Low output on T-Rex Squeezer

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Rik86
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Joined: 01 Jan 2021, 15:40

Post by Rik86 »

Hi all,

Could you help me out with my T-Rex squeezer? I got it as a defective "extra" to another purchase, so no idea about its history (wear on the screws indicate that it has been opened before!).
It has very low output. Only when cranking both gain and level to max do I get something close to the input signal level through. I think I checked some simple items (see below), but my electrical background is by no means sufficient to understand or follow the schematic... hopefully someone here can be my guide and teach me a thing or two :)

Schematic and gutshots here: https://music-electronics-forum.com/for ... for-repair

Tried replacing tube with known good one, no improvement. After disassembly I tested all the pots, they are good. Connected the unit to 12V powersupply, switched on (pilot lamp for tube + green LED are on). It did not have jacks inserted in input or output during the readings below (was this a mistake?).

Voltage on tube pins are as follows:
Plate 2: 5.3V
Grid 2: 12V
Cathode 2: 12V
Filament 2: 0.1V
Filament 1: 12V
Plate 1: 13.4V
Grid 1: 12V
Cathode 1: 12V
Filament tap: 6.2V

I'm not exactly sure how to check the transformer, but I would expect something other than what I'm reading on my DMM. I see 5 pins: 2 on one side, 3 on the other (center tap?)
Taking the outsides of the 3 pins gives 0.3V difference. Between the 2 pins on the other sides goes from 0.7 down to around 0.5V.
I would expect a high voltage around here somewhere?

The big 2 caps next to the transformer are at 12V (should also be in the high voltage section no?). There is a transistor right next to the transformer hidden between the 2 larger caps (you can just see it in one of the gutshots if you check the link above).
Center pin reads 0.4V (collector), outer pins both read 12V. Based on the schematic this is the only single transistor in this section, so least one of the pins should be ground, or am I missing something?
What is my next step?

Thanks in advance!
Rik

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Rik86
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Post by Rik86 »

Anyone?

Here is another link to an earlier posts on the squeezer from this forum:
viewtopic.php?f=1&t=22942

Thanks!

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stolen
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Post by stolen »

Hello!

your readings are a bit peculiar: Firstly, cathode and grid of one of the tube halves should be grounded (at 0V), which doesn't seem to be the case. Also the switching transistor indeed appears to have similar issues.

In this case, we would first visually inspect the PCB to make sure that there's no cracks or otherwise broken traces in it - if a ground section on the board has been disconnected from the main ground, it would explain some of the phenomena, but it could be something else too. With your DMM, you could check if all the grounds are intact by using it in continuity or resistance mode. Use one testing point, for example input jack ground or dc jack ground as a reference and check if the emitter of the BC639 is indeed grounded.

You can also bypass the tube circuit and check if the rest is working properly; the easiest way to do this would be:
- lift C8 at the side where it's connected to the tube plate, make sure it doesn't touch the pad anymore
- connect either side of C14 to the now lifted side of C8
This also bypasses the shelving filter @90Hz from IC2B, but there's a whole attenuation circuit there and it's way more inconvenient to include it. In this setup, all controls should be fully functional, albeit without the filter and the tube active. If this works, we know that the tube section indeed is the problem (the bypassed filter is unlikely to fail while the rest of the integrated circuits remain intact).

The 240V plate supply are generated as follows: The NE555 is an oscillator that switches T1 on and off, thereby generating AC from the DC power supply which the transformer can then convert up to much higher voltages. Some rectification and filtering later we're back at DC at 20x higher voltage. The oscillator is running at a surprisingly low clock*, around 2.5kHz if we didn't miscalculate, and you could check for it with an audio probe if you don't have a scope - but as a warning, this would be a full swing 12V signal and VERY loud, capable of damaging some typical audio inputs. If you can hack together an audio probe with say an 1:10 voltage divider this would be a good test however.

If the oscillator is not running, that plate supply voltage should be at 0V, which makes your measurements even more confusing. We'd suspect you'd accidentally taken a -12V power rail as a reference instead of ground, but from what we can see there isn't any, so the likely options are that either ground is disconnected somewhere, or, if you can't see any board damage, the transformer might be shorted - again, this is something you can check for with a continuity test.

We recommend that for these measurements you remove the tube entirely. It will be a bit of a puzzle until you have a working unit again we assume, but judging from the schematic this unit is a nice compressor and well worth it. Even if you can't get the tube section working, it is merely a colorisation gimmick and should make for a good compressor without.

All the best,
stolen

*it's kinda impressive that they made this work with a reportedly low noise floor. maybe ovnilab's unit was faulty there.

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bongoboystudio
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Post by bongoboystudio »

Hi,
My friend just bought one of these pedals on Craigslist on my recommendation, only to find
that it has this very low output issue. My friend couldn't try it out because the guy didn't have
a power supply. I got the proper 12V power supply only to find the pedal has this problem. If
you get your unit fixed, please let me know what you found. Could this be damage caused from
someone using a 9v power supply? (q to "Stolen")
Thanks,

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stolen
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Post by stolen »

Hey, nah, running it on 9V with proper polarity shouldn't damage it. Bypass the tube section, see if that fixes it.

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