800-C Restoration

[audmod01]

Yesterday the LED Fuse Lamps arrived. I removed the front panel and installed them. I tested it first and found that one side was brightly lit and the other side was darker. Tension against the contact in the holder was the problem. I re-tensioned the clip to provide more pressure against the new lamp ends. That corrected the poor connection. I added some new closed cell weatherstrip to block the light from showing through where it shouldn't. Here is a picture of the results:

This picture was taken in daylight but with overhead lights turned out. I have my favorite FM station tuned in. The type of LED fuse lamp was a warm-white per the advertisement. I had to replace the #1847 Stereo Beacon lamp as it unexpectedly burned out. Fortunately I had some on hand.

Joe

 

[larryderouin]

Looks Real Good Joe.

 

[notdigital]

Looks Real Good Joe.

My sentiments as well

 

[bhamham]

I agree! Beautiful 800-C Joe.

 

[dcgillespie]

So how many does this make that you've pulled out of the grave? Looks sharp as can be!!

Dave

 

[audmod01]

With this 800-C, that brings it to 11. I may have home destinations for several. My granddaughter is still enjoying her Electra system. I plan to get a wooden case for the 800-C, but first I want to add EFB and the TL82 preamp out circuit. I think I will leave the tone control circuitry as is and the same for the speaker wiring.

Here's the list: X-101-C, 500-C, 202-R, MF-300, RK-20, 90-R, 90-T, Electra IV/VI Combo, 300 FM MPX generator. Some work still remains on the 90-T and FM generator. I need to pick a power amp to use with the 90-T. This is a disease!

Joe

 

[dcgillespie]

You have a full blown case!

Dave

 

[audmod01]

I forgot the X-202-B and my 500-TX (which sees daily use). The X-202-B is awaiting attention soon. So the total comes to 11.

Joe

 

[audmod01]

Two days ago I took time to inventory my parts to see if I had everything I needed to add EFB to the 800-C. I had all the solid state parts, but not all the resistors. I also lacked some of the electrloytics needed to fully implement that modification and one remaining OEM electrolytic that has not been already changed. So I placed orders for the parts needed and am waiting. I found some of the resistors I needed on the auction site at decent prices and free shipping, so that helps keep the cost down.

Joe

 

[thornev]

Cool, Joe. You're on your way and I for one am interested in following your progress as I'm about to venture into EFB on my KX-200. Thorne

 

[audmod01]

This morning I was working on a tube type Zenith table top radio for a friend and listening to the 800-C. All of a sudden the 800-C popped in one channel and that channel went dead. I looked over and one 12AX7 was glowing purple. I knew that meant air inside. I shut it off and found another 12AX7 and inserted it, turned the set on again and soon had good sound in both channels. I looked at the base of the 12AX7 I had removed with a jeweler's loupe and spotted a crack in the glass around one pin reacing up about 1/4" in the side of the glass. It just shows that you never know when a tube may fail.

The 470 ohm 2W resistors I ordered came in yesterday along with some more 2SA1013 transistors (15 of them for more projects ahead). I still am waiting on a set of new 39K ohm 1/2W resistors before I begin building up the EFB board. Hopefully they will arrive next week.

Joe

 

[audmod01]

The order of 39K ohm 1/2W resistors arrived yesterday along with another order of 100 1N4007 rectifiers, so I can begin soon on the EFB circuitry. I will take a look at my 500-C to review how I mounted the circuit board and see if I want to change the method and place of mounting and go from there.

Joe

 

[audmod01]

I got sidetracked working on a Zenith R511V AC-DC All American Five radio for a friend of my youngest son. I had some new tubes for it that I knew I would never use and so installed them. I had to order a new 50C5 output tube (only one that tested really bad), replaced a broken wire to/from the AM loop antenna and then discovered that the electrolytics in its power supply were gone, so another order for those parts. I got around to replacing them this morning only to discover after turning it on that the speaker is bad - dragging voice coil. It does not make sense to have such a cheap 4" speaker re-coned. I found a replacement on eBay and ordered it - $14.95 with shipping. I was pretty aggravated that the speaker was bad after all the other work.

