10-29-2014, 12:53 PM
From my workbench:
Well, Arran,
I hate to admit it, but yes - I failed to follow my normal protocol which is to test tubes before applying power. The 45 to 90 seconds of operation with a shorted rectifier certainly did not help the situation, although I think the damage was probably already done.
The shorted Brand Z 6X5 was in the radio when I acquired it.
I just ran a resistance check of the power transformer windings. There are no resistances on the schematic, so I have nothing to compare the following readings to, but I think the following speaks for itself:
AC primary, two primaries connected in parallel for 115 volt operation - 7 ohms
6.3V winding - 0.5 ohms
One half of HV secondary - 188 ohms
Other half of HV secondary - 122 ohms
While the two halves would not have exactly the same resistance, I think the discrepancy is great enough to indicate shorted turns in the half reading 122 ohms.
I also measured from the main B+ line to ground. I get a measurement of 58K ohms. There are two resistors in series between B+ and ground - a 22K and a 33K which adds up to 55K. The 22K resistor measures around 23K and the 33K measures 35K which accounts for the 58K between B+ and ground. If there was a partial short or leakage between B+ and ground, I would expect the 58K measured resistance to be lower. So I feel confident in my diagnosis of a bad transformer.
Hammond sells a power transformer that looks like it would be a good replacement. The 270DAX has the same mounting footprint, and has a HV secondary of 520 VCT @ 104 mA and a 6.3V winding @ 3.5A. The original transformer is rated at 570 VCT @ 70 mA and 6.3V @ 2.5 A. Slightly lower B+ would not be a bad thing.
If I go the new route, though, it will have to wait a while. I might take a look at some other Philco sets of this time period and see if there might be another set that uses a similar transformer, and then look for a junk set using such a transformer.
I think that after I replace the power transformer, I will install silicon rectifiers and a dropping resistor in place of the 6X5 before I apply power again.
In other news...The flocking fibers arrived today. Perhaps next week I'll try that and see how it comes out.
Well, Arran,
I hate to admit it, but yes - I failed to follow my normal protocol which is to test tubes before applying power. The 45 to 90 seconds of operation with a shorted rectifier certainly did not help the situation, although I think the damage was probably already done.
The shorted Brand Z 6X5 was in the radio when I acquired it.
I just ran a resistance check of the power transformer windings. There are no resistances on the schematic, so I have nothing to compare the following readings to, but I think the following speaks for itself:
AC primary, two primaries connected in parallel for 115 volt operation - 7 ohms
6.3V winding - 0.5 ohms
One half of HV secondary - 188 ohms
Other half of HV secondary - 122 ohms
While the two halves would not have exactly the same resistance, I think the discrepancy is great enough to indicate shorted turns in the half reading 122 ohms.
I also measured from the main B+ line to ground. I get a measurement of 58K ohms. There are two resistors in series between B+ and ground - a 22K and a 33K which adds up to 55K. The 22K resistor measures around 23K and the 33K measures 35K which accounts for the 58K between B+ and ground. If there was a partial short or leakage between B+ and ground, I would expect the 58K measured resistance to be lower. So I feel confident in my diagnosis of a bad transformer.
Hammond sells a power transformer that looks like it would be a good replacement. The 270DAX has the same mounting footprint, and has a HV secondary of 520 VCT @ 104 mA and a 6.3V winding @ 3.5A. The original transformer is rated at 570 VCT @ 70 mA and 6.3V @ 2.5 A. Slightly lower B+ would not be a bad thing.
If I go the new route, though, it will have to wait a while. I might take a look at some other Philco sets of this time period and see if there might be another set that uses a similar transformer, and then look for a junk set using such a transformer.
I think that after I replace the power transformer, I will install silicon rectifiers and a dropping resistor in place of the 6X5 before I apply power again.
In other news...The flocking fibers arrived today. Perhaps next week I'll try that and see how it comes out.
--
Ron Ramirez
Ferdinand IN
