Military projector amp - last edited 9/26/22 - layout help needed

[yello]

Here are the PT voltages:

Primary: 122.4
Filaments/rectifier: 3.6 and 4.0, 7.6
HV: 301.8 and 300.4, 602.5

That seems normal.

What is throwing me is the PT troubleshooting instructions here:

http://www.geofex.com/ampdbug/pwrtrans.htm

I measure each primary to ground, and get a "UL" reading of resistance. The instructions state:

Step 3, part 2: Measure the resistance to the chassis from both ends of the primary winding. It should be more than 1M ohm.
If it is shorted or less than 100K ohms, the transformer is bad. If it is between 100K and 1M ohm, unsolder the primary leads from the terminals they contact and measure again.
If it is now less than 1M ohm, the transformer is failing, and should be replaced.
If it is over 1M ohm, there is a component connected to the wiring leading to the power transformer which is leaking to the chassis that needs to be traced down.

Then under step 5 part 2: Using an ohmmeter, verify that the resistance to chassis from the power transformer primary windings is over 100K ohms. If it's lower than that, you need to find out why before proceeding.
Make certain that no foreign objects or personal parts are contacting the amplifier.
Turn on the AC power switch. Leave it on, periodically touching the outer metal shell of the power transformer to see if it gets hot. If it makes a loud hum, gets noticeably hot to the touch, or emits a burning odor, it is defective. Leave the transformer on for ten minutes, or until it gets hot, whichever comes first.

I don't get why I should get a reading of PT resistance to ground, or why I don't get one. Does that mean the PT primary has a short?

My light bulb limiter shows a short when I have power tubes installed, but not when I have the rectifiers and preamps.

[yello]

Have you tested your OT yet?

Just tested it for resistance:

http://www.geofex.com/ampdbug/outtrans.htm

Primaries I get .84k ohms to each plate. And 2.5, 3.5 and 6.5 to ground for 3 primary wires.

Secondaries: .2, .9, and 1.3 to ground. No continuity between primary and secondary. 16 ohm tap is in range of 700k, 8 ohm tap in range of 580k, 0 ohm tap in range of 1M+ ohms. I say range as when I apply my DMM those numbers start high like several megohms, then slowly reduce down into those ranges when held there.

As I understand it from the above link, I think this seems normal.

I welcome feedback/insight/ideas on my PT voltage and OT resistance numbers.

[Bieworm]

Those values don't ring any alarm bells ...
To be sure you can compare them to your other amp, but I think they're fine.
I had that crackling sound once too.. it was a resistor with a broken off leg, still making contact. Soon after that I fried my power transformer.
There must be a faulty part somewhere, or a wrong connection you made while changing parts. Verify the circuit with the schematic and a highlighter, especially the swapped out parts.
I'm not going to reread this whole thread, but please verify you already tried othee tubes that are known to be working properly?

[yello]

Verify the circuit with the schematic and a highlighter, especially the swapped out parts

I've done that previously, but am in the process of doing it again to make sure I didn't miss something (see my next separate post below).

I'm not going to reread this whole thread, but please verify you already tried other tubes that are known to be working properly?

This amp was working briefly with these tubes. I have tested them for shorts, and they are fine. I have also tried several other tested tubes in the same slots so I am fairly certain it isn't tube related. Though I don't have a tube tester myself, or another of these amps up and running to test them in.

[yello]

This morning before work I started looking again at the turret board to trace and test potential problem areas in case I missed something, related to where I changed parts, or where that resistor blew up. I only had a brief time to test resistors in circuit, but when I tested R206, my tester says that resistor is damaged. It is connected to C203, which is a cap I replaced as the old one was microphonic. Wondering if I have a bad connection at that turret, or I damaged the resistor, or it was also originally bad, or if this issue is around C203 (Illinois Capacitor 630v 4700pf - maybe this is a bad part selection type to replace a ceramic disc cap?).

It was on the list of things to replace anyway per earlier recommendation in this thread, to replace R205 with 1k and R206 with 100k (and R208 with 1k and R209 with 100k) plus add bypass caps.

Worth replacing the bad resistor I imagine. Then test. Then if that solves it continue with mods above.

[yello]

I removed the C203 leg from R206, and both test to spec. I am soldering PT wires back on then did these additional tests, per amperage recommendation:

We will be working a process of elimination.

Please leave the tubes in and of course the amp off.

