800-C Restoration

1rebmem;

Yes, this one is an old Fuji camera and has no where near the resolution of many more modern ones today. I need to also look for a camera that will take better pictures of very small things if possible. My oldest son has a digital camera with a different lens he can install on the camera to take very detailed pictures of small things. He took the pictures of the ultrasonic remote control microphone in Matt's RK-20 remote control receiver shown in my thread on the Fisher MF-300 tuner. I may not need something quite that good, but it would be nice to have a better camera that could take focused pictures when held closer to the object of interest.

I installed the 12AX7s I had on hand and discovered I was one short! Three of the ones I installed are Telefunken units supplied by a friend. I put two in the driver/phase inverter stage and one in a magnetic preamp spot. Unfortunately I do not have another Telefunken for the other channel. I may just order some more 12AX7s and put ordinary units in the preamp stages and keep one Telefunken for eventual use in the driver/phase inverter position in case one fails in the future.

Joe
 
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1rebmem;

Yes, that is me in the title block as the drawing creator. I worked with Dave extensively to get the document right (11 revisions total ! ). That is my one other Fisher receiver, which I still have.

I spent time today installing the 1% 10 ohm 1/4 watt resistors in the cathode circuits of the output tubes. While doing that I noticed a small tubular electrolytic capacitor that was grounded at the tube socket ground lug at V13. It had leaked some of its electrolyte out of its case and was a dried crusty white color. I was not sure just what capacitor it was and saw it was a 2uF@70VDC. It turned out to be C65. In addition there is a 1.2K 7W wirewound resistor and a 1.2K 1W carbon resistor nearby. Those two are R128 and R123 which are B+ dropping resistors associated with the power supply. They just happen not to be located right at the can electrolytic capacitor sections they are connected to.
Components at V10 V11 V13 V14 web.jpg
I thought this picture might be helpful to others who may be working on an 800-C chassis.

The next step will be to install some 100 ohm 1/4W resistors at each screen grid for protection of the transformers. The 10 ohm cathode resistors do the same thing plus they also provide a way of measuring the cathode current of each output tube. With an IBAM circuit to adjust the negative bias to each tube the cathode current in each output tube can be adjusted to be very close to its companion tube in the push-pull output stage.

Joe
 
Yes, that is me in the title block as the drawing creator. I worked with Dave extensively to get the document right (11 revisions total ! ).

Joe...... you did a fantastic job on that 500C schematic!
I referenced it many times for the work on my 500B, so thanks!
Jef
 
Same here, Joe. I downloaded that schematic long ago and I intend to use it to get finally the Unity Gain mod installed. I've tried 3 times and hope the schematic will do the trick. Fingers crossed. Thorne
 
1rebmem and thornev;

I am glad to see that the information is being helpful to others. I worked in application engineering at Collins Radio and part of my job was creating schematics and parts lists for complete systems. I enjoy using my talents to be of help to others. The equipment may well outlast many of us, so for posterity, we all do what we can for future generations.

Joe
 
One question Joe. On the referred to schematic (see attached image), for the Unity Gain mod attached to the VOLUME control, why are the 10uF 35V cap and 47K resistor just hanging out, attached to nothing? In other words why are those 2 components not BETWEEN other components? Thorne
 

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They're both grounded. Remember on the heater circuit when you put the new resistor (27ohm 1W?) inline from the Negative P.S. and the cap off to ground. It helps stabilize the voltage spikes from the power supply to the heaters or something like that. I would think this does a similar job. i've been wrong before............
 
Ah. So those 2 components are simply there to run off to ground any excess current that would damage the op-amps?

UPDATE: Also, on the Unity Gain mod standalone schematic (Dave's 400 receiver, see schematic in post 31), after the 100 ohm coming off pin 7 of the op-amp, is shown a 220K resistor. But on the large overall schematic (for the 500-C), that 220K is not anywhere to be found. Why is that? On the large schematic the preamp out comes after the 100 ohm

UPDATE: I guess I answered my own question - the 220K is on the Fisher 400 and not the Fisher 500? I bet that has been my problem all along. I was trying to install the Unity Gain mod on my 500-C using the Fisher 400 schematic. They are different just enough to confuse me. So now the question is...

Does the Unity Gain mod for a 500-C have 220K resistors after the 100 ohm resistors? That would maybe explain why I had audio problems.
 
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Larry - Regarding the 10uF and 47K in post 28 and your response in 29... on the Fisher 400 schematic Dave drew (see attached image), it shows that after the 47K (next to the 10uF 35V cap) that is grounded on one side, the other side is supposed to go "to the other channel". When I built the board, I assumed "other channel" meant it connects to the 1M resistor that connects to pin 5 of the op-amp. Was I mistaken? Or is this another case of I shouldn't have used the Fisher 400 schematic?
 

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The parts you are discussing help establish the DC reference for the inputs of the two OP amps When OP amps are used in a single ended power situation their inputs have to be biased to about 1/2 of the supply voltage. In many applications OP amps are in applications where they are connected to balanced (equal) plus and minus supplies. In such a case the leads shown here connected to ground would be connected to a positive power source. You would have to study some OP amp application information to see examples of both cases.

