Fisher TX300

[fred soop]

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.

The bias networks and drivers put out close to 10 watts even at idle. That heat needs to escape or the build up will eventually kill the output stage. Been there, seen that.

 

[msh1999]

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.

The bias pots are the the circle things with screws in the middle near the back of the amp?

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.

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thanks


I think i will start try reccaping the amp, I got couple of questions of course:
1. I want to replace couple of 4uf capacitors, but there is no 4uf capacitors i can buy, I can put two 2.2uf capacitors instead?
2. When in the amp there is for example 100uf 25v capacitor i can replace it with 100uf 100v or more, i can do it? what does the V stand for?
3. There are parts that look in perfect shape that will be needing replacment? or just need to change the ones who look in bad shape?

 

[msh1999]

Here is pictures with all the capacitors that i could see that they are damaged.
The one in yellow sircil im not sure, i see white going out of there but cant tell for sure if I should be worried about it.

btw what about the output transistor in the end? i still need to replace them?

 

[fred soop]

Yes, the bias pots are the metal circles with the Philips head adjustments.

The 4 µF caps can be replaced with 4.7 µF or even 10 µF. I don't think it was well known in this era that electrolytics in the signal path should be large enough that they do nothing to shape frequency response. Any frequency limits should be done with film (preferably polypropylene) capacitors. Space will not be a problem as todays parts are much smaller.

V stands for volts. 40 volt rating will be sufficient in the preamp. 63 V is not overkill. If you are talking about C22, C23, C25 and C26, the 25V rating was probably ok back in the day (per the schematic) but with higher line (mains) voltage, I'd go with 40 V. My 600-T 36 volt supplies run at 39 volts without a load. If this unit turns out to be a keeper, then you should eventually replace ALL of the electrolytics, including the main filters. Those are rated at 40 V and if the supply is up to 39 V, there is no margin. I used 10,000 µF, 100 V caps in the 600-T rebuilds. The main reason for this choice was the physical size allowed for use of the same clamps.

When you say "output transistor in the end", are you referring to the 8 output transistors or the 3 next to the preamp boards?

When removing caps (or any parts) from the circuit boards, be aware that they sometimes squeezed the end of the leads flat so the parts wouldn't fall out. These can be difficult to remove. If that is a problem, then cut the leads close to the board on the top (component side) and let the wire ends come out the bottom. A GOOD solder sucker is a must if you are doing more than a couple of parts.

 

[msh1999]

Yes, the bias pots are the metal circles with the Philips head adjustments.

The 4 µF caps can be replaced with 4.7 µF or even 10 µF. I don't think it was well known in this era that electrolytics in the signal path should be large enough that they do nothing to shape frequency response. Any frequency limits should be done with film (preferably polypropylene) capacitors. Space will not be a problem as todays parts are much smaller.

V stands for volts. 40 volt rating will be sufficient in the preamp. 63 V is not overkill. If you are talking about C22, C23, C25 and C26, the 25V rating was probably ok back in the day (per the schematic) but with higher line (mains) voltage, I'd go with 40 V. My 600-T 36 volt supplies run at 39 volts without a load. If this unit turns out to be a keeper, then you should eventually replace ALL of the electrolytics, including the main filters. Those are rated at 40 V and if the supply is up to 39 V, there is no margin. I used 10,000 µF, 100 V caps in the 600-T rebuilds. The main reason for this choice was the physical size allowed for use of the same clamps.

When you say "output transistor in the end", are you referring to the 8 output transistors or the 3 next to the preamp boards?

When removing caps (or any parts) from the circuit boards, be aware that they sometimes squeezed the end of the leads flat so the parts wouldn't fall out. These can be difficult to remove. If that is a problem, then cut the leads close to the board on the top (component side) and let the wire ends come out the bottom. A GOOD solder sucker is a must if you are doing more than a couple of parts.

so 15v above the original volt rate is all I can go above?

i ment to the 8, but asking about the other 3 as well...

Im not sure I will change everything, i got plans for higher quality amp anyway (not sure if they will be vintage)

there is a chance u can give me a list of all the parts u ordered for the 600T? if u can mark some as "must" it will be even better.

 

[fred soop]

so 15v above the original volt rate is all I can go above?

i ment to the 8, but asking about the other 3 as well...

Im not sure I will change everything, i got plans for higher quality amp anyway (not sure if they will be vintage)

there is a chance u can give me a list of all the parts u ordered for the 600T? if u can mark some as "must" it will be even better.

You can go higher in voltage but shouldn't go too high. Dave recently posted good information with details as to what happens if electrolytic capacitors are rated too much higher than the applied voltage.

As for the other 3 transistors, they are not as likely to be bad, but should be checked anyway. Again, you can pull all the output transistors and connect headphones to the driver transformer secondaries to check that everything is good up to that point.

My restored 600-T had ALL electrolytic capacitors replaced along with a few modifications. Those capacitors were 48 years old and all are inexpensive other than the large main filters. If you search this forum with the serial number 43453K, you should find the details of the rebuild. I don't actually have a parts list of what was replaced.

 

[fred soop]

A question arrived via PM but it may be of general interest, so I'll post it here. The question is: Can the TX-300 (and probably other similar amplifiers) be used as MonoBlocks.

The short answer is: "yes" and "no".

Yes .....

First, any stereo amplifier can be used as a MonoBlock. It requires operating the 2 channels out of phase. My first attempt at this, over 50 years ago, was to simply reverse the connections of one channel of a stereo cartridge (channels now out of phase at the source) and play a mono record. A better method would be to insert a simple op amp and a few resistors, arranged for unity gain and phase inversion. This could be done internally or with an external unit via either the Tape Monitor loop or the Reverb connections. It could even be powered with a pair of 9 volt batteries to keep it simple.

No .....