Such is life! I won't get back to the 800-C until tomorrow. At least that unit is fully operational as it is now. I am cutting out a portion of the prototype board to assemble the EFB circuitry on. Once I get some depth to the perimeter sawed into the board material I can snap the board and get a nice size to work with. I just do not want to deal with it today after the Zenith disaster area.

Joe

 

[larryderouin]

Sometimes, Kids just spoil ALL the FUN! Grandkids are different tho. You can send them home!!!!!!

 

[audmod01]

Larry;

Yes, grandparents can spoil them and hand them back - payback! I have also heard that grandchildren and grandparents have a common set of enemies - the parents!

The tuning capacitor assembly of the Zenith had lost all three rubber mounting grommets - so another trip to a hardware store for replacements. Now the unit tunes without rocking the entire tuning capacitor and it now is much easier to tune to a station - another problem solved.

Joe

 

[audmod01]

This morning I finished cutting out the Proto-Board. The size is 3 3/8" X 1.5". That is just the size I decided upon. It could be made a bit larger, but I did not believe it was necessary. I used two 4-40 screws to mount two 4-40 X 3/4" standoffs under the chassis. I found it necessary to remove the bare filament wire that comes from the terminal strip with the Yellow filament wire from the power transformer. This bare wire would have been in the way of the Proto-Board mounting. I used a brown filament wire I had saved from a donor chassis and after soldering one end to the ceramic capacitor feedthru on the Front End, the rest of the brown wire was dressed against the chassis sheet metal and back up to the lug on the terminal strip where the Yellow wire is connected.


With that done I drilled two holes in the Proto-Board far enough apart to mount the board on the two 3/4" tall standoffs under the chassis. The height of the standoffs is enough to keep the board from touching the top of the wires and lugs of the terminal strip where the power transformer filament leads are connected. I will avoid placing any components on the Proto-Board near where the filament leads connect to the terminal strip. That should keep from causing any shorts there.

I took pictures with two 100uF@100VDC electrolytic capacitors positioned on the Proto-Board to show the relative size of the capacitors. Even with the board already 3/4" above the chassis, there is still room above them for the bottom cover of the chassis to clear the electrolytic capacitors.

I will begin mounting parts on the board this afternoon if all goes well.

Joe

 

[audmod01]

Today I am populating the EFB board that sits next to the AM/FM front end. I have all the parts mounted on the board except for the FET that actually does the regulating of the output tube screen grid. I could not remember how I handled this on my 500-C and did not want to pull it out of the storage box it is located in. I looked through all the pictures I took of the 500-C and finally discovered that I mounted the EFB FET transistor on the rib that runs from back to front of the chassis with the rear input jack cables secured along one side. It is above C91 and heat-sinked to that metal rib.

Dave Gillespie mentioned in the past that the FET screen regulator actually dissipates very little heat and runs at a very conservative internal current, so the heat generated is not really of very great consequence. I think I will attach the FET transistor in this unit at the same physical location in much the same way. This will keep interconnecting wires from the Proto-Board short and minimize any tendency for the wiring to pick up stray RF signals. Most people would probably not have to worry about stray RF signals in this circuitry, but in my case I have a 10KW FM station only 4.5 miles from my house. Any time I take a bottom cover off of a chassis there is RF from that station impinging upon every wire and component in a given chassis. Once I reinstall a bottom cover on a chassis I do not have to worry much about the RF signals, but mainly just while I am working on one with bottom cover removed.

It is time to stop for lunch and rest for a while. Then I will recheck my wiring of the Proto-Board and make sure I have all the connections correct.

