Fisher 600-T Restorations

[fred soop]

Pushbutton switches open but not cleaned


1405

Pushbutton switch assembly disassembled


1406

Pushbutton switches complete


1407

Chassis top. The final cleaning was done with silver polish.


1415

Chassis bottom


1414

Design goals:

Power supply will be modified similar to 43453K except some of the voltages will no longer be needed. Additional +/- 15 volts for op amps will be needed and should not share the regulators with the phono stage.

Power amplifier will be full complimentary. With 8 positions available for power transistors, several configurations are possible. A 4 channel amplifier could be built, or active crossovers with biamped channels, or 2 channels with a full bridge, but the power supply wouldn't support the increased power capability.

It will probably be a conventional design with parallel output transistors which will reduce beta droop and crossover distortion. Current sources will be used for the IPS and VAS and a current mirror for IPS loading. The power supply runs +/- 38 volts at no load and will hold +/- 30 volts at 5 amps continuous. That should be good for an honest 50 watts into 4 or 8 ohm loads.

Reverb jacks will be changed to preamp out, power amp in.

Control preamp will be redesigned around LM4562 op amps. Tone controls will be reduced to a more useful range along with the high and low filters. Filters will be switched completely out of the circuit when off by using a separate stage with an active filter, then selecting either the filtered or non filtered output. Loudness will be removed from the volume control and will be fixed response, similar to the Harman Kardon Citation 21. This will allow for using external resistors to try and better match volume control tracking between channels and reduce the sensitivity to small adjustments at low volume. Tape Out will be buffered. If this ends up being too much circuitry, multiple control preamp circuit boards can be stacked as was done with the phono preamp and its filter board.

Phono preamp will be replaced with the same design as 43453K. Tape head input will be high level, probably marked CD on the rear panel.

Tuner will be mostly unchanged. Still undecided on whether to do a redesign of the multiplex. I have an LM1870 decoder (out of production). The Heathkit AR-15 rebuild uses the LM1800.

What to do to replace the extra controls behind the hinged logo? The originals were to adjust brightness of the meter light and set muting level on the IF (redundant with on board adjustment).

 

[Fisherdude]

Fred, that's some amazing work. Really. :thmbsp:

 

[Brian]

Yipes. I can not believe how deeply you have dug into the 600Ts. You made me happy I decided to pass mine on along with my TX300 and TFM300. I could never have come close to doing what you did. I love these units and hopefully you will maybe go into more detail, offer consulting for others to follow in bring their back or, even monetizing your experience. I wish you had started the project before getting rid of my units. They worked but the transformers were running very hot and each had minor hum.

I was interested interest discussion regarding the 700-T power up issue and bass. I am having exactly the same 2 issues with my Sherwood SEL200. My speakers are pretty well placed and sound good with a variety of other amps. There is a thermostat (thermistor) on the secondary side of the transformer I am thinking may have shorted and the cause of the problem as there is neither a relay or other time delay system. I keep a set of old phones plugged in and this seems to have greatly reduced the issue but not eliminated it completely if I do not turn the speakers off before turning on. I can keep the phones and speakers online at the same time. When these receivers were new, I know there was no turn on thump. The unit has a lamp dimmer for all lamps so, it can be powered up, yet go dark. When not using FM, the tuner is not powered so, I can even keep it on similarly to what many do with their preamps and power amps. The offset after stabilizing is around 15mv per channel. The 700T discussion may have pointed me in a direction to figure out the issues I have.

At some point I do need to attack the issue.

 

[Kasanay]

My first system that could be considered "high fidelity" consisted of a Knight Kit KG-870 integrated amplifier, a Knight KN-990 record changer with Empire 880 cartridge, and home built speaker systems consisting of a 12 inch Allied full range speaker with whizzer cone (can't find a photo), eventually upgraded with Electro Voice HF-1 step up kits consisting of a 3500 Hz crossover and TW-35 horn tweeter. The MF-1 midrange kit was never added. A Knight Kit KG-765 AM-FM tuner was eventually added.

Fred,
Any chance you have the manuals/ schematics for the KG-765 or the KG-870?

Thanks,
Joe Y

 

[fred soop]

Fred,
Any chance you have the manuals/ schematics for the KG-765 or the KG-870?

Thanks,
Joe Y

No. Those units were sold by 1967 or so.

 

[tngan408]

No. Those units were sold by 1967 or so.

Are you still locking for the service manuals ?
I have full sets of manuals. I also had restored few units of the 600-T with difference serial number. Most of them have all small 4.7 uF electrolystic caps replaced.

 

[OABUKOVAC]

After restoring my 600T recently, the voltages for the amp ( with thermal diodes, 1N2326) section gave different readings than are in the schematics, when I was listening to 8 ohm speakers

The thermal germanium diodes 1N2326 gave Vf readings on my Fluke 115 diode tester section between 0.126 to 0.131 Since my experience with silicon diodes have always given me a consistent forward bias readings, I was considering putting a 1K trimpot, with the wiper connected to the anode, one of the sides of the trimpot connected to the cathode, and the other end connected to where the anode was connected I am thinking that this would make the forward bias more accurate per output transistor

I am also wondering if the 10 ohm resister in the circuit, should I return the 10 ohm trimpots?

Also, how would one adjust the bias, and what voltages should I be looking for?

Any thoughts would be immensely appreciated

 

[fred soop]

Post #31 of this thread shows schematics of my restoration and rebuild. Page 3 shows the power amplifier. That will show a completely revised bias setup along with the proper adjustments. This removes the diodes, which were not thermally coupled to the output transistors anyway, and replaces the troublesome bias adjustment rheostats with a small network that traps the bias adjustment between 2 safe values even if the pot should go open (unlikely with Bourns trimpots). Another post in this thread shows pictures of the small bias adjustment boards.

