Fisher tube chassis balance controls

[audmod01]

Preamp Equalization Circuitry

Saturday the parts came in for the RIAA equalization. I built up the networks and installed them. I was able to match the parts to within 1% which I think is as good as I can get them. That is based on the 1350pFd, 350pFd, 270K ohm and 2.13M ohm values that Dave mentioned in his thread on Fisher's RIAA equalization networks.

I did not chose to keep the tape head feature and instead just RIAA equalization for magnetic phonograph operation. So now both inputs will work for magnetic phonograph.

Next I will be working on the AC wiring and on-off circuitry.

Joe

 

[audmod01]

Getting Ready To Apply Power

Today I decided to go ahead and use (at least temporarily) the unused DPDT slide switch on the front chassis panel (behind the door) as an AC On-Off switch. I added the varistors on either side of the AC input to limit turn-on surge. Since the AC wires now have to traverse the front of the unit to reach the slide switch I am using I used a piece of 1/4 in. copper tubing as a shield to help keep down radiated 60Hz from the AC wires. The tubing is soldered to the same lugs that also ground an audio cable that runs along that edge. The wires to the switch then pass a short distance between the end of the shield and the switch body (about 2 inches) and are well away from low level wires for magnetic phonograph inputs.

I still have one plastic insulated white wire that I do not know where it went originally. It is connected to the second inboard row, second lug from the front when viewed from the rear of the chassis. Can someone look at their unit and let me know where it is connected underneath?

I have gone through the power supply checking for shorts and also checking the AC input wiring and do not see any shorts. Looks like the main amplifier power transformer input winding measures about 2.2 ohms DC with the ohm meter. The small power transformer used for the LV power supply measures about 69 ohms across its primary. Those measurements are of each transformer separately (not connected in circuit). I saw no shorts to ground from any B+ buss in the amplifier, so it looks like I am about ready to power the amplifier up after a few more checks.

Here are a couple of pictures.

Joe

 

[dcgillespie]

Fingers crossed for you Joe. Understand too that when it comes to installing your output tubes, since (or when) you have EFB installed, you can install one output tube at a time for testing if needed. The EFB circuits will largely automatically compensate as more tubes are added to the unit.

Let us know!

Dave

 

[fred soop]

Today I decided to go ahead and use (at least temporarily) the unused DPDT slide switch on the front chassis panel (behind the door) as an AC On-Off switch.
.....

I don't like that idea as slide switches only make and break as fast as you move the switch. Much more arcing than a toggle with a snap action.

 

[audmod01]

Right now I am fighting an apparent high leakage path in the LV power supply. The 25VAC to the bridge is held to about 18VAC and all the voltages are lower than they should be. The voltage feed to the 12AX7 filaments are only about 3VDC and that is true even if the leads feeding those tubes are disconnected at the LV power supply. It may turn out to be some leaky electrolytic capacitors. There was a loose wire that had fallen across a couple of connections that I found and there is still something wrong. I will be looking for bad new capacitors (possibly damaged by a short at turn-on) or something connected wrong

I did recheck the filament wiring changes and they are correct V1 and V3 have pin 4 fed by the LV board and pin 5 grounded. V2 and V4 have pin 5 fed by the LV board and pin 4 grounded as shown in the schematic we developed. Connecting the tube filaments to the LV board makes little difference, so there has to be a problem right on the board.

Although slide switches may not be the best of parts for switching AC current, they do quite well in most cases where employed in consumer products. My PAS has been in constant use over the years since about 1964 and it still has its original slide switch, still operating OK. It has been used with both ST-70 Amplifier for its first 16 years, then a Marantz DC-170 followed by an Onkyo M282 and now another ST-70 the last year.

Joe

 

[dcgillespie]

An electrolytic cap installed with its polarity reversed?

Dave

 

[audmod01]

Wired wrong?

Dave;

At this point I am not sure. I did find that I had wired CR106 from the end of R30 to the junction of CR102 & CR103. I don't know what I was thinking at the time I did that - most likely had been working on it too many hours. Now even after correcting the error and checking the negative voltage source for the -11.6VDC to the filaments I find that the 10 ohm resistors are getting very hot. I think I need to order some new electrolytics and replace them. The polarity appears correct, but they may have already been damaged. C123 and C125 both show their negative leads connected to circuit ground and C124 and C126 show their positive leads connected to circuit ground.

It can't hurt to get a new set of capacitors, they are inexpensive enough, and just change all of them out to make sure good parts go in. I suspect that several were damaged early on. The first time I turned it on I heard a distinct grunt of 60Hz and immediately turned the set off. I found a different bridge rectifier which is like the one you originally specified, so I plan to order one of those too. The first one I bought had leads so large they would not fit the holes in the prototype board material.

