Rough TX-300

[calman46]

I've had this TX-300 in my attic for 10 years now . It appears to have gotten wet . Anyway I checked the output transistors when I first got it ,they diode tested good. I stopped there because the on/off switch was frozen and because it seem such a improbable fix.I notice the other day that the switch had freed up so .... I disconnected the power transformer except for the primary (fuse is good). Did some ohm meter testing OK there. With 40w in the DBT I'm getting 33VAC on the lower secondary (SR5 bridge), 45VAC on the on the upper secondary with 22VAC from the center tap. Ohm test output transformers next .What ohm value should I expect to see on these ? If they test good could I dip these in wax ? the paper is flaking This entire TX-300 would have to be striped down to the frame, rust is everywhere . My old post of this may still be up , not sure .

 

[Catmanboo]

Sad to say, but that one's probably worth more as a parts donor than the time, effort & $$$ to resurrect, especially if the pots, bias rheostats & switches got as exposed to the elements as the rest shows in the pics. See my recent thread on my same unit regarding the bias controls, info from a knowledgeable ak-er Fred Soop. Early attempt at solid state by fisher, before they got the hang of it, not one of their more notable models. Germanium transistors throughout, for which substitution is about nonexistent.
Outputs easily poofed if bias is lost while adjusting due to the crappy mini-rheostats, which may well be as corroded on yours as is everything else is. But-if you choose to proceed, may it all work out!

 

[fred soop]

The 45 VAC (22.5 x 2) seems a bit low. Should be closer to 25 x 2, but it isn't a deal breaker.

Output transformers??? You mean driver transformers? The ratio is 1:1:1 (unless they changed transformers on later units). I don't know the resistance, but all windings on both transformers should be the same. The 600-T used the same transformers on the early versions but moved to a step up transformer of approximately 1:3 with serial number 39000 and above. The eft and right channel transformers are identical but have different part numbers due to different lead lengths. Make note of all lead connections before disconnecting. A reversal of one winding would put the upper and lower drive in phase and blow the outputs.

Before powering up, you can remove the output transistors, replace them with 100 ohm resistors between collector and emitter pins, then test for proper voltage. This will help ensure that the voltages are correct without risking blowing up the outputs. Also, if the power amplifier is disconnected from the power supply and driven from a variable dual supply, it will run on very low voltage (around ±5 V) which allows for making measurements with little risk of damage.

Most likely, ALL electrolytic capacitors should be replaced. In the 600-T, CDE 10,000 µF, 100 V main filter capacitors will fit the holes and clamps. All of those 4 µF caps in the preamp boards can be replaced with 10 µF or higher.

 

[calman46]

The both driver transformers are 4 ohms on primary side and each secondary winding is 2 ohms . There are slight differences but to small to matter I think . Can I just use masking tape on the driver transformer ? the outer layer of paper is gone . I have removed the power trans. and its end caps . It looks good inside there are no signs of over heating or moisture and just some surface rust on the inside of the bell caps . I re-check all the output transistors with a Eagle International transistor checker ,the meter needle didn't pin on any of them. The diode test did show some leakage . This unit has all 35144 with a green mark. I think checking all the pots and switches is next .
This is a crap shoot for sure but I'm going with it as far I can , hopefully to hear it play. With all the moisture damage the face plate is in good shape with all the lettering .

 

[fred soop]

Not masking tape, use electrical tape (available in various colors, brown would closely match the original paper). You can cut the pieces a bit long for each side and attempt to tuck the ends in the gap on the sides.

Were the resistance measurements made with the transformers disconnected (at least one lead of each winding)? The primary has a large capacitor in series but the secondaries have additional resistors that could cause a lower reading. Since both transformers are the same, they are probably ok.

 

[calman46]

I went with a combo of contact cement ,scotch tape and masking tape. I cleaned the transformers the best I could but were still far from ideal for using any kind of tape ,hence the contact cement. Yes resistance made with side lifted. The TX-300 has a AEG B60 C500 selenium rectifier I can't find any values for it other than the -38v on the schematic. My plan eventually is to get the power supply working without any load with maybe the exception of the indicator lamps. Do think it would be safe to run it that way ?

