LP 275M No Output

[homey68]

I was recently trying to listen to music after a storm and the output on one channel of my LP 275M amps was lower than the other channel and the sound was kind of degraded. I had been tube rolling with the input tubes and I decided to change out those tubes thinking that might be the problem. I was able to rebias all of the output tubes with no problem. One of the small tubes started flashing and I turned the amp off right away. When I turned the amp on again, there was no out put. Now I can turn the amp on and bias the tubes with no problem, except no ouput to the speaker. When this same thing happened with my Premier 140 a fuse had blown. Remembering that, I pulled the bottom plate and checked all the glass fuses on the bottom of the board, but none were blown. Hooked the amp back up and still no output. I did have to send the Premier 140 in for repair after I replaced the fuse and got it running again. Now I fear that the LP 275 also needs repair. I don't have the box or packing for either amp and havenl't had much luck getting in touch with CJ. Beginning to feel like I am sitting on a couple 25K boat anchors. Any suggestions as to why there is no output?

[admin]

I like to go with the low hanging fruit first. When you checked the fuses, did you test them or was visually looking? If just visually, I would pull them and actually make sure they are not blown. I would then also do a visual inspection of the internals looking for any clear signs of damage. Just to be clear, did you get rid of the flashing tube? If not, I would at least try with a substitute for testing purposes. It's very possible that the tube is bad or was damaged.

After that, it gets more complicated... luckily, we do have the LP275M's schematics so that potentially opens up the possibility of taking it to a local repair shop or attempting work yourself if you have the skills.

[homey68]

I did replace the flashing tube. I powered it up again and it is working sporatically with a lot of noise in the speaker. I suspect an output tube, tapped on all the tubes to try and figure out if one is bad. I don't have any spares to do a swap out. I will switch the tubes with the other amp to see if that solves the problem. More and more, I am suspecting a tube problem.

Thanks

[admin]

Let's hope it's a tube problem. That is the easiest to "fix."

[homey68]

I have isolated the tube position, but it isn't a bad tube as I have tried 3 or 4 and only 1 was really bad, but the noise was still there. Where can I find a schematic to try and trouble shoot this problem. The amp is working now, but the problem is still there, bad component, resistor, capacitor etc. It has been a long time since I got so involved with these problems. This amp looks to be fairly straight forward and simple, so maybe I won't have to ship it.

[admin]

https://www.conradjohnsonowners.com/viewtopic.php?t=128

[Big Dog RJ]

Hey mate, came across your post.

Looks like you've had two tubes go bad, one was a weak signal tube (the one that was flashing). And the other is a weak Output tube. This is what you need to do:

1. Replace the signal tube that was flashing (which you've already done). Don't go swapping bad tubes in the other amp, this will blow a fuse in the good amp. This is the very reason you need to have spares of every tube type on hand (at least a pair of each).

2. Replace the bad Output tube, since CJ Output tubes are individually biased, it's not necessary to get matched pairs (although that is a best way of replacing tubes). If you're going to replace a pair rather than just one tube, make sure the other pairs are also working perfectly, otherwise just replace the whole lot in that channel. When you get matched pairs or matched quad sets of 4, the tubes will have a similar lifespan... enabling you to better determine when Output tubes go weak after several 1000 hrs. It just makes identifying and replacing tubes easier.

3. ***NOTE*** please replace the Large Value Ceramic Fuse!!!

This fuse protects the Output Transfomer. The fuses that you've checked are NOT Output fuses for the Output tranny, rather those small glass fuses are for the mains signal board...nothing to do with the large Plate Fuse.

Refer to the Owner's Manual to locate exactly where this large plate fuse is. It's a Screw Type fuse and is located next to the Output tranny under a board section. These are big size high amperage fuses. In the case of the LP275M, I think rated at above 2Amps and above 600V DC. These fuses are not cheapo glass fuses, they cost around $50 each. You can source these fuses from a highend audio supplier or directly from CJ HQ.

***Note*** you need to replace this Ouput fuse first before anything else!!! Otherwise you're going to blow another Output tube. That waa the main reason why you heard dull and distorted sound. This very statement is clearly mentioned in the Owner's manual... which I'm suspecting that you haven't read! Please read your Owner's manual.

After reading the manual, you'll get a far better understanding of what the LP275M's are all about plus which fuses do what and where they're located. You don't know because you haven't gone through the manual, simple as that!

Cheers, RJ

[Big Dog RJ]

Ah! Now I remember!

That large Plate Fuse is located at the back panel right next to the amplifiers Output terminals. You can't miss it!

It also has a corresponding LED which lights up when the fuse is blown. 100% I'm sure that LED lights up when you power on the amplifier and that LED lighting up indicates a blown plate fuse, which indicates a weak/bad Output tube. This is a screw type plate fuse.

