Sounds like someone got two base pans swopped over!
Do you have the info on how to pop the lid and undo the plastic base pan?
I was rather wondering if it could have been a factory error or not!?
I've worked on several before so yes I know how to get the lid off.... Has to be the easiest access on any amp ever!
Have one plastic Dzus fastener missing on the bottom.
Most recent date is sept 77 on output caps so assuming 78 for year of manufacture. It has the Alps pots but the RCA decoder IC (I presume the TI decoder is not a pin compatible exchange?).
Smoothing cap and output caps are well dodgy looking and have blebed up rather with slight leakage... as usual they still work though! First try out, through headphones, and apparently the first time it's been powered up in 25 years, showed this it kinda works after a fashion.... vol control very noisy and only got output (on FM with a few foot of wire as a test aerial) on one channel but crackles from vol control present on both channels so it looks like the power amps work anyway. It seemed rather deaf even for just a few foot of wire as an aerial.
All I've done so far is give all the pots and switches a good squirt of contact cleaner and a thorough work out but I didn't give it another go after that.
Were encapsulated bridge rectifiers fitted to the last ones? I vaguely remember one with a bright green bridge fitted to the chassis around the power amp board area...
They are a typically British mix of "great!" and "what were they thinking?" in one unit.... For the time, impressive industrial design and styling.... but all sorts of femur aspects and obvious afterthought bodges as well!
Things like a single thin wire as the speaker earth return shared by left and right and speakers 1 and 2 are not good but easily modified.
plans after some more thought are now to not go as far as redesigning the power amp and going fully complementary etc but to re-cap it, including making the smoothing cap something like 15000uF as it should be about the same size as the original 7500uF item. Replace the leaky output caps but with standard value (4700uF anyway as 4000uF non standard today) to keep the clever "tuned damping factor" feature and find a way of mounting the new output caps as they will be a fraction of the size of the originals, then re-align the tuner section (which looks easy... famous last words! Only IF can on front end and discriminator can as Denco coils so not much to go wrong... plus front end itself of course... which has a coil in it with the ferrite core almost entirely unscrewed which looks a bit suspect!).
I'll also put right anything which is just not good engineering practice, ie the single shared earth for the speakers, and hopefully end up with a somewhat better than brand new one. May need to get another "spares or repairs" donor unit as the centre tune meter is buggered so I'll prob need to get another 625/6 just for a tuning meter!
I'd quite like a 626 for the novel and sophisticated AM section, even though I'm not likely to ever listen to AM!
I'm surprised we do not see more interest and demand for this British classic TBH. I love the styling, the features are well thought out, it's pretty powerful for the time (actually about 50WPC I believe), they were well reviewed, and I was impressed with the performance of a mates 625 which he had for years after he got it second hand in about 1984 ish and partnered it with B & W DM4 speakers. Excellent FM tuner performance from memory and amp to rival the likes of A & R A60 and similar in many ways.
oh, LOVE the styling of the 400 series! The pinky/purple circular FM tuning dial and all that! Power amp design looks rather dodgy though....