Low hum 50Hz in both channels SA-9800, SA-8800, SA-7800

[EWI]

Hi all,

I do have all three models of Pioneer amplifier (SA-7800, SA-8800, SA-9800) in everyday use. I bought them over the last 12 years and must have repaired the units with the help of Audiokarma forum (specifically thanks to Markthefixer). They all run fine, however I can find very little hum (50Hz as I live in Europe) when using sensitive speaker or sensitive headphones. When using not so sensitive headphones (e.g. my old AKG K-141) amplifiers are perfectly quiet. When I use more sensitive ones (e.g. Koss Porta Pro) the little level of hum is there. Even when volume is completely down, even if the inputs are short-circuited, symetrically in both channels. I do not know how to measure such low-level signals by scope. Is that the feature and I cannot achieve better result OR there is something I could repair to get rid of this "little level" hum. In case you would have some reccomendation how to measure it by scope and distinguish between the background noise and the hum I could share the results.

Thanks in advance for your suggestions and opinion

Ewi

 

[EWI]

Update for SA-9800

I disconnected the jumper between the pre-amp and power amp located on the rear panel of the amp. The hum is gone. So I suspect the control amp assy (preamplifier). And as the low level hum is presented in both channels I tend to suspect power source.
I made some measurement on the Power supply assy AWR-191 and everything seems to be fine except the ouput of the diode bridge (PIN 13 should be 68V and PIN 15 should be -68V). I measured DC values and also AC ripple over the condensator 100nF/100V. The values are as follows:


PIN3 (52,1V DC, 3mV AC) expected value 52V DC
PIN6 (32,0V DC, 3mV AC) expected value 32V DC
PIN 18 (19V DC, 3mV AC) expected value 18VDC
PIN 10 (-31,9V DC, 3mV AC) expected value -32DC
PIN 1 (-52,1V DC, 3mV AC) expected value -52V DC

PIN13 (70V DC 1,9 VAC !!) expected value 68V DC - unregulated
PIN 15 (-72V DC 1,2 VAC !!) expected value -68V DC - unregulated


I believe that the ripple values on the capacitors C2/ C3 (220uF/80V) are high and also the same ripple is propagated to the PIN9 and 12 (through the resistor R1 resp.R2

Any advice how to fix the ripple? The ripple after the regulated supply seems to be OK.

Thanks

EWI

 

[EWI]

Update_2 for SA-9800

Changes on the AWR-191 (Power supply assy)

- I changed all the capacitors and semiconductors according the thread and the list provided by Markthefixer https://audiokarma.org/forums/index.php?threads/sophisticated-sabotage-on-an-sa-9800.634147/page-2

- I have found and repaired many bad cold solder joints on the same board (specially around semiconductors stressed by heat (e.g. D10 Zener)
- I have changed the C2 and C3 from 220uF/80V to 1000uF/100V (Mouser No: 598-108TTA100M)
- the ripple measured on C2 and C3 decreased from 1,2V to 0,2V and also the ripple after the regulator was further improved

- the hum decreased and now is hardly hearable even with the sensitive headphones (Volume 0, inputs short-cutted)

I plan to repeate the same modification on the SA-7800 amplifier.

Any comments appreciated

EWI

 

[EWI]

Update_3 for SA-7800

- increasing capacitors C29 and C30 in the regulated power supply (+ - 48V) for pre-amp had no positive outcome. I left there the values 220nF/80V
- changing the snubber C38 (10nF ceramic) in the non-regulated power supply (+ - 46V) brought the decrease of the one hum element. I changed the snubber to WIMA
505-MKS4G024702E00KS (Mouser). Seems that the reason was not the increase of the capacity from 10nF to 47nF but the positive result was probably caused by the lower WIMA ESR (57ohm) in comparison to the former snubber (1 200 ohm) at low frequencies (compared by measurement, at high frequencies (100kHz) were ESRs equal)).
- Further improvement was achieved also by rerouting the headphone cable by the shielded one as described here https://audiokarma.org/forums/index.php?threads/pioneer-sa-8800-headphone-hum.936682/#post-14269094 (Thanks to Wally C)
- I changed also the relay to Omron 653-MY4-02DC24 (Mouser), but this had no relation to the hum issue - Thanks to Markthefixer for the recommendation I found in one old thread in AK

Now the hum has been decreased to the level which could be found on the very sensitive headphones only and only when the surroundings is completely quiet.
However if anyone would have any further recommendation, please share.

EWI

 

[hd-880]

Same problem here with SA-9800, hum with jumpers plugged on back. But if I remove the jumpers the hum is gone. I reccaped all pre capacitors and same. The hum is intermittent, sometimes works ok and sometimes humming. Did yopu find a solution? Thanks