Make sure the power is completely OFF and power cord disconnected before removing the plate fuse. All you need to do is replace this fuse, then all will be well. Not necessary to check anything on the schematic or internal circuits. You basically have a blown Output fuse.

***Note*** you need to replace that large ceramic fuse with the exact type! There are no substitutes or alternative kinds... only the exact same type with the correct voltage and amperage ratings.

It may also be advisable to keep a few spare fuses as well. After all, these are the LP275M's we're talking about, these are big power amps and require proper attention to detail for proper maintenance, especially fuses and tubes. Also, read the owner's manual!

Best, RJ

[AnotherJohnson]

The fuse RJ is pointing out is a KTK-2.

It is discussed on p8 of the manual, which I assumed you had read from your post.

FWIW, all will not necessarily “be well” if you replace the fuse. You may just blow it again, so have a handful of spares on hand.

It blew for a reason and if that reason hasn’t been fixed, it will blow again. Usually the reason is a bad tube. If this unit has not been retubed in a couple of years, it would probably be a good idea to retube the lot. If you do them one at a time as they blow, you’re going to have aggravating down time spread out, and the expense of more KTK-2s.

[Big Dog RJ]

Yes! Very correct stated by AJ there. In fact AJ owned the LP275m's as well. Those were the glory days eh?

Also, as advised on Output tube replacement... I would just replace the full set of both channels, plus have spares of those large ceramic fuses (KTK2).
Note: the OP hasn't checked these output fuses, only the glass fuses inside the unit. In any case, if the power amp was able to power up and still bias to a certain degree, then that's definitely a plate fuse, especially when the typical symptoms have been described by the OP (dull and distorted sound). If it were any of the glass fuses internal to the mains board, then the power amp wouldn't even turn on. Any of those glass fuses blown is a more serious problem... that needs to be verified by an authorized techie. The OP has checked these glass fuses... all seems to be ok.

*** again, all of this is clearly stated in the Owner's Manual***

Cheers, RJ

[admin]

All very good recommendations. I was under the impression that all fuses including the KTK2 in the back was checked by OP but if not, that really should be the first fuse to check.

When I had one of my KT120 tubes in my ART amp go, that was the fuse that went. As AJ mentions, even though they are costly, just buy a couple as those will tend to blow as tubes fail in the future and it saves a lot of time when you have them on hand.

If that fuse is blown, the truth is that is probably good news as it makes it much more likely that this was a typical standard tube failure and the design worked as it should. Replacing the tube and fuse should solve the problem.

[homey68]

Last night I replaced the flashing small tube and rebiased the outputs. The amp was playing fine with no noise at all. I only left it on for a few minutes though. Today I turned the amp on to see if it was stable and after a few minutes,a few of the bias led's started flashing. I turned the amp off and will try to rebias later. I checked that large tkt-2 fuse out and it is OK. I removed the fuse from the other amp to test. The red light does not come on. I am going to order a new replacement. I just purchased a tube tester, so I can test the KT 120 tubes that are in the amp. I have a set of 6550,s and will try those also as they were previously in the amp. First I have to figure out why the bias led's were flashing.

[admin]

Let us know how it goes. Also, keep in mind that the biasing is recommended to be done in two steps. Biasing after the unit is turned on and then again in 30 minutes of being on. So that initial biasing may be less stable or change. I only mention it as you may have some LED stability initially which can be normal.

[Big Dog RJ]

Ah! Ok good you've checked the plate fuse on both monoblocks and all is well. Usually, 9/10... when you hear dull and distorted sound, that's basically an Output tube going bad. It also can be just one tube or more than one.

Now that you managed to bias and it held steady for a few minutes until the LEDs started flashing again... could be:

1. There's another small signal tube playing up. Either the Input stage tube or the driver/phase inverter tubes, which are in pairs according to the LP275M design layout. When you said that you replaced one flashing small tube, which one? (Input or Driver tube?) There are 3 of these 6922's, the first one is the Input stage, followed by the other two, which are the driver/phase inverter stages. Any of these three could be faulty, which in turn will cause the main Output tubes LEDs to flash, that does happen.

2. If all three signal tubes are OK, then it could still be the case of another Output tube going south. Unless all Output tubes are stable and have plenty of hours remaining, those LEDs will flash... eventually causing the Plate fuse to blow. That's the design.

3. It would be advisable to just replace all the tubes in the monoblock that has the problem. Fit them with new tubes, including the Input & Driver tubes, rebias the amplifier properly and check that large Plate Fuse again, then things should be ok.

4. When a plate fuse is blown, its not very noticeable on the ceramic type design, mainly because you can't see through the ceramic. However, when one of these big fuses do blow, usually there's a strong burn mark on one end, like a tarnish stain but this sometimes doesn't happen. It just blows... the only way to know is if that corresponding LED attached to the screw type plate lights up! Then the fuse is no good. So replace this fuse immediately.

