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Yorkville 400B Bass Head Repair

10/8/2019

5 Comments

 
WARNING: ELECTRICITY CAN AND WILL KILL YOU. CAPACITORS HOLD THEIR CHARGE LONG AFTER AN AMPLIFIER HAS BEEN TURNED OFF. THIS IS NOT A TUTORIAL. WORK OF THIS NATURE SHOULD ONLY BE ATTEMPTED BY A QUALIFIED TECHNICIAN.

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We have a pretty straight forward solid state amp repair on the bench today. I've been cleaning up the shop and getting it organized the past few weeks. Someone gave me this amplifier awhile back after they gave up on trying to repair it. Let's dig in.

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Here is a view of the amp with the lid off. I didn't test the amp before I opened it up. I wanted to look for any obvious issues (burnt components, et cetera) before I tested the amp through my Light Bulb Limiter.

You can see that there are basically two boards: 1) the preamp closest to us & 2) the large board farthest from us that includes the power supply and output section. 


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DING, DING, DING! WE HAVE A WINNER. Look at D2. That big ol' diode in the output section took a HIT. You can see it's also burned the board underneath. We also can't help but suspect that the transistors on the heat sink and some of the components tied to them have also bought the farm.


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Once I pulled out the burnt diode (noting polarity) I started checking all the usual suspects (mostly diodes and transistors) with my multimeter. Look what we have here. R57 is also burnt, but the components around it read fine. There is an available schematic for this amplifier making figuring out the value of that resistor super easy.


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Here's the final kill count on parts. It turns out that one of the output transistors was shorted as well as that smaller guy. 


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When replacing transistors that are attached to a heat sink it's always a good idea to add a little bit of thermal compound.
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It's also important to note whether or not the transistors are insulated. All of these output devices on the Yorkville 400B have thin, clear insulators to keep them above ground.

With the big Motorola transistors, you should always tighten the screws or nuts holding the transistors to the heat sink and then resolder the connections. Those guys are susceptible to vibrations. 


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That diode basically burned a hole in the circuit board. You have to scrape that carbon of the board because carbon is conductive in certain circumstances. In my scraping, I had to pull out about a pea sized bit of board. Some traces had also lifted and burned.
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Here's the solution: lead stock, solder, and silicone. You scrape the coating off the copper traces and tin them. Then tack the component in and add lead stock as needed for strength and continuity. You have to make sure that you don't get any solder or lead stock over top of another trace - coated or not. It's just poor technique to do otherwise.


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After all that work I put the amp back together - save the lid - and tested it with my light bulb. It worked great except for a very small issue with the preamp: dirty potentiometers.
​I have fallen victim to the genius of Caig Laboratories and now only have one of the new shitty cans of DeoxIT D5. The can makes it damn near impossible to access pots except by pulling a board.
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I pulled the preamp board and sprayed the pots out with the DeoxIT. That gave me a chance to give the faceplate of the amp a good cleaning with some generic glass cleaner. I then put the thing all the way back together and tested it at full power. It works great! 

There ya go. All done. Thanks for following along with this repair journey.

​- Andy
5 Comments
André Gaudreault link
12/17/2021 07:52:23 am

Hello,

I just stumbled upon this article and I wonder if you could help me.
I have bought this exact amp exept that it is encased in a cab with an 8 ohms 15'' speaker.

Could you tell me if it puts out 400 watts at 4 or 8ohms? In other words, could I daisy chain another 8 ohms speaker with it?

Thanks for your help!!

AG

Reply
Andy Branton link
12/28/2021 10:55:35 am

It's supposed to put out 400 watts at 4ohms and 300 watts at 8 ohms.

I see no reason why you couldn't hook another 8ohm cab up to it.

Reply
Jim
2/27/2022 07:38:18 pm

I also just came across this and have one of these heads. It has a damaged 10k volume potentiometer I need to replace. Any idea where I can find a replacement that matches?

Reply
Andy Branton link
2/28/2022 07:13:10 am

Jim,
The schematic lists the Yorkville part no. for the pot at 4390. It also indicates that the 10k pot is linear.

A quick google search yielded this out of stock part at Full Compass: https://www.fullcompass.com/prod/268744-yorkville-4390-yorkville-volume-pot-for-ap2020-ap4020-sr300

I think you have enough info to source a pot from Mouser or DigiKey.

You know this about the pot:
- some physical measurements
- 10k resistance
- D / flatted shaft
- linear
- 4 pins
- through hole / pc pin
- vertically orientated

Both Mouser and DigiKey have really great filtering abilities. Good luck!
- Andy


Reply
Jim
2/28/2022 08:13:16 am

Thanks!
Going over the pots in my amp, looks like I have a V5.0 or earlier model. My pots have no 4th pin, support screw or detent on the shaft.
I am going to assume I have a P13 style (bottom right of page) Based on the revision history, looks like they made some modifications over time and it's possible it was a result of user issues that were similar to my problems.
In my case though, it was totally my fault. I had some scratchiness and I wanted to remove the knob to apply some deoxit. I did not notice that the knob had 2 set screws. I removed one and tried to pull off knob. Got it to move a little but could not. So I pushed back in and the force I was exerting essentially messed up internals.
I placed some photos of the pots here (in case this works)
https://photos.app.goo.gl/oL9sw2iJZfWFLSKW8
Looks to be a Noble B10K linear. There are a few soldered support posts which i am hoping are not critical to a replacement.
Anyway, thanks for the links and reply.




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    Andy branton

    I'm a country blues artist, multi-instrumentalist, and pro audio repair technician. I am always curious -  but never nosy.

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