60's projector amp - blowing fuses

[dayn]

Disconnect the end of r228 that does not feed r240.

Will do. That would be a wire going from the R228 turret to the tube socket for the C216 multi-cap. I think its easier to remove the wire end at the tube socket rather than the wire fed through this type of turret.

Also please hook up your meter set to read amps so you can report back with accurate numbers and not just a level of bulb brightness.

I've never done that. Can you describe more what to do? Am I setting the red probe in the red mA socket (as opposed to the 10A socket)? Then am I measuring between the areas I unconnected, for instance between R228 and the filter cap, or between power tube pin 5 and the OT, or 6x4 pin 7 and OT center tap? With rectifier and power tubes in, amp on, through limiter? Will the fuse eventually blow if the light bulb is glowing brightly or does it last long enough to do these tests?

I want to learn the process and concept correctly so I can test things safely and without damaging the amp (or me).

[dayn]

Also please hook up your meter set to read amps so you can report back with accurate numbers and not just a level of bulb brightness.

What am I measuring between? Am I inserting my meter between the power cord, and the primary input of the PT? To measure if power from the wall is correct? Or is it between the PT primary and something else, like the places where I removed the OT wires?

[dayn]

I need to measure voltages, but while I had the PT wires disconnected I measured and came up with this:

Primary to Primary: 4.6 ohms
Primary to ground: UL
HV secondary: 63.3 and 68.0 each to CT, 131.2 between
Filament secondary: .3 between, .2 for both to CT

No continuity to ground, and no continuity between primary and secondary.

Do all the numbers seem normal/right? I want to verify before I rewire it and start testing voltages through it.

[martin manning]

Those resistances don't look unreasonable. You could hook up just the primary and measure voltages on all the other windings to test the transformer by itself. If you don't want to deal with the high voltages, you can put 12 VAC (from an AC adapter) on the primary and measure, expecting to get 1/10 of the actual levels.

[dayn]

Those resistances don't look unreasonable. You could hook up just the primary and measure voltages on all the other windings to test the transformer by itself. If you don't want to deal with the high voltages, you can put 12 VAC (from an AC adapter) on the primary and measure, expecting to get 1/10 of the actual levels.

Thanks martin, I'll do that this evening and report back the voltages I get.

[dayn]

You could hook up just the primary and measure voltages on all the other windings to test the transformer by itself.

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.

[dayn]

In addition to the PT voltages above, I tested my OT 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.

[Stevem]

Ok, let cut this Elephant down into bite size pieces!

If you have any PT or OT wires disconnected please hook them back up.


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.

[dayn]

Please report back with your findings.

Thanks Stevem, I'll do that this evening and report back. I really appreciate you breaking it down into easy steps to try to track down the problem.

On a related note of trying to trace things:

This morning before work I started looking again at the schematic and turret board to trace and test potential problem areas in case I missed something, related to where I had changed parts, or where resistor R210 had blown up at the same time I started blowing fuses. 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?

I imagine it is worth replacing either before or after doing the suggested other tests.

[Stevem]

I am having a hard time understanding why r210 would have burnt up in the first place, but anyway what do you mean that you had R206 test bad?

R206 could be shorted of left out of the circuit and a fuse would not blow, however if C203 was shorted, then that could be a issue.

[dayn]

I am having a hard time understanding why r210 would have burnt up in the first place, but anyway what do you mean that you had R206 test bad?

R206 could be shorted of left out of the circuit and a fuse would not blow, however if C203 was shorted, then that could be a issue.

I don't know what happened to R210. I didn't notice a visible problem with it prior to turning the amp on after modding it, other than after I started blowing fuses, I looked inside and R210 was cracked in half. I replaced it.

R206 - I tested it this morning with my resistor tester, while still in circuit, and I get no reading - it says "damaged or missing part" which is what my tester says when a part is bad. Being in circuit, it could be that is the issue for the reading. It also is connected to C203, which is a replaced part (Illinois Capacitor 630v 4700pf, tested prior to install as in spec - maybe this is a bad part selection type to replace a ceramic disc cap?). I do have an original spec ceramic disc cap I can put in instead if needed. Maybe that solder joint between C203 and R206 is bad, or the cap I put in there is bad/shorted for some reason. I'll double check all of that this evening.

