Replacement transitor advice needed.

[mmerig]

What is an acceptable replacement for each of the following transitors:

2SC1980
2SA921
2SA913
2SC1913

These are in a power amp and thay compare voltage and stabilze current for the front end. Voltages were out of spec, and the existing transitors tested as bad.

I tried NTE 289A, NTE 91, NTE 398, and NTE 375 for the above, respectively, but the voltages are even further out and uneven. The complementary pairs may have to be matched, and there may be other problems, but I thought I would make sure the NTE's are okay before doing a lot of checking in the amp. There are some differences on the spec sheets of the putative replacement devices, but the NTE's were the only matches I could come up with from suppliers.

I could supply the specs for each, but maybe someone knows right off that these would/should work or not.

While searching the diy forum, MJE15030 and MJE15031 were recommended for the 2SA913 & 2SC1913. Somehow I missed this before getting the NTE's. (see Squalo's Technics 8080 post)

Any ideas would be appreciated.

 

[jmateus]

NTE's no good for that type of replacement....
Try to get originals

 

[Leolabs]

try these:

2SA913:2SA968,2SA11122SB628

2SA921:2SA941,2SA970,2SA1016,2SA1038

2SC1913:2SC2238,2SC2344,2SC2592,2SD608

2SC1980:2SC2088,2SC2240,2SC2362,2SC2389

 

[mmerig]

Thanks, jmateus and Leolabs. I'll try your suggestions and post the results.

 

[mmerig]

After replacing as follows:

2sc1980: 2sc2389
2sa921: same
2sc1913: 2sc2344
2sa913: 2sa940

There is still about 1/10 the voltage on the front end rails than there should be. Voltages leading into it are okay.

The vendor sent me an 2SA940 instead of either the 2SA968, 2SA1112, or 2SB628.

Circuit-wise, the problem seems to be where the 2sa913 was. The original was broken, and voltage was too high, by about 15% or so.

The spec sheets show differences betwen the 2sa913 and 2sa940. Should I try to get a better replacement?

Thanks.

 

[Nigel Goodwin]

Generally power amplifiers are basically high gain, high power, opamps, with 100% DC feedback around them.

So different transistor types should make very little difference to the voltages - certainly 15% out sounds like something faulty, rather than the wrong type of transistor.

 

[mmerig]

Thanks. Nigel.

In my first post, I state:

"These are in a power amp and thay compare voltage and stabilze current for the front end. Voltages were out of spec, and the existing transitors tested as bad."

The transistors, at least some of them listed, appear to be voltage and current regulators rather than AC signal amplifiers. In other words, they seem to be a discrete version of say an LM337.

If the above is true, then would the circuit be sensitive to transistor specifications?

 

[Nigel Goodwin]

The transistors, at least some of them listed, appear to be voltage and current regulators rather than AC signal amplifiers. In other words, they seem to be a discrete version of say an LM337.

If the above is true, then would the circuit be sensitive to transistor specifications?

I wouldn't have thought so, not to that extent, do you have the circuit for it?.

 

[anatech]

Hi mmerig,
What is the make and model of the amp you are repairing?

There will be a zener diode involved (at least one). Some of the resistors may have opened possibly.

Typically, if a pass transistor shorts, the parts "behind" it get zapped as well. It would be a good time to replace the electrolytic caps. Regulators run warm at best.

-Chris

 

[mmerig]

Hello Nigel and Chris:

Thanks for the advice. I did some more measurements and realized that the present problem is probably ahead (feeds into)of the circuit I have been focusing on, although some devices were broken in it was in its original state. The first set of replacements (the NTE's) tested okay when I pulled them.

I'll do some more measurements to find the problem.

The amplifier is a Technics SE-9060. I have the repair maual, and will send a schematic if you like ,or if I need to if I have trouble.

 

[anatech]

Hi mmerig,
Wonders never cease, I have that manual.

If your positive output goes low, the negative one will too. It's set to track the positive rail. R407 is a 10 ohm 1/8 watt resistor. Confirm it's good by measuring it. TR405 / D402 form a constant current source (CCS) and TR401 is the error amp. D401 is your voltage reference, should be close to 6.4V.

-Chris

 

[mmerig]

Replacing D401 (6V zener diode) did the trick. This probably broke along with the transisters I mentioned in earlier posts.

Thanks for the advice!

 

[anatech]

Hi mmerig,
Yes, if the other parts fail, the diode in that position is sure to fail.

Good going with the repair.

-Chris