PL 700 Pro Build

[George S.]

Been reading all week about impedance and seems straightforward. Fired up my new to me very cheap Panasonic signal generator to provide a signal to measure preamp output impedance and ran into a issue.
As I increase amplitude I don't see any AC but do see DC. I'm thinking the output transistor is shorted. Going to open it up and take a look.

 

[Gepetto]

R

Been reading all week about impedance and seems straightforward. Fired up my new to me very cheap Panasonic signal generator to provide a signal to measure preamp output impedance and ran into a issue.
As I increase amplitude I don't see any AC but do see DC. I'm thinking the output transistor is shorted. Going to open it up and take a look.

Rule#1
It’s your test equipment

 

[George S.]

It's a very clean pristine unit inside. Going to search for a repair manual. I do have the operators manual.

 

[George S.]

Unfortunately it's all surface mount. Found a schematic and zeroed in on the RF module that is directly at the BNC out connecter. There are two surface mount transistors just to the right of a relay.
Per the schematic these are the "driver and final" and from what I've read, easily shorted with the test cable.
Would really like to test them out of circuit, but given they're surface mount, probably best to just replace them and hope for the best.
Going to take a lot of precision work to get the module apart so I can get to them without damaging other components. Think I'll mix some ChipQuick into the joints before I pull them with tweezers. Bummer. Oh well, another project!

 

[Gepetto]

Unfortunately it's all surface mount. Found a schematic and zeroed in on the RF module that is directly at the BNC out connecter. There are two surface mount transistors just to the right of a relay.
Per the schematic these are the "driver and final" and from what I've read, easily shorted with the test cable.
Would really like to test them out of circuit, but given they're surface mount, probably best to just replace them and hope for the best.
Going to take a lot of precision work to get the module apart so I can get to them without damaging other components. Think I'll mix some ChipQuick into the joints before I pull them with tweezers. Bummer. Oh well, another project!

Aren't you planning on tracing out signal paths with your scope before you begin shot gunning things?

 

[George S.]

Aren't you planning on tracing out signal paths with your scope before you begin shot gunning things?

Scope quit working last weekend. Think one of those weird minature 4 legged rectifiers died.
Would make better sense to get it up and running as I have a extra power supply for it.
Will do. Going to clear off the bench and fix the scope first. Thanks for keeping me straight.

 

[George S.]

I lost the horizontal on it. Really didn't want to deal with it as I put so much work into it last winter. Probably a easy fix, I have another parts unit.

 

[Gepetto]

Scope quit working last weekend. Think one of those weird minature 4 legged rectifiers died.
Would make better sense to get it up and running as I have a extra power supply for it.
Will do. Going to clear off the bench and fix the scope first. Thanks for keeping me straight.

Makes more sense to get that working first George. The scope is your eyes and ears...

 

[George S.]

All power supply voltages are spot on. Replaced the horizontal amplifier with another I rebuilt last winter. No joy. Now pulling the vertical amplifier to clean all the pots and switches.

 

[George S.]

At this stage it's apparent I needed a new digital oscilloscope to replace and repair the old HP when I get around to it later.
Been putting this off for a long time. Went up to Microcenter in Cleveland and got a new Siglent
Set the box down in my work room and turned the work room system on not noticing the preamp volume was turned up. The turn on thump blew something out in the preamp. ********* it! The amp is a old bipolar output power amp from the early 70s.
So here I am tracing signal . Funny how things often work out.

 

[George S.]

With the opamps out, I lose the signal at Q1. That's a TIS97 or GES97. Have a bunch of them in old Cylon boards. Going to print out that PL plastic transistor sheet Don has on his site and get orientation correct.

 

[George S.]

My error, unit wasn't powered. Q1 conducted once B+ came up.

 

[George S.]

The Z2 opamp was blown, that's the one closest to the power supply.
Gotta say, that scope is so easy to use, I love it! I followed a 500 Hz signal from a tuner input jack right to the dead opamp.
Really didn't even need to take the preamp apart. Live and learn.

 

[D-500 Flogger]

Many generations of phono preamps have run straight off of DC for quite a while now. A top seller for Radio Shack was their battery powered phono pre. Had quite a following until they discontinued it.

vintage Radio Shack Cat. No 42-2111 Stereo Phono Pre-Amplifier stereo switch box | eBay

Gepetto,

Nice, no drama finish to your pointer - this is what arrived via the USPS:



Since you are a SPICE devo (and I wanted a copy for future reference) ...I found a legible copy (purportedly) of this battery-powered RIAA phono pre-amp. (Radio Shack model #42-2111)



At the moment I've got to assemble a decent lab space & then fix what I've already got. But once I've got my system up & running *this* will be the first audio windmill that I will tilt at... Can't wait! :0)

Thanks again for the tip!

 

[George S.]

Making progress with the Panasonic AM FM Stereo signal generator. It appears to work, just needed a good scope to see it. Now to figure out how to see the difference in AM and FM modulation, and the FM pilot frequency this is supposed to generate.

 

[George S.]

Been reading about preamp output impedance and how to measure it.
Measured it two ways. One way gives me 33 ohms impedance using the calculation.
The other way was to use a resistor decade box to exactly half the unloaded voltage seen on the scope. Unloaded the peak to peak was 4 volts. Loaded at 2 volts, the resistor decade box says 38 ohms. I am using the built in peak to peak voltage measurement function on this new scope to measure voltage.
So, a average of 35 ohms impedance on the output of my modded PL2000 S2. From what I've been reading, this seems rather low. Low appears to be good from what I've read, just surprised it's so low.
I verified the resistor box reading with both Flukes. It's accurate.
Watched two YouTube videos on doing this.

 

[Gepetto]

Been reading about preamp output impedance and how to measure it.
Measured it two ways. One way gives me 33 ohms impedance using the calculation.
The other way was to use a resistor decade box to exactly half the unloaded voltage seen on the scope. Unloaded the peak to peak was 4 volts. Loaded at 2 volts, the resistor decade box says 38 ohms. I am using the built in peak to peak voltage measurement function on this new scope to measure voltage.
So, a average of 35 ohms impedance on the output of my modded PL2000 S2. From what I've been reading, this seems rather low. Low appears to be good from what I've read, just surprised it's so low.
I verified the resistor box reading with both Flukes. It's accurate.
Watched two YouTube videos on doing this.

What frequency? It may well grow higher at low frequencies.

 

[George S.]

500 Hz sine wave 3 Volts peak to peak at input. Turned up the volume some to see 4 Volts peak to peak on the scope.
Left it all on the bench so I can try some other frequencies today and make sure I'm not overlooking something like I always do.
Seems like a relatively simple measurement. But I do see more complicated formulas for testing equipment that's highly reactive to the load. Going to play around with this today and see if results average.

 

[George S.]

Using the simplest measurement scheme, of halfing the the unloaded voltage with a load on the decade box. At 50 Hz the impedance is 386 ohms. At 500 Hz the impedance is 38 ohms. Interesting how the factor of 10 appears.
Time to do some more reading.

 

[Gepetto]

Using the simplest measurement scheme, of halfing the the unloaded voltage with a load on the decade box. At 50 Hz the impedance is 386 ohms. At 500 Hz the impedance is 38 ohms. Interesting how the factor of 10 appears.
Time to do some more reading.

Not a mystery at all George, you are seeing the impedance of the output coupling cap come into play. At high frequencies, that cap approaches a short, as you go down in frequency, that cap approaches an open...

That is why I asked about what frequency you are using.

Increase the value of the output cap and your low performance will improve in a linear fashion with the increase in C.