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Emitter Resistors Location - Parasound HCA-1500A
Emitter Resistors Location - Parasound HCA-1500A
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Old 21st October 2019, 01:45 AM   #1
hongrn is offline hongrn  United States
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Default Emitter Resistors Location - Parasound HCA-1500A

Hello All,

I just acquired a Parasound HCA-1500A with an almost dead left channel, as volume was barely perceptible. After taking things apart, I found 2 issues: two blown fuses on the main board, and a cold solder joint on the stereo-bridge switch. The left channel came to life, but runs warmer than the right channel. I'm attempting to adjust the bias, and need your help to identify the emitter transistors correctly. On the schematic, I highlighted the trim pot TVR101, so am I correct to assume that a measurement should be taken across R130 and R127? Parasound's recommendation for the bias voltage for this amp is 15 mV, +/-2 mV. I built Rod Elliott's P101 so I am somewhat familiar with bias setting (not a pro by any means).

Thanks in advance for your guidance.

Hong
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Old 21st October 2019, 01:48 AM   #2
rayma is offline rayma  United States
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That's right. First I would also clean the trim pot, and verify that all the emitter resistors
are still the right value.

Last edited by rayma; 21st October 2019 at 01:51 AM.
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Old 21st October 2019, 04:36 AM   #3
hongrn is offline hongrn  United States
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Thanks @rayma.I was able to set the bias on the left channel - the one that didn't originally work - to 15 mV.

However, the right channel shows 0 mV, and turning the trim pot either direction didn't do squat. Checked the emitter resistors on that channel and values are normal (0.33R). I guess I need to replace the 2K trim pot. Does anyone know which brand or model it is? Thanks.
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Old 21st October 2019, 04:41 AM   #4
rayma is offline rayma  United States
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That may not be the problem, but you can replace it anyway.
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Old 21st October 2019, 04:52 AM   #5
hongrn is offline hongrn  United States
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Quote:
Originally Posted by rayma View Post
That may not be the problem, but you can replace it anyway.
The channel works fine but I haven't put it through extensive testing, so it may exhibit more issues later. What are your thoughts? Thanks.
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Old 21st October 2019, 06:07 AM   #6
hongrn is offline hongrn  United States
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I ordered new trimmer pots and will report back. In the meantime, please give me your thoughts. Thank you.
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Old 21st October 2019, 08:30 AM   #7
mjf is offline mjf  Austria
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try to measure the pot with a DMM (ohmmeter).....turn it up and down and see what happens............
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Old 21st October 2019, 03:41 PM   #8
rayma is offline rayma  United States
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Quote:
Originally Posted by hongrn View Post
The channel works fine but I haven't put it through extensive testing, so it may exhibit more issues later. What are your thoughts? Thanks.
I would compare DC voltage readings between the good and bad channels, and post the results here.
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Old 21st October 2019, 03:58 PM   #9
analog_sa is online now analog_sa  Europe
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Emitter Resistors Location - Parasound HCA-1500A
Quote:
Originally Posted by hongrn View Post
I ordered new trimmer pots and will report back.
And your conclusion the pot needs replacement is based on?

You should check it with a meter and if ok solder it back on and make sure turning it does produce a change in collector-emitter voltage.
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Old 21st October 2019, 10:29 PM   #10
hongrn is offline hongrn  United States
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Quote:
Originally Posted by analog_sa View Post
And your conclusion the pot needs replacement is based on?

You should check it with a meter and if ok solder it back on and make sure turning it does produce a change in collector-emitter voltage.
I hate it when you guys are right. Both trim pots are fine and vary from 15 ohms to 1.9K ohms. I think pulling the main board out and check for solder joints is my next step.

One poster PM'ed me about the same issue he had with the stereo-bridge switch on his Parasound amp, but I can say that after I corrected the cold solder joint, continuity checks are fine between the poles when switched up or down.

Thank you all.
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