[dayn]

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?

What is next in the process of elimination?

[dayn]

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...

[R.G.]

I've been reading the thread, trying to soak it up. I'm going to just say that I got from this, so correct me if I've missed something.

> PT and OT resistance measurement seem normal. I think that "UL" reading may just be what your meter says on a complete open circuit.

> PT and OT don't seem to have external shorts. Overcurrent seems to be related to just the output tubes being in.

That seems to me to indicate that something is wrong with the output tube circuits (um.... duuuh... ) and you have reasonably eliminated a bad tube. New electrolytics and checking components/shorts seems to eliminate a failing electro.

Observations and questions:
One of your posts said: "Primaries I get .84k ohms to each plate. And 2.5, 3.5 and 6.5 to ground for 3 primary wires." about the OT. Is that right??? If any of the OT primary wires are under 10 ohms to ground, you're going to get overcurrents through it. Can you re-check that and try to verify that? Was that taken with the tubes in or out? Ideally, put the tubes out and with the line cord unplugged, measure from output tube socket pins to ground for the two "outside" connections.

On the output stage itself:
This circuit relies on the current through the shared cathode resistor for reverse bias on the output tube grids. A failure of the bypass cap on the cathode resistor R241 would likely cause what you're seeing. So would getting the polarity of the bypass cap C224. So would a short to chassis of the cathode leads of the output tubes; however, your test for shorts should have eliminated that.
> Is the cathode bypass cap C224 absolutely, positively soldered in correctly, not backwards?
> With the AC line cord unplugged from the wall, what is the DC resistance to ground from the plate, grid and cathode pins in the sockets for the output tubes?
> With the output tubes out, IIRC you can power it on and it does not overcurrent. With only the output tubes left out, and it powered on, what are the DC voltages on the filter caps?

[dayn]

I've been reading the thread, trying to soak it up. I'm going to just say that I got from this, so correct me if I've missed something.

> PT and OT resistance measurement seem normal. I think that "UL" reading may just be what your meter says on a complete open circuit.

> PT and OT don't seem to have external shorts. Overcurrent seems to be related to just the output tubes being in.

That seems to me to indicate that something is wrong with the output tube circuits (um.... duuuh... ) and you have reasonably eliminated a bad tube. New electrolytics and checking components/shorts seems to eliminate a failing electro.

Observations and questions:
One of your posts said: "Primaries I get .84k ohms to each plate. And 2.5, 3.5 and 6.5 to ground for 3 primary wires." about the OT. Is that right??? If any of the OT primary wires are under 10 ohms to ground, you're going to get overcurrents through it. Can you re-check that and try to verify that? Was that taken with the tubes in or out? Ideally, put the tubes out and with the line cord unplugged, measure from output tube socket pins to ground for the two "outside" connections.

On the output stage itself:
This circuit relies on the current through the shared cathode resistor for reverse bias on the output tube grids. A failure of the bypass cap on the cathode resistor R241 would likely cause what you're seeing. So would getting the polarity of the bypass cap C224. So would a short to chassis of the cathode leads of the output tubes; however, your test for shorts should have eliminated that.
> Is the cathode bypass cap C224 absolutely, positively soldered in correctly, not backwards?
> With the AC line cord unplugged from the wall, what is the DC resistance to ground from the plate, grid and cathode pins in the sockets for the output tubes?
> With the output tubes out, IIRC you can power it on and it does not overcurrent. With only the output tubes left out, and it powered on, what are the DC voltages on the filter caps?

Thanks for checking out the thread, I appreciate you chiming in to help figure out the issue.

My mistake on my DMM, it says OL, which is as you pointed out is what it says when open circuit.

I think my OT measurements were tubes out, to ground. I am currently waiting for new resistors as I'm updating a few things as part of the process, but once they arrive and are soldered in I will test the OT and update the thread. I do have another OT I can put in if this one is bad.

The original cap C224 was in and in stock working form, and the short was there. I then as part of my mods put in a different value cap for tonal difference, and it is oriented correctly, and the short is still there.

I'll grab the resistance measurements too once I have the resistors in and the circuit in completely (old resistors are already removed at this point).