View Full Version : question about a 8-t-241


stusnyder
04-27-2011, 07:45 PM
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
04-29-2011, 05:47 PM
using the grid voltage or variac?

stusnyder
04-29-2011, 06:20 PM
Grid voltage

timmy
04-29-2011, 07:39 PM
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
04-29-2011, 07:55 PM
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
04-29-2011, 09:06 PM
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
04-29-2011, 09:57 PM
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.

radiotvnut
04-29-2011, 10:13 PM
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
04-30-2011, 03:26 AM
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
05-09-2011, 10:40 PM
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.

stusnyder
05-10-2011, 04:18 AM
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.

I'm having trouble deciphering that resister, to test use a ohm meter with a lead on one end and work down the line with the other lead?? Test out or in circut??? I have no formal training in electronics, just what I have read and experienced thru the years...radios I have no problem....damn televisions are so much harder.

stusnyder
05-10-2011, 08:30 PM
I located the resistor on my sams, and checked it. It's in specification, nothing open on it.
But voltage does not match. Looking at the bottom of the chassis, transformer on the right, I hold 1 lead of my meter on the first resistor terminal, moving to the left with the other lead I get 139v, -200v, -188v, and the last to the left -176v

Penthode
05-10-2011, 09:21 PM
The high voltage on the 5U4 cathode suggests that one of the resistors in the chain may be open.

If you check the resistance from pin 8 of the 5U4 to chassis, it should measure about 6400 ohms. If you check the resistance of the entire divider chain from pin 8 of the 5U4 to the metal can of C147 (located under the chassis under the power transformer), it should measure 7700 ohms. These resistances may vary +/- 20% but if they are considerably higher, then one of the resistors is likely at fault.

Lastly, try measuring from the metal can of C147 to chassis and it should be 1300 ohms. A short circuit to chassis of the C147 can could also lead to a high voltage on pin 8 of the 5U4.

Penthode
05-11-2011, 05:42 PM
I located the resistor on my sams, and checked it. It's in specification, nothing open on it.
But voltage does not match. Looking at the bottom of the chassis, transformer on the right, I hold 1 lead of my meter on the first resistor terminal, moving to the left with the other lead I get 139v, -200v, -188v, and the last to the left -176v

It is difficult to understand what you are measuring. Can you measure the overall chain resistance as I first suggested?

Further, could you identfy what you are measuring corresponding with what it should measure?

Correct ......... vs .......... Measured
Voltages ....................... Voltages

+215
+135
+88
-60
-120

stusnyder
05-11-2011, 08:05 PM
The high voltage on the 5U4 cathode suggests that one of the resistors in the chain may be open.

If you check the resistance from pin 8 of the 5U4 to chassis, it should measure about 6400 ohms. If you check the resistance of the entire divider chain from pin 8 of the 5U4 to the metal can of C147 (located under the chassis under the power transformer), it should measure 7700 ohms. These resistances may vary +/- 20% but if they are considerably higher, then one of the resistors is likely at fault.

Lastly, try measuring from the metal can of C147 to chassis and it should be 1300 ohms. A short circuit to chassis of the C147 can could also lead to a high voltage on pin 8 of the 5U4.

resistance pin 8 to chassis 6460 ohm you said 6400
pin 8 to can c147 7720 ohm you said 7700
can c147 to chassis 1242 ohm you said 1300 ohm

Penthode
05-11-2011, 09:17 PM
resistance pin 8 to chassis 6460 ohm you said 6400
pin 8 to can c147 7720 ohm you said 7700
can c147 to chassis 1242 ohm you said 1300 ohm


Hmm. That rules out the divider at least. If the voltages are then wrong, it appears that something is conducting heavily when the set is running if the voltages are incorrect.

Can you provide the as measured voltages against the voltages the schematic advises? This may provide the clue.

stusnyder
05-11-2011, 09:21 PM
Hmm. That rules out the divider at least. If the voltages are then wrong, it appears that something is conducting heavily when the set is running if the voltages are incorrect.

Can you provide the as measured voltages against the voltages the schematic advises? This may provide the clue.

I can try to test some of the voltage points....if I can figure out where by the schematic...

Penthode
05-12-2011, 06:00 PM
I can try to test some of the voltage points....if I can figure out where by the schematic...

If you look at the schematic near where the power transformer is drawn, you will see the sections of the divider resistor. Measure the junction nodes indicated with respect to chassis and provide us the readings against what it should be. For example, downstream from the 5U4 at the first node (section A of C148 elctrolytic positive it should read +215v to ground. You could measure from the focus control as that is a convenient access point.

The next point should read +135 volts to chassis.It may be accessed at C148B positive connection.

The voltage at the C147C positive should read -60 volts to chassis and the can of C147 should read -120volts to chassis.

stusnyder
05-12-2011, 07:38 PM
If you look at the schematic near where the power transformer is drawn, you will see the sections of the divider resistor. Measure the junction nodes indicated with respect to chassis and provide us the readings against what it should be. For example, downstream from the 5U4 at the first node (section A of C148 elctrolytic positive it should read +215v to ground. You could measure from the focus control as that is a convenient access point.

The next point should read +135 volts to chassis.It may be accessed at C148B positive connection.

The voltage at the C147C positive should read -60 volts to chassis and the can of C147 should read -120volts to chassis.

Ok I measured directly on the divider resistor, here are the readings from right to left:
1st term (under trans) 202v
2nd term 340v
3rd term 0
4th term 15
5th term 31

measured in dc scale, black to chassis

Penthode
05-12-2011, 10:01 PM
Ok I measured directly on the divider resistor, here are the readings from right to left:
1st term (under trans) 202v
2nd term 340v
3rd term 0
4th term 15
5th term 31

measured in dc scale, black to chassis

I assume that the DC voltages on the 4th and 5th terminals are negative with respect to chassis?

The second terminal which is supposed to be +135 volt measures 340 volts: is that correct? I assume you have changed the electrolytics. If the 135 volt line is so high, it may be that the wiring may be incorrect. Please check the wiring closely around these points.

In addition, the -15 volts and -31 volts show the correct division ratio of about 2 to 1. (It should be -60v and -120v). This would imply that the other end of the divider is open or high resistance. Otherwise something is pulling down the -120v.

Please check the wiring.

stusnyder
05-13-2011, 03:06 AM
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
05-13-2011, 08:37 PM
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
05-14-2011, 04:55 AM
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
05-15-2011, 06:03 PM
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
05-16-2011, 10:02 PM
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.

Penthode
05-16-2011, 10:23 PM
Are you familar with the 6-t-71? I got a odd problem with one.

I have worked on a few.
What is the problem?

stusnyder
05-17-2011, 02:27 AM
I have worked on a few.
What is the problem?

On the bench hooked up to a 5" test crt I get a decent picture. Back in the cabinet hooked up to the 16" crt I get no vertical lock.....

Penthode
05-19-2011, 11:42 PM
On the bench hooked up to a 5" test crt I get a decent picture. Back in the cabinet hooked up to the 16" crt I get no vertical lock.....

Note that on the design of the kcs47 chassis that the sync take off point is after the contrast control at the video amplifier. If the contrast is reduced (less video amplifier gain) the sync amplitude is reduced. This may aggravate an already weak (problem) sync condition. Note that the kcs66 reverted back to constant amplitude sync (sync take off before the contrast control).

I assume that you have replaced the capacitors in the sync separator section? Do you have a scope to check sync amplitude? Do you notice with the test CRT if the contrast control setting is higher? Does adjusting the control to increase the contrast help with vertical sync?

Otherwise it is checking voltages, waveforms and substituting tubes in the sync section?