700B blew one Right channel 5A supply fuse

[62vauxhall]

Seems to be that like others, I was led to this forum while searching for causes of problems with Phase Linear 700B amplifiers. The example I have was made on May 2, 1974 or at least the PC board was. Serial number is 574-0878. I've had it since 1990 and early on, used it to power 2 Peavey PA speakers in a band rehearsal room. It was never pushed and the meters barely moved.

Within the first 10 years I had it, it was taken for service 3 times during which, some output transistors were replaced. What's in there now are (10) RCA MJ10534 (made in Mexico) and (2) RCA 410 for the right channel. The Left channel contains (10) Phase Linear XPL909 and (2) RCA 410.

For 10 years after it's last repair, it sat unused in a closet. In 2010, I thought to put back into service powering a home stereo but I anticipated it was in need of some lamp bulbs. With no source or speakers connected, I plugged it into AC but immediately upon turn on, there was a spark and flash from the inside. It went to a tech for repair and the bill was $25 parts and $135 labour. The part was (1) Mallory 10,000 Mf 100V capacitor in the right channel. I asked why only one and was told that's all that was needed.

The amp seemed to work satisfactorily for a couple of years but from time to time, a "snap" sound would be heard. It occurred randomly and I could not tell if it was coming from the speakers or if it was something arcing within the amplifier. The last occasion this occurred, the "snap" was a little louder and the right channel quit. I first examined the fuses, found the uppermost right channel 5A supply fuse blown and put the amp back in the closet until now.

Would there be an informed guess as to what causes this fuse to blow? I'd like to have it work again but cannot afford the $75 per hour or higher labour cost.

My diagnostic ability is rudimentary and I have nothing but an analog multi meter for equipment. However, I can solder and comprehend schematic and wiring diagrams somewhat enough to have re-built a tube amplifier and re-capped several tape deck pre-amp boards.

I understand there is now a loaded PC board for these amplifiers. Could something on the existing board have the blown that fuse and would a new loaded board be beneficial? Or is it more likely power transistors again and/or the associated 1W carbon composite resistors?

 

[Northwinds]

Just sounds like it's time to get her rebuilt and rebuilt right. Most everything in it is past it's sevice life. Stock or upgraded, you came to the right place. Someone will be along to help you

 

[laatsch55]

Seems to be that like others, I was led to this forum while searching for causes of problems with Phase Linear 700B amplifiers. The example I have was made on May 2, 1974 or at least the PC board was. Serial number is 574-0878. I've had it since 1990 and early on, used it to power 2 Peavey PA speakers in a band rehearsal room. It was never pushed and the meters barely moved.

Within the first 10 years I had it, it was taken for service 3 times during which, some output transistors were replaced. What's in there now are (10) RCA MJ10534 (made in Mexico) and (2) RCA 410 for the right channel. The Left channel contains (10) Phase Linear XPL909 and (2) RCA 410.

For 10 years after it's last repair, it sat unused in a closet. In 2010, I thought to put back into service powering a home stereo but I anticipated it was in need of some lamp bulbs. With no source or speakers connected, I plugged it into AC but immediately upon turn on, there was a spark and flash from the inside. It went to a tech for repair and the bill was $25 parts and $135 labour. The part was (1) Mallory 10,000 Mf 100V capacitor in the right channel. I asked why only one and was told that's all that was needed.

The amp seemed to work satisfactorily for a couple of years but from time to time, a "snap" sound would be heard. It occurred randomly and I could not tell if it was coming from the speakers or if it was something arcing within the amplifier. The last occasion this occurred, the "snap" was a little louder and the right channel quit. I first examined the fuses, found the uppermost right channel 5A supply fuse blown and put the amp back in the closet until now.

Would there be an informed guess as to what causes this fuse to blow? I'd like to have it work again but cannot afford the $75 per hour or higher labour cost.

My diagnostic ability is rudimentary and I have nothing but an analog multi meter for equipment. However, I can solder and comprehend schematic and wiring diagrams somewhat enough to have re-built a tube amplifier and re-capped several tape deck pre-amp boards.

I understand there is now a loaded PC board for these amplifiers. Could something on the existing board have the blown that fuse and would a new loaded board be beneficial? Or is it more likely power transistors again and/or the associated 1W carbon composite resistors?

Do you have a service manual. Even with rudimentary troubleshooting skills the manual walks you through many standard tests that will narrow down the problem. Are you in the UK?

 

[62vauxhall]

No, not in the UK, I live near Vancouver, BC.

This morning I downloaded the service manual and found the Blown Line Fuse portion in the Troubleshooting section. It describes "...setting the meter to lowest resistance scale, connecting the negative lead to the Collector Bus and measuring resistance at the Emitter. If the outputs in that bank are good, you will get a reading slightly lower than a typical semiconductor junction reading and go on to the next bank."

I do not know what a typical reading would would be.

After reading that, looking at the schematic and reading details about output transistors I concluded the RCA MJ15024's and Phase Linear XPL909's have two pins. And the Emitter is the one that has the 1W resistors connected to it and I believe I've identified the Collector Bus. If this is correct, I got 12 Ohms at those points on all the RCA MJ15024 transistors in the Right channel and within a 1/2 Ohm of that on the Phase linear XPL909's in the Left channel.

