Fisher TX300

[msh1999]

Aksed my cousin to pick up for me this Fisher amp and I will come take it next week, of course asked for pictures, so this is what he sent:

Anyway its a a FIsher amp named TX300, i have absolutely no idea what i should expect from it, and the guy who sold it to me said it wasnt woking last time he tried, might be only fuze, or it might be needing some serious work. either way i will make it work again.

please if you know anything about this amp let me know!

btw he gave me another thing with it, is anyone here familiar with this device?

 

[larryderouin]

More than likely the output's and drivers are SHOT! And they are Germanium.(MUY EXPENSIVE!!!) It can be converted to Silicon output's but the bias needs to be totally revamped. Not for a beginner. Even experienced techs won't touch it sometimes. Basically it's the Amplifier section of the 600-T receiver. And a good amp when it was running. But the output's are very sensitive to shorting speaker wires.

Fred Soop would know all about it, as he's done a couple 600-t's as a hobby. It's from FISHER's 1st Genration of Solid State stuff, when they were using Germanium transistors because Silicon transistors couldn't handle the power needed to be developed.

 

[msh1999]

More than likely the output's and drivers are SHOT! And they are Germanium.(MUY EXPENSIVE!!!) It can be converted to Silicon output's but the bias needs to be totally revamped. Not for a beginner. Even experienced techs won't touch it sometimes. Basically it's the Amplifier section of the 600-T receiver. And a good amp when it was running. But the output's are very sensitive to shorting speaker wires.

Fred Soop would know all about it, as he's done a couple 600-t's as a hobby. It's from FISHER's 1st Genration of Solid State stuff, when they were using Germanium transistors because Silicon transistors couldn't handle the power needed to be developed.

not good, i had a feeling this was too risky, how good is this amp? i really want to know if its worth even trying

 

[fred soop]

How good is the amp? The sound is good but the design is hopelessly obsolete. I agree with Larry that the output transistors are probably bad. If the fuses are blown, the transistors were most likely the cause. They are similar to 2N2148, but those are also unobtanium. Changing the bias for modern silicon transistors may not be all that complicated, but it would no longer be necessary to stack 4 transistors in series to handle the voltage.

Before applying power, I'd pull all the transistors and check them with an ohmmeter. If they are somehow still good, then the unit should be tested with resistors between collector and emitter in place of the transistors (shown in the service manual) until all voltages appear good. If you have a dual power supply, you can actually electrically separate the power amplifier from the rest of the unit, thanks to the driver transformers. The power amplifier will actually work with as little as ±5 volts.

I am presently testing an up to date power amplifier design based on recent info published by Douglas Self in the Power Amplifier Design book to go in the second restored 600-T, but, it will be a while (too many hobbies, too little time). The power supply in the 600-T (and the TX-300) is good.

 

[msh1999]

How good is the amp? The sound is good but the design is hopelessly obsolete. I agree with Larry that the output transistors are probably bad. If the fuses are blown, the transistors were most likely the cause. They are similar to 2N2148, but those are also unobtanium. Changing the bias for modern silicon transistors may not be all that complicated, but it would no longer be necessary to stack 4 transistors in series to handle the voltage.

Before applying power, I'd pull all the transistors and check them with an ohmmeter. If they are somehow still good, then the unit should be tested with resistors between collector and emitter in place of the transistors (shown in the service manual) until all voltages appear good. If you have a dual power supply, you can actually electrically separate the power amplifier from the rest of the unit, thanks to the driver transformers. The power amplifier will actually work with as little as ±5 volts.

I am presently testing an up to date power amplifier design based on recent info published by Douglas Self in the Power Amplifier Design book to go in the second restored 600-T, but, it will be a while (too many hobbies, too little time). The power supply in the 600-T (and the TX-300) is good.

hi again

I have a chance to put my hands on another not working TX300 for pretty cheap price, i was thinking that i can combine the 2N2148 from the two amps...
the question if useally all the 2N2148 are damaged or there is a good chance that only 50% of the 2N2148 from each amp is gamaged?

 

[fred soop]

I would not bet money either way on how many transistors are bad. But, if you are combining transistors from 2 units, the beta needs to be somewhat matched. The color dot on the flange indicates the beta group. If your transistors are from different beta groups, first choice is to keep the same beta in one channel. If that is not possible, then match the 1st and 3rd, or 2nd and 4th transistors. For example, Q1 and Q3 should match and Q2 and Q4 should match.

 

[msh1999]

i guess i wil not buy the other tx300, they look very bad (or that it doesnt say anything how they look from the outside?)

first pic is the one i got and second is the other tx300

 

[larryderouin]

Your's already has one channel worth of output's replaced with 2n301's dated 52nd week of 1978 (as close as I can tell on the transistors themselves.) The other channel looks original.

