Solid state Fender Yale Reverb

[Fred G.]

Recently, I was asked by a friend to try and repair a solid-state Fender Yale Reverb (I'm NOT experienced in solid state gear, but I decided this was a chance to learn). The problem, as stated, was that the amp was turned on, blew a fuse, smoked a little. I took out the chassis and put it on a dim bulb tester. Indeed it did smoke, and I suspected that the filter caps had failed. I should mention that I couldn't isolate the smoking component yet, and that the amp motor-boated before I replaced the components I mentioned above when I initially lit it up with the dim bulb, but not nearly as loudly. I've replaced the filter caps, as well as some potentiometers that were broken. Upon turning on the amp, now, with a dim bulb tester, the amp is motor-boating loudly, with the dim bulb brightly lit and the bulb brightness pulsates in rhythm with the the motor-boating. Where do I go from here? Thanks in advance!

[Enzo]

My first suspects would be the power amp. Is it a power amp IC in this model?

[Fred G.]

Hi Enzo,

No, these appear to be power transistors - RCA TIP30B, RCA TIP29B. Does this sound right? Four of them are set in heat sinks and apparently have a bias trimpot? two other are located away from these, not heat sunk.

[The Dude]

Does this schematic look right? It shows the outputs as TO3 style transistors. Maybe mounted on the back of the chassis under plastic isolators? Or maybe it's the wrong schematic.

https://reader020.staticloud.net/rea...a2a762/bg1.png

Do you measure any DC at the output?

[vintagekiki]

_{little help}

https://vdocuments.site/fender-yale-reverb-schematic.html

http://www.stratopastor.org.uk/strato/amps/twoseriessolidstatefenders/twoseriessolidstatefenders.html#yalereverb

[Enzo]

The TIP29/30 types appear to be drivers. Are there not two larger transistors perhaps bolted right to the chassis. TO3 types - the ones shaped like a cowboy hat.

[vintagekiki]

little help

[Fred G.]

Sorry about the slow reply. I wasn't getting email alerts and I got busy with some other work. But in reply to your responses - YES. I flipped over the chassis, and removed the black plastic covers - two nice shiny transistors, just like the ones in the photo above. Do I need to remove them from circuit to test them - and can I test them with a DMM? I had completely overlooked them because I hadn't completely flipped the chassis over - they're easy to miss!

They are RCA branded, 2N6254 HXQ 8345

[Enzo]

MAke it easy on yourself. Check them for shorted right where they sit. If they show short, then pull them and recheck to be sure it is THEM and not something else. On the other hand, there is NOTHING in the circuit that can make a shorted transistor appear not to be.

[Jon Snell]

schematic, try here; https://www.stratopastor.org.uk/stra...tml#yalereverb
This was made when Fender had a lot of special value components that will cost a small fortune to replace if you don't own an E64 set of values!
Have fun but always look for the leaky/faulty power transistor first, then replace the faulty bias set pre set to stop it from cooking the new set you just put in.

[g1]

Just in case you don't know, those are sockets so no soldering required. Just remove the 2 screws each and pull them out. Be careful not to damage the mica insulators.
To re-install you will have to hold the sockets in place while inserting the transistors.

[Fred G.]

OK...I unsoldered the two transistors from the circuit. I executed the following steps on each one, from this website;

https://vetco.net/blog/test-a-transi...04-12-25-37-07

For BOTH transistors, Steps 1,2,4, and 5 yielded "OL" results (overrange). for transistor 1, step 3 gave a value of .471, transistor 2 a value of .458. These results were observed on my Micronta DMM, set to the Diode Test setting (2k on the ohms range). I'm assuming that both transistors, then, are bad. Is this aassumption correct? I'm learning solid state on the fly, so please be gentle, lol. If these are defective, what should I replace them with?

Please note that I wasn't even sure which pins were which - I assumed the collector was the case (the bottom pin), and the base was the top left pin, the emitter the top right pin. I hoped that the results would be revealing either way due to NPN vs PNP, one way or the other. Thank you for your patience.

Thanks!

Fred G,

[The Dude]

I strongly suspect an "orientation error" in your measurements. I think you may have gotten the base and emitter pins mixed up for your testing. Your measurements seem to say so.

Edit: Uploading pic for reference.

[Fred G.]

Uhm, yeah, but...this image "nearly" symmetrically reverses itself depending on which way you are oriented to it, if you aren't visually very careful. I see that the two pins are somewhat oriented to one side in the diagram, but the bottom of this transistor looks nothing like the image above. I've searched for something that looks like the bottom of these, with zero luck. It's a damned guessing game so far. Note that I have NOT removed them from the chassis - I was testing from the lugs inside the chassis. Is there further dissassembly needed? I don't know, never opened one of these up before.

Again, remember, I have no solid state experience. It's all tube.

[The Dude]

You can test them in the sockets, as it seems you did. IMO, the easier way is to leave the sockets soldered and remove the transistor from the socket. This requires no soldering or desoldering. I think the reason you can't find something that looks like the bottom of these is because you're looking at the socket and not the transistor itself. There should be a couple of screws that mount the transistor to the socket. Remove the screws and the transistor should pull right out of the socket. That also insures that you don't accidentally solder the wires back to the wrong pins when you are done testing.