PL 200 Help Please!
- Thread starter bobdod
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I saw that smoked 220 Ohm resistor on the display board. I had some of those on 200 Gen 1 amp also. Each channel has three of them that form resistor voltage divider for the display. I replaced them with "flameproof" versions. In my case, I traced the fault to overvoltage coming from the one of the amp modules. I was getting about 63 VDC on the ouitput line with no input applied. I found a number of things, including a brocked tracew and a bad ground. I had previoulsy changed output tnasistors, drivers, anfd the inverter transistor as well as 3 shorted diodes. I am not the first to work on this unit, and I am still not done with it.
I have output as seen on the LED meters for each channel, which tracks the input. The transistor do not get warm and there is a small offset voltage (blelow 10mV if I recall() on the outputs, under no load and no input. I will re-measure it again, and if okay, proceed with bias and stability checks. The protection relay does nothing yet - Apparently no voltage at the coil, but the diodes and transistors assocated with the circuit have tested okay. I also need to find a pair of 3mm LED's that need more power than Vishays' I got from Mouser. Two wer mecahnically damaged, and the replacements are much brighter, needing less current.
Stay tuned. I am getting close with mine.
I have output as seen on the LED meters for each channel, which tracks the input. The transistor do not get warm and there is a small offset voltage (blelow 10mV if I recall() on the outputs, under no load and no input. I will re-measure it again, and if okay, proceed with bias and stability checks. The protection relay does nothing yet - Apparently no voltage at the coil, but the diodes and transistors assocated with the circuit have tested okay. I also need to find a pair of 3mm LED's that need more power than Vishays' I got from Mouser. Two wer mecahnically damaged, and the replacements are much brighter, needing less current.
Stay tuned. I am getting close with mine.
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Okay - I replaced all of the LEDS - I like the brightness.... I have run audio through both channels into an actual speaker - and that part works!!!!
What doesn't work is the protection circuit. The cause - an open coil in the relay. Anyone have one of those they's like to sell? It is a 1/4 HP/10 Amp DPDT relay, but only the wipers and Normally Open contacts are used. Coil says 120 VAC 60 HZ, but its being fed by a full wave rectifier of around 55 VDC. Funny about that stuff.
It was very cool to hear this thing sing after all these years. I cannot wait to finish the repairs, button it all up and pair it with my C-4000 premp.
What doesn't work is the protection circuit. The cause - an open coil in the relay. Anyone have one of those they's like to sell? It is a 1/4 HP/10 Amp DPDT relay, but only the wipers and Normally Open contacts are used. Coil says 120 VAC 60 HZ, but its being fed by a full wave rectifier of around 55 VDC. Funny about that stuff.
It was very cool to hear this thing sing after all these years. I cannot wait to finish the repairs, button it all up and pair it with my C-4000 premp.
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I know that relay, you are going to have a hard time replacing that.
Unless you look here:
https://tinyurl.com/3rsrpf62
Unless you look here:
https://tinyurl.com/3rsrpf62
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The transistors connected to the relay test fine. There is a suspect 10 uF / 100 V electrolytic (swollen seal) that I will probably replace too - I do not have an LCR meter or capactor tester (yet), so I can only do the coarsest of test with a DMM, on them. I did see the charge/discharge curves on my scope as I was probing this section during operation.
I did notice something unusual during frequency generator tests - The LEDs will pulsate in brightness at certain applied frequencies. I suspect this to be a hetrodying beat against the power supply, but it could be something totally different. There were no indications of anything amiss with either applied input or measured output waveforms on the scope or as heard from the speaker. If the filter cap on this part of the circuit is weak I guess it could be responsible, especially given the new LEDS.
Any ideas on this or is it normal operational behavior?
I did notice something unusual during frequency generator tests - The LEDs will pulsate in brightness at certain applied frequencies. I suspect this to be a hetrodying beat against the power supply, but it could be something totally different. There were no indications of anything amiss with either applied input or measured output waveforms on the scope or as heard from the speaker. If the filter cap on this part of the circuit is weak I guess it could be responsible, especially given the new LEDS.
Any ideas on this or is it normal operational behavior?
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I'm gonna try one of these things from that 'Bay place. It's an add on to the back of 'any' amp, and remotely 12VAC powered.
A short term 'fix' maybe.
Don't all burn me at once, but interested in opinions.
Bottom line is, it's an affordable way out and I won't have to pull stuff to bits otherwise and it can 'float' between several amps that I 'play' with.
A short term 'fix' maybe.
Don't all burn me at once, but interested in opinions.
Bottom line is, it's an affordable way out and I won't have to pull stuff to bits otherwise and it can 'float' between several amps that I 'play' with.
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I did that before I knew about Watts Abundant and his most excellent DC Protect Relay/Delay Board. I even installed a small separate 12V transformer to power the Chinese speaker disconnector in my PL700B (the 6V lighting voltage was not high enough). It worked until the relay got stuck and then no more tunes...
You get what you pay for.
You get what you pay for.
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How long should these relays last? Since I received the unit in a failed state and others have obviously worked on it, I have no idea if the part I found in there is correct. The installed one is an apparently unsuppresed coil designed to work on AC voltages, but is fed with a DC voltage of apporximately 50% of the coil rating through current limiting resistors on both legs. I may have measured that voltage with respect to the wrong reference point, but it still seemed strange to me.