Like Fred said. The percentages of 1/2 of them working at all is very low. These are germanium transistors and had a very poor track record when it came to fault resistance. You could look at them crosseyed from 50ft, and they'd give up the ghost. Personally I would part out the one you have, and let it live on as a parts donor.

 

[msh1999]

Your's already has one channel worth of output's replaced with 2n301's dated 52nd week of 1978 (as close as I can tell on the transistors themselves.) The other channel looks original.

Like Fred said. The percentages of 1/2 of them working at all is very low. These are germanium transistors and had a very poor track record when it came to fault resistance. You could look at them crosseyed from 50ft, and they'd give up the ghost. Personally I would part out the one you have, and let it live on as a parts donor.

sell the one I bought? :\
How much its even worth like this?

 

[larryderouin]

If this goes further into pricing and such it's required to take it to the Dollars and Sense Forum.

 

[fred soop]

Before turning this into a parts unit, how is the cosmetic condition? If it's reasonably clean, then some sort of rebuild should be attempted. If it's a beater, then, yes, parts unit.

Also, if you remove the output transistors, which you will need to do to check them anyway, you can connect headphones to the secondary of the driver transformer. This will allow you to determine whether or not the rest of the unit is working. You do not need the rail fuses for this test.

If the problem is only the output transistors, and if you are good with a soldering iron and following schematics, this might be a good test case for replacing germanium with silicon. This would only require adding a few jumpers, and inserting a few diodes.

My proposal would be to jumper the collector and emitter pins of Q8 and Q6 to bypass them completely. Modern devices with higher voltage ratings would not require 4 transistors in series to handle the voltage. Next, break the lower connection of R58/R86 and R64/R88 and insert a diode in series. This should change the bias enough for silicon.

A further refinement would be to replace R68 and R70 with 0.33 ohm resistors.

Transistors, if PNP, could be MJ4502 (MJ802 compliment), MJ15016 or similar, or MJ21195. If you wanted to use NPN transistors, the power supply leads would need to be reversed, along with the added diodes. Do not use 2N3055 unless you want to test at a low volume. At full power, the devices would be exposed to voltage above their rating.

 

[vendo81]

For a couple years I used a TX300 that I recapped as my music source in the garage. Worked great and sounded very tube like. I was lucky and the output transistors were good as it was working fine before the recap. It was in a president console and had a factory fan cooling it at all times. I've heard that helps.

 

[msh1999]

I dont really have any experience with those kind of things or good soldering skills, though if the repair wont cost too much I will try to brind it back to life, not sure that i will be even able to sell it in my country anyway (and selling it on ebay wont be worth it...)

For a couple years I used a TX300 that I recapped as my music source in the garage. Worked great and sounded very tube like. I was lucky and the output transistors were good as it was working fine before the recap. It was in a president console and had a factory fan cooling it at all times. I've heard that helps.

thanks, that what i wanted, good vintage amp that sounds like tubes that wont cost much.

I will pick up the TX300 today, and will open it to see what is the condition inside...

 

[larryderouin]

msh1999; add a general location to your profile, please(country is fine). It helps everyone out with time differences, when asking and answering questions, and who know's there may be an AK'er near you (less than 500miles/800 KM).

 

[msh1999]

I will add soon.
im from Israel

still not at home but had a chance to see the inside for a second, things doesnt look good, very very dirty though didnt look like there is rust, but im still not 100% sure because i did it very fast...

 

[larryderouin]

AK'er OMRI617 is in Israel. He may be just around the corner. PM him and see if he can possibly help (2nd set of eye's and hands.

 

[msh1999]

AK'er OMRI617 is in Israel. He may be just around the corner. PM him and see if he can possibly help (2nd set of eye's and hands.

thanks. i will pm him

 

[msh1999]

so the tx300 turn up with the output transistors on, though it only make very slight noise (using phone output)
when connected to the Tape output it sounds much much clearer, but only when i turn on the filters.

I guess i need to start by cleaning everything...
omri617 still didnt answer me so he will soon so I can decide if to really try fixing it or not... (hopefully he can help me)

 

[larryderouin]

There is some serious capacitor replacement needing to be done. Don't turn it on again until the caps on the board have been replaced along with the Main filters.

For Example the caps circled in Yellow have opened up and dried out. NO GOOD! and there will probably be others that are just as bad or worse on this unit. Go ahead and clean up the pot's and switches, then recap it. Sounds like it's trying but it's fighting a loosing battle with those bad caps.

 

[fred soop]

Don't touch the 4 bias pots. Or, if you do want to clean them, measure the resistance, mark the setting, and make sure they are at the same resistance as before when reinstalling (not open wiper) before reapplying power.