Fisher 60 chassis voltages

[malloyt]

Hello, first, thanks to all who answered my questions regarding my rebuild of the A-69 hybrid. I now have questions about a 60 monaural console. I rebuilt the chassis replacing all electrolytics, paper caps and any out of spec resistors (about 75%). All vacuum tubes read "good" on my budget heathkit tester. All inputs sound fine, including AM & FM. I power the chassis through a bucking transformer which brings the line voltage down to about 116VAC. Transformers don't get too hot, output tubes don't red-plate. Everything seems fine except the voltage on the anode pin of V-10, an EL84. If I clip my voltmeter to the pin, on power-up it will read correctly, but then diminish down to 0 volts by the time the tubes warm and the music sounds. The reading on the other EL84 is fine throughout, at a steady 337v. I have included a copy of the schematic as well as the voltage reference values. My recorded voltages are in red print, the red asterisk indicates the reading I'm having trouble with. All readings were taken with a digital voltmeter, on FM input, volume at low, bass at low, and treble at high as intructed. I appreciate any and all troubleshooting tips. Thanks.

 

[larryderouin]

Swap the EL84's and take the voltages again. If the voltage drop follows the tube, replace the tube. If not you've got a wiring or component problem somewhere upstream. Include the Filament voltages and pins 2 & 3 Voltages.

 

[dcgillespie]

I'm afraid this problem is quite major, as all indications are that the half of the primary winding in the OPT that serves V10 is open. Further, if you continue to operate the unit in this condition, it is extremely hard on the tube installed in the V10 socket, as with no plate voltage, it means that the screen grid element is handling ALL of the current flow through the tube -- which is WAY more than the screen grid is designed to handle. Ergo, the great likelihood of damaging the tube.

With the unit turned off, unplugged, and the power supply caps discharged, measure the resistance of each half of the primary winding in the output transformer. It is not uncommon for there to be a slight difference in these values, but we're only talking 10Ω or so. Likely, if you even do get a reading in the side connected to V10, it will be extremely high -- likely many thousands of Ohms -- IF you get a reading at all.

Other than a bad connection at pin 7 of V10, this is about the only scenario that fits the symptoms given.

Sorry to be the bearer of bad news, but the chances of this diagnosis being correct is very high.

Let us know!

Dave

 

[malloyt]

Thanks for the quick replies. I swapped the tubes, same result. I then measured the resistances as directed. The readings are 314Ω (V9) and 322Ω (V10).

I tested the output transformer prior to rebuilding by running a very low ac voltage though the 8Ω secondary and then measuring the voltage on the primaries as well as the 16Ω secondary tap. The readings were typical of what I've seen before. The voltage on the 16Ω tap was about double the input voltage, the red and blue primaries were close to each other in reference to the center tap and about double that voltage for red-blue. I don't know the number of windings, so I don't know the expected voltages, just that they were in sync.

I checked the wiring of the output tubes and they match each other but differ in the way they wire to the 12AX7 just like in the schematic. I guess I would have expected them to be wired similarly because I think of the 12AX7 as two triodes.

I will check all the tube sockets for oxidation and measure the values of the new C39, 40 and R50, 51 to make sure that I didn't replace with the wrong values and that they are within spec (something I guess I should do prior to installing them). I will let you know what I come up with. If anyone has an explanation for the different wiring patterns for the 12AX7 (even if it doesn't relate to this problem), let me know. It's nice to learn the theory behind the schematic. Thanks again, Terrence.

 

[dcgillespie]

Terrence -- OK. Information at this point is conflicting to the point that some of it must be questioned. But first, in checking the OPT, please confirm the DC resistance readings taken:

1. Between the B+ lead and pin 7 of V9.
2. Between the B+ lead and pin 7 of V10.
3. Between pin 7 of V9 and pin 7 of V10.

There are only two windings in the OPT:

1. A primary winding with one end connected to pin 7 of V9 and the other end connected to pin 7 of V10. This winding has a center tap connection which is the B+ lead.
2. A secondary winding with one end being the Common, and the other end the 16Ω tap. The 4Ω tap is made at the center of the winding, and the 8Ω tap between the 4Ω and 16Ω connections.

For a transformer in this application, if you applied 1.0 vac between the Com and 8Ω tap, the transformer should produce:

1. About 1.414 vac between the Common and 16Ω terminals.
2. About 32 vac between the two plate leads of the primary (those connected to pin 7 of each EL84).
3. About 16 vac between the B+ lead and either plate lead.

The DC resistance of the primary winding in particular needs to be verified as:

1. If the resistance between the B+ lead and the pin 7 plate lead of V10 is 322Ω, then there must be a similar DC voltage present there as is present at pin 7 of V9. OR
2. If there is no DC voltage on pin 7 of V10, there there cannot be 322Ω DC resistance between the B+ lead and the pin 7 plate lead connected to this tube (V10).

Gotta chase down which one it is, as both cannot be true at the same time.

