Technics SU-V9 bias adjustment clicking sound

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Hi

Yesterday, i waited around 20 minutes before adjusting the amplifer - i set at 5.0mv. Certain the manual has a misprint at 1mv at 1min?


I was monitoring it this morning with leads and meter on
2.7 mv ~ 2mins, 5.1mv ~ 5mins, then 5.8 when i came back around 5 mins later and heard a clicking sound in the amplifer so i turned it off and then the meter

??
 
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Technics SUV series is not conventional class AB so don't expect conventional bias levels. They are current dumpers (distant cousins of Quad 405, 606, 909, 36 etc). There is only a small amount of fixed bias used for the output stage and the rest is taken care of with their "synchro-bias" system ( a type of sliding bias controlled by the input signal level). You can do damage if you attempt to set bias as a static current for some reason. This begs the questions why you are adjusting it in the first place.
 
long story short - trying to fix the amplifer , probe slipped and took out thyristors. Recaps, replaced thyristors with diodes as a friend suggusted. Also got new bourns 3296 trimmers as old were dancing around the place

Amplifer is running and plays well but stays cool - oinly heat really is the transistor on the heatsink which is hot, after sometime the heatsink gets warm. Sound clear, bright on the test speakers. This was with trimmers at almost zero ohms ( my meter said 00.2ohm)

i will lowest the trimmers and try to set as per manual at 1mv at 1minute
 
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From what I can deduce of the power supply circuit, the thyristors are necessary to provide soft starting and safety cutout features, triggered by over temp, low imp, shorts or DC at the speakers. The circuit is integrated with a speaker impedance sensing circuit which automatically compensates for the speaker load too. If they are not replaced with similar type SCRs, there won't be any effective protection from the safety circuits. You probably realise that the diodes you fitted effectively bypass all the major safety devices and this isn't good.

First, I'd try to search what data you can find on (presumably) the SVD6RAM-4 TO220 package thyristors. The maximum current and gate sensitivity are the leading parameters here, others won't be much of a problem, AFAIK.
 
The impedance detector simply drives tap select relay based on impedance detected during speaker selection or power on.

Thyristors were for puristically soft start.

Overcurrent detect is based on current through Emitter resistor -see Q501/502 and TA7317P (also does DC detect)
 
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.....Thyristors were for puristically soft start......
'appreciate the help. So diodes should be OK at least for testing then. As as it seems the thyristors are long obsolete Sanyo types, could you suggest any substitutes?

Perhaps also the clicking noise is the transformer tap relay, after the load sensing circuit has tested the speaker load impedance but not come up with a valid result?
 
I’m not sure about thryistor replacements... - never come across them other than V9/V909 and hadn’t had any of those fail.

You’re asking the right question about relay clicking.... we need to identify which one is the culprit.

If there’s a intermittent transistor in cascade/drive - it could be triggering DC protect... or could it be tap select relay doing this - which will come back to comparator circuit
 
I have a Technics Home Theatre Receiver at a holiday house which is closed up for weeks at a time. This is quite old however it developed a problem with the dc protection relays clicking while playing music usually an indication of a dc problem.

After I took the cover off the Receiver I applied a little pressure to the pcb close to the relays and noticed the slight flexing of the board caused the clicking to stop. The relays themselves were quite large and it seems enough to transmit some vibration into the pcb and that over time this had compromised the solder joins. The solution was to remake these. The ones for the relay connections that is.
 
From what I can deduce of the power supply circuit, the thyristors are necessary to provide soft starting and safety cutout features, triggered by over temp, low imp, shorts or DC at the speakers. The circuit is integrated with a speaker impedance sensing circuit which automatically compensates for the speaker load too. If they are not replaced with similar type SCRs, there won't be any effective protection from the safety circuits. You probably realise that the diodes you fitted effectively bypass all the major safety devices and this isn't good.

First, I'd try to search what data you can find on (presumably) the SVD6RAM-4 TO220 package thyristors. The maximum current and gate sensitivity are the leading parameters here, others won't be much of a problem, AFAIK.

The Mitsu transistor is discontinuted, Reanas or something now has some thyristors CRM 8s. The thyristor i had was messing with the power, wasnt a good subsitute

The impedance detector simply drives tap select relay based on impedance detected during speaker selection or power on.

Thyristors were for puristically soft start.

Overcurrent detect is based on current through Emitter resistor -see Q501/502 and TA7317P (also does DC detect)

I checked a factory SU-V9 (no repairs or fiddling) 17mV after 20 mins both channels

Thanks shall set it both left and right chan to 17mv after 20mins - start with 1mv as manual says at 1 minute then monitor at 10 if a bit how slightly lower bias and - after 20min set at 17mv

I’m not sure about thryistor replacements... - never come across them other than V9/V909 and hadn’t had any of those fail.

You’re asking the right question about relay clicking.... we need to identify which one is the culprit.

If there’s a intermittent transistor in cascade/drive - it could be triggering DC protect... or could it be tap select relay doing this - which will come back to comparator circuit

Yes is the speaker relay that is clicking madly Baily as you and Ian mentioned. OPamp for the comparator is NE5532 TI ( do have original modern replacement RC4558 also on hand)

I have a Technics Home Theatre Receiver at a holiday house which is closed up for weeks at a time. This is quite old however it developed a problem with the dc protection relays clicking while playing music usually an indication of a dc problem.

/QUOTE]

The board did have some flex due to the new banana terminals but i used some foam and hotglue to make sure board was flat as possible

 
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** i feel like a spaceboy lol, i found a blob of solder (in future dont rest a box of spares on top of amp - solder blob mustve droppped out) touch resistors and possible cause of problem, as didnt click after waiting 2 mins , it would be pretty much on power up other times randomly

fingers crossed that is all it is

Q707 and Q707 with probes C / E1615 dropping and reverse 1 Q706, and Q707 C/E 1 and reverse 1
 
* update *

started amp, speaker selectors not engaged, just one click then not anything else

turned on amp
checked at 10mins around 8mv
checked a little after 20mins 22.9mv and only 8.8mv left

adjusted both to 17.0v

made a few adjustments after 20mins maybe another 10 min or so to get them roughtly the same bias

heatsink nice and warm to touch, it would never get warm only if played for a long time

14.6 and 15.2 around before turning off?

should i turn it on and monitor if 1mv at 1min and 20min is still 17v and turn off and check do the same for the other channel or just flip across to check channels as i was doing above
 
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1mV after 1 minute is OK for an initial indication of operation. However, that measurement will depend a lot on ambient and local temperatures which will be changing as you monitor the current. A stable reading at 20 mins, as quoted by SV12004A is going to be the better indication of correct operation and for trimming at temperatures closer to working conditions.

I haven't adjusted bias on early series SUV models but trimming the current will cause a change of heating and temperature on its own, so I suggest only making small adjustments and wait for settling, to avoid overshooting and having to keep twiddling up and down to track it.
 
Thanks Ian and Baily

Will mostly concentrate on the 20min settings and yes it did settle more after 20mins - suppose the 14mv 15mv can get brought up to 17mv

since right channel was the higher starting with that

1min (littler over 1.1mv)

report and update at 20min

once i sort right move to left
 
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17.8 highest around 20mins , so did a few adjustments 30mins plus

i got both sides around 17.0mv (slight fluctuatons 16.9 17.0 and holding -17.3 during monitoring)

amp has never been so nice and warm - heatsink never been warm even after long play i could touch it! The repair place in Germany didnt do bias or something?

? check DC offset before hooking up speakers and playing some music
 
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