Tremoverb switching issues - LDR Source needed, Truth table?

[Norse]

Greetings,

I'm looking at a friend's '94 Tremoverb which had no sound due to bad J175 transistors in the perma-mute circuit. I now have volume (but still have some broken attack) I am now looking into the channel switching LED's: Red is on in both modes, but is dim on the orange channel. Orange LED never comes on, and I am testing the LDR's.

Anyone know where I can find replacement LDR's? I've tried searching at Mouser, but get no results.

Also, if anyone has a truth table, I would greatly appreciate a copy.

Many thanks,
Greg

 

[94Tremoverb]

I don't have a truth table but the switching matrix is in the standard schematic (I just got mine online somewhere) and it's not too hard to work out. But to be honest, it sounds like you may have multiple failures and it *might* be best to just change all of the LDRs - some of the dead ones are likely to be lower down in the stacks, so you'll have to remove the top ones first to get at them anyway. I got mine from Mesa - certainly not the cheapest but considering that they aren't that expensive, I preferred to go for the no-question correct parts than go through the specs to get an exact match (there is more than one spec) or unknown quality (even Mesa had batch problems once) from elsewhere.

Are there J175s in a Tremoverb? I don't remember any - there are three transistors in the main channel switching (not sure if they're 175s or not) but as far as I remember there's no mute circuit... and it's not on the schematic.

 

[Norse]

Wow- this thing has got 25 of them! Do you recall what Mesa is asking for them?

The J175's have all been replaced, but I am suspicious of them... I'm getting ~ 15 volts on the center of two of them, depending on red/orange channel selection, but the third only gets to ~13.

In addition to the LED issue, I also have an intermittent condition with volume/sound:
1. I find that, in Red Channel mode, the Orange Master & Gain will have an effect on the red channel - but only when the blues/Modern switch is in the modern position.
2. The amp sounds a bit like when you turn off the main power switch, leaving stand-by on (in order to drain the filter caps). This straightened-up when i measured voltage at the OT primary feed, there is a cap to ground (1000 vdc orange drop). Coincidence? I may need to revisit the J175's, but I still suspect LDR issues.

 

[94Tremoverb]

Wow- this thing has got 25 of them! Do you recall what Mesa is asking for them?

I think they were about a couple of dollars a go, but it may be different because I'm in the UK.

The J175's have all been replaced, but I am suspicious of them... I'm getting ~ 15 volts on the center of two of them, depending on red/orange channel selection, but the third only gets to ~13.

I think that's right. Q1 and Q2 have 10K resistors plus a common 3K3 in series to the supply, Q3 has 22K, so I would expect it to have a lower base voltage.

In addition to the LED issue, I also have an intermittent condition with volume/sound:
1. I find that, in Red Channel mode, the Orange Master & Gain will have an effect on the red channel - but only when the blues/Modern switch is in the modern position.

This does sound like something up with the switching, possibly to do with the Channel Cloning switch - which (intentionally) doesn't operate in Blues mode.

2. The amp sounds a bit like when you turn off the main power switch, leaving stand-by on (in order to drain the filter caps). This straightened-up when i measured voltage at the OT primary feed, there is a cap to ground (1000 vdc orange drop). Coincidence?

I've heard of this cap causing trouble before. Check it's not got a cold joint on it, I think the person who reported the same thing said it was loose at one end.