Rock-ola Capri 100, amplifier.

[transistor73]

Hello Everyone.

I open this post discussion because I found the sound totally lack of TREBLE, and poor of BASS, only middle frequencies seems to sound good.

What do you think about the capacitors replacement at amplifier, the Juke box has been more than 30 years without turning on.?

I have read the better thing to do is changing all capacitors, others think only some of them?

what are more common type due to fault?


I have thought to measure with multimeter all capacitors and change only the fault ones...

What do you suggest please?

[Richardgr]

I think a circuit diagram and/or pictures helps. Then the circuit can be assessed for the issue, and the pictures can be reviewed for general condition of the apparatus.

[transistor73]

thank you Richard for your quick reply.

I have attached the diagram, but not the amplifier, because is not disassembled yet, Ill do it ASAP.

Carlos

[transistor73]

I have attached two amplifier pics...

[joebog1]

I can see a couple of VERY pregnant electrolytic caps there. I cant see values or labels.
There are also a lot of "Dart " branded capacitors, I have never seen them before but they look like paper capacitors. These should be checked on a decent ESR meter that measures at the actual voltage in the circuit.
ALL the resistors I see can be very drifting in value. Mostly they get higher in value as they are carbon composition resistors. Very low values will probably pass a test, but anything over about 22k or 47k will be very suspect.

Apart from value shift, they can also be very noisy!!.

Joe

[transistor73]

Thank you Joe; I would,nt like to change randomly the caps...

but I got inform electrolytic caps should be recommended to change after twenty years even so, if the machine has been without connecting... due to these caps get dry quite often...

Close to the tones knobs there are four 0,1 caps value... these look like not very good, with a CAP MULTIMETER this kind of values are difficult to test because of a fluctuating measure...

do you suggest to change almost all the electrolytic caps?

[PJL]

Before you continue to use the amplifier you need to replace the audio coupling capacitors that connect the phase splitter to the output stage grids. Faults in the output stages can cause expensive damage to the transformers and valves.

Old paper capacitors fail by electrical leakage and this affects the DC operating conditions of the valves. Simple capacitor testers do not measure leakage which must be measured at the working voltage of the capacitor.

If you post the picture of the full amplifier we can advise you on the parts to replace.

[vinrads]

Hi Carlos can you give us the voltage readings on the output valves there are four 6973's, be careful you should have around 350/370 on two of the pins, 8 and9 and a negative voltage around 25 volts on pin six.as Joe said the electrolytics may well need replacing especially the 200uf smoothing , Mick.

[transistor73]

Thank you everyone, i dont want to take any step without making sure what I,m doing... appreciate your help.

The thing is; there are two top treble speakers which only makes hum.

and bass sound is quite poor... only middle frequencies seems to work ok...

[transistor73]

I, m think I,m going to wait up to I get more information.

I dont want to changes capacitor without test them, my multimeter has a basic scale and consider it is not very reliable.

[Radio Wrangler]

To test the paper capacitors, you need a high voltage leakage tester. Ordinary multimeters won't show anything wrong with a part that would destroy your output valves and transformers.

To test electrolytic capacitors, you need to do leakage at the appropriate voltage, and ideally also check the equivalent SERIES resistance (ESR).

For the DC blocking capacitors from phase shifter stages to the grids of the output valves, you might just as well replace them. These are the ones which can do real damage.

David

[transistor73]

Thank you Dave

""""""""""For the DC blocking capacitors from phase shifter stages to the grids of the output valves, you might just as well replace them. These are the ones which can do real damage""""""


do you refer to the marks with white arrows; please?


Sorry for my knownledge, its the first time I open a valve amplifier... Thats why I want to make sure every step I take

Regards.

[Radio Wrangler]

I don't know the specific amplifier so I can't say from the picture. Assuming the amp has a push pull output stage, find the output valves. There will be four of them. `then look up the data for their type and find which pin connects to grid number 1. Trace connections to each of these and look for a capacitor of about 0.1 microfarads. These will be the ones

David

[vinrads]

This amplifier uses 6973's x4 with fixed bias I have never had to replace the coupling capacitors the brown ones like in this amp ,the 25volt neg is derived from the power supply I allways replace the neg smoothing capacitor. and the two 200uf in the HT doubler cct. Mick.

