Author Topic: My Forum Vibe building journey - Need help with Vibrato boosting mids  (Read 2662 times)

warioblast

Hi folks,

Here's the whole story. I finished my Forum Vibe v5 yesterday. It's the vintage build, using a 24V regulated power adapter, 24V to the LFO and 15V to the audio via 7815 regulator.  I used Banzai Univibe LDRs/bulb kit, put the photocells flat. 2N5088 all around. Added Redhouse Gain mod and Bias-Offset trimmer. The vibe worked on first try.

The first thing I noticed was the limited range of the speed pot. I fixed that by the reducing the 4k7 limiting speed resistors to 1k5. I tweaked the trimmers and started to do A/Bs with my trusty Mojo Vibe. The FV sounded good, only lacking a bit of intensity to my taste. That's when I noticed the Vibrato mode was louder. At first it sounded like a volume boost but it's a actually a mid boost.

Here comes drama part 1
I decided to try to add a speed indicator led, Bob Sweet style.

(For the first time I ordered clear purple LEDs, and when I received them I was surprised the light was so dimmed with a 3k9 resistor I found out I could loose the resistor and have the same amount of intensity as a regular red LED + resistor.) So I was toying around with LED over the board and the speed pots, I managed to lit the LED a few times I think there were some sparkles involved too. I realized the bulb wasn't flashing any more. I immediately plugged in and of course the vibes have gone. Luckily, I had taken voltage readings of the first 10 transistors shortly before. They all looked fine, Q13 too but Q11 & Q12 looked dead. Trying to desolder those turned out to be a PITA; even with a desoldering pump; I gave up and clipped them. I used 2 old 2N5088 I had around and the vibe started working again.

Here comes drama part2
The mid boost was still there but what bugged me the most was the slight lack of intensity. I then decided to toy with the photocells. Once again a PITA to desolder. I desolder the bulb first, it looked very fragile. I had already clipped the leads  of the photocells so I had to solder new ones. I put sockets on the board while I was at it and put back the photocells this time facing the bulb. I didn't gain intensity as I thought. It only made the vibe more choppy. I tried to tweak the trimmers but it was helpless. I don't know if it's the changing of the 2 dead transistors or the over-heating of the LDRs & bulb but the offset and lamp bias trimmers have lost a good part of their range.  :icon_question:
I tried to angled the photocells but it didn't changed a thing. So back to putting them flat. But before that I needed to desolder the leads. I took care of removing the solder of the remainings leads in order to put them in the sockets. What was bound to happen happened. I broke a photocell lead straight at the base. After desperate attempts at soldering a lead back I caved in and faced reality. I'll be stuck until I got new parts.

So until I'll be waiting for them, do any of you guys have an idea regarding the mid boost. It may not be useful, but the boost happens in the cancel mod too. The electro caps and transistors are oriented correctly. I measured each resistors with a DMM before I soldered them. I'll give another closer look.  :icon_confused:

And finally do you think it's safe to give another shot at the speed indicator with my purple LED. Did and /or will I fry Q11 & Q12 again  by putting this LED directly from Q12 emitter to ground  :icon_question: :icon_redface:

antonis

Re: My Forum Vibe building journey - Need help with Vibrato boosting mids
« Reply #1 on: September 16, 2015, 05:25:48 AM »
And finally do you think it's safe to give another shot at the speed indicator with my purple LED. Did and /or will I fry Q11 & Q12 again  by putting this LED directly from Q12 emitter to ground  :icon_question: :icon_redface:
I can't see Q11 bias so, assuming that the Darlington pair act at saturation, any current less than 25mA should be OK..
(for 24V & 625mW ignoring Vce & LED voltage drop)

Try with a resistor not lower than 1k in series with the LED.. :icon_wink:
« Last Edit: September 16, 2015, 05:30:13 AM by antonis »
"I'm getting older while being taught all the time" Solon the Athenian..
"I don't mind  being taught all the time but I do mind a lot getting old" Antonis the Thessalonian..

warioblast

Re: My Forum Vibe building journey - Need help with Vibrato boosting mids
« Reply #2 on: September 16, 2015, 07:08:43 PM »
I tried a 1k with the Ultra Violet LED but the light isn't bright enough. So I give up the idea of using the UV LED with Bob Sweet method. The UV LED I experimented with the vibe is dead. 

I also ordered some Silonex NSL-5542 photocells and a spare bulb.

warioblast

Re: My Forum Vibe building journey - Need help with Vibrato boosting mids
« Reply #3 on: September 25, 2015, 05:14:25 PM »
So I received my parts yesterday. Took out my old photocells and their sockets. Put the new ones flat.

Drama 3:
While I was tweaking the trimmers my screwdriver slept off my hand, landed I don't know where and it created another shortcut, the bulb stopped blinking. I took the power on and off, didn't change a thing. I wanted to try other transistors from Q10 to Q14 so I took them out and put sockets. I fiddled with 2N3904 and MPS13, the bulb started blinking again. I measured the 2N5088 (Q10 > Q13), they looked fine so I put them back, the bulb was still blinking. I didn't spend much time trying out those differents transistors but I expected a bigger impact. I'll give another try.

The good news  :icon_biggrin:
I checked my resistors and solderings and all looked fine.
[img]http://classicamplification.net/fv/pix/OEM_schematic.gif[img]
Looking a the schematic I thought my mid boost problem should involved R35 or R36 or R37. I reflow those, didn't change a thing. I look at the discrepancies between Univibe models on the ClassicAmp website. "The R36 discrepancy, the OEM schematic shows 47k resistor but many production vibes have a 100k resistor in that position". I had no idea what difference it made. So I took R36 out, put sockets. Put a 100K resistor in and the mid boost was gone.  :icon_surprised: A 68K still boosted the vibrato.

My forum vibe sounds a bit choppy but I think the bulb didn't like the shocks he received and as a result the range of the bias trimmer has considerably diminished. I'm guessing a new bulb will fix all this.

I decided to have a go at testing the rate LED again, so I kept my old bulb, just in case...
Used a 5mm red LED and Bob Sweet method and it worked straight up.

Tomorrow I'll be soldering the new bulb in. And then working on the light shield...