getting a hammer and smashing the thing, not much can really damage these old transformers. I've heard similar said by others. Of course, this is me, so knowing my luck I probably have damaged it, but it gives an idea of how strong these transformers are.
I look forward to hearing what you suggest next anyway, I'm quite confused by this now! Could it maybe be any old carbon composition resistors by the 56 stage?
I've tapped the transformer with it switched on. The thing literally doesn't heat up the slightest, none of the transformers do actually. Hammonds are incredibly light on their valves (hence why most still have the original 80 year old valves) and transformers. If I tap on it with it switched on, nothing happens at all. No sound, no change. There's no sound from tapping the transformer with the 56 either in or out.
Incidentally, the transformers in the older models such as the BC are particularly robust, much more so than any other Hammond or nearly any other amplifier made. As my engineer said: short of getting a hammer and smashing the thing, not much can really damage these old transformers. I've heard similar said by others. Of course, this is me, so knowing my luck I probably have damaged it, but it gives an idea of how strong these transformers are.
I look forward to hearing what you suggest next anyway, I'm quite confused by this now! Could it maybe be any old carbon composition resistors by the 56 stage?
Yes, it certainly could be a carbon comp that's playing up. Having done all the tests thus far, and it still being somewhat elusive, then I think the next step would be to just replace all the parts around that stage. You can measure the old parts for interest... it might show something... it might not... but I think replacing them has to be the next step.
The resistors aren't too critical on value, the 0.35meg would be a 330 or 360k in todays money. You do need to watch the voltage rating of these... yes resistors have voltage ratings too, as well as the more common power dissipation value. I would think that most 2 watt components would be up to the job. The cathode resistor, the 2.5k would need to be a similar wattage to whatever is in there now although that is perhaps one of the less likely suspects. The caps are all suspect too. The 0.05uf would be a 0.047 (47nf) in todays values.
Something like,
BFC233610105 - VISHAY BC COMPONENTS - CAPACITOR, CLASS X1, 1UF | CPC
and,
BFC233820473 - VISHAY BC COMPONENTS - CAPACITOR, CLASS X2, 47NF, 275VAC | CPC
MFP2-330K JI - WELWYN - RESISTOR, 2W 5% 330K | CPC
MFP2-1M JI - WELWYN - RESISTOR, 2W 5% 1M | CPC
The resistors aren't too critical on value, the 0.35meg would be a 330 or 360k in todays money. You do need to watch the voltage rating of these... yes resistors have voltage ratings too, as well as the more common power dissipation value. I would think that most 2 watt components would be up to the job. The cathode resistor, the 2.5k would need to be a similar wattage to whatever is in there now although that is perhaps one of the less likely suspects. The caps are all suspect too. The 0.05uf would be a 0.047 (47nf) in todays values.
Something like,
BFC233610105 - VISHAY BC COMPONENTS - CAPACITOR, CLASS X1, 1UF | CPC
and,
BFC233820473 - VISHAY BC COMPONENTS - CAPACITOR, CLASS X2, 47NF, 275VAC | CPC
MFP2-330K JI - WELWYN - RESISTOR, 2W 5% 330K | CPC
MFP2-1M JI - WELWYN - RESISTOR, 2W 5% 1M | CPC
Thanks for the links. I bought replacement caps already, although I bought them in a rush and 3 have actually turned out to be enormous! I doubt these will be able to fit in, so I'll probably have to order some smaller replacements for these.
I see there's some programs and sites which can convert values to in circuit values. How reliable are these? Is there any way I could measure these with them being in circuit? Also, is tapping the components (I guess with say, the rubber end of a pencil?) a good test to see which is making any noise, or is it not worth doing?
I see there's some programs and sites which can convert values to in circuit values. How reliable are these? Is there any way I could measure these with them being in circuit? Also, is tapping the components (I guess with say, the rubber end of a pencil?) a good test to see which is making any noise, or is it not worth doing?
As I have no real practical experience of building and working with valve equipment, I threw the question of whether the phase splitting transformer could have suffered to one of our moderators who actually designs valve circuitry and is very experienced in this field. His immediate thought confirms my suspicion that there is a very real possibility that it has indeed developed interwinding shorts. So that could be a major problem.
I would still say to press on and do the checks of the other parts and replace them but be prepared that it might end up being a winding problem with the tranny.
How do you mean, convert circuit values ? Do you mean things like micro farad to nano farad and so on.
The way I remember it now (without thinking) is that 0.1uf equals 100nf equals 100,000 picofarad. So 1nf equals 1000pf for example.
Is that what you meant...
