418S with squelch problem...

[frankb]

I'm working on this 418S Rhapsody II: amp is rebuilt and sounding great, using a P132 cartridge and the AVC is working.  When the tonearm initially starts playing the record, volume level is quite loud and quickly reduces to a normal level.  Say, in the middle of the record the tonearm is lifted off the record for a few secs, and put back down, the volume is again loud and comes back down to normal.  I believe that means AVC is working but every record starts out loud which I believe is a squelch problem.  I have the manual for it but can't find any info regarding squelch.  Can anyone point me in the right direction regarding this?  Thanks in advance.

Frank B

[jukenorman]

Hi Frank,
Is the mute relay opersting OK? There is a time delay on it also to be considered. Try operating the relay manually; when you release it, does the volume start low and gradually increase?

Norman.

[frankb]

Yes, the mute relay is working. The amp comes on about 1/2 second after the stylus hits the record. I really don't know how I could try to operate the relay manually - I don't have a workbench set up to power and play an amp like this and it would be pretty tough to do inside the machine. I normally work on Seeburg and AMI tube amps. Is that 300 uF cap across the mute relay involved with squelch? I assumed it was strictly for timing the amp start. I just looked at the schematic for this amp again and I can't find anything that looks like squelch circuitry. Everything is so perfectly "bilateral" between the two sides. Usually there will be an oddball tube that functions as a diode to rectify a bit of the audio to a DC voltage used to control amplifier gain at a given stage. I don't see that anywhere on this amp.

[jukenorman]

Hi Frank,
The mute relay contacts appear to have two functions, straight muting and some "squelch" action via the volume control and AVC. I have never had to do any real fault finding on that section though. The 300uF capacitor would have an effect because it holds the mute relay energised for a short time after micro switch 3 operates. So if it was bad, the AVC has more time to ramp up before music starts. A long lead in groove would also contribute. Manually operating the mute relay would provide some answers but I agree that sometimes what looks straightforward may not that be easy in the real world.

Norman.

[frankb]

Hello Norman, thank you for your response. I did manage to rig up a pair of jumper wires to run to the outside of the amp to switch the mute relay off & on. I disconnected the black-orange wire coming from pin #3 of the 10 pin connector at pin #3 of the 6 pin volume control socket and soldered a jumper wire to it that runs external to the amp. A second jumper was soldered to pin #3 of the volume control socket and run externally. Now, at idle with no record selected, when I connect those 2 jumpers I can actually hear the mute relay click on and off. With the jumpers connected (normal operation) and a selection made, the record comes out; amp turns on about 1/2 second after the needle hits the record and is at first loud then comes down to normal volume. If the jumpers are left open, the record comes out and the amp is already on before the needle touches the record and again, plays loud and quiets down to normal level. The only difference between those 2 scenarios is the time when the amp turns on. I also tried disconnecting and reconnecting the jumpers while the record plays - I couldn't detect anything! The volume stays the same & the amp stays on. I'm starting to get suspicious of those 2 thermistors in there. I don't know much about them. I did measure their resistance - 3.8K & 4.2K ohms. What do you think? Thank you so much!

Frank B

[jukenorman]

Hi Frank,
What about the contacts on the mute relay, are they clean? When you operate the mute relay with a record playing, you would expect no sound because you are applying a ground to the 0.01uF capacitors at the balance pot. And when you release the mute relay, you would expect to hear the sound ramp up.

Norman.

[frankb]

Hi Norman, The relay contacts are very clean. I got the thinking, with the way I rigged up those jumper wires when a record plays, micro switch #3 has already killed the power to that relay. And, of course, when no power to the relay - the amp will be on. Connecting or disconnecting those jumpers will do nothing while the record plays. I guess I would have to rig power to the relay that I can switch off and on to actually power the relay on while a record plays.

Frank B.

[frankb]

One more thing - just to be clear - when I wrote that the record starts out playing loud, I don't mean "knock down" blasting loud, just noticeably louder than it should be.

Frank B

[jukenorman]

The only other thing I can suggest is to temporarily attach a wire to each end of the balance pot and ground these to simulate the mute relay.

Norman.