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FRX 7710 power transistors

Started by bloke66, October 21, 2017, 05:06:16 AM

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bloke66

Hi
The amp in my Mk2 suitcase does not have the 2N6254 power transistors installed.....so can I assume this is the 80 watt version of the amp and if the power transistors were installed it would be the 100 watt version??
I'm thinking about installing the 4 power transistors to hopefully get a bit more volume with less drive.
Is this something that others have done??.....or could I damage the power supply and/or speakers??
cheers

bloke66

I'm getting the feeling that the question I asked is a bit dumb!!!
The reality is that I have a Mk 2 suitcase (73 key) that has a FRX 7710 amplifier in the speaker cabinet.
The amplifier is working fine after having some work done on it by a local Fender repair agent.
The Fender tech suggested to me that I could add the 4 output transistors......which plug into sockets wired from the 2 power amp circuit boards and mounted externally onto the case of the amp. These 4 transistors have never been there since I've owned the Rhodes and do not stop the amp from working....albeit at a lower volume level.
Is this a weird situation not to have the 4 transistors installed???
A bit of advice would be much appreciated.
I'm keen on installing the 4 transistors but would just like to confirm with you guys that I;m not going to damage the amp and speakers.
Cheers

vanceinatlance

Greetings,
I don't think the question is dumb. Just not sure how to answer other than, "No!", Unless someone else knows more about doing this with your amp and can chime in.
I am not aware of external transistors being used on a fr7710 amp. Maybe the amp tech was confused with the peterson amp that does mount the amp boards in the cabinet?
You state that you have a frx7710 amp. I can't remember if the X identified the unit as wired for 220 or if it was the super satellite amp or other. Regardless, I don't believe it was much different than the fr7710 in basic design.
Maybe if you can post some pics where these transistor sockets or the connector is on the amp board?
In general, I would not try to mess with the amp if it is working well and just serviced. This amp should be able to get very loud, (my neighbors would testify to that). I would be more concerned with the amp board sockets and if their solder joints are solid. This is a problem most of these amps experience.

vanceinatlance

 Just realized that you might not see the power transistors because they are mounted to the heat sink facing outside between the sets of fins. They are usually covered with a shielding that is about 3 1/2" by 1" per pair.

bloke66

After talking to Vintage Vibe I now understand the situation.
For some reason the output transistors were removed before I bought the Rhodes.... with the transistor sockets left under each channel PCbd....still connected into the circuit.
The transistors should be installed.
V V were surprised that there was enough power to drive the speakers.....I could get a reasonable volume level out of it so should be interesting when the output transistors are installed.
It amazes me that due to the circuit design the output transistors can be removed and the circuit still works!!!!
V V said that I will have to adjust the DC Offset and Bias when the transistors are installed...these adjustments are on each channels PCBd.
Any tech heads who can help with these adjustments would be appreciated....I understand the electronics but have never done much audio amplifier work.
Cheers

vanceinatlance

That's crazy!! I have never heard of that happening before. Vintage Vibe actually sold, at one time, a great little video that walked you through setting the offset voltage. I would recommend that if it is still available.

bloke66

Just thought I'd let you guys know the end result of this one.
I had a closer look at the output transistor sockets and found 2 of them had a transistor leg stuckin them.
I'd assume at some stage the output transistors needed to be replaced and the transistor legs broke off when trying to seperate the transistor from the socket. I couldn't remove the legs so had to buy 4 x new sockets.
The ones I found on ebay are excellent....gold plated connectors....silicon insulators....mounting screws....all for about $10 US delivered to Oz. You have to be very careful that no part of the output transistors touch the metal case.
The DC Offset and Bias current adjustments are easy to do with a digital multi-metre.
Just follow the dude on the VV youtube clip.....
https://youtu.be/GOdCPVJpzu4
https://youtu.be/Cnsf0lITQmg
So the end result has been a much louder and better sounding Rhodes....I'm still astounded at the volume level the amp produced without the output transistors....but it was definitely not a clean sound.
Cheers