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Yes, the 55 VDC value I stated earlier was a measurement at the coil. The coil of mine measured as open circuit.
The coil I got to replace it has series chokes on its terminals. This one is apparently from one of the later models that do not have the LED meters, so it has the protection circuit on a small PCB with the same parts as appears in the center of my unit’s display board.
I wonder if I should graft those chokes into my unit.
The coil I got to replace it has series chokes on its terminals. This one is apparently from one of the later models that do not have the LED meters, so it has the protection circuit on a small PCB with the same parts as appears in the center of my unit’s display board.
I wonder if I should graft those chokes into my unit.
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I was almost right. The LED circuits are fed by their own tap from the power transformer, via an (unfiltered!!!) full wave bridge constructed from discrete 1N4004 diodes on the high side (B2+), and the low side is connected to ground. The output voltage is not specified but we can do some homework:
The signal from the amp channel is applied to a voltage divider network. The divider consists of 5 resistors, two 820-ohm resistors in parallel, series connected to two 220-ohm in parallel, and a single 220-ohm that can be shorted out by the range selector switch. Effectively, this is (220 or 0) ohms in series with 110 ohms and 410 ohms.
The junction between the 220-ohm and 820-ohm resistors in that divider is then halfwave recified, and filtered by a 10 uF 30 VDC electrolytic capacitor. The filered signal is then applied to a chain of individual resistive voltage dividers which feed the bases of the LED switching transistors. The ratio of each pair of resistors sets the base voltage for each transistor (and with it, the LED connected to its collector), to turn on. The emitters of each transistor are connected to ground.
I replaced all of the LEDs in the PL 200 (original series) I was working on, because some were physically broken. I did not want an inconsistent appearance with newer, brighter and more power efficent LED mixed with older, dimmer, less efficient ones. The part I used was here:
https://www.vishay.com/docs/83494/tlhk46q1r2.pdf.
It has a 2.0 volt typical and 2.6 volt maximum forward voltage drop. The LEDs probably drop less than 2.6 volts when turned "ON".
It has a 20 mA typical and 30 mA maximum forward current.
Since the 1K resistors are in series, the drop across them is 20 volts typical to 30 volts maximum, from ohms law. The value can be less than this, if full brightness is not achieved. The minimum voltage is for the part is not specified.
What is left is the voltage across the transistors. These are TIS97s. the data sheet is here:
https://www.mouser.com/datasheet/2/308/TIS97-1125550.pdf
From the data sheet, it appears that about 5.0 V will appear between collector and emitter in the "ON" state.
A range of estimated total voltage is:
[ ?(min) 20(typ) 30(max)] + [2.0(typ) to 2.6 (max) ] + 5] volts.
Looks like 27 volts using the typical values.
Hopefully, I have neither bored you with my detective work and analysis nor led you astray with unfounded assumptions, defective logic and bad arithmetic!!!
What I got wrong in the prior post??? Surprisingly, there is no regulator and no filter capacitor on that LED anode bus!!! I think I now solved the mystery of the pulsating LEDs I observed when testing the one I repaired with a signal generator!!!!
Good luck in repairing your PL 200!!!!
The signal from the amp channel is applied to a voltage divider network. The divider consists of 5 resistors, two 820-ohm resistors in parallel, series connected to two 220-ohm in parallel, and a single 220-ohm that can be shorted out by the range selector switch. Effectively, this is (220 or 0) ohms in series with 110 ohms and 410 ohms.
The junction between the 220-ohm and 820-ohm resistors in that divider is then halfwave recified, and filtered by a 10 uF 30 VDC electrolytic capacitor. The filered signal is then applied to a chain of individual resistive voltage dividers which feed the bases of the LED switching transistors. The ratio of each pair of resistors sets the base voltage for each transistor (and with it, the LED connected to its collector), to turn on. The emitters of each transistor are connected to ground.
I replaced all of the LEDs in the PL 200 (original series) I was working on, because some were physically broken. I did not want an inconsistent appearance with newer, brighter and more power efficent LED mixed with older, dimmer, less efficient ones. The part I used was here:
https://www.vishay.com/docs/83494/tlhk46q1r2.pdf.
It has a 2.0 volt typical and 2.6 volt maximum forward voltage drop. The LEDs probably drop less than 2.6 volts when turned "ON".
It has a 20 mA typical and 30 mA maximum forward current.
Since the 1K resistors are in series, the drop across them is 20 volts typical to 30 volts maximum, from ohms law. The value can be less than this, if full brightness is not achieved. The minimum voltage is for the part is not specified.
What is left is the voltage across the transistors. These are TIS97s. the data sheet is here:
https://www.mouser.com/datasheet/2/308/TIS97-1125550.pdf
From the data sheet, it appears that about 5.0 V will appear between collector and emitter in the "ON" state.
A range of estimated total voltage is:
[ ?(min) 20(typ) 30(max)] + [2.0(typ) to 2.6 (max) ] + 5] volts.
Looks like 27 volts using the typical values.
Hopefully, I have neither bored you with my detective work and analysis nor led you astray with unfounded assumptions, defective logic and bad arithmetic!!!
What I got wrong in the prior post??? Surprisingly, there is no regulator and no filter capacitor on that LED anode bus!!! I think I now solved the mystery of the pulsating LEDs I observed when testing the one I repaired with a signal generator!!!!
Good luck in repairing your PL 200!!!!
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