rockola amp 41710-a

forom experts

Hi me again with an amp buzzing querie, when my rockola 430 has played a recod and is on the runnout gooves before operating the tonearm switch there is a loud hum thro the speakers ,the main caps have been replaced and the amp was ok , checked grounding ,cartridge/stylus , is it a possible tube fault
any info please

regards to all

mick

Forom-Experten

Hallo ich nochmal mit einer Amp-Summen-Frage, wenn meine Rockola 430 einen Recod abgespielt hat und auf den Runnout-Gooves ist, bevor der Tonarmschalter betätigt wird, gibt es ein lautes Summen durch die Lautsprecher, die Hauptkappen wurden ersetzt und der Amp war in Ordnung, überprüft Erdung, Patrone / Stift, ist es ein möglicher Röhrenfehler
irgendwelche Infos bitte

Grüße an alle

mick



..

Hello mick

I would suspect that the rubber grommets of the turntable-motor mount are hardened. ( [www.jukebox-world.de] )

Best regards - charly49

Hi Charly49
thanks for reply ,i will check the grommets but i fairly sure they are new but not 100% sure so i will test them

regards

cheers
mick

charly49
i have replaced the grommets on the tt with my spare set but no change, i have notice that with ful bass the hum is a lot more noticeable, (love that bass) could it be a cap or valve again any advice would be a great help

best regards

mick

Hello Mick,
a simple test is to unplug the 110V TT-supply while a record is playing. If the hum is at the same level that bothers you in the runout groove (same volume and bass setting), the problem is electrical nature and you can further isolate it easily because the amp remains unmuted. If the hum comes from the TT-motor, worn out motor shaft bearings can also cause high vibrations of the motor which are transferred to the turntable. Grommets mounted correctly (soft side up) ?
Regards
Jürgen



1 mal bearbeitet. Zuletzt am 19.08.20 11:48.

Hi Jurgen
thanks for help, i will try all you say and reply with results, the grommets are in correct

regards

mick

i have completed the check suggesed byJurgen with the TT unplugged during the ru-off the speakers stil have a loud hum until the tone switch is activated,could i adjust the hum control or is it an amp problem?

cheers
mick

Hello Mick,
with the TT motor unplugged while a record is on the TT, you can lift the tonarm and move it around. If the hum is only in the inner position of the tonearm, the cable could have loose contact. You can also pull the cable at the amp. to rule out the cable. The HUM control pot. at the amp has only little effect but you can try. Switch off the neon lamp to see if it radiates into the amp. If all these can be ruled out, your amp needs repair.
For reference, the hum of my 1484 (max volume, mid mass, TT motor off, record on the TT) is about the level of the mechanical hum generated by the neon ballast. For me this not audible if a record is playing, also not bothering in the runout groove. There is some noise during runout and runin but needle scratches and very low frequency rumble is dominant over the 50Hz and 100Hz hum.
Regards
Jürgen

Hi Jurgen
again thanks ,i will try all you suggest and hopefully solve the fault, if not i will remove the amp and see if i can see a problem,i will post after i do the checks you mention

best regards

mick

i have carried out all the checks suggesed by Jurgen i also tried another volume contol but no luck ,
so i tried the balance control on the amp and found that when on part/full right hand speaker there was a breakup on the speaker output , the left speaker ok ,so it looks like a problem with the right channel in the amp
again could this be a tube fault

regards

cheers
mick

Hello Mick,
yes, a bad tube can be the reason for hum. If one channel is ok, you can swap the tubes one by one from one channel to the other (swap the 6BQ5 in pairs) . Only the first stage (12AU7) is common for both channels. You can take it out and if te hum dissapears, it worth a try to replace it with a new one. Of course, other components in the amp can be the problem.
Best regards
Jürgen