I will recheck the polarity of the caps again just to make sure.

Joe

 

[audmod01]

Sanity Check on LV Power Supply

I decided to take another look at the LV power supply. I did recheck polarity of electrolytic capacitors and on the + supply all the - leads to go to chassis ground. On the - supply all + leads go to ground so that should be correct. There is a white band on the side of the capacitors (Nichicon) with a minus sign in the band that I interpret as the negative lead.

I also disconnected the 11.6VDC filament feeds from the LV power supply board and measured resistance to ground. For V1 and V3 filaments in parallel to chassis ground I measure 6.21 ohms. For V2 and V4 filaments in parallel to chassis ground I measure 6.15 ohms. There is no real difference if measured with the filament leads connected to the LV power supply board.

Those are DC measurements of course. By the book stated 12.6VAC filament current of 0.15A the calculated resistance of a 12AX7 would be 84 ohms with the filament hot. A cold filament measures about 12.7 ohms using the multimeter. This shows that there is considerable difference in resistance after the tube filament warms up.

I ordered a new set of electrolytic capacitors for the board and also a new correct bridge rectifier. The resistors I am using for the 10 ohm 2 watt resistors are metal film 2W resistors rather than the wire wound resistors shown on Dave's board, but that should not matter since both have the same wattage rating. The 10 ohm resistors get hot enough to smoke with the filament leads disconnected from the LV board. To me that indicates that the electrolytic capacitors are breaking down under applied voltage. One thing I have not done yet is to look at the DC there with the oscilloscope to see if it really is DC or if perhaps it is AC instead. It is possible a rectifier in the bridge has failed.

One thing that could be an overall issue is that I have two CL-80 varistors on the AC input to both the main power transformer and the added LV power transformer. I could connect the LV power transformer directly to the AC main leads if necessary to get full 25.2VAC at the secondary of the transformer. Right now it is less than that. Measured with the multimeter I show about 19.8VAC at the input to the bridge with a line voltage at the wall socket of 120VAC.

Joe

 

[audmod01]

Oscilloscope check at 10 Ohm resistors

I decided to do one more check at either side of the 10 ohm resistors referenced to the chassis ground. I see about 2.5VAC ripple at the input to C123 or C124 and essentially none at the output side at C125 and C126. So there is no raw AC getting to the electrolytic capacitors and the appearance of the ripple at the input to C123 and C124 appears symmetrical, not distorted.

Joe

 

[dcgillespie]

Smoking resistors mean they are dissipating way more than their two watt rating. They normally dissipate just under 1 watt each. It sounds like there is some sort of serious short on the output side of these resistors. That would also cause no ripple on the output side too -- which there would normally be with such an overload placed on the supply. Is the buffer stage connected into the system yet? The short could be there, too.

Dave

 

[audmod01]

Buffer Stage?

Dave;

By buffer stage do you mean the Op Amp? I did check resistance to ground of the plus and minus supply leads to the Op Amp and each showed around 2 K ohms to ground. I need to take a closer look at the wiring of the LV power supply board again. The resistors smoke even with the filament leads to V1-V4 disconnected, so the short has to be right there somewhere.

Joe

 

[audmod01]

Making Progress on the LV Power Supply

Dave;

This morning I disconnected the Op Amp board leads. When I did that the plus and minus voltages came back up and the 10 ohm resistors remain relatively cool. The filaments on the 12AX7s also lit up OK, so the problem seems somehow related to the Op Amp board. At the same time the minus voltage for the grids of the output tubes went from negative to positive! I need to do more troubleshooting to find the source of that issue with the Op Amp board disconnected. The negative voltage for the grids should still be there. Perhaps I inadvertently disconnected something in the circuit that generates the negative bias voltage. More checking lies ahead.

Joe

 

[audmod01]

Egg on my face

Well, my last post was only partially correct. The minus bias voltage for the grids of the output tubes is there, but only while the unit is powered up! If the unit is turned off it changes from negative to positive due to bleed back of the B+ circuitry which holds a charge for a long time. Perhaps I should consider adding a high K ohm resistor to the B+ to discharge the B+ capacitors after the unit is turned off.

In any case it turned out the reason the plus and minus 11.6VDC feeds were overheating the 10 ohm resistors was because I had reversed the plus and minus leads going to the Op Amp board! All I had to do to clear the apparent short was to swap the leads to the correct polarity at the power supply. The Op Amp is probably history, since it had reverse polarity power applied to it, however I do have a spare that I can install since I ordered two of them. The first TL082 I installed does not cause a short condition currently, but I do not trust it, so it will be changed.

