Adcom GFA-555 II issue... - diyAudio
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Old 16th January 2014, 04:14 PM   #1
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Unhappy Adcom GFA-555 II issue...

I bought a GFA-555 II amp last night for one and half centuries.
Seller said it was installed in a church and they upgraded system.
Anywho, I checked DC at the speaker terminals and it is 1.4 and 1.44 constant without anything connected.
When speakers are connected without input, it's around 0.03.
I connected pre-amp and there is no output from the speakers.
Checked the fuse and the leftmost fuse is blown. Other fuses are okay.
It has AGC-7A/250V label on the fuse.
Should I get fast blow or slow blow fuse? Click the image to open in full size.
If it's something else, should I return it and get money back or have it fixed professionally?
Thanks for information.
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Old 16th January 2014, 04:20 PM   #2
kevinkr is offline kevinkr  United States
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Get an AGC 7A fuse.. (a fast acting type) Seriously I would return it and get your money back, out of the box not working is not a good starting point if it was represented as being in working order. Repairs will be expensive.

https://www.egr.msu.edu/eceshop/Part...3AG%20Fuse.pdf
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Old 16th January 2014, 04:39 PM   #3
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Thanks for comment.
Is time delay same as fast blow?m.homedepot.com/p/Cooper-Bussmann-MDL-Series-7-Amp-Silver-Electronic-Fuses-2-Pack-BP-MDL-7/100176781/

I want to replace the fuses at least and have to do it soon so need to find fuses locally.
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Old 16th January 2014, 05:27 PM   #4
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Okay found matching fuses@ radioshack.
m.radioshack.com/radioshack/product/detail.do?itemId=2103755&op=%22http://www.radioshack.com/product/index.jsp?productId=2103755%22

7.0A 250V Fast-Acting 1x" Glass Fuse (4-Pack)
Catalog #: 270-1013 | Model: 27-1013

I'll try tonight and update.
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Old 16th January 2014, 05:53 PM   #5
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Another question.
I adjusted BIAS pots and measured speaker terminals.
I could not find the terminal points in service manual to measure BIAS so just went with speaker terminals.
I marked the location before changing and adjusted it back to original.
It may be off slightly but should be close.

From service manual:
Quote:
1. set bias controls (R119 and R169) to midpoint.
2. Connect a millivolt meter across TP201 and TP301.
3. Turn amplifier on and allow 3 to 5 minute settling period.
4. Adjust BIAS control R119 to obtain either a + or - 10mV(+-1mV) indication on the millivolt meter.
5. Connect a millivolt meter across TP251 and TP351.
6. Adjust BIAS control R169 to obtain either a + or - 10mV(+-1mV) indication on the millivolt meter.
7. To check for proper bias setting, remove millivolt meter and appy input signal to obtain 66 watts into 8 ohms for 10 minutes with cover on.
8. Remove input signal and connect the millivolt meter as in Step 2 and step 5. Let amplifier idle until bias stabilizes and readjust to 10mV(+-1mV).
Service manual doesn't show where the TP201, TP301, TP251 and TP351 are.
Is there a better diagram that shows where to check bias and how to adjust DC offset?
Also, how do I get 66 watts with input?
Thanks for info.
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Old 16th January 2014, 06:06 PM   #6
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This has become confusing.

After you replace the fuse check the DC voltage out on each channel. Use a power resistor (not a speaker) as the load (a 10 watt 10 Ohm resistor will only cost a dollar at radio shack). If you then measure 1.4 volts DC then DO NOT hook up a speaker to the amp. There could be a number of possibilities. None of them would be simple or cheap.
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Old 16th January 2014, 06:37 PM   #7
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Quote:
Originally Posted by WithTarragon View Post
This has become confusing.

After you replace the fuse check the DC voltage out on each channel. Use a power resistor (not a speaker) as the load (a 10 watt 10 Ohm resistor will only cost a dollar at radio shack). If you then measure 1.4 volts DC then DO NOT hook up a speaker to the amp. There could be a number of possibilities. None of them would be simple or cheap.
Okay will get the resistor.

Now back to the test points.
Are these highlighted in red the test points?
If yes, do I test on two pins of the same part or across?
Thanks
Click the image to open in full size.
Attached Images
File Type: png Adcom test points.png (435.2 KB, 108 views)
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Old 16th January 2014, 09:04 PM   #8
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NO
The test points you identified are for adjusting the bias. The problem you are having (I thought) is DC at the output (the speaker jacks). There should be minimal DC voltage there (my guess is less than 20-40 mV).

Where did you measure the DC voltage you mentioned above?
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Old 16th January 2014, 09:14 PM   #9
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Quote:
Originally Posted by WithTarragon View Post
NO
The test points you identified are for adjusting the bias. The problem you are having (I thought) is DC at the output (the speaker jacks). There should be minimal DC voltage there (my guess is less than 20-40 mV).

Where did you measure the DC voltage you mentioned above?
I measured DC voltage at the speaker terminals that showed 1.4 and 1.44 without connecting anything.
When speakers are connected, it showed 0.03 on both.

I also wanted to check the bias and make sure it has good numbers.
I think I am looking at wrong board.
May be I should look at the side vertical boards, not the board where the bias adjustment is.

Click the image to open in full size.
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File Type: jpg P1050420 -small.JPG (484.8 KB, 104 views)
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Old 16th January 2014, 10:59 PM   #10
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There is a procedure for measuring/adjusting the bias. It is detailed in the Adcom service manual (there are free copies if you do a Google search). You will need a voltmeter and a power resistor.

However, I would not bother with the bias until this problem about DC voltage at the output jacks is solved. Given your voltmeter, experience, etc. are you sure you are measuring 1.4 volts DC at the output jacks? Forgive me if this sounds patronizing, but we do not know your level of experience.

IOW, 1.4 volts DC is not normal.
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