800B Power Transformer too hot?????

[sloane freeman]

Got the unit working after replacing all electros in both units.  Checked the remaining caps for value and leakage and they SEEM OK.   However the transformer gets HOT!  After about two hours it radiates heat.  Is this normal or am I missing something.

I measured the plate current at 80 mils but did not check the screens.   With the B+ disconnected, the transformer stays cool.

[Norman S Braithwaite]

It is normal for the transformer to get warm or even a bit hot but not so hot that you cannot hold your hand on it for 5 to 10 seconds.  If it is so hot that you cannot hold your hand on the transformer, something is amiss.  Check the DC resistance from B+ to ground.

Norman

[sloane freeman]

Resistance to ground at the high side of the 4mf cap, C95, runs over 35M ohms as the filters charge from the meter.

Please straighten me out on this:  the schematic shown in the EH Scott Dean of DX book shows the speaker plug and a tapped inductance, one portion 670 ohms the other 9K ohms. The 670 ohm section is hooked to pins 2 and 4 of the speaker plug and completes the B+ circuit.  The 9K ohm section is hooked between pins 4 and 6, continuing thru R93, a 3K ohm 10 watt resistor to ground.

I hooked a field coil  of 700 ohms between pins 2 and 4 of the speaker plug and a 9K ohm 10 watt resistor between pins 4 and 6.  Using this setup the amp chassis and power transformer became exceedingly hot in about two hours of low level radio output.

I turned it off and the following day made measurements of the current drawn by the 13K ohms between pins 4 and 6 of the speaker plug and found it drew some 20 ma and reduced the voltage by around 18 volts.  I opened the connection between pins 4 and 6 and the set seemed to play just as well, this time after two hours, the transformer was quite warm to the touch, but not nearly as hot as before.

Help!!!!

Thanks!

sloane

[Norman S Braithwaite]

With the speaker plugged in, the resistance between the 4-mfd capacitor and ground should not exceed 12.7k, the sum of the 9k section of the field coil (or 9k resistor in its place), the 700-ohm field coil, and the 3k resistor in the power amplifier chassis. I presume the resistance measurement was made with the speaker unplugged.  If so, the test only shows that the filter capacitors are not excessively leaky.  We need to find out if there is a low resistance path in the power amplifier chassis after the field coil and in the tuner chassis.  Here is a suggestion and a couple more tests.

Disconnect or replace C91, the 25-mfd, 50-volt electrolytic capacitor from the 6L6 cathodes to ground.  If shorted or excessively leaky, this will cause the 6L6 tubes draw to much current.  It is OK to run the set with this capacitor disconnected but make sure R88, the 250-ohm, 10-watt resistor is good.  Run the set with the 9000-ohm resistor in circuit and see if the transformer gets to hot.

Does the voltage regulator tube fail to illuminate when playing either an AM or FM station?

With the speaker plug disconnected and the tuner plugged in, what is the DC resistance between pin 4 of the speaker socket and ground for the set on AM and on FM (operate AM/FM solenoid switch by hand)?

Again but with the tuner plug disconnected?

Norman

[sloane freeman]

Replaced C91.  R88 measures 247 ohms.  With the 9K resistor in circuit, the transformer heats up rapidly.  OD3 glows at all times.  Pin 4 to ground with receiver plug out is around 5M ohms, and with it plugged in  is about 30K after the cap charges some from the meter. No appreciable difference between AM/FM mode.

What is the purpose of the 9K plus 3K  between pin 4 and ground?  It sinks a lot of current, 20ma by my measurement? 

It is too low in resistance to be simply a bleeder.  Since the set works with the 700 ohm field alone connected between pins 2 and 4, what does the extra 13K to ground do besides loading down the B+.

Thanks a lot for your guidance!  

[Norman S Braithwaite]

Originally, the speaker required the 9k winding for magnetism although the original speaker will work satisfactorily even if the 9k winding is open.  The 12k resistance path to ground improves voltage regulation of the power supply and prevents B+ from being to high.  If B+ is not to high without the 12k resistance path to ground, the set can certainly be operated as such but the cause for B+ being lower than it should will not be resolved and may come back to haunt the set.  It is better to figure out what is causing the problem and address it.  What are the cathode, screen, and plate voltages on the 6L6 tubes with the resistor in circuit?  They should be approximately +20-, 270-, and 340-volts respectively.

Norman

[sloane freeman]

I put the resistor back in the circuit and hooked up another receiver that I have that nothing has been done to so far.  It is playing OK on AM, the 9K resistor is warm to the touch.  I will let this setup run for a bit and see if it overheats the amp also.  If it does not, I will hook up the previous receiver and make the voltage checks and try to determine if the temp is the same between the two units.  BBL

Thanks!

[sloane freeman]

9K resistor hot to touch.  Re-connected original receiver and with 9K resistor in circuit , I measured 33.2 vdc on the cathode, 267.9 on screens and 322.2 on plates.

sloane freeman said:

I put the resistor back in the circuit and hooked up another receiver that I have that nothing has been done to so far.  It is playing OK on AM, the 9K resistor is warm to the touch.  I will let this setup run for a bit and see if it overheats the amp also.  If it does not, I will hook up the previous receiver and make the voltage checks and try to determine if the temp is the same between the two units.  BBL

Thanks!

[sloane freeman]

9K resistor hot to touch.  Re-connected original receiver and with 9K resistor in circuit , I measured 33.2 vdc on the cathode, 267.9 on screens and 322.2 on plates of the 6L6's.

AM sounds fine. FM sounds a little distorted.  I am using the Scott loop antenna and a short piece of wire for FM....maybe I need more FM antenna?

[Norman S Braithwaite]

OK, it sounds like the 6L6 tubes may be drawing a little to much current (266-ma total) but I'm not sure this may not be the sole cause of overheating.  Make sure the cathode resistor measures 250-ohms after a few minutes of use when warm or hot.  If not, replace the cathode resistor.  If it does, try replacing the 6L6 tubes with known good new tubes.  The 9k resistor will get hot.  It is dissipating approximately 5-watts over a small surface area.

Norman

[sloane freeman]

K resistor 247 ohms when hot.  Brand new matched JJ 6L6GC's .  Measured AC voltage at rectifiers...389 and 397 to ground.  787 across. Having trouble with AM FM relay not wanting to switch.    It plays for hours relatively cool without that resistor and gets too hot to fool with in an hour with.  What is the chance of an internal short in the HV windings?

I have another amp chassis of unknown condition.

[Norman S Braithwaite]

It is very unlikely that the transformer has any shorted windings.  If so, the transformer would get hot without tubes or anything plugged in but fuses.  Plus the transformer would release the stinky magic smoke.

I would not trust the new JJs.  They are marketed for use in musical instrument amplifiers and may not spec out the same as original American 6L6 tubes.  According to Scott service data the pair of 6L6 tubes should be drawing 80-ma total (cathode: 20v/250-ohms).  The 6L6 tubes in your amplifier are drawing a total of 134-ma (33.2v/247-ohms - prior estimate incorrect because I had erroneously doubled).  That is 54-ma to much, almost three times what is being drawn through the 9k resistor when it is in circuit.   Do you have some older American made 6L6 tubes to try?

Try cleaning and lubricating the AM-FM solenoid switch, make sure all pins on the tuner plug are clean and making good contact, and make sure all contacts on the pushbutton assembly are clean and making good contact (especially if some FM station buttons operate the switch better than others).

Norman

[sloane freeman]

I have 2 GE 6L6's.  I will give them a try. I will check out the tubes and redo the voltage measurements. Thanks so much for your help!  Have a great Christmas!