question about a 8-t-241

[stusnyder]

I'm working on a 8-t-241 and going over voltages at the 5u4 tube. Pin 2 is suppose to have 275 volts and I'm getting 402.
Pins 4 and 6 are right on, Pin 8 is suppose to have 275 volts, I got zero.
According to the diagram, the line to pin 8 goes directly into the transformer....so does this mean that section of transformer is dead???
Update: Located both leads off that transformer tap and I do have voltage. I found 2 bad ecaps (these were new, recently replaced) and replaced them. Now got voltage on pin 8 but its too high, should be 275 I got 390....anybody have any ideas where to check next? I went pretty far into the circut and could not find the cause of the higher voltage.

[timmy]

using the grid voltage or variac?

[stusnyder]

Grid voltage

[timmy]

do you have a photo fact for it, maybe someone tried to repair it an didnt know what they were doing. something could have been indirectly put in the wrong place. wrong cap somewhere, shorted electrolytic?

[Eric H]

The voltage is high because something is probably open and not drawing current like it should.

Look for an open power resistor, there is a metal strip riveted across the back of the chassis that contains a long wirewound resistor with multiple taps, it's quite common for these to open up.

I would check your B+ voltages at some of the tubes, knowing the section missing the B+ would make it easier to know where to look.

[stusnyder]

hi eric,
yea I"ve seen that resistor mounted on the back of the set. I'll have to check it against the SAMS...
If it's bad, is there something modern that it can be replaced with?

[Eric H]

You can replace the individual sections with Wirewounds under the chassis, depends on how many watts it is, you may need to get some that mount to the chassis for a heat sink.

I wouldn't worry about it just yet, the voltage could also be high because some parts of the set aren't working right, tubes not conducting due to bad caps and hence no load on the Power Supply.

[stusnyder]

Originally Posted by Eric H

You can replace the individual sections with Wirewounds under the chassis, depends on how many watts it is, you may need to get some that mount to the chassis for a heat sink.

I wouldn't worry about it just yet, the voltage could also be high because some parts of the set aren't working right, tubes not conducting due to bad caps and hence no load on the Power Supply.

Eric, the set has already been recapped, both e caps and paper caps.

[Penthode]

Because the voltage at the cathode of the 5U4 (pin 8) is high, I see one of two possible scenarios: either the resistive divider chain is open (R193A, R194 R210//R177, R193B and R193C) or else there is a short on the negative end of the chain (-120v to ground).

I suspect you may find an open resistor chain (bad connection or bad resistor). I have seen R193A open on a couple of these chassis. It is the big fella mounted on the rear chassis apron. In each case, I successfully replaced the one bad section with a big wirewound (ceramic tube type) mounted high up on the HV compartment cage on a home made bracket.

[radiotvnut]

I agree about there being an open somewhere that is causing the proper load to not be placed on the power supply.

I will add a word of caution here. Normally, when the set is working properly, the load placed on the power supply is sufficient enough to discharge the filter caps, shortly after the set is turned off. When the proper load is not placed on the power supply, there is often nothing there to discharge the filter capacitors. In this case, the full power supply output voltage can remain on the filter caps and this charge can get your attention in a not so nice way. So, be careful. Use your DMM to make sure the caps are discharged when the set is off. If there is a charge; discharge the caps, preferably through a high wattage/high value power resistor, to ground.

[stusnyder]

The set is recapped, ecaps and paper caps. There was no negative readings. Checked e cap wiring many times over already.
I think this chassis has more than one problem and I won't find them all.
I have another chassis on the bench that is practicly the same thing, using as reference. That one did display a test pattern,weak and washed out, but after replacing 3 paper caps I lost it. Got a vertical problem with it now.
Ecaps were replaced.

[Penthode]

Originally Posted by stusnyder

The set is recapped, ecaps and paper caps. There was no negative readings. Checked e cap wiring many times over already.
I think this chassis has more than one problem and I won't find them all.
I have another chassis on the bench that is practicly the same thing, using as reference. That one did display a test pattern,weak and washed out, but after replacing 3 paper caps I lost it. Got a vertical problem with it now.
Ecaps were replaced.

Both chassis should be repairable. It is important to follo the schematic closely. Part of this game is patience and perseverence. The RCA design is more complicated than most.

[stusnyder]

Originally Posted by Penthode

Both chassis should be repairable. It is important to follo the schematic closely. Part of this game is patience and perseverence. The RCA design is more complicated than most.

Figures I picked a real complicated one to start with. The latter one is really the one I'd like to get going, because it has a nice wooden case.

[Penthode]

Originally Posted by stusnyder

Figures I picked a real complicated one to start with. The latter one is really the one I'd like to get going, because it has a nice wooden case.

I would be glad to help. I have owned and used an 8T243 for 42 years and have fixed a number or these chassis since. It is my favorite design.

[stusnyder]

Originally Posted by Penthode

I would be glad to help. I have owned and used an 8T243 for 42 years and have fixed a number or these chassis since. It is my favorite design.

Are you familar with the 6-t-71? I got a odd problem with one.