 

[OABUKOVAC]

Post #31 of this thread shows schematics of my restoration and rebuild. Page 3 shows the power amplifier. That will show a completely revised bias setup along with the proper adjustments. This removes the diodes, which were not thermally coupled to the output transistors anyway, and replaces the troublesome bias adjustment rheostats with a small network that traps the bias adjustment between 2 safe values even if the pot should go open (unlikely with Bourns trimpots). Another post in this thread shows pictures of the small bias adjustment boards.

After studying the many write ups on this amplifier design, that was used in a few Fisher products, I intend to use an AC125 transistor with a 1K trimpot instead, to replace the 1N2326 diode, as documented on another site In the late sixties and early seventies, I understand that there was a service bulletin to replace the 10 ohm trimpot, and replace the circuit with the temperature compensating diode In fact, someone stated in another thread, that Fisher used a couple of trimpots in addition each diode

As the designer of this trimpot/transistor circuit explained to me, Joe Sousa (from RMorg), the transistor will also have a , "safety" diode in the circuit, if the emitter side goes out

 

[fred soop]

There needs to be a "safety" diode or at least a fixed resistance in series with the anode side of the pot. As shown, if the pot were adjusted with the wiper all the way to the anode side, the AC125 would be at cutoff and the output stage would go to a very high bias and possibly blow up.

 

[OABUKOVAC]

There needs to be a "safety" diode or at least a fixed resistance in series with the anode side of the pot. As shown, if the pot were adjusted with the wiper all the way to the anode side, the AC125 would be at cutoff and the output stage would go to a very high bias and possibly blow up.

Per circuit designer, Joe Sousa,

"The schematic is only a suggestion derived from data sheet values and has not been verified with simulation or measurement. During all experimentation you should have the speaker disconnected until the output DC voltage has been adjusted to zero volts. Mounting the AC125 on the heat sink with the power transistors takes the most advantage of thermal stabilization.

The AC125 are cheap and common in auction web sites. I mention this particular type only because the AC125 data sheet on Radiomuseum.org is extensive enough to specify the forward Vbe drop. It is very likely that a comparable low power audio germanium transistor will have a low enough forward Vbe drop to work. The advantage of the Ge transistor is that the total voltage drop can be adjusted precisely with the trim pot circuit to get the amp perfectly biased.

The forward drops are specified in the data sheets as:

1N2326 135mV at 2mA
AC125 105mV at 2mA

In theory, the 25mV lower forward drop of the AC125 wired as a diode with base tied directly to the collector as the cathode and emitter as the anode, will run the output Germanium transistors at a little less than half the desired current, unless the Vbe multipler trim pot is added to increase the forward drop slightly as shown in the diagram.

The 1N2326 data sheet explains the advantages of using the 1N2326 to bias ouput power transistors as improving thermal stability and immunity from supply variations, as outlined in the earlier post.

Excessive germanium diode leakage is not a serious issue with the diode in forward conduction. It may however indicate that the diode is rusting out, which may eventually lead to a short. In this particular circuit, if one diode shorts out, it will turn off it's associated power transistor and the 30mA that is available from the other half of the circuit will appear at the output. This should not kill the speaker, but the sound will be very distorted. The max current that a speaker can handle is simply the Sqrt(power rating/impedance). For a speaker rated at 10Wrms max and 8 Ohms, the max speaker current rating is 1.1Amp. A slow acting 0.8A fuse would be recommended in series with the speaker to guard against catastrophic shorting failure of the power transistors. The built-in fuses in the amplifier limit the output current to 2A."

 

[rickl]

@fred soop, what a great thread. I realize this is almost 4 years old but I'm working on a buddy's 600-T (serial #26346F) off and on for the past few months. I did replace the selenium rectifiers, safety cap, PS caps, ... but need to re-read this again for more tips.

for some reason, this is my 5th Fisher this month. And I have 2 more 80az's in the queue (i did a couple last year).

Thanks for posting this great resto thread.

 

[fred soop]

Never realized that the serial numbers got that high. I don't recall seeing anything above 55000 or so.

 

[rickl]

Looks like I transposed a number

 

[cferry1]

Never realized that the serial numbers got that high. I don't recall seeing anything above 55000 or so.

Hi Fred / others -
I own one slightly above your peak serial number of 55000.
I have also recently completed repairs / caps of one " 55073L " for a close audiophile.

 

[malden]

This is a great thread. I have a 600-T that was professionally restored several years ago, it's one of several 1960's era amps and receivers in my collection that gets rotated in and out of my system. I like this receiver so much I bought a couple of parts units in case of emergency.

The receiver sounds great but I'm having an issue with FM, there is an audible hum/buzz when there is dead air or quiet passages of music. This only occurs when listening to FM (Automatic, Stereo, Mono) and will increase if the volume is turned up.

Any ideas what would cause this?

 

[fred soop]

There was a service bulletin published shortly after these units were released. It was to move the SCA coil on the multiplex board to a separate terminal strip further away from the power transformer and run wires back to the mpx board. If this has not been done, then it could cause hum.

Since any hum in the IF ahead of the detector is not going to be audible, then the problem must be on or related to the multiplex board or wiring connecting it.

If the unit is a low serial number (10000 series) the multiplex is completely different and actually had a Nuvistor and relay.

 

[malden]

Thanks for the reply. Mine is serial no. 24xxxE.

I'll take a look over the weekend and take a photo or two and post it here.

 

[4really]

Great stuff~