SOLVED X2 vhs problems.

[Teddeeh]

So i have 2 VHS’s. One the signal only goes through the chorus and reverb, and the other there is no signal atall.
the boxed one has the chorus and reverb signal, the out of box one has nothing. I changed the eeprom around and different tl072’s but no change.

 

[Robert]

Regarding the one that does pass signal... Does it still pass audio when the boost is on? Does the boost LED light?

The boost circuit is always in the signal path, it just switches from a boost to a unity gain buffer. If signal is passing but you have no boost the most likely culprits (aside from solder splash, etc) would be the toggle switch or the boost pot itself.

 

[Teddeeh]

Here is the one with no boost. Checked for bridges, checked the switch, and changed pot. Nada.

 

[Teddeeh]

Boost led does light up if i place one in there.

 

[Robert]

Turn the boost pot all the way up then measure the resistance across the outer two lugs with the toggle on and off.

 

[Teddeeh]

No resistance atall 0.00

 

[Robert]

There's a short of some sort across the boost pot then...

I'd remove C13 (470pF) just to eliminate that as a problem, but it's unlikely.

If there are no solder splashes pretty much the only thing left is the toggle switch. Switch it back and forth several times quickly and see if maybe the contacts are stuck on one side.

 

[Teddeeh]

Yes removing the 470pf cap did the trick. Boost working.

 

[Robert]

Well I'll be damned.

Put a good one in there and you should be good to go.

 

[Teddeeh]

Yup, new cap working fine. First dodgy cap ive had in a long time.
1 down. 1 to go.

 

[Robert]

The other one might be a little trickier to diagnose...

Does the noise floor change when you turn up the boost knob?

 

[Teddeeh]

My sincere apologies- its been a hell of a week. I will be investigating the second vhs asap so i will reply here when i can. I dont think the noise floor changed atall.

 

[Teddeeh]

The other one might be a little trickier to diagnose...

Does the noise floor change when you turn up the boost knob?

Yes the noise floor does change.

 

[Teddeeh]

Any ideas?

 

[Teddeeh]

Bumpity? Any guesses?

 

[Robert]

Do you have an audio probe to confirm if audio is going into the FV-1 on Pin 1 and leaving the FV-1 on Pin 28?

 

[Teddeeh]

Do you have an audio probe to confirm if audio is going into the FV-1 on Pin 1 and leaving the FV-1 on Pin 28?

I dont have an audio probe, shall i make one? Or any suggs for a purchase?

 

[PJS]

To make one you pretty much just need a jack socket, piece of insulated wire and a cap. If you also have an alligator clip that will certainly help. A bit of heatshrink and an old multimeter lead will make it really fancy. Just google for audio probe and you will find heaps of posts on how to make it and use it.

 

[Teddeeh]

Do you have an audio probe to confirm if audio is going into the FV-1 on Pin 1 and leaving the FV-1 on Pin 28?

No audio atall. The probe didnt pickup any signal atall.

 

[zgrav]

you can trace the incoming audio signal and find out where it stops before reaching pin 1 on the FV-1.