Hi Jurgen
thanks ,i will swap tubes as you say and see if this is the problem with the right channel and hum, will post reults

thanks and best regards


mick

Hi Jurgen and all

i have replaced the tubes the EL84 in pairs , the 12at7's and 6cy7's seperate it appears the righr hand pair of EL84's were the channel poblem ,stil got the hum so i will remove the amp and have a look , could it be a cap?

regards

cheers
mick

Hello Mick, did you connect 2 speakers and switched to "stereo" when you swapped the tubes ? If you just listened to the machine speakers in "monoraul" , swapping the tubes doesn`t make a difference. I should have mentioned this. If the volume or balance setting changes the hum level, the problem is likely in the preamp. If it`s really just one channel humming, you can rule out the filter caps of the power supply because they are common for both channels. Other than that, it's not possible to point to a specific component.
Best regards
Jürgen

Hi Jurgen

i have 2 door speakers connected to the 16ohm left + right terminals and on (20w) stereo all the time, i will check the balance to see if it hums on both channels, the hum supsides on lowering the volume
thank
i have just checked with the balance control and it does hum on both channels

best regards

mick



1 mal bearbeitet. Zuletzt am 21.08.20 16:18.

If the hum is on both channels, check if it's 50Hz or 100Hz.(60/120 in US) . A simple test is to touch the input. Your body picks up 50 Hz, so if the hum is one octave lower when you touch, your problem is 100Hz hum likely caused by bad power supply filter caps having lost capacity.
Regards Jürgen

Hi Jurgen, when you say touch the input can this be the stylus or am i completly wrong

regards

mick

It's easier to pull the input cable and touch the input terminal at the amp.

no problem Jurgen

cheers

Hi Jurgen

with the tonearm unplugged and record playing (so amp on) touching either channel at the input socket gives a loud hum at the speakers does this mean anything

regards
mick

Hello M,
that means the amp is doing his job.
Regards PitinWHV.

Play a record, pull the 110V TT suppyl and the tonearm cable.
You hear the same level of hum that bothers you in the runout groove - right ?
Then touch the input a couple of times.
If you hear a frequency change, the problem is very likely the power supply filter caps.
If you only hear a volume change, then there is no easy way to find out the cause, your amp. needs professional help.
Best regards
Jürgen



1 mal bearbeitet. Zuletzt am 22.08.20 12:29.

Hi Jurgen
thans again for your help, i will carry out advice and check thew results,i have just aquired another amp it is complete with valves but has not been re-capped, so after doing the checks as stated i will replace the amp and test the filter caps on removed amp although these are replacement caps (6months old) i will post after all done
thanks again for your time

cheers
best regards
mick

Hi Jurgen
i have carried out the 2 checks unplug TT and remove tonearm plug while playing a record what i noticed was when i removed the input plug the hum stopped,when i touched the input channels just a buzz thro speakers, i plugged the input back and the hum returned, i will swop the amp this weekend

best regards

mick

Hello Mick,
I assume there is a bad ground connection between the pickup and the amp. For example dirty pins at the input plug or the pickup connectors, a bad cable, a bad pickup or a problem with the ground connectin inside the amp.
Regrds Jürgen

Hi Jurgen

i have started checking all possible causes and will post if i find the problem, thams

best regards
mick

Hi Jurgen
i removed the complete tonearm from the JB to test the wire /plug checked the cartridge/stylus all ok then i noticed a wire (tail) from the ground on the cartridge to the fixing screw on the base for the cartridge, removed it re-installed the tonearm played record and no hum don't know why but it's gone
have you a reason why?

best regards

mick

Hello Mick,
the tonearm makes a connection to the chassis which has a slightly different ground potential than the amplifier. When you connect the cable shield on both sides, current flows through the shield adding just a few millivolts of hum to the tonesignal. See "ground loop".
Best regards
Jürgen

thanks Jurgen, it shows how a small problem has you spending hours looking for a fault, so it is a big thanks for the all info and your knowledge

til next problem

best regards

mick