[Joe_Lorenz]

I've got a 1967 Rock Ola 434 Concerto in my dining room. Due to rather frequent usage it has eaten up rather fast the 6CY7 drivers twice, and finally all electrolytics were beyond trustworthiness. Yes, some resistors can go high value but mine have not yet.
Check all pots thoroughly: If the machine has spent it's first life in a smoker's pub the pots can have become from cracklin' to o/c.
In healthy condition those machines give a full and warm sound (such as the power consumption does suggest!).

Joe

[DMcMahon]

Originally Posted by transistor73

Sorry for my knownledge, its the first time I open a valve amplifier... Thats why I want to make sure every step I take

Regards.

Because you are not experienced with working on valve equipment, please be cautious and very careful if/when you do any live measurements like voltage.

As well as the AC mains voltage and AC voltages from mains transformer, there will be numerous DC voltages at components and valve electrodes, these DC voltages can be well in excess of 200 volts, maybe as high as 300V, these voltages can seriously bite !

Edit Update - 350/370V from Post 8.

David

[transistor73]

Thank you Everyone for the help offered!!

David the safe is the main thing to take care about due the high voltage at main power and amplifier comes, thanks a lot for your info and very useful advice.


I can say the jukebox sound is not bad, but should be better than it is due the kind of machine it is... So maybe the reason for that, is a couple of components that should be worn out, or maybe an easiest thing...

When started, changed the Astatic cartridge for another ceramic one... but I did not get any sound improvement...

I,m going to do a spotlight cleaning of pots as Joe_lorenz has explain, because the machine has surely been in a pub at first time.

Always point at capacitors according what I have read about filters and frequencies, That is why I asked what are they the more common due to fault, In all recommendations for changing them, exist different opinions.

Thanks a lot, i will keep inform.

Regards

[transistor73]

Hello Everyone!

Two points;

1. The treble sound issue has been solved. I opened the top speakers and there is a quite hide pot to set up these speakers... Sorry about that, but thats was the problem, i,m not acquainted in these kind of machine.


2. It probably has to sound with a deeper bass, but I,m not going to "touch" the amplifier anymore

it probably is working at 80%... now, my opinion at this point is better no operate the amp.

Some filter, some cap, some resistor... are not working at 100%, but the sound quality is not bad. I do not owned a proper cap multimeter, so I think is better to enjoy the rest of restoration and maybe in the future, it the sound quality turns bad, I will sort out...

I think so...


I would say thank you to everyone who has replay and interested in this issue, and hope it helps also.


Regards.

[DMcMahon]

Originally Posted by transistor73

Always point at capacitors according what I have read about filters and frequencies, That is why I asked what are they the more common due to fault, In all recommendations for changing them, exist different opinions.

Yes there will always be a wide range of opinions/advice/recommendations when someone asks for help replacing components, especially capacitors, this will vary from "replace them all" to "only replace any that are proven to be faulty" with various intermediate recommendations between these two extremes.

This is normal and there normally will be valid reasons for the various recommendations given.

I admit depending upon one's experience it can be confusing at times.

David

[DMcMahon]

Originally Posted by transistor73

Some filter, some cap, some resistor... are not working at 100%, but the sound quality is not bad. I do not owned a proper cap multimeter, so I think is better to enjoy the rest of restoration and maybe in the future, it the sound quality turns bad, I will sort out...

Carlos I understand your reasoning above but I would advise you still consider the advice given in Post # 7 which advised replacing the coupling capacitors from the phase splitters to the output valves grids because as stated, if the capacitors are bad (high leakage) then worst case is that the output valves and/or the output transformers could become damaged.

For the Forum to advise which capacitors these are, the full amplifier schematic or a section showing the output valves should be posted.

You could measure (bear in mind your personal safety) the control grids of the output valves to make sure there is no +ve DC voltage present, but even if there is no +ve voltage, it may still be best to replace the coupling capacitors for future long term reliability.

David