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Some help repairing a power amp Kenwood M2
Some help repairing a power amp Kenwood M2
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Old 12th November 2020, 09:48 PM   #1
K-amps is offline K-amps  United States
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Default Need some help repairing a power amp Kenwood M2

Got this Kenwood M2 power amp off eBay, hoping for an easy fix. Does not look that way. Hoping to get some helpful tips on this amazing forum.

Its a weird issue that I have not experienced before.
  • All outputs are ok. Amplifier powers on fine and relays click in etc.
  • At very low volumes there is sound (less than 1vac RMS), then gets distorted in both channels as volume is raised.
  • Pushing a 1kHz sinewave yields a distorted waveform that traces all the way back to the upa68h input differential.
  • There is No bias in the output stages, little to no DC offset.
  • I have replaced the upa68h and it makes no difference.
  • The distortion is in both channels (slight variation) not just one.
  • please see attached pics from my scope. At 1vac the sinewave looks good, and above that starts to clip assymmetrically.
  • Here is a hint, with NO load, the sinewave looks like a sawtooth, but with an 8 ohm load connected (my speakers), it starts to look like a sinewave.

The service manual is available here: Kenwood Basic M2 Stereo Power Amplifier Manual | HiFi Engine I can't attach it, its too large for this forum.
Attached Images
File Type: jpg IMG-2630 copy.jpg (750.8 KB, 175 views)
File Type: jpg IMG-2629 copy.jpg (831.8 KB, 173 views)
File Type: jpg IMG-2628.jpg (835.5 KB, 166 views)
File Type: jpg IMG-2627.jpg (887.5 KB, 163 views)

Last edited by K-amps; 12th November 2020 at 10:26 PM.
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Old 12th November 2020, 11:49 PM   #2
rayma is online now rayma  United States
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Have you tested the power supply? That should be first.
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Old 13th November 2020, 12:15 AM   #3
K-amps is offline K-amps  United States
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Thanks, Yes, replaced the PSU sections with spares (Transformers, Bridges, Caps) no difference. Then added power caps onto existing caps, no difference. Inputted directly to input diffs from sig gen, no difference. Core voltages are spot on.
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Old 13th November 2020, 01:27 AM   #4
rayma is online now rayma  United States
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Are all the measured power supply and circuit quiescent DC voltages correct, including the input stage DCV
across the Zeners (which are fed by current sources)?

Last edited by rayma; 13th November 2020 at 01:31 AM.
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Old 15th November 2020, 03:25 PM   #5
K-amps is offline K-amps  United States
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Zener voltage are good. I measured the input diff (upa68) voltage to a good channel and have done variations...
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File Type: jpg A746329C-60B9-4599-BFD7-FCD448D59C83.jpg (435.1 KB, 137 views)
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Old 15th November 2020, 03:53 PM   #6
Regenpak is offline Regenpak  Netherlands
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This is a flakey design. Look at the first page of the manual. If something has to kick in then in your case it prolly doesn't. Your best bet is to check the voltages marked in the schematic. From the bias V(BE) multiplier Q19 through IC1 (that seems to do something magical) to the weird output transistors Q29-35. No idea what those designers have been smoking...


Edit: the fact that it occurs in both channels is an important clue. Look for things that are shared, like some aux voltage that is missing.


Edit2: This procedure requires differential probes. Or scope with math capabilities (Ch1-Ch2).
Attached Files
File Type: pdf m2_pa_check.pdf (93.4 KB, 29 views)

Last edited by Regenpak; 15th November 2020 at 04:13 PM.
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Old 15th November 2020, 05:08 PM   #7
K-amps is offline K-amps  United States
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Vbe voltage is absent.

There are two relays that click in, both work ( I get signal at the outputs)

The design seems ok. These amps have been working for close to 40 yrs. Cant fault it that much. The OP is class G. Dual rails, lightweight. The OP devices are dual BJT’s (2 substrates on single package) custom Sankens.
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Old 15th November 2020, 05:11 PM   #8
K-amps is offline K-amps  United States
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Quote:
Originally Posted by Regenpak View Post
This is a flakey design.Edit: the fact that it occurs in both channels is an important clue. Look for things that are shared, like some aux voltage that is missing.


Edit2: This procedure requires differential probes. Or scope with math capabilities (Ch1-Ch2).
Exactly, itís some shared supply... just need to figure out where.
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Old 15th November 2020, 05:28 PM   #9
Regenpak is offline Regenpak  Netherlands
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So you have no bias on both channels? As in base voltages of Q21/23 and Q22/24?
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Old 16th November 2020, 03:35 PM   #10
K-amps is offline K-amps  United States
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Yes
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