national glenwood model 90

[davohilts]

Hi 52 Bill,
I really appreciate what people are trying to tell me, I guess I'm just not understanding what is being said. I am using extreme caution when ever I do anything inside the amp and rightfully so. What I am doing is trying to understand the correct way to wire the primary circuit. Is there a schematic that you could point me to that could help?
Thanks and I'm sorry if I have been annoying you, that is not what I am trying to do.
Dave

[52 Bill]

Dave:
You're not annoying me, you are frightening me. Start by answering this question.

Do you know the three wire ac cord color codes?

[davohilts]

52 Bill,
Yes, at least I think I do. Most are black for hot, white for neutral, green for ground.
Dave

[52 Bill]

Okay, so far so good. The hot should connect to the power switch and to the line fuse.

You apparently have a double pole switch for the power, so you have both hot and neutral switched, which is fine. The hot lead then goes to the fuse or do you have the fuse connected first?

After the switch and the fuse, the hot and the neutral both connect directly to the power pilot light. This light turns on whenever the main ac switch is turned on.

The hot and neutral also connect to pins 5 & 6 of the connector cable. This cable sends the 120vac to the primary winding of the power transformer that is located in the power amp chassis. The original schematic also shows two caps connected to the primary winding, one to ground and one across the winding. I think that you stated that you have removed the cap to ground. Is this correct? What about the cap across the line?

The standby switch should do two things. One half of the switch is used to turn on and off the standby light and the other half grounds out the audio signal that feeds the driver stage. Your drawing shows the hot ac line connecting to the standby switch ground connection. Is this in fact true? If it is, then disconnect that before doing anything else.

The standby light needs to connect across the hot and neutral lines when the switch is turned on. This is just a simple series switched circuit.

Once you are done there should be no continuity between either hot or neutral wires and the metal chassis. Test this with your meter set to read resistance.

[davohilts]

Okay, so far so good. The hot should connect to the power switch and to the line fuse.

After the switch and the fuse, the hot and the neutral both connect directly to the power pilot light. This light turns on whenever the main ac switch is turned on.

The hot and neutral also connect to pins 5 & 6 of the connector cable. This cable sends the 120vac to the primary winding of the power transformer that is located in the power amp chassis. The original schematic also shows two caps connected to the primary winding, one to ground and one across the winding. I think that you stated that you have removed the cap to ground. Is this correct? What about the cap across the line?

The standby switch should do two things. One half of the switch is used to turn on and off the standby light and the other half grounds out the audio signal that feeds the driver stage. Your drawing shows the hot ac line connecting to the standby switch ground connection. Is this in fact true? If it is, then disconnect that before doing anything else.

The standby light needs to connect across the hot and neutral lines when the switch is turned on. This is just a simple series switched circuit.

Once you are done there should be no continuity between either hot or neutral wires and the metal chassis. Test this with your meter set to read resistance.

The way it is laid out the hot goes to the switch, then the unprotected side of the fuse. From the protected side of the fuse the hot feeds the pilot light, one pole of the double pole standby switch and the #6 pin feeding the power section.

You are correct. The neutral feeds the power switch, then goes on to the #5 pin feeding the power supply. There is also a shielded cable running from pin 5 to the pilot light.

I removed both caps, the one across the primary winding and the one to ground. I still have them, should I have left the .02 ceramic disc cap across the switched side of hot and neutral?

What I meant to show was that the double pole standby switch has the hot connected to one switchside with the feed to the lamp on the other switchside there is a connection to ground with the switched feed to one of the 2 pin plugs feeding the #5 pin of one power tube. When the switch is in the off position it the standby light is on and grounds the #5 pin of one power tube. In the on position it turns off the standby light and ungrounds the power tube 5 pin.

I have tested continuity and have none except for the grounded side of the double pole standby switch that grounds the power tube pin5 when in standby mode.

52 Bill, I really appreciate you taking the time to get my head on straight.
thanks,
Dave

[52 Bill]

I have tested continuity and have none except for the grounded side of the double pole standby switch that grounds the power tube pin5 when in standby mode.

Okay, now does the fuse still blow when you plug it in and turn it on?

[davohilts]

Okay, now does the fuse still blow when you plug it in and turn it on?

52 Bill,
following your description that was exactly how I have the amp wired. I didn't make any changes and yes the fuse still blows.
the only thing not discussed is the .02 cap across the primaries, I removed that cap along with the death cap to ground. Should I have left the cap? It doesn't see likely that it should be there because I eliminated the reverse polarity feature.s

[tim]

Is it possible that your variac is acting as an isolation transformer (isolating your amp from the mains)?

[davohilts]

Is it possible that your variac is acting as an isolation transformer (isolating your amp from the mains)?

