Crate TD-70 Schematic

[HotHead]

Hey Y'all,

Newbie here in need of a schematic. I found a couple attachments/links but they do not work.

Just for reference, channel A was dead when I got the amp. Channel B worked but was noisy as hell.

I replaced the 12AX7 and cleaned all the jacks and their switching contacts. Viola! Channel A is alive. However, Channel B now has MORE noise and internal feedback. Also developed weird microphonics. Swapped the tube again, feedback and microphonics gone (tossed the tube in the trash).

Now, amp is very hissy. Worse on B channel. I have inspected all the solder joints and reflowed a few that looked iffy.

Can somebody please email the schematic or post a link that works?

Thanx

[Jazz P Bass]

Crate TD 70 Schematic

Can't resist a 'Please'.
Here you go.

[HotHead]

Outstanding! Many thanks...

Anybody got hands-on experience with a "Tube Driver" Crate (TD35, TD50C, or TD70)? This is my first hybrid.

[Jazz P Bass]

As with any amp, check the power supply first.

The IC power supply, above all, must be free of any Vac ripple.

[HotHead]

Ripple would manifest as hum. I have excessive hiss and low gain on the B channel.

I'm not new to electronics or amplifiers, just this forum. I do appreciate the advice.

[nashvillebill]

I follow what Jazz is saying, though. With the FET switching, it would be prudent to rule out any problems in the 15 volt rails, and it's not going to be that hard to measure them just to be sure. TP 11, 12, 14 and 15 would be the points to check. Might not be a bad idea to put them on a scope to see if anything's riding on the DC.

[Jazz P Bass]

Alright then, look at the TP Chart on page 2.
Follow the input parameters & go directly to TP10 & measure it for ChA & ChB.

This will tell how far off ChB's amplitude is.

Then working back you can find where it is failing.

Nail the 'ChB low volume' & the hiss may go away.

Maybe.

[Audiotexan]

Outstanding! Many thanks...

Anybody got hands-on experience with a "Tube Driver" Crate (TD35, TD50C, or TD70)? This is my first hybrid.

Yep, but it's been a few years. Serviced quite a few of these as well as owned a TD-70.

Quite common in that era/series of SLM for jacks to induce issues (cheap parts), and also the opamps seem to drop like flies in them. Especially when you mention hiss/low gain, which to me screams 'bad opamp'.

Also, deoxit would typically help for a short-term quick fix, but more often than not as I recall, I had to either burnish the jacks' contact points (especially TRS "switching-jacks") or replace them completely on those.

Rarely would have the odd PSU issue, but usually cold-joints/bad jacks/dying or dead opamps. Like most of Crate's stuff IME, mostly just annoyance repairs. Rarely a complex fix.
Don't let the "hybrid" mess with you, just treat it as you would any other SS amp.

Hope this helps!
Audiotexan


Edit: correcting fingers fighting with the keyboard. "hyrbid" lol

[HotHead]

So, nothing I haven't seen before regardless of the "Hybrid" nature of the beast. Muchos gracias!

Looks like IC1 is failing. Factory part is an RC4558P. I have JRC4558D's and TLO72's available. I think I'll install a socket for future experimentation.

Crate, Fender, Marshall, Maxon, Peavey, VOX, the name doesn't matter. 9 out of 10 of the amps and pedals I get have mechanical issues. Broken/dirty jacks, bad/broken solder joints, broken wires, old/bad tubes, and DIRT! OMFG doesn't anybody take care of their gear? And, of course, too often a previous "technician" has inserted his phallus into the works and left a mess...

[HotHead]

This thing is kicking my ass. Channel B passes signal but, has poor volume until the volume is turned up past about 70%. Then it comes on strong. Channel B also has increased noise over Channel A. Channel A works like a champ. Swapping tubes makes no difference.

So logical troubleshooting says anything common to both channels should be okay. The problem is, everything but the tone sections are common to both channels!

Power supply outputs are dead on. Power amp is fine. Tone controls work as they should. Reverb is fine.

IC1 and IC4 have been replaced, no change. So, I'm looking at the pre-amp switching JFET's (Q1, Q17, Q20) and thinking WTF?

[Enzo]

I am an old man, save me some work. Channel A is the strip through IC2 and channel B is the strip through IC4A?

Channel A works.

"The volume" means what, P6?

We are talking about night and day differences, not subtleties, right? So brand of OP AMP doesn;t matter.

Until someone corrects me, I am assuming the above.

The upper strip is common to all channels, and extends through the tube. Only differences are the JFETs, so a couple strategies: Since the signal pops in and out at 70%, apply a test signal and monitor it at the tube output, C15. Does the signal pop in and out the same way there? Or does it work just follow the control up and down.? That tells us if the problem is in the top row or not.

You have the two controls that the JFETs enable, P1 by Q1, and P6 by Q20,17. First, verify the control signal to the JFET gates, signals A and B. TP14,15 on the drawing cover those, so when channel switching do BOTH those control signals behave? Does TP14 toggle 0 ti +15v? Does TP15 toggle 0 to -15v? More important, do those control signal make it to the gate pins on those JFETs?

A JFET is a low resistance until turned OFF at its gate with a voltage. So with the amp running, measuer resistance with a meter from source to drain across each of those JFETs to see if they are being controlled. Q1 has 1meg in parallel, so I expect to see toggling from 1 meg down to maybe a couple hundred ohms. Same deal at Q20.

All that should tell us if the problem is in the top row.

The next row down is split into two paths. JFET Q2, Q3 shunt signal to ground to mute the unwanted channel path. Are they both turning on and off appropriately? A marginal JFET could even be reacting to the signal voltages across it, and one is getting kicked on at higher levels.


In any case if this symptom is repeatable, then we ought to be able to turn the level up and down and have signal popping in and out, and a scope or signal tracer along the signal path should find the point where signal falters. A signal tracer is especially effective hunting down noise sources.


And I cannot assume, so has that 70% control been taken out and checked for cracks in the element?

[HotHead]

Thank-you, Enzo! The volume of Channel B doesn't really "pop" in and out. It's more of a "knee" around 70%. Quiet and slowly increasing then jumping up.

Troubleshooting this SOB is slow as there are no designators ANYWHERE on the board. I have a hand drawn layout but, it's hard to read. I am also limited to my DMM and signal generator at the moment. My o'scope probe has gone missing. But I digress...

I checked the control voltages at the gates of Q1, Q17, Q20, Q3, and Q2. All performed as expected. Next I checked the resistance across each from source to drain. All showed expected results except Q2. I get 68k ohms (R35) regardless of the channel switch position. Looks like a dead (open) JFET.

I put a jumper across Q2 source to drain. Channel B gain and noise improved. I need track down a replacement J176.

Again, thank-you for the help.

[HotHead]

I removed Q17 and installed it in Q2's slot. Amp works like a champ! I'm sure those of you with some engineering knowledge will cringe...

However, Channel B is still too hissy and bright. Push the "Bright" switch and dogs start howling...

I'm looking at the feedback loop on IC4A (C31, R47). Seems to me I could apply some "Pedal "Mod" tech and roll off some high end here. Any suggestions for cap and resistor values? My RC math skills are dismal! I'd like to attenuate frequencies above 6kHz and add a little gain below that.