PL Model 700 Series II with LEDs that just won't work!

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bhuskins

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I have a really nice model 700 series II that's working great other than the LEDS. Basically they don't work. Me and member laatch55 have troubleshooted it quite a bit on AK...see thread here: http://www.audiokarma.org/forums/showthread.php?t=643045

I'm focused currently on getting the main board working 1st which is the one on the right that includes the rectifier diodes and the additional 470uf 25v cap on the back. This board has been fully recapped and all the diodes have been replaced. All 4 transistors have been checked to be good. Voltage inputs at the UAA170 ICs (pin 10) and the RC4558 opamp (pin 8) correctly read 13.4vdc and the input signal fluctuates as volume increases at the switch as well at the board input...the attenuator switch seems to be working as well by reducing the AC signal by about 10 fold when engaged.

So, I'm a bit stumped to know where to go from here. The amp is working wonderfully otherwise so it's just the displays that are non-functioning.

Maybe I have bad UAA170s or a bad RC4558...options for replacements if needed? Are the UAA170 on eBay ALL bad? There's a lot of RC4558 replacements but not sure what specs matter for a proper replacement. I'm physically about 15 minutes from mouser so that's obviously my preferred vendor :)

Is there a good option for replacing the LED bars if need be?

I've also verified all the resistors...
 
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laatsch55

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So, we ran out of my expertise so I invited him here. So, you IC and logic folks need to chime in here...

Welcome to Phoenix...

So there are LED driver IC's on E-Bay??
 
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gadget73

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Silly question, but someone didn't crank the calibrate pots all the way down did they ? If thats set to full nothing, the input to the whole circuit will be grounded and it won't do anything.
 
Gepetto

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bhuskins said:
I have a really nice model 700 series II that's working great other than the LEDS. Basically they don't work. Me and member laatch55 have troubleshooted it quite a bit on AK...see thread here: http://www.audiokarma.org/forums/showthread.php?t=643045

I'm focused currently on getting the main board working 1st which is the one on the right that includes the rectifier diodes and the additional 470uf 25v cap on the back. This board has been fully recapped and all the diodes have been replaced. All 4 transistors have been checked to be good. Voltage inputs at the UAA170 ICs (pin 10) and the RC4558 opamp (pin 8) correctly read 13.4vdc and the input signal fluctuates as volume increases at the switch as well at the board input...the attenuator switch seems to be working as well by reducing the AC signal by about 10 fold when engaged.

So, I'm a bit stumped to know where to go from here. The amp is working wonderfully otherwise so it's just the displays that are non-functioning.

Maybe I have bad UAA170s or a bad RC4558...options for replacements if needed? Are the UAA170 on eBay ALL bad? There's a lot of RC4558 replacements but not sure what specs matter for a proper replacement. I'm physically about 15 minutes from mouser so that's obviously my preferred vendor :)

Is there a good option for replacing the LED bars if need be?

I've also verified all the resistors...
Click to expand...
Pretty straightforward circuit so we should be able to fix you up pretty quickly. What do you have in the way of test equipment, lets start there?
 
Gepetto

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Gepetto said:
Pretty straightforward circuit so we should be able to fix you up pretty quickly. What do you have in the way of test equipment, lets start there?
Click to expand...
If you have a sine wave signal generator it is easier to troubleshoot.

Start with a measurement at pin 3 of Z301. You should see about 7V. Likewise you should see about 7V on pin 2. Check that you have approximately 7V formed by the voltage divider between the R302 and R301 resistor node. This is the reference voltage for the front end amp.
 
grapplesaw

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The UAA 170 chip are now available from Chinese suppliers. I prefere to get the original Siemens chips before they are no longer available. i got four from Germany with quick delivery and not bad price. I think the seller has some left on eBay listing 160940762365 eBay ID: jW-versand

The thing to look for is the dimple at the end and correct markings. Siemans uses a letter S with a letter h across the S and UAA 170 with a code sideways on the uaa170 at its end. If it said Seimans all spelled out , probably not real. Lots of counterfit ones out there. Compare markings carefully with what you have on your OEM chip and what is being purchased if you want good stock. I have no experience with the counterfit ones, they may work well.
 
Gepetto

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grapplesaw said:
The UAA 170 chip are now available from Chinese suppliers. I prefere to get the original Siemens chips before they are no longer available. i got four from Germany with quick delivery and not bad price. I think the seller has some left on eBay listing 160940762365 eBay ID: jW-versand

The thing to look for is the dimple at the end and correct markings. Siemans uses a letter S with a letter h across the S and UAA 170 with a code sideways on the uaa170 at its end. If it said Seimans all spelled out , probably not real. Lots of counterfit ones out there. Compare markings carefully with what you have on your OEM chip and what is being purchased if you want good stock. I have no experience with the counterfit ones, they may work well.
Click to expand...
Have you found failed ones in your amps Glen or did you just stock up in case?
 
grapplesaw

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Gepetto said:
Have you found failed ones in your amps Glen or did you just stock up in case?
Click to expand...
Well Joe I do not think these fail very often. I needed one for a amp that I bought that had one missing. It was a pa unit so had a hard life. I just bought four because Siemens has not made them for years and wanted to have a few spare ones. I may never use them if I am lucky.
 
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bhuskins

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gadget73 said:
Silly question, but someone didn't crank the calibrate pots all the way down did they ? If thats set to full nothing, the input to the whole circuit will be grounded and it won't do anything.
Click to expand...
No, I've checked that and I've cleaned the pots as well as returned them to their preset level. I also tested it while adjusting the pot up and down to make sure it didn't react at somewhere on the spectrum.
 
grapplesaw

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bhuskins said:
Funny thing about mouser is I get impatient having to wait 1 hour for the will call order to be ready for pickup!
Click to expand...
Some people have all the luck!!! Good on you. Welcome to the group. I joined this year and have made great friends here already.
 
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bhuskins

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Gepetto said:
Pretty straightforward circuit so we should be able to fix you up pretty quickly. What do you have in the way of test equipment, lets start there?
Click to expand...
I've got a pretty deep test bench...signal generator/scope/multimeters galore and so on.
 
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bhuskins

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Gepetto said:
If you have a sine wave signal generator it is easier to troubleshoot.

Start with a measurement at pin 3 of Z301. You should see about 7V. Likewise you should see about 7V on pin 2. Check that you have approximately 7V formed by the voltage divider between the R302 and R301 resistor node. This is the reference voltage for the front end amp.
Click to expand...
Do this with a signal present? Or just with the amp powered?
 
Gepetto

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grapplesaw said:
Well Joe I do not think these fail very often. I needed one for a amp that I bought that had one missing. It was a pa unit so had a hard life. I just bought four because Siemens has not made them for years and wanted to have a few spare ones. I may never use them if I am lucky.
Click to expand...
Figured that Glen, I doubt those ICs fail very often. Not much going on in there
 
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bhuskins

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Gepetto said:
Start with a measurement at pin 3 of Z301. You should see about 7V. Likewise you should see about 7V on pin 2. Check that you have approximately 7V formed by the voltage divider between the R302 and R301 resistor node. This is the reference voltage for the front end amp.
Click to expand...
Z301
Pin 2 - 7.02vdc
Pin 3 - 6.32vdc
Node between R301 and R302 - 6.99vdc
 
Gepetto

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bhuskins said:
Z301
Pin 2 - 7.02vdc
Pin 3 - 6.32vdc
Node between R301 and R302 - 6.99vdc
Click to expand...
That looks good. Pin 3 is probably depressed because it is a 1M resistor providing the bias and your meter probably loads it down. Is pin 1 at 7.02V as well?
 
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