Odd how things do work right if you hook them up correctly! I have no excuse for what I did, but perhaps it will teach me to double check polarity of connections more closely in the future.

Joe

 

[audmod01]

Output Tube Measurements

After I managed to repair the LV power supply and replaced the Op Amp, the rest of the unit settled down and I had it running idle with just the preamp and line amp tubes installed. After lunch I decided to put the output tubes in and check them out.

I installed the Electro-Harmonix tubes which were supposedly a quad matched set. Here is how they checked in circuit:

#1) cathode resistor measures 10.14 ohms, cathode voltage climbs to over 2V if left on.

#2) cathode resistor measures 10.08 ohms, cathode voltage reaches 0.06V if left on.

#3) cathode resistor measures 10.10 ohms, cathode voltage reaches 0.17V if left on.

#4) cathode resistor measures 10.06 ohms, cathode voltage reaches 0.32V if left on - normal.

Above voltages were measured with -31.5VDC applied to grids. The tubes maintain the same current values as they are moved from socket to socket. Only one tube out of the set of four seems to adjust properly. There is not enough difference between the cathode resistors to account for the differences seen. Looks like I will be sending these back. I really hate this, but I cannot tolerate such conditions in the amplifier. I even tried resoldering the grid and cathode pins of the tubes and there was no change. Tube sockets were checked for loose connections and were found to be OK.

Joe

 

[audmod01]

Bottom Chassis Cover Cleaning

I spent some time yesterday at the Vintage Antique Radio & Phonograph Society in Irving, TX to visit with members and also to pick up some tube shields for the 12AX7s and some more Fisher knobs. I put the last of the Fisher knob caps with dashes on the Mode and Volume knobs, so now the knobs look correct to the best I can do them.

I also cleaned and sanded the chassis bottom cover to prepare it for painting. I will be using some of the left over Krylon Satin Nickel paint which matches the OEM chassis plating. This will help the appearance of the unit when it is out of the cabinet.

I have the original metal cabinet that came with the amplifier. It is not much to look at, but does provide pretty decent protection against physical blows etc. I might just put some wood veneer on the outside of the cabinet and add some picture frame pieces to the front, stain them and finish as an oiled walnut finish.

Joe

 

[dcgillespie]

Joe -- there is no way the tubes should have given you those numbers with a control grid bias of -31.5 volts. Normal current draw (about 35 ma) is typically established with about -18 to -22 volts or so depending on where the tubes fall within the tolerances for the manufacturer you are using, and the manufacturer itself. You specified that you are using EH tubes, so typically the bias voltage will run around -18 to -20 vdc to produce a target cathode voltage, such as shown by tube #4. With -31.5 volts however, all should produce results like that of tube #2!

Either that is one messed up quad of tubes, or the bias voltage is all over the map for the four different tubes. Try removing the tubes and see what voltage you are getting at pin #6 of each output tube. If the EFB grid bias regulator is operating correctly, it should be much closer to the values indicated above.

Dave

 

[audmod01]

Negative Bias Recheck

Dave;

I can get a swing from -30VDC to -45VDC at the grids with the tubes removed. The tubes have been returned to the company I ordered them from. It seems to point to a problem with the two transistors that are part of the feedback circuit. I will take some measurements there. It is possible that one or both transistors have been damaged by the problem that was going on in the LV power supply.

I did double check the variable control value and the 10K to ground from one end of the control, but they are within tolerance. The Q101 could be shorted. I will start looking there. Hope I can have this last issue solved before the replacement output tubes arrive.

Thanks for your input!

Joe

 

[audmod01]

Found the problem!

It turned into another case of egg on the face! The Q102 had an open emitter, so I changed it. Then I still had not less than -30VDC at the grids of the output tube sockets. I discovered that I had tied the high end of the bias control to the full negative voltage coming off of C128 instead of the emitter of Q101. Once I corrected that I now have a swing of -16VDC to -24VDC at the grids. This is with all the output tubes removed and on their way to the vendor.

I think now it has a good chance of working once I get the new output tubes.

Thanks again Dave for spotting the potential problem areas!

Joe

 

[dcgillespie]

Glad you found it! When I read your first post, I was going to suggest that an effectively open Q102 would produce the same symptom -- but you found it first, which is great!!

Sounds like you're well on your way now!

Dave

 

[audmod01]

Dave;

Yes, I believe now it has a very good chance of working correctly when I get the new output tubes.

In the meantime I can hook up my audio generator and do some simple troubleshooting to verify signal through all the circuits ahead of the outputs.

I am really looking forward to a really nice Fisher for listening to good music! None of this would be possible without your expertise and your engineering improvements. Thanks once again.

Joe