Vox Pathfinder 15 Doesn't seem quite right.

[ZMAN]

Just got a schematic from a person, with whom I've dealt with on CL. I wish he'd make an account here, he knows a ton. In any case, he added reverb to a Epiphone Valve Jr., and I was thinking of maybe trying something similar with this Vox (as long as I have it apart), using a Belton module. Just gotta figure out where to get 5v from the Pathfinder.

Help me out on this one, I have no knowledge of electronics. That schematic shows how to hook up this Belton module. What does the belton module consist of? I see a 12ax7 which I know is a tube. I was wondering what the difference in price would be to use a reverb pedal instead, or do you just like to tinker? The reason I am asking is that I have seen guys do a ton of mods to a Blues Junior, and make it unsellable because they have too much in it.
Glad to hear you diagnosed the problem right off. The trem seems to be a common area of failure. Is the 15R basically the same design with a reverb added?

[Commodore 64***]

That Belton is a digital reverb module. I don't know how much it costs off hand.

So I think the tremolo in the Pathfinder isnt' working as it should. The optocouple fixed all of the volume issues, but tremolo is very very weak. I was not able to get it to do much. You can hear a trem effect, just not much. So I'm looking into other things at them moment, and thought I'd add the schematic to this post, since they aren't super easy to find.

[Duffy]

I really like the Pathfinder 15R and a brand new one costs 119 thru MF, no affln.

Mine has been a workhorse and never been a problem, with the reverb and tremolo circuits always working strong.

I would want to get it right.

I bought a Cambridge two twelve a couple years ago used and when I got it home it sounded really bad, with no volume, no tremolo, etc. I read all the fixes on the internet, octocoupler, etc., etc., and decided within hours to bring it back for a refund. That turned out to be really difficult to get. I wound up having to put the money I paid for it toword another purchase. The purchase I then made was a very solid brand new amp of very high quality.

I have a considerable amount of electronic experience, but not that much with amps in particular and I had a gut feeling that trying to fix that Cambridge was going to be a project that was going to cause me a lot of stress and result in very little reward. Plus my confidence that I could actually fix the amp the way it needed to be fixed was very low. It would have been a wild goose chase and getting it fixed by a tech would have cost way more than it was worth.

I hope you can fix that Pathfinder without stressing out too bad or wasting a lot of time and money. They are so inexpensive to begin with that you can get a factory clean one for a very small amount and have a great little amp.

I gotta say I really love my little DA5 Vox that runs off C cell batteries or wall current. Great for trips and camping and so forth and has some of the "Chromie" modelling and effects. Really a cool 139 dollar amp. I got mine for less, new.

[MoustacheMod]

My Allied order came today. Replaced the optocoupler, and the Vox Pathfinder rides again. All better now. I can't say for sure yet, but I think I might like the Vox tone better than my SF Champ (*gasp*).

Very simple, unsolder 4 leads remove bad part, put in new part and solder back on 4 leads.

Should anyone else stumble across this thread whilst troubleshooting or googling "Cambridgitis", shoot me a message, I ordered 3 extra optocouplers, and it won't cost much for me to drop one in a regular old envelope (they are very small).

Do you still have any extra optocouplers?
I have a Cambridge 30 Twin that has THE problem.

Cheers

[Commodore 64***]

Do you still have any extra optocouplers?
I have a Cambridge 30 Twin that has THE problem.

Cheers

Yep. Send me a PM with your address and I'll throw one in an envelope.

[MoustacheMod]

swapped out the bad opto-coupler in the cambridge and its sounding great!
Thanks Commodore 64!

[Commodore 64***]

No problem. I also sent out optocouplers to withnail and someone else, so I'm all out of them now, but I'm glad I could help out.

In any case, the link to the part from Allied should still be good, if anyone else finds this thread via troubleshooting (google).

[sub006]

I have the 15R and it is plenty loud. Mine is new and was impressed with how good it sounded. There must be something wrong and I am sorry I can't help with the electronics. If it doesn't work I take it to the shop. I can fix just about anything mechanical but electricity scares me!

I've heard the 15R speaker actually "breaks in". It loosens up after about 20 hours of use and sounds even better!

[jim p]

I was just looking through this thread and would figure the quick test for a bad opto would be to parallel pins 3 and 4 with a resistor to bypass it. If you have good clean sound then either the opto or the drive on its LED would be your problem. I found a schematic for the Pathfinder on the web and posted a link here http://www.thefret.net/showthread.ph...er-15R...-help! It is not a very good one hard to read but still gives you something to go by.
That drawing for adding reverb is a bit vague and I see some things that could be a problem. The input impedance of the Belton module is only 10k by there data sheet and I find it to be closer to 5k an impedance that low is a problem in a tube amp. You can hook it up after the first coupling cap in a VJ with a 220k resistor in series for a fixed input level that will still cause you to loose gain in the amp but not too much. The drawing shows a LM7805 for 5 volts with no added decoupling. The DC filament supply in a VJ is really pulsating DC because the filter caps are too small by a factor of 10 to 20. Also the LM 7805 needs 2 to 2.5 volts greater then 5 volts on its input so the 5 volts out of it will probably have a high ripple content. If you use an LDO 5 volt regulator that will solve part of the problem. Also a Shotkey diode anode to the plus heater supply with a filter cap of 2200uf connected to the cathode will give you a clean raw supply for the regulator. There should also be a filter cap on the output of the regulator another 2200uf there should work fine.

On the tremolo side just as a way to add phase shift as well as volume level change you could add a cap to ground after the variable resistor in the optocoupler. I can’t tell the resistance range of the opto from the schematic but figure 1k to 100k as the range would mean a cap of 1nf to 22nf added between the opto and C18 on the schematic would give you a phase shift as well as a volume change. This is more like the trem on a Magnatone amp if I am correct from what I remember. Another way to go would be to connect the added cap to ground through a pot to vary the degree of phase shift my guess a 100k audio pot would be a good choice.

For adding reverb using a spring tank would probable come to the same amount of parts and cost as the Belton digital module plus you will not need a 5 volt supply.