Orange Crush 35RT drive channel gone quiet!

[Stratfordade]

I'm very fond of my Orange 35RT but the drive channel has gone very quiet relative to the clean channel which is as loud as ever (it only needs to be turned up a couple of notches to be too loud for home). I also notice the gain control needs to be wound up much more than usual to create overdrive. Normally I rarely need to go past the 12 o'clock setting to get the tone I need, but when the drive channel is in the fault condition it needs to go further round.

Two issues make this a much trickier fix than it should be — the problem seems to come up after the amp is turned on from cold, after which the Drive channel seems to lose volume within a minute or two of playing through it. The second problem is I've been unable;e to find a schematic. I've found one for an Orange Crush 20L solid state but it's nothing like it — the 20L circuit only shows a couple of TL072 op-amps whereas the 35RT has eight! Oh and the Orange pub is a bear to work on as I discovered when I first got it and had to replace a voltage regulator IC. Being double sided makes desoldering a real pain as solder sticks in the through holes.

So far I've removed the chassis and can't find any issues with the drive and gain pots. I did remove a TL072 op amp right behind the drive gain pot (dual 1 Meg) that I thought had a short between pin 1 (output) and 2 (- input) but that was my mistake as I didn't spot a tiny track linking them! The new op amp has made no difference — drive channel started fairly loud but lost a lot of volume after a minute or so.

Realise it's not helpful asking for advice with no circuit diagram or test readings, but perhaps someone has seen a similar fault with this amp and could point me in possible direction? Just having the schematic would be huge help as at least I'd know which components were associated with the drive channel. Thanks in advance.

[Stratfordade]

I’ve put it on the scope and it confirms that unless the drive volume and drive gain are turned a long way up, the clean output is a lot higher with the clean volume set very modestly by comparison. Flexing the board doesn’t make any cold joint show itself but all the joints look bright and shiny anyway. There’s some issue with gain in the drive/dirty channel but without the circuit diagram I’m struggling. I think what I’ll do next is look at the op-amp outputs on pins 1 and 7 of each TL072 and see which are associated with drive and clean channels.

[DrGonz78]

I read somewhere on another site where the amp had similar problems. Turned out to be cold solder joints. Maybe some freeze spray on a few solder points?

[Stratfordade]

Thanks Dr Gonz if you have a link to where you saw that thread I’d love to read it.

I’ve prodded and wobbled components while watching dirt channel output wave and couldn’t find any problem joints that way. The issue with your suggestion is that it’s difficult/impossible to power the amp up without the PCB mounted with no access to underside, given the transformer and power amp connections (though problem clearly in pre-amp stage so may be able to monitor output with power amp disconnected - again a schematic would be lovely!). Till then I’ll keep looking for dry joints by wobbling things.

I can see on the scope that the gain only pushes signal into clipping very near end of its range, which seems wrong.

[DrGonz78]

Yeah here is the link. Not much other than the last post says what happened. Either way dry joints are the hardest to find.

[Stratfordade]

Thanks

[Stratfordade]

No joy so far. Went over board with magnifying glass. Reflowed anything slightly suspicious but nothing stood out as possible cold joint. Replaced two 072 op amps that had done role in Dirty channel but problem persists - unless gain is wound up very high the dirty channel is pretty quiet compared with the clean which is very loud just a fraction into its travel. Stumped for now and in absence of schematic may consider asking an authorised Orange repairer to take a look as I’m working in the dark as it is.

[g1]

I wonder if there are jfets for switching and if maybe one for the drive channel is failing after warm up or losing it's control voltage.

[Stratfordade]

Thanks that’s a good suggestion. I only remember seeing one discrete transistor on the board (right at the top of the only pic I took of the board in attached photo) so it could be a FET and it’s in vaguely the right place close to the input. The amp is all buttoned up now but I’ll keep playing with it and if it shows that warm-up behaviour still I’ll take it apart again and check whether that’s a switching FET. Oh for a schematic!

[g1]

Looking at other orange schematics, they seem to favour relays for switching and it looks like there is one in your picture. Poor relay contacts might also cause your issue.

[Stratfordade]

Another good shout. I had same thought about that relay and I tested the coil and could hear it switching, but didn't check that the contacts it drove were actually opening/closing. But if the relay was involved then I don't see how it could create condition where the amp switched from clean to dirty, but the dirty channel had very reduced volume. I was using the amp again last night and it was obvious that in the dirty channel gain was anywhere below about 11 o'clock, the output was very quiet unless the Dirty volume was turned way past 12 o'clock! I'm sure when I first got the amp there was plenty of Dirty channel volume with the volume just off the lowest position, as long as the gain was anywhere off its lowest position.

Anyway thanks for the advice. It does motivate me to disassemble the amp again and be more thorough checking the relay contacts and look into your JFET idea too.

[g1]

But if the relay was involved then I don't see how it could create condition where the amp switched from clean to dirty, but the dirty channel had very reduced volume.

There will be 2 sets of contacts, NC (normally closed) when the relay disengaged, and the other set get closed when the relay is engaged. If only one set is oxidized, you will get issues only in one (engaged or disengaged) condition.

Tapping on the relay when fault is present might give some response.

[Stratfordade]

Wanted to update this with the fix, for anyone searching for similar problem.

Recently I picked up very cheaply an identical 35RT, but with blown speaker.

In absence of schematic, I simply injected signal into all 9 (count ‘em) TL072 op-amp inputs (pins 2, 3, 5, 6) with the chassis out of cabinet and connected to working speaker, and made a note for clean and dirty channels of where there was output in speaker, and if it was particularly loud.

Comparing the two sets of results suggested that the U4 and U5 op-amps in the amp with quiet Dirty channel were suspect.

Replaced both of them (desoldering is much harder than usual with double sided board with plated holes) and the problem amp is now running perfectly, with a Dirt channel to match the always loud clean channel.

Will find a good 4 ohm speaker to replace the blown Voice of the World and sell that amp eventually.