Measuring in circuit, well you have to look at what else can interact with the parts. Providing the caps are not leaky which they should not be (that is something that should be checked and if they are then it will affect reading) then it looks as though you can check those resistors with them in place but any slight residual charge on the caps will confuse and affect the reading on a DVM. So in theory yes, in practice make sure its all off and discharged. If any read strangely then they must be isolated and measured again. Best to replace them all and be sure though.
I would still say to press on and do the checks of the other parts and replace them but be prepared that it might end up being a winding problem with the tranny.
How do you mean, convert circuit values ?
Do you mean things like micro farad to nano farad and so on.The way I remember it now (without thinking) is that 0.1uf equals 100nf equals 100,000 picofarad. So 1nf equals 1000pf for example.
Is that what you meant...
Measuring in circuit, well you have to look at what else can interact with the parts. Providing the caps are not leaky which they should not be (that is something that should be checked and if they are then it will affect reading) then it looks as though you can check those resistors with them in place but any slight residual charge on the caps will confuse and affect the reading on a DVM. So in theory yes, in practice make sure its all off and discharged. If any read strangely then they must be isolated and measured again. Best to replace them all and be sure though.
Thanks for doing that for me. The knowledge on this forum is absolutely brilliant and I'm now much calmer with all your help. It's taken nearly 2 months for someone to help me get the slightest idea what that problem could be, so thanks for all your help here.
My first thought as soon as this happened was the output transformer. I think it's definitely best trying to go through everything else first, but I'll be prepared for the worst! Finding one of these transformers is very difficult. I can get one custom made, but it will cost several hundred pounds I expect.
As for the "in circuit" values, I was referring to measuring the components with a DVM while soldered in circuit and then converting that value to what it's value should be - someone on another forum said there was an Android app that done that, but it wasn't too reliable. I'll discharge the capacitors and see what I get, I may be able to eliminate some that way perhaps. As for the uF and pF conversions etc, I cheat and use a table on the Internet!
To possibly see any noisy capacitors and resistors in the preamp, could I go under the chassis with it switched on and being very careful, using an insulated object such as a pencil eraser end of plastic case of a biro pen, tap on a capacitor or resistor and see if the noise worsens or goes away? This is one test I've heard of, I was wondering how useful it might be.
Also, are there any readings I could take of the output transformer T4 on my DVM that might be helpful? I know really an oscilloscope is required to eliminate it, but may any voltage/resistance readings be an indicator of it's health? If so, how do you recommend I do this?
My first thought as soon as this happened was the output transformer. I think it's definitely best trying to go through everything else first, but I'll be prepared for the worst! Finding one of these transformers is very difficult. I can get one custom made, but it will cost several hundred pounds I expect.
As for the "in circuit" values, I was referring to measuring the components with a DVM while soldered in circuit and then converting that value to what it's value should be - someone on another forum said there was an Android app that done that, but it wasn't too reliable. I'll discharge the capacitors and see what I get, I may be able to eliminate some that way perhaps. As for the uF and pF conversions etc, I cheat and use a table on the Internet!
To possibly see any noisy capacitors and resistors in the preamp, could I go under the chassis with it switched on and being very careful, using an insulated object such as a pencil eraser end of plastic case of a biro pen, tap on a capacitor or resistor and see if the noise worsens or goes away? This is one test I've heard of, I was wondering how useful it might be.
Also, are there any readings I could take of the output transformer T4 on my DVM that might be helpful? I know really an oscilloscope is required to eliminate it, but may any voltage/resistance readings be an indicator of it's health? If so, how do you recommend I do this?
The transformer (and the one we are talking about isn't an output transformer, its an inter stage coupling transformer that generates two out of phase signals for the output stage) can't really be checked on a diy level using the equipment you have. Shorted turns and problems with the insulation (where the applied high voltage might have caused it to burn) tend to show up when the full operating voltage is present. The very low test voltage of a meter probably won't show anything amiss. That said, you could compare resistance values of each secondary winding using the centre tap as a common. They should be similar to within a couple of percent. Also, with the transformer isolated, there should be absolutely no continuity (even many meg ohms) between primary and secondary. To test that you must isolate all leads from the surrounding circuitry and make sure your meter is set to read "high ohms". All those tests can show a definite problem, but they can't prove there isn't a problem if it passes those tests.
I would also guess that a transformer that could be suitable should be available (says he who never builds valve gear) and that could be made to work with only very minor changes to components around the valve driving it.
I would also guess that a transformer that could be suitable should be available (says he who never builds valve gear
) and that could be made to work with only very minor changes to components around the valve driving it.- Status
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