The connection of the EL84 tubes to the 12AX7 is very typical for Fisher designs. The 12AX7 does contain two separate triode tubes within it. In this case however, they are connected in cascade, where one section (pins 6, 7, & 8) drives the second section (pins 1, 2, &3). The signals to the two EL84s are taken at pins 1 and 3 of this second section (the plate and cathode respectively), because the signals that appear at these points are balanced in amplitude, but 180 degrees out of phase with each other, forming the two balanced "push-pull" signals that are then coupled to the output tubes. The output tubes then amplify the two signals and present them to the primary winding of the push-pull output transformer. There the two out of phase signals are combined in the primary winding, and presented as a combined signal in the secondary winding, which connects to the speaker.

Let us know what you find!

Dave

 

[malloyt]

Thank you for the insights Dave. I did as you requested measuring the resistance of each pin 9 to the B+, my earlier readings were to chassis ground. V9, pin 9 read 166Ω, V10, pin 9 read 175Ω. The reading between V9 and V10 was 341Ω. Since these seemed fine, I cleaned the sockets and tube pins and also borrowed a better voltmeter. The voltage on V9, pin 9 read 332V. For V10, pin 9 read 336V. I'm going to presume the problem is solved and am leaning toward blaming the voltmeter.

Thank you for the explanation of the 12AX7 wiring, that was the first time I'd seen that.

I do have a question regarding the voltages you stated for the output transformer. How did you come up with them? I typically feed about 6vac in because that's the output of a filament transformer I have. When I don't have any specs on the output transformer, I just look for parity, between each primary and the center tap and ratios of the 6v between the secondaries. In this case between the 8Ω and 16Ω taps. It would however be nice to know what voltages to expect. Were your values based on your experience with EL84 output transformers or based on the voltages specified on the schematic?

Thanks again and let me know whether you believe the problem solved. Terrence.

 

[dcgillespie]

OK. Now we're getting to the bottom of things: Either you have a typo, or are checking the wrong pin on the output tubes. The pin you need to be concentrating on is PIN 7 on each output tube socket. In your last response, you are always referring to pin 9. Pin 9 is the screen grid terminal -- not the plate terminal that the OPT lead connects to. The plate terminal that the OPT leads connect to is pin 7 on the EL84. THAT is the terminal you need to be measuring to per my last post. This is the same terminal you were having the issue with measuring the voltage at.

As for the expected transformer voltages:

1. There is one assumption made, but it is a rather safe one, and that is that the primary impedance of the transformer is likely 7500 - 8000 Ω. For ease of calculation, I used 8000Ω.

2. The secondary impedance is factored out by dividing the primary impedance by the secondary impedance in question. You used the 8Ω tap. 8000/8 = 1000. This means that if you placed a 1Ω load on the 8Ω tap, the reflected impedance back to the primary would be 1000Ω.

3. Since impedance is equal to the square of the turns ratio, that means that the turns ratio of this transformer is the square root of 1000 or 31.62. This means that if a 1 vac signal is applied to the 8Ω tap, then the primary will produce about 32 vac across the full winding, or about 16 volts across each half of the winding. If you applied 6 vac (or there abouts), then all voltage levels will be raised by a factor of 6.

4. In an output transformer, the relationship between the 4, 8, and 16Ω taps is a factor of 1.414. That is, for any given voltage applied to or appearing at the 4Ω tap, the voltage at the 8Ω tap will be higher by a factor of 1.414. for any given voltage applied to or appearing at the 8Ω tap, the voltage at the 16Ω tap will be higher by a factor of 1.414. Same thing for a 32Ω tap over the 16Ω tap, etc. Therefore, if you apply 1.0 vac to the 8Ω tap (relative to the Common terminal), then the 16Ω tap will produce 1.414 vac. Again, if you are applying 6 vac, then all voltage levels will be raised by a factor of 6.

Let us know!

Dave

 

[malloyt]

Dave, Sorry it was a typo, all values were measured at pin 7, which in this case were wired with either blue or red going to the output transformer. The voltages were measured with a line voltage of 117VAC reduced by a bucking transformer. I am reluctant to run any of the old tube amplifiers, radios, etc at mains voltage (about 124VAC) since the transformers tend to get warm enough even with the reduction.

Thanks for the explanation of the output transformer voltages. The information will come in handy for future testing. I did not know the relationship of impedance = square of the turns ratio. A while back I had to test a 10kΩ output transformer from a Stromberg Carlson SR-405. Using my 6.3vac transformer resulted in extremely high voltages at the primaries. Having known this information back then would have prepared me for the very high values I recorded. Your explanation also tells me its probably time to use a lower ac voltage source.

Thanks again, Terrence.

 

[dcgillespie]

It is usually easier -- and what I do -- to apply 100.0 vac to the complete primary winding, and then measure the full secondary winding, and compute the impedance from there -- assuming of course that you know what the rated secondary impedance is.

But after all of this, there is still the discrepancy as to no voltage being measured on pin 7 of V10. You need to get that resolved for the continued health of the unit.

Dave