Joe
 
Thorn -- On the Buffer schematic, the "To Other Channel" designation does in fact refer to connecting to the 1Meg resistor in the other channel that connects to pin 5 of the IC. I drew the schematic this way because only one channel of the buffer is shown, yet the biasing rail developed by the two 47K resistors and 10 uF cap serve both channels -- ergo, the To Other Channel designation. As Joe explained, for an op amp to operate linearly in this application, its output voltage need to be able to swing both positively and negatively relative to ground. Normally, that then requires a positive and negative power supply (relative to ground) to power the op amp. But in the Fisher receivers, there is only (conveniently) a negative low voltage source available. Therefore, to satisfy the power requirements for the op-amps, they are powered by only the negative source, but then a voltage divider (made up of the two resistors and cap mentioned) is used to bias the normal ground reference of the op amps to a point midway between ground, and the value of the negative supply. That reference voltage is the -7.5 volt designation you see on the schematic, being on half of the -15 volt level applied to the op amps at pin 4. Finally, because the input and output of the op amps both operate at a DC value below ground level, both input and output coupling caps are required so that the input bias voltage, nor the resulting DC output voltage is shorted to ground, and the DC levels are isolated from the input and output audio signals.

As for the missing 220K resistor at the output of the buffer circuit on Joe's schematic, that resistor should in fact appear in ALL installations of the buffer (and on any schematics the buffer circuit is applied to as well). Without it, there is no ground reference for the other side of the .22 uF output coupling cap from the buffer IC. When the new Preamp Output and Power Amp Input Jacks are strapped together as they were when they operated as Reverb connecting jacks, there is no problem with this resistor missing from the circuit, as the input 1Meg resistor in the revised power amp design will perform this function in this case -- assuming you have also done the power amplifier driver stage modifications as shown in the modified 400 power amplifier schematic! If you did not do these modifications and simply installed the buffer circuit in your 500C -- leaving the driver circuit of the stock Fisher design in place, then NO ground return exists for the input AF Amplifier stage of the power amplifier section. The result will be very significant distortion!

For those who DID implement the power amplifier driver stage modifications, the 220K is still required -- becoming important when the jack straps are removed to allow the receiver to operate as a separate tuner/preamp and power amplifier (or both) for use with other pieces of equipment. In this case, without the 220K resistor installed, there will be a significant "pop" in the speakers when a different power amp is connected to the preamp output jacks, created from the charging of the .22 uF output coupling cap when the external power amp is connected (assuming it is powered on). But also, even if the 220K was installed, if the power amplifier driver stage modifications were not performed, then the input AF Amplifier stage will loose its reference to ground when the jack straps are removed, again, potentially causing distortion in the power amplifier section (if a return is not provided in the preceding equipment), and harm to the 12AX7 driver tube as well.

I hope this helps!

Dave

Edited to reflect the comments in my following post.
 
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OK -- Referencing my previous post: In researching the above information further, I find that the schematic I was originally referring to when checking this out and showing the 220K resistor in place is NOT Joe's schematic of the modified 500C, but a schematic drawn by a different AKer who kindly drew up the modified Fisher 400 receiver. On that schematic, the 220K resistor does in fact appear as it should. On the modified 500C schematic, the 220K resistor does not appear like it should, leading to the problems noted in my earlier post if the modified power amplifier circuit is not also implemented. Joe, if you can make the change on your schematic when you get a chance, it would be greatly appreciated. I have edited my last post to reflect this new information!

Thorn -- If you did not implement the power amplifier changes, then the distortion you were hearing was almost certainly due to the missing 220K resistors if not installed -- a fact exposed entirely by the particular way you decided to install the buffer circuit into your unit -- if this is what you did. If this is the case, I'm sorry for all the problems you had, but your symptoms, noting of the missing resistors, and decision to install the circuit the way you did all worked to bring the schematic error to light, which is a good thing! Again, if this is the scenario of your installation, then adding the resistors will no doubt resolve the issue.

Dave
 
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Dave;

I will indeed make the change! Sorry for overlooking that necessary part in each channel output of the Op Amp and input to the driver circuit! At the output of each channel of the OP Amp there is first a .22uF@50VDC coupling capacitor followed by a 100 ohm series resistor. There should be a 220K ohm resistor from the end of the 100 ohm resistor that is connected to the jack labeled "Preamp Out". The drawing will go to Rev 13.

Joe
 
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thornev;

If you will check your Private Messages, I have completed the necessary changes to bring the drawing to Rev. 13. I can send you a copy, but need your e-mail address as the PM function of this website does not permit me to add the file which is 2.3MB.

No need to post your e-mail address here, send me a PM using the "Start a Conversation" function by clicking on my member ID.

Joe
 
Dave - Thanks so much for the detailed explanation. I have to admit it is over my head. What I got out of it is that I cut out the 220K resistors from the Unity Gain mod. I did not use the preamp out jack. I left it untouched. I'm not sure what the power amplifier circuit is and as such I have to assume I did not install it. But from reading "the resistor should in fact appear in ALL installations of the buffer", I conclude that cutting out the 220 K's was a mistake. Thorne
 
thornev;

The Power Amplifiers are J12 and J16 and everything following them to the speaker outputs for each channel. In other words the V12 & V13 plus V8, V9, V10 & V11 circuitry is considered the Power Amplifier in this drawing. If you examine other tube type power amplifiers you will see that it typically includes a driver stage followed by a phase inverter and then two push-pull output tubes per channel. You may see some tube power amplifier designs with push-pull parallel designs using 4 output tubes per channel, though those are not as common as typical push-pull output stages of most tube type power amplifiers.

Joe
 
I just sent a copy of the revised drawing to the AK Database monitor for posting. I noticed that the document they had was Rev. 11 and I had a Rev. 12 in my files, so with changes that were made in Rev. 12 plus the 220K ohm resistors being added the proper Rev. for the drawing becomes Rev. 13. I do not know how long it will take to get the new file posted. The *.pdf version of the file is 8.23MB while the *.jpg version is 2.31MB.

Now I need to get my 500-C out and check to see if I left off the 220K ohm resistors when I modified it.

Joe
 
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