Stupidly, I was using a ground lift adapter because the outlet on the variac was a 2 prong. So that's why it wasn't blowing fuses the ground was already lifted. I have tried everything to resolve this issue but have come up empty.
Thanks for the help.
Dave

[DrGonz78]

I checked the PT and got the following readings:
2.2 ohms across the primarys
55 ohms from one primary to chassis
53.4 ohms from the other primary to chassis

Earlier on you noted that there was some sort of continuity from some winding to the chassis, which would be true if the center tap was connected to ground.

If the transformer is sitting out free from the chassis, check it for any continuity from any winding to the core lamination.

I think you need to take 52 Bill's advice and pull the transformer out from the chassis to check the continuity/resistance of windings to the core.

So, what convinced me that I had a shorted PT was that I disconnected all the secondaries from the amp but left the primaries connected. I then plugged the amp into the wall outlet and turned on the switch. The fuse blew so I figured okay the PT is toast. I disconnected it completely from the amp and removed it completely.
I then considered that maybe the switch could have a short in it. I didn't have a double pole switch so this morning I hooked the PT primaries to my variac and installed a fuse holder in line with the hot side leading from the variac to the PT primary. I then turned the variac on and slowly brought the voltage up to 120 volts. When I got to 120 volts the fuse didn't blow, this matched my test with the 15 amp breaker on the plug strip but with a 4 amp slow blow fuse. Maybe my PT isn't toast! I picked up a new double pole single throw on/off switch and wired it into the amp then installed jumpers from the switch to PT primaries and installed a lamp across the 6.3 v winding on the secondary side. I then turned on the amp and the fuse didn't blow and the lamp came on. Looking better, I thought.
So I reinstall the PT and reconnect all the secondaries plug the amp back into the variac and turn on the switch, slowly bring it up to 120 volts and everything works. I then unplug the amp from the variac and plug it into the wall outlet and turn it on again. The amp powers up for about 10 seconds and then the fuse blows again.

To me measuring 55 or 53 ohms from primary to the chassis seems very low. Obviously you realize now that the variac was working as an isolation transformer and that might explain the strange AC volts you measured at pins 5 and 6 on the preamp plug thing. One thing is certain, when you removed the transformer from the chassis for testing the fuse did not blow, but it had no ground reference.

[52 Bill]

Yes, I thought that during the last go round, Dave had checked the resistance from the chassis to either side of the ac line. I assumed that all of this was checked. If there still is resistance reading from the primary wiring to the chassis, the transformer needs to be tested.

As for the two caps, one is the traditional ground cap to the chassis and the other is a line filter to reduce noise from the ac line. Leaving them out is not the cause of your problem.

[davohilts]

DrGonz78 and 52 Bill,
Thanks for your help on this problem. I have pulled the transformer out and now have taken the time to completely test it and it has show to failed.
I used the advice of another forum on this site, http://music-electronics-forum.com/t26848/, and have the following to report:
Primary Leads:
I measure 2.1 ohms across leads, no connection to chassis ground (note I pulled off the CT for the heaters and secondary HV windings)
Secondary Leads:
across heaters: .1 ohm
h1 to hCT: .1 ohm
h2 to hCT: .1 ohm

across 5V winding: .1 ohm

across HV secondary: 117.7 ohms
HV1 to HVCT: 60.5 ohms
HV2 to HVCT: 57.3 ohms

Now measurements between windings:
From HV1/2 to 5V1 and 5V2: no connection
From HV1/2 to Heater1, Heater2, Heater CT: no connection
From HV1 to Primary1: 5.4 ohms
From HV1 to Primary2: 7.0 ohms
From HV2 to Primary1: 113.5 ohms
From HV2 to Primary2: 115.1 ohms
From HVCT to Primary1: 53.1 ohms
From HVCT to Primary2: 54.7 ohms

Conclusions: the HV Secondary has internally shorted to the Primary windings making the transformer unusable.

During my testing earlier I showed very high AC voltage on the Primary and now it is obvious to me why this was.

Now I want to say thanks to everyone who helped me through this problem and especially 52 Bill for his patience with my lack of understanding.
I have learned a lot troubleshooting this amp and have ordered a replacement transformer that I will install when it arrives.

Dave

[olddawg]

This makes no sense. If the secondary was shorted to a primary it would blow a fuse immediately or smoke. What is the resistance from ANY lead to the metal case of the transformer?

[davohilts]

This makes no sense. If the secondary was shorted to a primary it would blow a fuse immediately or smoke. What is the resistance from ANY lead to the metal case of the transformer?

I've tested all leads to the PT body and there is no connection for any of them. It has left me confused.
I have an old Chicago Standard Transformer that is 270/0/270 with 3A 5V and 3.5A 6.3V windings (the 6.3V winding is a little small to actually use) so I temporarily wired it in place and turned on the amp and it came on and played without a problem. I've ordered the correct size transformer and will install it when I get it.

Thanks for your help, I really appreciate it.
Dave

[olddawg]

Did you earth the chassis ground when you replaced the PT? The old one worked fine with the ground lift? I would hate for you to buy a new PT and have the same problem. Very odd indeed.