 

[calman46]

 

[larryderouin]

A silicon diode brige rectumfrier of 400v 4A will be plenty enough with a good amount of over head. Voltages are set by the transformer winding anyway, so it's ok with that one.

 

[fred soop]

.....
My plan eventually is to get the power supply working without any load with maybe the exception of the indicator lamps. Do think it would be safe to run it that way ?

No problem running it with no load. Good plan to make sure the power supplies are good before connecting anything else. If the power supply is not correct, nothing else can be fixed properly.

Check the main supply voltages. The main filter capacitors are rated at 40 V. With todays slightly higher mains voltages, the supply could be close to the limit. The 600-T is around ±39 V with no load, way too close to the limit for a 40 V cap.

 

[larryderouin]

Give yourself some headroom and get 63V caps. I've done that with most all of my FISHER Solid State units and it works out well.

 

[calman46]

So I won't need a dropping resistor to the bridge ?

A silicon diode brige rectumfrier of 400v 4A will be plenty enough with a good amount of over head. Voltages are set by the transformer winding anyway, so it's ok with that one.

63V on caps got it. Thanks guys

 

[larryderouin]

Probably. Won't know until you install and test. Probably in the neighborhood of 100 ohm 2 or 3 w.

 

[fred soop]

The best fit for the 600-T capacitors was 10,000 µF 100 V. Overkill, but the diameter was right for the existing holes and clamps. Length was shorter but that just leaves extra space. The TX-300 uses different mounting but the clamps are probably for the same diameter. Those could be drilled out and replaced or cable ties used instead.

 

[calman46]

The TX -300 has two 3,000uf 40v caps on the bridge rectifier and one 1,000uf @ 45v on the selenium rectifier . I was thinking 3300uf or 4700uf (63v or 100v) I haven't look in options on the 1,000uf yet . Transformers all set .I think I will test the power trans. one more time. Back plate cleaning next then on to the bias circuit. I will be studying Fred's mod.What do you guys think about adding Freds "safety" resistors to the original trimpots? Bubble gum on a leaking pipe.

 

[larryderouin]

Fred's mod is the best chance you have of running the TX-300 safely and reliably. IF you look at the bias network now it's like an OPEN KNIFE SWITCH with absolutely no protection on the output transistors, where FRED's Mod is like a REGULATED POWER SUPPLY in that if something goes wrong, the Output transistors are effectively bias to a minimum level to protect them. I wouldn't say bubble gum on a leaking pipe. More like a modern re-design with overload protection. Granted it's not a relay system, which could be incorporated, but it's the best that can be done without a major reconstruction of the unit. And it will save the transistors if it lets goes. Germanium transistors are extremely pricey, so it's best to try to keep them as long as possible. Plan on over $125. for a quad of 35144's, IF you can find them.

 

[EngineerNate]

A silicon diode brige rectumfrier of 400v 4A will be plenty enough with a good amount of over head. Voltages are set by the transformer winding anyway, so it's ok with that one.

+1 for rectumfrier. Makes me giggle every time.

I commend the OP for trying to ressurect this one. Sometimes the roughest pieces are the most satisfying when completed.

 

[larryderouin]

I learned that one from an old Navy Reactor Operator. HE ran S3W and A2W Reactors on Subs and Enterprise in '60's.

 

[EngineerNate]

I picked it up from AvE on YouTube.

Working as an engineer, my favorite coworkers are often the salty old technicians, many of whom are ex-military. Their lexicon can be equal parts vulgar and hilarious at times.

 

[calman46]

I was referring to re-using the original trimpots with leaking pike reference . Meaning its not a good idea to reuse the old pots.

 

[larryderouin]

AHHHH! Yeah definately put some NEW Bournes Sealed pots in.