Crate V33 head powering issue (gut shots included)

[megatrav]

This is a "fun" side thing for anyone who might have some insight or is interested in helping.
As usual I will call upon the great and powerful @vigilante398 since he is our resident tube/amp/all things high voltage guy!

My nephew has a Crate V33 head. Its not the higher end made in USA ones. Its the ones that were made later and didn't stick around for long.
Last year he noticed an issue where when he would turn it on, it would stay on for about 5 minutes, then it would shut off.

Like any reasonable person who doesn't know much about amps, he bought a new matched set of el84s for it. He put them in, same issue.

He reaches out to me and asks if I can fix it. I tell him I probably can't do it myself, but I'm happy to help. Upon doing research, I find out that these amps have some known power supply failures. This is because they don't use a traditional power transformer, but they in fact use a SMPS (switching power supply) to power the preamp tubes, power amp tubes, and relays.

So, I took it to a local amp tech, he keeps it for a couple weeks and informs me that it won't be worth it for him to fix it. He would charge $400 per his bench fee, time, and replacement tubes. I didn't argue with him, but I know that the tubes are brand new and likely not the issue.

I find the schematic, read on some forums, and even join a few Facebook groups to ask for insight. The first thing everyone says is to be careful and not die, but secondly to open it up and check for any burned/broken parts and replace them. So, I opened it up and as far as I can see, nothing looks messed up to me:



I am only allowed to upload 10 photos per post, so here is a link to the rest of the gut shots Crate V33 gut shots (Google Drive)

So, I guess now I think it might be easier/simpler/the better move to think about replacing the SMPS with a linear power supply. Not knowing much about it, I assume it would need to be more than one transformer. One for the tubes and one running off of that one to power the relays. As much as I try to research it, I can't seem to find out exactly what I need.

I know that this is not a high end amplifier and it would probably make more sense to just build a new one in the chassis, but its not mine to make that call. My nephew would like to try to fix it. Unless its unreasonably expensive to do so, but if I get the parts myself, I assume it wouldn't be.

Here is the schematic and board layout:


If anyone has any suggestions for what parts I should get or can help me figure what voltages/ratings for transformers, my nephew would really appreciate it!

 

[PunchySunshine]

This is a "fun" side thing for anyone who might have some insight or is interested in helping.
As usual I will call upon the great and powerful @vigilante398 since he is our resident tube/amp/all things high voltage guy!

My nephew has a Crate V33 head. Its not the higher end made in USA ones. Its the ones that were made later and didn't stick around for long.
Last year he noticed an issue where when he would turn it on, it would stay on for about 5 minutes, then it would shut off.

Like any reasonable person who doesn't know much about amps, he bought a new matched set of el84s for it. He put them in, same issue.

He reaches out to me and asks if I can fix it. I tell him I probably can't do it myself, but I'm happy to help. Upon doing research, I find out that these amps have some known power supply failures. This is because they don't use a traditional power transformer, but they in fact use a SMPS (switching power supply) to power the preamp tubes, power amp tubes, and relays.

So, I took it to a local amp tech, he keeps it for a couple weeks and informs me that it won't be worth it for him to fix it. He would charge $400 per his bench fee, time, and replacement tubes. I didn't argue with him, but I know that the tubes are brand new and likely not the issue.

I find the schematic, read on some forums, and even join a few Facebook groups to ask for insight. The first thing everyone says is to be careful and not die, but secondly to open it up and check for any burned/broken parts and replace them. So, I opened it up and as far as I can see, nothing looks messed up to me:

View attachment 69623

I am only allowed to upload 10 photos per post, so here is a link to the rest of the gut shots Crate V33 gut shots (Google Drive)

So, I guess now I think it might be easier/simpler/the better move to think about replacing the SMPS with a linear power supply. Not knowing much about it, I assume it would need to be more than one transformer. One for the tubes and one running off of that one to power the relays. As much as I try to research it, I can't seem to find out exactly what I need.

I know that this is not a high end amplifier and it would probably make more sense to just build a new one in the chassis, but its not mine to make that call. My nephew would like to try to fix it. Unless its unreasonably expensive to do so, but if I get the parts myself, I assume it wouldn't be.

Here is the schematic and board layout:
View attachment 69632View attachment 69633View attachment 69631View attachment 69630

If anyone has any suggestions for what parts I should get or can help me figure what voltages/ratings for transformers, my nephew would really appreciate it!

Yes, this high voltage can kill ya. Take the proper precautions and BE SAFE please!

I would check the voltages at the power supply first. I can't read the schematics you posted (might be my old eyes) but it looks like the voltages are listed at the appropriate test points.

Second, test and make sure the output transformer is good as well as all the resistors tied to it. Uncle Doug on yt has a good vid on this.

Third, inspect and reflow the tube socket solder joints as well as any other that look suspect. Psionic has videos on this.

Again, be safe.

 

[megatrav]

Yes, this high voltage can kill ya. Take the proper precautions and BE SAFE please!

I would check the voltages at the power supply first. I can't read the schematics you posted (might be my old eyes) but it looks like the voltages are listed at the appropriate test points.

Second, test and make sure the output transformer is good as well as all the resistors tied to it. Uncle Doug on yt has a good vid on this.

Third, inspect and reflow the tube socket solder joints as well as any other that look suspect. Psionic has videos on this.

