Realistic SA-2000 amplifier burning resistor.

[UNITRAX]

Hello. Can anyone help with the burning resistor?

Realistic SA-2000 amplifier. The problem is with the resistor R 625b

I changed it for a new one, but it burned out immediately..

R 625b is in the Meter Circut, in the diagram page 30 in the upper right corner.
I checked the diodes D605, D607, D609, D611.
Variable resistors VR601a and VR601b have almost identical resistances.
Then I checked all the other diodes on the pcb.
I changed to new electrolytes C615, C602, C603, C802, C801.

After switching on for a while through a light bulb connected in series, the resistor still smokes.

What could be the cause of burning resistor R 625b?

There is one more problem with this amplifier. In the diagram, resistor R625a and b have values of 27k. In fact, they are 100 ohm metallised 1W resistors. The PCB doesn't look soldered here. All solder pads looks as from the factory. The values of few other elements are also different between the schematic and reality. I searched the web for information on this subject, but found nothing on the Internet.

[redwire]

Re: Realistic SA-2000 amplifier burning resistor.

What year is the amplifier? There are differences 31-2075 or 31-1985 so you might be using the wrong schematic?
STA means stereo tuner+amplifier, SA means stereo amplifier.

I see it as part of the power amp driver stage R625a,b 100Ω 1/4W carbon. Which makes more sense than it being a part of the power meters.
It would burn up if TR610b E-B or R629b are open.

P.S. -> Radio Shack Catalog Archives is excellent for finding pricing etc, kind of a heartbreaker too. I loved that store.

[UNITRAX]

Re: Realistic SA-2000 amplifier burning resistor.

Thank you.
This sheds a whole new light. All the time these differences with the schematic drive me creazy and I knew that I was making a mistake somewhere.
I will check TR610b E-B and R629b and write what resulted.
In the meantime, I'll look for a proper schematic.
I have saved this page to my favourites.

[UNITRAX]

Re: Realistic SA-2000 amplifier burning resistor.

It does not look good, probably the transistors have already been replaced. The problems multiply.
TR 612a, TR 601a TR 608b are motorola 2N3773.

TR 610b is PMC 2N3773.

If these Motorolas were factory fitted, but I'm not sure about that either. PMC is the replacement and he just behaves differently than the other three.

test the diode with a multimeter

TR610b measured on pcb.
emitter - base = 1.177 V
base - emitter = 1.177 V
emitter - collector = 0.156 V
collector - emitter = 0.156 V
base - collector = 1.237 V
collector - base = 1.237 V

TR 612a, TR 601a TR 608b

emitter - base = 0.114V
emitter - collector = loads into overload
collector - emitter = 0.454 V
base - collector = loads into overload
collector - base = zero to 0.455 V
base - collector = loads into overload

I took out the R629b for measurements. resistance = 0.40Ω

[redwire]

Re: Realistic SA-2000 amplifier burning resistor.

It looks like the power amp has blown transistors and somebody replaced a few.
Output transistor originals are 2SD339, substitute as 2N3773, MJ802 etc. is OK. Hopefully the PMC 2N3773 is not some cheap chinese ripoff.

TR610b looks shorted which could also take out TR612b or TR611b or R630b. TR612a looks OK but the readings with a multimeter in-circuit can get fooled i.e. the 100Ω R625, R627 can make it look like a shorted E-B but it can be fine. Out of circuit is the best test.
When I get a power amp to repair, I test all the transistors because the shorted part can overload and damage many others, including new parts. You have to work back. I expect TR608b (small one with bias trimpot) to also be open-circuit. When an output transistor shorts, somewhere something acted as a fuse and went open-circuit. So hunt for that too.

This is a well built vintage amplifier, well worth it to repair.
If one channel works then you can use comparison readings between the two channels to some extent.

[UNITRAX]

Re: Realistic SA-2000 amplifier burning resistor.

TR 608a,b.
Diode test ok, both measurements are close.

At first, I didn't notice them on the pcb at all, because they were drowning in grease. I don't know if it should be like this. It doesn't look like thermal paste. The same grease is used under the other transistors on the heatsink.

TR610b is garbage.
The tester shows 104pF
With a multimeter
Emitter - collector = 143 Ohms
Emitter - Base = O.L
Base - Collector =O.L

For the first time I deal with transistors in the TO-3 package. What is their proper insulation and installation? someone before me used plastic washers under the transistors. However, there is no insulation on the screws. The screw washers are metal. There is grease instead of thermal paste.

I ordered transistors from two sources. One seller claims they are from old stock. The other had some positive comments on ebay so I took a chance.
Waiting for the shipment will be a few days for further study and cleaning SA-2000.
Thank you for your advice. I try to verify all of them to fix this equipment.

[R_J]

Re: Realistic SA-2000 amplifier burning resistor.

That grease that you call it IS thermal grease, it is just not white, that is what was used in those days. The screws are insulated from the heat sink but are used to connect the collector to the socket.
I do not see this ending well if you are not familiar with mounting these type of transistors.

[jiroy]

Re: Realistic SA-2000 amplifier burning resistor.

Plastic washers and thermal grease are very good for temperature dissipation , especially with TO-3 transistors .
You shouldn't be worried when transistors of the same type and rail are on the same heat sink , when speaking insulation . On the same heat sink , transistors are supposed to be mounted either on the negative rail or the positive one .

You should be worried if you're designing/building an amplifier while mounting a negative rail's transistor collector to a positive rail's transistor collector ..In that case , a unified heat sink , transistors and screws are supposed to be insulated and in the same time , you take a wire from the collector through the screw to the main pcb .

[eccerr0r]

Re: Realistic SA-2000 amplifier burning resistor.

If you're getting 143Ω b-e and b-c and ∞Ω e-b and c-b, chances are that transistor is still good, but you should be using diode check mode. As an aside, was playing with one of my spare 2n3771's and saw these resistances only if i used my analog multimeter, my DMM produced much larger numbers... of course this is due to input differences.

Depending on how the TO-3's are attached, chances are there should be/are plastic shoulder washers in the heatsink to prevent the screw from touching the heatsink. Wherever those plastic shoulder washers are, do not lose them and assemble exactly as before! Inspect carefully, they may still be in the heatsink. from what I can tell from your pictures, it looks like screw head on bottom - shoulder washer - heatsink substrate - mica insulator - TO3 transistor - lock washer - nut on top?

if r625b is burning i'd make sure TR612b and TR611b are not shorted. And TR609b as well...

[UNITRAX]

Re: Realistic SA-2000 amplifier burning resistor.

I've read a bit about TO-3 assembly. It's like R_J wrote: The screws are isolated from the heatsink, so no insulating washers are needed on them. Washers are only needed under the power transistors.

I checked twice most of the transistors on the main amp pcb.

I changed the R625b to 100 Ohms 0.5W
I put the TR611b on a supposedly original Japanese production and the amplifier seems to be working.

A light bulb connected in parallel to the power supply turned on and then went out. I wanted to measure the voltage across capacitors C851 and C852. They are connected together to a symmetrical voltage of +- 43.5V

I noticed that one cable from the GND line is disconnected.
There are 12 cables coming out of this line.
I quickly turned off the power and I'm looking for where it should be connected.
I can't deal with the diagram.

It is a short cable compared to others coming out of this rail.

[UNITRAX]

Re: Realistic SA-2000 amplifier burning resistor.

Thread to closed.
The amplifier has been working for a week, transistors in the final stage are fine. I compared the voltages with the schematic, they all match. Idling current set to 70mV.