Jet City 50 Head

[bsco]

Hi. Has anybody here worked on Jet City amps?? I have one here, with KT66 duets and I am trying to figure out some of the schematic designations...there are two relays on the one I have here...VT1 and VT4...but there are other designations called VT1, VT2 and VT3......and with these last three, it shows a square box with a diode and a resistor in each one......the resistors are in the circuit but the diodes are not as shown on the schematic.....which I will post here.......There is no sound but I can get sound from the send jack....I have traced the signal to the top of the treble control, VR5 but there is nothing on the wiper....however the pot checks fine and the solder connections are also fine as well....power supply voltages look good.......no signs of overheating....I wonder if I have the wrong schematic??anyway if somebody can take a look at the schematic that I have that would be great........
CheersJet City JCA50H Schematic.pdf

[52 Bill]

That's confusing using VT1 for a relay as well as an optoisolator.

The resistor with a diode represents and photocell and an LED, being used as a switch just like Mesa amps.

[mozwell]

The sch i have for this (found in my travels) doesnt have the optos, it has relays instead, so maybe this matches your amp better
The date in the title box is different to the sch you linked to

good luck on the repair

[bsco]

That's confusing using VT1 for a relay as well as an optoisolator.

The resistor with a diode represents and photocell and an LED, being used as a switch just like Mesa amps.

Yeah...there are no optoisolators in this one...only two relays....

[bsco]

The sch i have for this (found in my travels) doesnt have the optos, it has relays instead, so maybe this matches your amp better
The date in the title box is different to the sch you linked to

good luck on the repair

Thanks for the schematic.......the only difference I can see off hand, is that the head I have uses KT66's....the schematic shows 6L6's which is no big deal.....outside of that, it looks the same.....with two relays....

[bsco]

I had a look at this last night......I can trace the signal right up to the top of VR5 which is the treble control....there is also signal at the bottom but nothing at the wiper....I removed the pot and checked it and it is fine...I also checked for broken traces in that area and couldn't find any......I will take another crack at this later today...any suggestions anybody?? The schematic that moswell posted is the right one with the only difference being my amp uses KT66's and the schematic shows 6L6's......

[g1]

Jumper the wiper of the pot to the top lug, does signal now get through?

[bsco]

Jumper the wiper of the pot to the top lug, does signal now get through?

Thanks g1....I just fixed my signal tracer that I had made from an old computer speaker....put a 600V coupling cap in series with the tip and it works great....I just probed the pins on the tone pots.....ther is a tone on both ends of the treble pot but nothing on the wiper....I had removed the pot to check it with an ohmmeter and the pot works fine.......I'll jumper that out for a quick test and let you know what happens......most of the traces on this one are on the bottom of the board which makes it difficult to power up the board out of the unit with the tubes installed......very dangerous.....

[bsco]

Thanks g1....I just fixed my signal tracer that I had made from an old computer speaker....put a 600V coupling cap in series with the tip and it works great....I just probed the pins on the tone pots.....ther is a tone on both ends of the treble pot but nothing on the wiper....I had removed the pot to check it with an ohmmeter and the pot works fine.......I'll jumper that out for a quick test and let you know what happens......most of the traces on this one are on the bottom of the board which makes it difficult to power up the board out of the unit with the tubes installed......very dangerous.....

Ok. I just tried that g1.....no sound.....funny when the pot is removed, it checks fine...when the unit is put back together, it doesn't get to the wiper.....I don't suppose the wiper is broken inside in such a way as to prevent it from working...but that don't make sense as I jumped the top pin to the middle pin.....and I did check the traces underneath for breaks but they checked ok......unless the trace opens up but if that was the case, there would be a signal on the wiper contact.....but would not get through the break on the trace......this is a weird one.....seems like the wiper or something connected to it is killing the signal.....and the only thing connected are the top of the master pots and the relay and I have already removed the relay and checked it with my lab power supply....the relay switches fine...the master pots checked fine as well.......I will have to take the board out again and see what I can find without connecting power.....

[g1]

Do you measure a short from wiper to ground?

[bsco]

Do you measure a short from wiper to ground?

I just checked it..Yes..there is a short from wiper to ground....I unbolted the pot from the chassis to isolate it and checked it again...still shows a short of 22 ohms...either the pot is bad or there is a short on the board in that general area.....I will have to pull the board again and re-check everything...If the short disappears once the pot is removed then it is the pot......I'll keep you up to date......
Cheers

[bsco]

OK. I removed the board again and removed the pot and took it apart...I noticed that the wafer was sort of on an angle.......it wasn't perfectly flat....don't know if that caused any issues but I reassembled it and re-installed it......checked fine...re-installed the board and just slid it in place with finger pressure and re-checked...short was gone...so I secured the board, installed all the jacks, washers and nuts, etc......checked....no short....Re-installed all the tubes and powered up....It worked.........but..........I found that I had to have the foot switch connected in order to get sound......is that the way this amp works?? The plug on the end of the foot switch is only mono......I would think that the amp can be used without the foot switch....anybody have any ideas on this???

[g1]

Is it that the channel that is on with no footswitch is dead?
Or do both channels work when footswitch is connected?

[bsco]

Is it that the channel that is on with no footswitch is dead?
Or do both channels work when footswitch is connected?

Both channels work only when the footswitch is connected...if you disconnect it there is no sound.....the foot switch is connected through both relay coils....But...before that, I had that short to ground on the treble pot...That is gone now.....The owner does not know if the amp is supposed to work without the footswitch....he said he had always used it with the amp....I broke a few years back and he took it out of storage and dropped it in to me.....

[olddawg]

Both channels work only when the footswitch is connected...if you disconnect it there is no sound.....the foot switch is connected through both relay coils....But...before that, I had that short to ground on the treble pot...That is gone now.....The owner does not know if the amp is supposed to work without the footswitch....he said he had always used it with the amp....I broke a few years back and he took it out of storage and dropped it in to me.....

As Enzo is fond of saying... Call them! What have you got to lose?
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