Joe

 

[audmod01]

I managed to get the Proto-Board just about ready to mount. I mounted the FET transistor to the metal ridge using 4-40 hardware. The 4-40 hardware also secures the clamp holding the audio cables on the other side of the ridge. I began readying the parts to attach to the FET transistor and discovered that I did not have the .47uF@630VDC bypass capacitor, so I had to order some. It will be a few days before I get them, so another delay. I thought I had some of these capacitors, but I did not.

Joe

 

[audmod01]

I had taken a break from the 800-C to work on a Zenith AM radio for a teacher friend of my youngest son. I finally finished that yesterday.

Today I completed installation of the STF10NK50Z FET transistor that does the regulation of the output tube screen grids in the EFB circuitry. I also disconnected R128 (a 1.2K 5W resistor) and R123 (a 1.2K 1W resistor) from the output tube screen grid bus that I had installed previously. I replaced R123 with a 2.7K 2W resistor and secured their junction to a nearby terminal lug that had been emptied of previous components in other work. This should help downstream circuitry to avoid excessive B+ from the prevalent AC input voltage over 120VAC here at our house. I took some pictures of the FET installation so that you can see where I placed it. I had given a written description, but there's nothing like having pictures.


The rectangular large blue cap is a .47uF@630VDC and the nearby electrolytic with the white cable tie is the 22uF@450VDC. I regret that I could not get better pictures of the actual FET which is heat sinked to the rib or partition that runs from front to back. The components somewhat hide it in the first view. In the second view you really cannot see the FET at all, only the end of the screw with nut that is securing it to the rib/partition metal. The spiral shield over the audio cables attached to the rib/partition obscures it from view. The 22uF and .47uF caps are grounded at the terminal strip ground lug next to C91. It was the closest convenient ground point. The original orange wire from C87 to the R128 R123 junction was removed and a new longer orange wire was run from C87 to the new junction point of those two resistors close to the output tubes. That orange wire no longer will provide the screen grid voltage for the output tubes. Instead the EFB FET will provide the screen grid voltage.

The EFB circuitry that is on the Perf-Board near the FM Front End is pretty much complete except for adding some wires to interconnect it with the FET and the IBAM board. I should get that done tomorrow and take some more pictures.

Joe

 

[audmod01]

As promised, I made final wiring connections to/from the EFB Perf-Board and took some pictures:

To help avoid making mistakes as I adjust the bias for each output tube, I added the V numbers at each tube socket and also at the individual bias controls at the IBAM board.

I added a label to identify Q103, the EFB Screen Grid FET transistor.

Here you can see the EFB Perf-Board installed. I identified the Q numbers. I used a yellow wire to identify the output tube screen grid wiring, so the yellow wire goes to Q103 at the middle metal rib of the chassis. There is a white wire going to the IBAM controls connected from underneath Q101. I used a brown and an orange wire to connect to the same color of wires at the bridge rectifier to make connection to the AC for the rectifiers in the power supply for the EFB circuitry. It is not necessary to use the same colors, but I believe it helps to avoid confusion during construction and troubleshooting.

I removed each output tube one at a time and put V10, V11, V13 and V14 labels on the phenolic base of each tube so that I can get each one back to the same position they were originally set up in.

Dave Gillespie;
A couple of questions - was the 1M ohm resistor at the base of Q102 (2SA1013BU) a 1/2 watt or a 1 watt? (I used a 1/2 watt).
What is the best current range to set the new Tung-Sol 7591 output tube to for the best results?

I am going to take a rest break and will do some resistance checks and recheck wiring before applying power tomorrow. The output tubes will be left out until I verify that bias voltage is getting to the IBAM controls and all the way to the output tube signal grid sockets. Negative bias will be set back to deliver the most negative voltage to each output tube grid and gradually backed off until each tube reads the range of negative voltage that Dave recommends.

If I remember correctly, the new Tung-Sol 7591 tubes are capable of the full 35 watts per channel.

Joe