Fisher 690-T Chassis Questions

[malloyt]

Hello, I have read many of the threads relating to this tuner chassis, but have concerns regarding the one I own (Model 690-T, Serial 33680D) which is installed in a 1964 Ambassador console. I have used this console for many years but now need to update out of spec components. My questions relate to the mismatch between the schematics for the 690-T listed in the 1964 Ambassador A-69 Service Manual, the Sam's Photofact 977, and the tuner itself. First, the MPX unit is the #65 (3 12AX7 Tubes), which matches that listed in Sam's schematic but not that in the A-69 Service Manual. However the Sam's schematic does not match the rest of the tuner. My 690-T more closely matches the Model 680-T tuner schematic serials 10001 to 19999 inclusive, listed in the Ambassador V Service Manual A-68. However my 690-T includes a number of modifications which because of the consistent use of ROE(derstein) branded capacitors appear to be original. Past posts stated that Fisher mismatched chassis depending on available inventory, etc. My question is whether an accurate schematic is actually available or is it best to simply replace electrolytics and out of spec resistors as they are in the tuner or revert to the closely matching 680-T schematic? I have included an image of a portion of the 680-T schematic with a variation from my tuner shown in red (the 680-T is the only chassis I found which uses a pair of 820K resistors (R82-83) near the MPX as in my 690-T).

This is only one of several variations which I have found. If more information is required, let me know, Thanks. Terry.

 

[gadget73]

In my experience when faced with a piece of equipment that doesn't match the documentation, I scribble on the documentation to make it match whats in front of me. Parts replacements are driven by the gear, not the paper. It worked when it left the factory with those values, and it will work again if they stay there.

I don't have an answer about this unit specifically, but its not all that uncommon to find gear that had running production changes and now, 50 years later, those changes are no longer documented.

 

[larryderouin]

I concur with Gadget. Use the schematic that is closest to your unit. BTW What is the serial # of your 690-T itself. It will have a different serial than the console. Same with the Amplifier. IIRC the console serial should match one of them.

According to the A69wa manual the MPX is the 125 (1 12ax7and 1 12at7 with the Relay next to the tubes.) Look at the A68 manual and look at BOTH tuner schematics (10001-19999 and 20001 to 29999) They are different due to how they switch the relay The early one has the old style switching system and the later one has the matrix board like the 800c. If yours matches the early one and the .05uf caps were installed, they could have been installed to cut down on switching noise. Line changes almost never got into the manuals, and if a critical item, was put out as a service Bulletin, or on non critical stuff as periodic updates to service personnel.

Go thru the manual, and compare your tuner with the manual, and markup all changes. Sam's is ok when you don't have a manual, but they do make mistakes too. I tend to believe the factory manual more and note changes on my gear as they crop up when occasional stuff doesn't match up. I'm betting you have an early 680-T instead of a later 690-T. Actually the 65 mpx is probably the best one of the 3 tube mpx. The 125 is not 1/2 as good. I should change my 59-T (65 Futura) to a 65 mpx rather than the 125.

Take a couple of pictures of the tuner (top and underneath) and post them. One shot of the info section is helpful.
Larry

 

[malloyt]

Hello and thank you for your assistance. I have attached the images you requested as well as an image of the console's identification sheet. I assumed there were many undocumented changes during production, though I would have thought the components would closely match the schematic, since the tuner is labeled as a 690-T.

 

[gadget73]

Ah, the hybrid amp.

 

[larryderouin]

You've got the Matrix switcher board, like that found in the 800C. This would be the 20001-and above schematic. Use that one as it most closely identifies with your tuner. Having said that, when you find a component that doesn't fit the schematic, make a note or draw in the component on the schematic. Also if a component value in the unit does not match it's schematic identity, GO WITH THE ACTUAL PART! Line changes after the manual went out generally did not make it back to the engineering dept (drafting), but instead were included in Service Bulltetins or manual addendums for Service Centers.

The other thing going on at this time was that FISHER was incorporating the MPX internally in their units, to alleviate the need for an external unit, plus each had to be designed to work with the MPX, so many changes in the tuners had to be made, not all of them made the manuals. And FISHER was changing over to Solid State during the period of 1964 to 1966/67. Most of the line became hybrid units (the engineers didn't have CAD, computers to do all the esoteric math, etc. All they had was pen and paper, and sliderules (slipsticks), gray matter between the ears, and endless cups of coffee and packs of cigarettes to keep them up and working on problems). So there was a lot of change going on, Most good, some not so good. Compared to a lot of Co's. at the time, FISHER came in to Solid state slowly, and tested each step endlessly to produce the best they could. Hence the Hybrid amps. Transistor technology was still in it's infancy (they still used Germanium transistors which had poor heat resistance and if bias'ed slightly off could go chernobyl on you). As the technology matured a change to silicon NPN transistors was made, and when Silicon PNP transistors matured to the point where they would hang together reliably, that change was made also. But it took time. FISHER didn't want a lot of returns or shop queens because the tech wasn't matured enough. So they took it on in steps. This is one of the reasons for FISHER having many different types of amplifiers in such a short time between 1964 and 1967. By 1967 the all in one chassis (full transistorized) was used in consoles. By 1968, silicon powered receivers were reliable enough that they offered consoles with the main unit being a receiver suitably changed to allow Auto off of the Phonograph, with a molex plug in the back for the phono power, and a different larger heatsink to compensate for the lack of a cabinet. By 1970 the consoles were ALL powered by variants of receivers that had grown in power, up to 65W RMS, (the 500TX in the 1969-70 President IIRC.).

Getting back to yours, I'd use the 20001 and up schematic on the A69 manual and make changes on the schematic as needed to fit your particular tuner. But as for part replacement use the parts in the UNIT as the blueprint on what to get. If it agree's with the parts list, ok. If it differs, change it on the parts list.

That's the best I can offer without having the unit and the scats in front of me.

Larry

 

[gadget73]

Sort of interesting how companies approached solid state. Fisher had their tube tuners with a hybrid amp that had germanium outputs, Sherwood went straight from tube amps to silicon transistor amps and tuners with FET front ends. The mid-late 60s was an interesting period in the evolution of technology, lots of things were making the shift from well known and understood tube circuitry to the new and mysterious transistor.

 

[fred soop]

Wasn't Fisher ahead of their major competition (Scott, HK, Sherwood) with introducing transistor components? That would explain the tube front ends and germanium transistors.

 

[gadget73]

Probably so. I think Sherwood's first solid state integrated was 1967 or 68.

Its just interesting how some jumped right on and sort of developed it as they went where others waited a bit longer and started off with a more mature solid state product. Probably says something about the management structure more than anything, some companies are willing to take risks with newer tech, others are more conservative and prefer to prove it out some more first.

 

[larryderouin]

Look at Magnavox. In '62 they were all tube, but BANG! 1963 comes in and they have a New Transistorized line called ASTRO-SONIC (undoubtedly based on a Space Age hook). Every one of their amps was germanium all the way up to about 1968. And there are a lot of them still running on the original Tr's, because even tho they had magnavox numbers on them, you could walk down to the local ET shop and get some based on the Maggie #. Even ten years ago I could cross ref magnavox #'s directly to The RCA Numbers and get RCA Old Stock Transistors. Probably not now, except on E-prey and most of them are probably fakes. FISHER took a slower tact, and broke in to it slowly, and developed the technology to their needs.