While you should be able to theoretically get 4 times the power of a single channel, the power supply will probably not be able to supply the necessary current unless the amplifier was designed for MonoBlock operation. Additionally, with an 8 ohm load, each channel will "see" 4 ohms. In the case of the TX-300 (and 600-T), the 4 ohm setting will add series resistance in both channels, placing additional limitations on the available power. The end result is that I would not recommend doing this as the added power would not be that much and the added stress on these amplifiers should be avoided.

Alternate (BiAmp connection).....

Build a 4th order active crossover and connect it using the Tape Monitor or Reverb connections to separately feed high and low frequencies to the 2 channels. Do the same with the second amplifier. Next, disconnect the woofer of the speaker systems from the crossover network and drive it directly from the low frequency channel. Since the lower frequencies require more power, splitting the channels such that each amplifier is using one channel only for lower frequencies will ease the demand on the power supplies and result in less heat buildup.

More details at:

http://sound.westhost.com/bi-amp.htm

I have done this with a pair of Harman Kardon Citation 24 power amplifiers (100 watts per channel) and AR-3A speaker systems, but, quite frankly, I can't really hear any improvement. My environment rarely requires high volume and that could be where the difference starts to become audible.

 

[msh1999]

You can go higher in voltage but shouldn't go too high. Dave recently posted good information with details as to what happens if electrolytic capacitors are rated too much higher than the applied voltage.

As for the other 3 transistors, they are not as likely to be bad, but should be checked anyway. Again, you can pull all the output transistors and connect headphones to the driver transformer secondaries to check that everything is good up to that point.

My restored 600-T had ALL electrolytic capacitors replaced along with a few modifications. Those capacitors were 48 years old and all are inexpensive other than the large main filters. If you search this forum with the serial number 43453K, you should find the details of the rebuild. I don't actually have a parts list of what was replaced.

thanks!

as I said that i have cheked the amp with headphones without the output transistor and with them and i could hear that the preamp was working...

about the output transistors:
all I need is change 2 resistors in the right places and replace the 4 original output transistors with MJ4502\MJ15016\]MJ21195?

main filters:
as long as they look good there shouldnt be any problem right?

 

[fred soop]

thanks!

as I said that i have cheked the amp with headphones without the output transistor and with them and i could hear that the preamp was working...

about the output transistors:
all I need is change 2 resistors in the right places and replace the 4 original output transistors with MJ4502\MJ15016\]MJ21195?

main filters:
as long as they look good there shouldnt be any problem right?

The change in transistors is an experiment. It can be done but I have not actually done it. Adding a diode at the bottom of each of the bias pots and tuning for around 30 mV across the emitter resistors should get it close to correct.

The main filters can be used until you determine that you want to keep the unit. If you are going to sell it, no point in spending the money, but they should be replaced for the long term due to age and voltage rating too close to actual voltage.

 

[omri617]

You can go higher in voltage but shouldn't go too high. Dave recently posted good information with details as to what happens if electrolytic capacitors are rated too much higher than the applied voltage.

As for the other 3 transistors, they are not as likely to be bad, but should be checked anyway. Again, you can pull all the output transistors and connect headphones to the driver transformer secondaries to check that everything is good up to that point.

My restored 600-T had ALL electrolytic capacitors replaced along with a few modifications. Those capacitors were 48 years old and all are inexpensive other than the large main filters. If you search this forum with the serial number 43453K, you should find the details of the rebuild. I don't actually have a parts list of what was replaced.

Would love to have a link to the post that Dave wrote.
Thanks
By the way , me and the op talked on the phone and he decided to sell it as is . So I offered to purchase it and soon I will get Into fixing it hopefully .

 

[daj-kh]

آمپش چقدر خوبه صدا خوب است اما طراحی به طرز ناامیدکننده ای منسوخ شده است. من با لری موافقم که احتمالا ترانزیستورهای خروجی بد هستند. اگر فیوزها سوخته باشند، به احتمال زیاد ترانزیستورها علت آن بوده اند. آنها شبیه به 2N2148 هستند، اما آنها نیز غیر ابتانیوم هستند. تغییر بایاس برای ترانزیستورهای سیلیکونی مدرن ممکن است آنقدرها هم پیچیده نباشد، اما دیگر لازم نیست 4 ترانزیستور را پشت سر هم برای کنترل ولتاژ قرار دهید.

قبل از اعمال برق، تمام ترانزیستورها را می کشم و آنها را با اهم متر چک می کنم. اگر به نحوی هنوز خوب هستند، باید واحد را با مقاومت‌هایی بین کلکتور و امیتر به جای ترانزیستورها (که در دفترچه راهنمای سرویس نشان داده شده است) آزمایش کنید تا زمانی که همه ولتاژها خوب ظاهر شوند. اگر منبع تغذیه دوگانه دارید، به لطف ترانسفورماتورهای درایور، در واقع می توانید تقویت کننده برق را از بقیه واحد جدا کنید. تقویت کننده قدرت در واقع با ولتاژ 5± ولت کار می کند.

من در حال حاضر یک طراحی تقویت‌کننده برق به‌روز را بر اساس اطلاعات اخیر منتشر شده توسط داگلاس سلف در کتاب طراحی تقویت‌کننده قدرت آزمایش می‌کنم تا در دومین 600-T بازسازی‌شده استفاده شود، اما مدتی طول خواهد کشید (سرگرمی‌های بسیار زیاد، زمان بسیار کم ). منبع تغذیه 600-T (و TX-300) خوب است.

سلام آقا فرد من خداداد هستم آمپلی فایر فیشر TX-300 دارم. همه ترانزیستورهای قدرت تنها جایگزینی بودند که من پیدا کردم 2Sb449 بود و من می خواهم Bias را تحت پیگرد قانونی قرار دهم.