Again, be safe.

The main issue is when the tech looked at it, he said that it will not power up!

I can reflow solder joints and check for anything that looks bad. Upon first inspection, everything looks fine.

I will look up those videos and see if they help me out.

I uploaded a different schematic with better pictures. The PDF is also attached

 

[PunchySunshine]

The main issue is when the tech looked at it, he said that it will not power up!

I can reflow solder joints and check for anything that looks bad. Upon first inspection, everything looks fine.

I will look up those videos and see if they help me out.

I uploaded a different schematic with better pictures. The PDF is also attached


View attachment 69648

I can read that schematic!
What I would do is start at the bottom left and work up to the diodes checking the inductors and every single component and trace. If you make it to the transformer without finding anything, I would start from the output voltages on the right and work backwards from there.

 

[cliffs_trip]

Almost seems easier to get a power transformer and build a power supply

 

[megatrav]

Almost seems easier to get a power transformer and build a power supply

Is the laugh in sarcasm or because the thought of trying to fix the stock PSU is comical? Lol

Even if it is difficult, I think a linear power supply would be the solution for long term reliability

 

[cliffs_trip]

Yeah, the stock power supply circuit looks exceedingly over complicated.

You still have a transformer in the mix, and it’s fairly standard layout on the secondary side, but the junk before the primary just seems a little excessive when it comes to a power supply for an amplifier. Or maybe I’m just used to looking at themPSU circuits from tube amps.

I’m probably wrong and old school power supplies are for troglodytes. Get off my lawn!!!

 

[Robert]

The tricky part about troubleshooting this type of power supply is that you usually can't just bring them up slowly with a variac.

The circuit will sit dead as you turn up the power until you reach a threshold, then everything will come up in an instant. It has a feedback circuit (PH101, U102) that is used to regulate the power supply. If that circuit is defective the power supplies can "run away" and go far beyond their intended voltages... taking out transistors, capacitors, and your pants with them.

My guess is that "value" wasn't the issue, more that the tech possibly didn't know how to repair it.

With no power applied and all capacitors discharged, check to see if R103 or R109 is open. If so, Q103 is likely shorted (possibly among other things)

 

[megatrav]

The tricky part about troubleshooting this type of power supply is that you usually can't just bring them up slowly with a variac.

The circuit will sit dead as you turn up the power until you reach a threshold, then everything will come up in an instant. It has a feedback circuit (PH101, U102) that is used to regulate the power supply. If that circuit is defective the power supplies can "run away" and go far beyond their intended voltages... taking out transistors, capacitors, and your pants with them.

My guess is that "value" wasn't the issue, more that the tech possibly didn't know how to repair it.

With no power applied and all capacitors discharged, check to see if R103 or R109 is open. If so, Q103 is likely shorted (possibly among other things)

I very much appreciate that advice. I’m tempted to look at it and test it, but I think I should plan to replace the power supply with an old school type.

If I wanted to do that, can you tell from the schematic what the specs should be?

I see the 330v for the power tubes. So, I’m guessing it should also have some other windings available. Like I said above, I’ll probably need to run a second smaller transformer (or a small SMPS) for the relays.

I assume I will also want to think about filtering. So, I’ll probably need a small board for those caps, resistors, and diodes.

I don’t expect it to be easy, but I’m willing to try if I can get some help from you smart folks! Lol

 

[megatrav]

Bumping

 

[cliffs_trip]

As a starter, I calculated the secondary for a replacement pt should be around 225-0-225. That is taking the rectified voltage and dividing it by 1.414. You also need to make sure the new PT will handle the current of the power tubes. Also, you will have to add a new rectification circuit to the secondary.

You will need a tap on the secondary for the 6.3v heaters. They are using a 12.6v tap on the secondary, but only using half, which is rectified. Taking into account the heater current for the preamp and power amp tubes. The components for the rectification are located on the old power supply board, so a new board with those components will need to be added.

Lastly, you will need an additional tap to add the 17v+- circuit. There is a discrepancy between the power supply schematic and the main schematic. The power supply says it's outputting 17v and there aren't any components listed as requiring 17v. There are, however, plenty of components requiring 14.7v. Replacing this will require adding components originally found on the power circuit board, just like the 6.3v circuit. I believe the voltage for that secondary should be around 12 volts.

The majority of filtering is already on the main PCB, so a lot of that you wont have to redo. The amp is cathode biased so you don't have to worry about replacing a fixed bias circuit(This is according to the schematic presented).

So a PT I would scout for would have three secondary taps :
Main Voltage : 225-0-225 With 4 EL84s I would think somewhere around 250 ma
Heater Voltage : 6.3-0-6.3 With around 4 amps for the heaters.
SS comp Voltage : 12-0-12 Not really sure how much current all the SS stuff will pull, including the reverb section.


All this said, I'm not an amp tech, just like to fix stuff. I can try to answer any questions, but I can't guarantee I'm correct in any aspect .

 

[RetiredUnit1]

Basics first. Is the fuse blown? It may have broken from vibration, and that's why it would go on and off, and now doesn't power up.

 

[megatrav]

Basics first. Is the fuse blown? It may have broken from vibration, and that's why it would go on and off, and now doesn't power up.

No blown fuses and everything appeared to be in place. I supplied a link in a post above which has detailed photos of the amp including all of the fuses.