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Vintage Audio (record players, hi-fi etc) Amplifiers, speakers, gramophones and other audio equipment.

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Old 22nd Feb 2018, 5:34 am   #1
retailer
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Default Help with Akai AM-U04

My daughter inlaw's Uncle has asked me to repair his 1980's vintage Akai amp an AM-U04, it runs twin power supplies one for each channel (powered by one transformer) with Sanyo STK pre and power amp modules a STK 3042 voltage amplifier and 2 x STK 0050 II power amplifiers. Even though I've done my own amps in the past it's not something I've had a lot of experience doing. I'm posting this in the vintage section as the amp is close to 35 yrs old so I guess that is now vintage.

The ON indicator lamp came on when powered up but the rest of the amp was dead, both power amp module 4A fuses had blown. The right channel output module measured short circuit on a few of the pins and once this was unsoldered and the amp powered up - the speaker relay clicked on and I could play music through the left channel - ha! easy I thought. I replaced the faulty module with one sourced from Ebay, once fitted I noticed low volume and distortion in that channel this was confirmed with a scope and signal generator. The distorted waveform was on the input and output of the STK3042 voltage amp and so was also visible on pins 1 and 7 of the fet differential pair. These were also sourced from Ebay, however once fitted there was no difference - the distorted weak signal was still there.

I lifted all of the right channel STK 0500II pins from the board and bridged the PCB pads for pins 7-8 and 3-4, this coupled the STK 3042 directly to the speakers/headphone socket - taking the right channel STK0500II out of the picture the distortion was still there. I went over the complete board checked voltages and did a diode type check of all semiconductors, diodes - I know they should be lifted from the board but at least this shows up a shorted or open component everything checked out ok except the output pins of the STK3042 these are 5,6 for the left channel and 10,11 for the right. Voltages here were +5 and -5 on the good channel and +3.4 and -3.4 on the distorted right channel. In addition I had + and - 53V on the output of the bridge rectifier D3, with 43V AC in I should have around 62V DC, D3 and its associated components provide + and - 48V for the STK3042 chip.

I also went over the board and checked all of fusible resistors, those that did not measure the correct value were removed from the board for checking nothing seems obvious. All things seem to point to the new SDTK3042 chip being defective - I'm reluctant to try a another though if it just turns out to be something else, I've just about run out of ideas and now need to ask for help has any one any ideas of things to check or try, I am in the process of breadboarding the old STK3042 and dual fet to see if I can duplicate the problem - I suspect the original STK voltage amp was ok. The scope trace shows the sine wave distortion as the bottom half clipped off,I should mention the "spikes" on the trace aren't normally there I'm still coming to grips with my new DSO so far the only thing I could say it is better than my old TEK453 is the size - much smaller.

I couldn't shrink the circuit pdf enough to include it in the post so here is a link to it: https://drive.google.com/open?id=1ms...R7n9L8JTUU-apT
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Old 22nd Feb 2018, 8:28 am   #2
Ted Kendall
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Default Re: Help with Akai AM-U04

I'm inclined to agree that the new chip is probably faulty - if YouTube gossip is to be believed, many of the chips of this broad type now available are inferior. Finding a decent replacement may now be a matter of luck.
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Old 22nd Feb 2018, 11:53 am   #3
tri-comp
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Default Re: Help with Akai AM-U04

A very long time ago when AKAI was still in business in this country, they recommended replacing the voltage amplifier STK-3042 with an upgraded version, the STK3042III, still readily available also through non-eBay sources.
This I did numerous times and had no problems with the replacements installed.
Nowadays I would think doing a re-cap is a must. At least the electrolytic cap's should all be checked.

Yesterday I finished repairs to another AKAI amp., the AM-U210. This one features a stereo-amp. chip, type STK-463 that had blown an output-transistor inside.
Just to be sure I checked each and every electrolytic cap. and they were all fine with low ESR. That shouldn't be possible but there you are !

EDIT:
I'm possibly wrong about the AKAI recommendations to replace the original STK-3042 with an upgraded version STK-3042III.
The /III variant is much newer, I think and fully compatible. What they DID recommend was to use the now obsolete STK-3042II.

Last edited by tri-comp; 22nd Feb 2018 at 12:08 pm. Reason: Variant confusion.
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Old 23rd Feb 2018, 9:59 am   #4
retailer
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Default Re: Help with Akai AM-U04

I decided to unsolder the power amp module from the good working channel (left side) and bridge the board pads 7-8 and 3-4 so the left channel is connected in the same way as the right channel, I had previously hesitated to do this as I wanted a good working channel for comparison. On powering up and running a signal through I now have the same distortion on both channels, on checking the new power amp module with an ohm meter I get a dead short between pins 3 and 4 so it would seem fairly certain the new module was faulty right from the word go, I had assumed as it was new there would be no need to check this out, and so I was chasing a faulty component on the board that probability does not exist, and probably replaced all of the electrolytics for nothing. I went over all of them with an ESR meter and they all check out ok but I thought for only $10 or so I may as well replace them.

Removing the new STK 0500II module and checking pins 3&4 with an ohm meter I find that pin 3 has a very small amount of wobble - push it forward and the short between 3&4 goes away, push it back and the short returns. I messaged the seller and they immediately issued a full refund no questions asked. I'm reasonably confident that a new good power amp module will bring the amp back to life.

Will have to wait until my new power amp module STK 0500II arrives but I suspect that the original STK 3042 voltage amp module is ok
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Old 28th Feb 2018, 5:28 am   #5
retailer
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Default Re: Help with Akai AM-U04

For those that are interested the fault turned out to be the NEW STK 0050II darlington power amp module, it had a pressure sensitive short between pins 3 and 4 , when used in differential mode as in this Akai amp pin3 is input and pin 4 is the corresponding output, as I predicted (and hoped) a new module and the amp was performing great. With my lack of experience it caused me a great deal of head scratching to the point where I was bordering on desperation.

Just about all components now come out of China and the quality control on a great many is just about non existent, I believe this has been discussed on this forum before. I spoke to a friend and colleague who actually earns a living doing repairs and he says that it is getting very difficult to actually make money with new components and in particular power transistors, darlingtons and mosfets giving up the ghost while under test on the bench.
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