We are are going to be making resistance test with the tubes in, but first unhook R226 from where it joins C216C.

Next do the same with R230 where it joins C220.

At the locations where you have unhooked these two resistors make a test from there to ground.

If these two test work out then unhook R228 from C216A and make the same test.
With all of these test your meter should read a brief short and then jump up to a very high resistance.
If you do not read this high resistance then something is loading down that filter node and creating a near short or full short condition.

Please report back with your findings.

I removed the C203 leg from R206, and both test to spec. I must have had a bad reading as they were in the circuit still. I soldered PT wires back on and did the additional tests.

It is worth noting that the oscillator section of the circuit has been entirely removed. Everything in that box on the schematic is gone, including R230 and C220. Clipped at C216C.

Before unsoldering the wire going to the filter cap tube sockets, I just unplugged the filter cap trio C216ABC. I tested resistance from R226 to C216C and R228 to C216A and I get a high resistance, 6M ohms. It then slowly drops down.

That sounds like the expected/normal result? Or do I need to plug cap trio back in and remove wires so C216B stays in circuit?

[yello]

Any more tips on what I can test next? I started blowing fuses on Feb. 23rd - and I still haven't figured out why after countless hours prodding and testing. I have traced wires countless times, looked at the schematic until I'm dizzy, etc. I REALLY want to build my 18-watt project, but I gotta finish this thing first!

It obviously has to be something, are there logical places to test or replace parts beyond what I have done and described in previous posts given it is in the power section somewhere? Could it be something in the preamp or PI causing the short that only shows up when power tubes are in?

R210 was the resistor that was either already cracked, or blew at same time as fuse - anything near there I should be checking?

[JMPGuitars]

There's a chance that there's a little gremlin hiding in there that is well hidden. It could be a frayed wire making contact where it shouldn't. Some components might fail under load (heat) but appear fine when cold.

You know the issue is in the power amp section, so maybe rebuild that section.

You need to think systematically and logically. There's simply not that many parts in that section to fail. Replacing them all would be less effort than what you've been dealing with so far. If you replace everything in the power amp section, and inspect the hell out of it, and it still blows fuses, then it's highly likely to be (one or both of) the transformers. That's assuming you put in all new, tested components, and verify nothing else is shorting out. That's also assuming you verify the work against the schematic, and that there's no error in the schematic.

Edit: also check and verify the speaker output connection is working correctly, and that neither it, nor the speaker are shorting out.

Thanks,
Josh

[yello]

There's a chance that there's a little gremlin hiding in there that is well hidden. It could be a frayed wire making contact where it shouldn't. Some components might fail under load (heat) but appear fine when cold.

You know the issue is in the power amp section, so maybe rebuild that section.

You need to think systematically and logically. There's simply not that many parts in that section to fail. Replacing them all would be less effort than what you've been dealing with so far. If you replace everything in the power amp section, and inspect the hell out of it, and it still blows fuses, then it's highly likely to be (one or both of) the transformers. That's assuming you put in all new, tested components, and verify nothing else is shorting out. That's also assuming you verify the work against the schematic, and that there's no error in the schematic.

Edit: also check and verify the speaker output connection is working correctly, and that neither it, nor the speaker are shorting out.

I appreciate your input. As much as I can in a cramped amp, I've traced the amp to the schematic, no wires or components look out of place that I have noticed.

I think I have replaced every component in the power section a few weeks ago. Short of R241 which is a 220 ohm 8w ceramic ohmite resistor bolted to the chassis and connected to C224 which I replaced with a 1000uf 50v. The resistor tests at 220 ohms while in circuit, could it be bad even if it appears fine and tests fine? I replaced C212 and C213, R220 and R221, R228 and R240. All components were tested prior to install. Are there other components that are part of the power section, that maybe I'm assuming are PI as they connect the two or are sharing a turret? I'd love to start replacing stuff as I now have every single component value in the amp on hand for that purpose if needed.

If you or someone doesn't mind taking a look at the schematic quickly, maybe there are suggested parts I can focus on replacing first?

Schematic on page 21:

http://www.acofs.org.au/part_5_files/Be ... 614EVM.pdf

I did test speaker output today, rather than using the 16 ohm speaker jack and external cabinet, I used the internal 8 ohm speaker wires. No difference, still a power section short.