If you've already tried that with the other monoblock's plate fuse and all is well, then both the Plate Fuses are fine. You can swap the plate fuses, no harm in doing so. If one works it will work! And if one's bad it won't work, as simple as that. You'll know straight away, there's no guess work unlike trying to identify a bad Output tube. That is a bloody nightmare, especially when you've got 16 of those big buggers.

So, let's say best case scenario is to re-tube the full monoblock that's giving issues. Once done, and things settle in just leave it alone after final biasing. Then, since you've bought a tube tester, now go ahead and test all your old tubes... and you'll come across the good- the bad & the ugly. Obviously keep only the good as future spares and discard the rest!

I know this is a real pain in the Butt with a capital B to go through...we've all been there. However, it's just the way tooobs are, and once sorted out, you'll be enjoying those fine tunes in no time! It's the effort, time and nonsense we put up with just to look forward to that glorious tube finesse. And no, I don't think you've got 25grand boat anchors... they just require some attention that's all.

Note: bigger the tooob amp, the more tooobs = more maintenance. There's no denying that. CJ tube gear is all about the high performance of recorded music. And this comes at a price including a bit of fuss, so not to worry mate. With a little patience you'll manage just fine.

Cheers, RJ

[AnotherJohnson]

And no, I don't think you've got 25grand boat anchors...

I don’t think so either. In 2018 they were $10k boat anchors. Today, probably $7k if they’re not humming. And these are per pair, so at most maybe $3.5k per anchor.

A yacht needs two anchors for stability, and a long line to each. You do not have to use AudioQuest Dragon anchor chain. Any strong chain will do.

[Big Dog RJ]

WOOF!!!

[SolderSlinger]

As an FYI - the CJO LP275M is not a CJ production schematic. The production LP275M amp has 6922 V2, V3. The schematic indicates 6n30 for V2, V3. Component value and number of parts placed are suspect. The CJO schematic LP275M is a good reference for overall circuit design, so it's better than no schematic.

To comment of AJ's hum post, my $0.02 is all LP275M's hum. No such thing as a LP275M that does not hum.

Odds are the Vishay resistors for V1, V2, or V3 are wounded. Might be only one resistor at one tube, or multiple resistors associated with all 3 tubes. Because it's an older amp, odds are that a previous owner caught the tube bug and decided to try a NOS 6DJ8. Early NOS 6DJ8 tubes have a lower plate voltage, leading to arc flash failure when used in a circuit optimized for a 6922. The transient voltage of an arc flash will cause a Vishay resistor to change value. It's a byproduct of the resistor design to allow factory trimming to very high precision.

Another failure mode from arc flash is damage to the PCB under the output tube sockets. Repeated arc flash will cause a spark between traces routed between the output tube socket pins. It causes carbonization of the PC board material, which is a conductor. Carbonized pcb material allows arc flash at lower voltages. You will need to disassemble the amp to the point that you can clearly see under the tube sockets and verify no carbon tracks between foils and pins. If you find carbon damage, remove the output tube socket, grind out damaged area (use a vacuum to keep carbonized PCB material from contaminating other areas), then coat repaired area with "Super Corona Dope" available from DigiKey.
 
The arc flash damage sets off a domino effect. The amp is wounded. Replacing with new 6922 tubes might seem like it's the fix, but it's not. The plate voltage has changed. Might not fail at first, but at some point in time even a new 6922 will arc flash in the amp. And the sound between the amps is not the same. Some folks may never notice the difference because their system does not have the resolution hear it.

While it's possible to DIY repair the amps, it's best to send them to CJ for repair because you can't find the Vishay resistors. Not cheap, but you will know that the amps are in perfect working order, hum and all.

[roberto]

Hola,
You can get and buy any value of through hole metal foil resistors in eBay.com
https://www.ebay.com/sch/i.html?_nkw=Vi ... QFEALw_wcB

[SolderSlinger]

Hello Roberto,
I respectfully disagree with your statement. There are many resistors available on eBay, but not direct replacements. Dale bought Vishay a long time ago. Some of the eBay titles have Vishay and Dale, when the parts are truly a Dale resistor. If you narrow criteria to Bulk Metal Foil, correct wattage, correct package size, correct precision, correct temperature coefficient, correct lifetime value shift, correct voltage, and correct resistor value you'll get zero listings.
The world wide demand for components of this quality is very low. Typically used for extreme high reliability instrumentation and reference control electronics.

Besides, based on OP's description, I truly believe the PCB has arc damage. My recommendation is to send it back to CJ to restore to factory specifications.

Just my $0.02. Not trying to open a CJ service policy good/bad can of worms. Just want to point out, there's something more than just a bad tube at work.

[roberto]

Hola,
And did you try here?

https://www.vishay.com/en/resistors-fixed/through-hole/