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Toshiba 2290 testing info.

R

RichardHartman

Active Member
Feb 7, 2019
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I am currently working on an amp that does not have any power output. It is running a 2290 into 4 2879. When I started to work on it, was producing 200-300 watts. It should be around 800 I think. I got inside and found a few small issues, replaced a bad solder joint on the hi/lo power switch, replaced the electrolytic caps and 2 of the mica caps that were bad. Closed it up tried again and no difference still 200-300w, while I was testing it I probably only spoke about 2 full sentences and then the output power dropped to nothing. In it again and removed the 2290 and all 4 2879. Hooked up the component tester and they all tested ok-no short. My question is since they still show ok on the tester is it possible they wont produce any output? Is there a way to test it while its in circuit? I don't have any experience with amplifier pills and with how expensive they are I don't want to just buy new ones and throw them in there. Any advice / direction would be appreciated.

73

KC8DVE
 

Not enough information to provide you with help. If the transistors are good and you need help with the circuit, we need to know what circuit it is and to see some detailed pictures of the inside, in order to spot potential problems. Do we even know if the relay keys or if the amplifier is drawing any power from the 12 volt supply?
 
How did you check the transistors after it laid down? Did you measure the hfe?

Sounds very much as if this amplifier is built in what they like to call the "competition" mode.

I call that design method "DFX", for DaveFatXforce.

They use the same design features, saturating the transistors at the highest possible power-supply voltage. Square law explains it. Increasing the power supply by half-again from 13.5 to 20 Volts will double the peak power the transistor can show.

More or less.

The math is not complex. Power is voltage times current. Increase the supply voltage by half and the circuit current should also increase to half-again more. Half-again times half-again is roughly two. Or a little more

I hear a subtext that this amplifier ran for some time with Toshiba transistors before blowing them out. The recent replacements are no doubt chinese parts that will match the conservative 100-Watt nominal peak power level at the rated power supply voltage.

Those same transistors most often do NOT last long in the 20-Volt hot-rod environment of a DFX amplifier.

Not long, like maybe a few keys of the mike, max.

What's needed for this kind of amplifier now that you can't get the super-tough Toshiba parts is a rating method.

I vote for a performance-rating number we'll call the "KBK".

As in "Keys Between Kabooms". How many times can you key the mike between sets of transistors.

Kinda like the unlimited alcohol/nitro drag-racing motors. Needs a total rebuild at the end of every 1/4-mile run. Valves are warped, electrodes burned off the spark plugs, etc.

Turning down the power supply to the rated 13 or so Volts is the only way to make the modern-day (post-modern?) transistors last very long at all.

73
 
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Shockwave said:
Not enough information to provide you with help. If the transistors are good and you need help with the circuit, we need to know what circuit it is and to see some detailed pictures of the inside, in order to spot potential problems. Do we even know if the relay keys or if the amplifier is drawing any power from the 12 volt supply?
Click to expand...
 
Thanks for your reply.. I am working on a LA900 I attempted to reach out to Paul on the forum but with no response. My apologies I thought I gave enough info to ask how to check the transistor. I was using 3w from the radio into the amp. I was using a car battery with #6 power wire only 12.8v. I did not think of checking for an amp draw. What I do know is that the relay does kick in and there is very little movement on the meter but some rf does get out like 1-2w. The tx light did work until the amp quit producing power now it does not light up with the relay. I also do know that in bypass everything is ok and the swr don't change a bit. The preamp works fine. I have removed every component and replaced anything that was bad or the value was to far off. I checked the transistor with the component tester. I am new at this and trying to learn. How I was using the tester was to check to see if the transistor was shorted. It did give Hfe reading but I did not pay attention to that. Is the Hfe reading what tells me if the transistor still has amplifier properties? I guess I need to hook it back up and check for an amp draw and then remove the transistors and get the Hfe readings again. I will also take some pics and add them on here. So what should the unit be actually producing if it was working correctly?
 
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RichardHartman said:
My apologies I thought I gave enough info to ask how to check the transistor. I was using 3w from the radio into the amp.
Click to expand...

You can test the transistors with only the base (bottom) lead lifted from the board. From the base to the collector and to either emitter tab, should test like a pair a diodes. Froward conduction in one direction with a voltage drop of about .6 and open with the leads reversed. YouTube has several videos that can help if confusion exists. You don't need a transistor checker to perform this test either. Any analog ohm meter or a digital one with a diode test scale, will work here.
 
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RichardHartman said:
Thanks for your reply.. I am working on a LA900 I attempted to reach out to Paul on the forum but with no response. My apologies I thought I gave enough info to ask how to check the transistor. I was using 3w from the radio into the amp. I was using a car battery with #6 power wire only 12.8v. I did not think of checking for an amp draw. What I do know is that the relay does kick in and there is very little movement on the meter but some rf does get out like 1-2w. The tx light did work until the amp quit producing power now it does not light up with the relay. I also do know that in bypass everything is ok and the swr don't change a bit. The preamp works fine. I have removed every component and replaced anything that was bad or the value was to far off. I checked the transistor with the component tester. I am new at this and trying to learn. How I was using the tester was to check to see if the transistor was shorted. It did give Hfe reading but I did not pay attention to that. Is the Hfe reading what tells me if the transistor still has amplifier properties? I guess I need to hook it back up and check for an amp draw and then remove the transistors and get the Hfe readings again. I will also take some pics and add them on here. So what should the unit be actually producing if it was working correctly?
Click to expand...

Years ago people would claim some pretty strong numbers from that LA900. If I had one running at 14.5 volts and I didn't drive the crap out of it I would be very happy with 500 watts. Are the 2879's still Toshiba's? If so I hope they are all still good or you will end up buying four new HG2879's. To buy a matched quad of the Toshiba 2879 today might run 600 bucks if you could even find them.
 
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Thanks everyone for trying to help me out here. I know how to check what I call a regular transistor but for whatever reason I was thinking that the 2290 / 2879 had some kind of "superpowers" that had to be tested differently (my ignorance). I did not know I could just lift the base off the board I have always removed them and then tested. That is some great information. So now onto the rest of my ignorance or lack of knowledge there of. I am looking at the data sheet If the transistors are rated at 60Wpep at 12.5v which rating/symbol tells me the input drive to get the 60Wpep? Shockwave mentioned 3 times 6W required drive-with that said if the 2290 is providing 60Wpep to 4 2879 then I would assume that each of the 2879 are getting 15Wpep input. So if the 2879 only needs 6W to drive but are getting 3 times the input does the output also increase 3 times its output power? I have went and watched some videos on the LA900 output power performance looks like 400w / 800w pep. So know I am trying to understand if the 4 2879s are rated 400Wpep how are we seeing 800Wpep in the videos? I understand that the increase in voltage has something to do with it but just having a slightly tough time wrapping my head around this.
 
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Ranch55 said:
Probably more like $800 to $1000 for four of the original Toshiba 2879's .......
Click to expand...
If anyone wants to pay me that much, I'll modify your 4 pill to match the impedance of the bigger Mitsubishi 2SC3240. NOS is still available for $40 or less. They are rated for 20 watts more dissipation and 2 more volts on the collector.
 
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