The two RCA 410's look to have different topology so I measured at 2 points on each with the black test lead still connected to the Emitter Bus. One point on both Right channel transistors was 14.5 Ohms but at the other point I got different readings - 100 Ohms for the one in the inner row and 180 Ohms for the one in the outer row.

The RCA 410's in the Left channel were different again. Readings for the one in the row closest to the transformer were 2 Ohms and 190 Ohms and the other row, 1 Ohm and 160 Ohms.

Does that mean that all 4 RCA 410's have gone funky and be replaced or should I be looking elsewhere?

 

[laatsch55]

All the outputs are good if you got 12 ohms between the collector bus bar and the emitter common tie point. The 410's are in backwards relative to the 24's. TO-3's are offset with the pins. When holding one in your hand with the pins up and the shorter distance between the pins and hold down hole on your left the base is closest to you. Put your positive lead on the base and the other on the emitter. Ih diode test mode should be about .200-.265 dc volts. In ohms check around 200-300 ohms. The collector is the case of the transistor, negative on that should yielkd the same reading. Then reverse leads and run the same 2 measirements....

 

[laatsch55]

On the blown fuse, did it go nuclear or just separate the element with no scorching of the inside of the glass??

 

[laatsch55]

The 410's have resistors hanging off the sockets. Pull the 410's and test out of circuit. OUT OF CIRCUIT they should test B-E . 600 plus DC volts B-C the same.... Infinity when reversing the leads.

 

[62vauxhall]

The fuse did not suffer a violent end, just melted a portion of the element - no scorching or blackness on the glass.

I was prepared to unsolder those 410 transistors and pleasantly surprised to find them in sockets.

As mentioned before, I only have an analog multi-meter powered by a single AA battery so could only do the resistance check. There is no Diode test mode. Not even sure if the 1.5V battery is sufficient to perform a resistance test on these transistors.

Be that as it may, I pulled them and followed your directions * pins up * shortest distance pins to hole on left side * connected Positive meter lead to the closest pin (base) * connected Negative lead to the other pin (emitter).

With the meter in the lowest range, the needle did not move on any of the four 410 transistors. Likewise when moving Negative lead to transistor's case.

However, and maybe I have a misunderstanding of the procedure or term, reversing the leads to the pins did not produce Infinity. I got 25 Ohms, 21 Ohms, 24 Ohms and 20 Ohms. I thought Infinity meant dead short - zero Ohms - needle hard right.

 

[laatsch55]

On my Fluke 179 DMM, I'm showing 336K ohms on the BE jct and 285K ohms on the BC junction..... was the amp in use when the fuse went??

 

[laatsch55]

Try the 100K scale...

 

[laatsch55]

Sir, what is your first name??

 

[laatsch55]

Nav!!! You hgot a Simpson 260 layin around??

 

[NavLinear]

Nav!!! You hgot a Simpson 260 layin around??

Yep - just measured a 410 with a Fluke 179 and my numbers were very close to what you saw. Give me a few and I'll see what a 260 does.

 

[laatsch55]

Good man Navo!!

 

[62vauxhall]

My name is Gary and thanks for all the assistance.

The amp was operating when the fuse went.

My meter is not sophisticated - it's scales are X1, X10 & X1K. Using the X10 scale, there is still no needle movement with the Positive meter lead connected the the nearest pin (base) and negative lead connected to farthest pin (emitter).

Reversing the meter leads (Positive to farthest pin and Negative to nearest pin) still in X10 scale I get:
#1 120 Ohms, #2 120 Ohms, #3 140 Ohms and #4 125 Ohms.
Moving the positive meter lead from to pin to case produces the same on all but #4 where it drops to 110 Ohms.
Moving the negative lead from pin to case produces no reading.

 

[laatsch55]

I don't think your meter can turn on a semiconductor. Your fuse did not blow from a short. Put a fuse back in it, with no load or speakers attached, and set the meter to DC volts and measure accross the speaker posts for DC offset. You can have good outputs and drivers and still have LOTS of Dc on the speaker outs....

 

[laatsch55]

Gary, all that is after you replace the 410's.....

 

[NavLinear]

I haven't had much time to dick around with the 260 but it just doesn't do what a modern digital meter can. With the 179 you can easily verify the transistor type and get a reasonable idea that it is good or not. The 1.5 V battery that Gary's meter has will allow some simple measurements but... I don't think it's cut out for semiconductor measurements other than shorted or open.

 

[62vauxhall]

Put the transistors back in and must now locate a new AGX 5A fuse. Can't test without one so will be visiting a couple of good ol' electronics parts stores in Vancouver tomorrow to try and find some.

Guess it's time to get a digital multimeter. I looked into them them once some time ago but got sticker shock at what some of the "worth having" ones sell for. Any thoughts on finding one for $50 or less new or used?

 

[laatsch55]

I have a little "Craftsman" (sears) one I got a few years ago for 39.00 In running it against my Fluke 179, not enough difference to count , the frequencies at which we work , that little DMM would work just fine. That analog should be able to measure small DC at the speaker outs, have you tried?