Phonoland

Hi Guys: I am stumped on the amp for My Wurlitzer 1800. I have totaly recapped it and replaced the tubes that were weak. Because I totaly refurbed all the electromechanical bits to get it restored to working order. Everything works, coin mech, player, keyboard and seletor. cobra cartidge replaced. No sound out of the amp at all, except for an ocasional pop or click. So i know the speakers work. I have gone through the pots and rotary switches with De-Ox it.
Sooo because i have gone over all the usual suspects I have to move on to bench testing all the voltages. I am not sure what i will need to do to power it up out of the box, as the placement of it does not allow you to get to the component side of the amp.
I do not have a signal injector to test out the diferent sections of the amp. (would like some suggestions of what a good one would be)
Any other help would be great as i was nearing the end of this restore and the box looks beautiful when powered up. Thanks Mark

Have you checked the muting switch on the stack relays below the mechanism motor? Perhaps the blade switch is out of adjustment, sticking or dirty........

Only other thing I can think of is if you had a spare amp laying around or someone local could borrow you one. That would isolate the amp from the rest of the machine.

Also, make sure the solenoid in the amplifier is working to provide full power to the output tubes. You can jumper it in the mean time to make it go full power all the time, just for troubleshooting.

Thanks for the reply. i have checked the mute switch at the amp plug end and get the correct continuity. So i know that is ok. I do not have a doner amp, but i connected a line output from an old cassette deck to the amp input to isolate the cartidge, nothing. If you are referencing the heater voltage diference when the juke is on standby, the diference is obvious as the heaters get all pretty when a selection is made.
I am still on square one and the next thing I can think of is voltage checks on all the tube pins. Any suggestions on what to be careful of when going through this process would be appreciated. Mark

Bad tube sockets or connectors?

mb9513 wrote:Thanks for the reply. i have checked the mute switch at the amp plug end and get the correct continuity. So i know that is ok. I do not have a doner amp, but i connected a line output from an old cassette deck to the amp input to isolate the cartidge, nothing.

Hold on here, if you connected a source to the same RCA plug where the pickup goes -you will get --nothing--. the Cobra is an FM oscillator/detector design. It does not generate voltage but instead varies the frequency of the oscillator which causes it to change the maount of Dc that it draws. the result of that change corresponds to the audio content of the record.

FWIW: This is a super-simple for of FM called "slope detection".

There is another RCA plug for a booster amp (IRRC) It is meant as an output, but should also be useable for a test here. Make sure all is in play mode.

If you get sound there, the suspicion falls on the oscillator tank circuit and the connection to the Cobra. Set volume at mid point and in play mode try jiggling and then removing the Cobra. This -should- make a lot of noise and may indicate poor connection..

An ohmmeter across the pickup RCA should show a low impedance which goes infinite when the Cobra is removed.

I suggest that you consider replacing that obsolete pickup with a magnetic and outboard preamp. it is more difficult with the 1700-1800 arm design, but the results are definitely worth it.

Here is a pic of one 1800 I modded with a Pickering DAT magnetic:

http://s1192.photobucket.com/user/Rob-N ... ort=3&o=41

Rob/NYC

Thanks Rob, I am trying to understand this kind of pickup. i have tried to find anything on line but din't have much sucess. I was leaving the Cobra in it(got a nos one as well) I will be selling the box when restored to get seed money for the next project and wanted to keep it as origonal looking as possable.
I tried your suggestion of connecting an audio line output to the aux amp output to see if i could get anything out of it, nothing.
I did voltage checks on the tubes and found most were in line with the schematic. the off value ones were the plate of 12bh7(pin1) just up from the 6l6 tubes. it was at 200v rather than 144. pin 1 on 12ax7 was 175v rather than 150.The ocillator tube 6j5 was goofy. it had 80v on plate (pin 3) the schematic notes say 45. and 0v on the screen (pin 3) rather than 4.5. this was with the cartridge connected but empty spot w/o a record in play mode. i am not sure what the schematic notes mean, on these pins "input removed. whith pick-up in and circuit oscillating"
so i am at a loss as to how to get the oscillator circuit to "oscillate"
one other thing i noticed is when checking the plate and screen on 6sj7-2, i got sound out of the speakers when the meter probe touched them. Mark

I tried your suggestion of connecting an audio line output to the aux amp output to see if i could get anything out of it, nothing.

If you did this with machine in play mode with 5U4 lit and mute plug removed and had no output, the problem is after this point. Check things like volume control wiring, From there-on look for mistakes in rebuilding.

The odd voltages around the oscillator tube may be due to it not oscillating which it won't do w/out the Cobra attached properly.

In other tubes an abnormally high plate voltage indicates the tube is either very weak or not conducting at all.

Rob