[Bieworm]

Is your OT primary center tap still connected to B+? If you accidentally connect that to ground, which doesn't seem strange since most transformer CT are, then you get a fault when plugging in power tubes..

[yello]

Is your OT primary center tap still connected to B+? If you accidentally connect that to ground, which doesn't seem strange since most transformer CT are, then you get a fault when plugging in power tubes..

I just checked, the OT center tap goes to the rectifier tubes and then to C216, no continuity with ground.

[yello]

Does having my light bulb limiter there, with a 75 watt light bulb, limit power for an extended period of time without blowing fuses so I can get voltage readings in the relevant sections? Or is it useless given the limiter will change the numbers?

Last night I started going through the turret board, removing components and testing them. I got 1/3 done. Everything so far was the correct spec part, though many had drifted. I will be replacing all with new parts as I rebuild it. Cleaning every turret, inspecting and tracing every wire, and testing every new part before installing. Hopefully by going through methodically I will find a bad part or whatever is causing the short.

I also brought two books to work today to read on my breaks, "Electric Guitar Amplifier Handbook" by Jack Darr and "Inside Tube Amps" by Dan Torres. I'm hoping to expand my circuit and troubleshooting knowledge. The first thing I noticed in the troublingshooting section of Darr's book is that after tube failure the power supply is the most common area to have problems.

[Bieworm]

You need a variac to do that. Then you can do the math... the lightbulb limiter is just for safety to cover the excess current draw with shorts. Different things...

[yello]

I went through the turret board and removed every component. Everything tests with no shorts and the values were correct, albeit many had drifted 10-30% from original spec. I have then traced wires, and am checking everything for continuity to ground where it should or shouldn't be.

Everything looks and tests in order as far as I can tell. Still have no idea where the short would be...

[JMPGuitars]

After rebuilding the circuit, do you have a different OT you can test?

[yello]

After rebuilding the circuit, do you have a different OT you can test?

Yes, I have extra of these amps with the same circuit. The PT and OT in those amps are from a different year and manufacturer, but should be the same spec.

Is it the different OT to try first? And why?

[JMPGuitars]

Is it the different OT to try first? And why?

Because, some things may only fail under load. You know that you only have a short with the power tubes connected. You can have everything else loaded, and no short, so less likely the PT. You can have only the power tubes and rectifier loaded, and the amp shorts. If it's not the circuit components, it's logically the OT. If with another OT it still shorts, then you either have 2 defective OTs (highly unlikely) or you missed something else.

Thanks,
Josh

[Bieworm]

Josh is stating things I think too are suspicious.
Some years ago I had a very expensive amp (still have it) doing weird stuff. It had volume loss , made crackling noise and started whining sometimes. It didn't do so immediately, but after warming up.
This turned out to be the choke. The amp tech explained this to me as the choke being under load for some time started warming up and the windings were malfunctioning due to the miniscule swelling. There was a tiny fault in the winding that would appear after behaving in certain circumstances.
I know this is not the same thing , but just to point out these hidden problems are in fact a possibility.. even in other parts, such as resistors and stuff.
My advice is to swap the OT and see if that solves your problem. You're bound to radical things since you need and lack lab equipment to look for it at tech level.

[yello]

Josh is stating things I think too are suspicious.
Some years ago I had a very expensive amp (still have it) doing weird stuff. It had volume loss , made crackling noise and started whining sometimes. It didn't do so immediately, but after warming up.
This turned out to be the choke. The amp tech explained this to me as the choke being under load for some time started warming up and the windings were malfunctioning due to the miniscule swelling. There was a tiny fault in the winding that would appear after behaving in certain circumstances.
I know this is not the same thing , but just to point out these hidden problems are in fact a possibility.. even in other parts, such as resistors and stuff.
My advice is to swap the OT and see if that solves your problem. You're bound to radical things since you need and lack lab equipment to look for it at tech level.

After I finish getting the new circuit parts in, I will take a closer look at the OT, and swap in the other one if the short remains with the new circuit parts all updated. I just gotta order some resistors tomorrow, I thought I had 560k but I don't.

[yello]

While waiting for parts, I'm exploring removing the final parts from the turret board that are for the now unused film input. You can see it/them on the schematic here:



Looks like I can remove:

R201, C202, R204, R223, and R224. Does that look right?

I already took out the tube socket thing with its resistors, R203 which is a pot and R235 which connects pot to v1.