I’ve got this beautiful teak mcm hifi console I’m looking to restore. It works fairly well now, just doesn’t get great FM reception, and I worry about the selenium rectifier every time I turn it on.
It’s 2watts single ended ELL80 (2xEL95 in one) powered. Wondering if there’s anything you might suggest to modify while I’m in here replacing the rectifier and any bad caps. I’m considering bypassing the bass pot and the two capacitors with an appropriate coupling cap. Would 0.022uF be a good starting point? And how would you go about bypassing the treble pot?
Also, I’m considering not using the tuner portion, and putting a DAC output right where C201 is, removing C201, and replacing R201 with 100k. Would this be blasphemous (or incorrect)?
I’ve got a pair of sound dynamics speakers with high sensitivity that should fit perfectly in the stock speaker locations that I’m hoping will yield good results. They need to be refoamed, but are otherwise in nice shape.
Any other inputs or suggestions?
Thanks
It’s 2watts single ended ELL80 (2xEL95 in one) powered. Wondering if there’s anything you might suggest to modify while I’m in here replacing the rectifier and any bad caps. I’m considering bypassing the bass pot and the two capacitors with an appropriate coupling cap. Would 0.022uF be a good starting point? And how would you go about bypassing the treble pot?
Also, I’m considering not using the tuner portion, and putting a DAC output right where C201 is, removing C201, and replacing R201 with 100k. Would this be blasphemous (or incorrect)?
I’ve got a pair of sound dynamics speakers with high sensitivity that should fit perfectly in the stock speaker locations that I’m hoping will yield good results. They need to be refoamed, but are otherwise in nice shape.
Any other inputs or suggestions?
Thanks
I was in a similar position to you here ... Pioneer SM-R150 ... and came to the forum for some guidance. It is still sitting on the bench, partially dismantled, taunting me because I ended up losing sight of what I wanted to do with it.
I replaced the old electrolytics and coupling capacitors to remove those concerns. There were a lot of old Paper in Oil caps that originally I thought were a good thing, but turned out to be leaky in more ways than one, so it was quite a lot of work to carefully tease apart the tightly wound connections on tag strips and replace those old parts.
Then I started to look at the overall block diagram - order of the stages - the RIAA, superfluous switches (switch left right, mono, mono one channel), the tone controls, the old potentiometers, the fact it was 110V supply in a 235V country, the suboptimal output stage (although the transformers are fairly good I think), and then thought about what my goal would be, what it could be worth to another enthusiast, and my enthusiasm waned.
Looking at that schematic, I think it is easy to underestimate the brilliance of some of the designers of these old consumer electronic devices, with a fixed goal of a quality level at the best price to manufacture possible. The tone controls and how they are embedded into the circuit are a good example, also the choice of output tube, ELL80, which is not that cheap to replace. The circuit has been optimised, and another level of performance is unlikely to be possible with a tweak here and there.
Is your goal to give it a new lease of life? Or do you have you imbued it with some expectations?
I replaced the old electrolytics and coupling capacitors to remove those concerns. There were a lot of old Paper in Oil caps that originally I thought were a good thing, but turned out to be leaky in more ways than one, so it was quite a lot of work to carefully tease apart the tightly wound connections on tag strips and replace those old parts.
Then I started to look at the overall block diagram - order of the stages - the RIAA, superfluous switches (switch left right, mono, mono one channel), the tone controls, the old potentiometers, the fact it was 110V supply in a 235V country, the suboptimal output stage (although the transformers are fairly good I think), and then thought about what my goal would be, what it could be worth to another enthusiast, and my enthusiasm waned.
Looking at that schematic, I think it is easy to underestimate the brilliance of some of the designers of these old consumer electronic devices, with a fixed goal of a quality level at the best price to manufacture possible. The tone controls and how they are embedded into the circuit are a good example, also the choice of output tube, ELL80, which is not that cheap to replace. The circuit has been optimised, and another level of performance is unlikely to be possible with a tweak here and there.
Is your goal to give it a new lease of life? Or do you have you imbued it with some expectations?
Thanks for the response. I was assuming the bass and treble controls were added to look more attractive to the consumer. Bypassing the bass control with a single coupling capacitor would also be cheaper than replacing the existing circuit with new. I will take your statement into consideration however, and may change my mind on this.
My hopes for this unit is just to have something nice to listen to in my living room. I’ve moved it up to my living room since taking the picture I shared in the first post. I’m mostly interested in convenience with this, so a streaming device with built in DAC has been the plan.
I got looking at it, and I’m questioning whether this is a selenium rectifier or actually silicon (or germanium).
My hopes for this unit is just to have something nice to listen to in my living room. I’ve moved it up to my living room since taking the picture I shared in the first post. I’m mostly interested in convenience with this, so a streaming device with built in DAC has been the plan.
I got looking at it, and I’m questioning whether this is a selenium rectifier or actually silicon (or germanium).
The shape is characteristic for Selenium Rectifiers made by SEL (Standard Electric Lorentz) and SIEMENS in Germany.
Yours is a bridge with something like 32 selenium pads (8 in series per leg) like in this picture.
By the way, the veneer is walnut, not teak ...
Yours is a bridge with something like 32 selenium pads (8 in series per leg) like in this picture.
By the way, the veneer is walnut, not teak ...
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We didn’t restore the one we got… i sold the drivers and we built this with the ELL80 and the Iron.
dave
dave
Thanks. I will replace the rectifier then.
I’m pretty sure these came in walnut and teak veneer, and mine is teak. The speakers and amplifier in the photo I posted are both walnut, most all of my other furniture is walnut, and this is more red with different grain; I could be wrong though.
The first stage uses grid leak bias; 4.7 to 10 megs is the normal grid resistor, and a coupling cap is required to keep the bias voltage from draining off. A 100K grid resistor would be OK, but you would need a cathode resistor to bias the stage, and a bypass cap across that to get the same gain as the original stage. The feedback from the output transformer to the volume control and the tone controls all interact to shape the frequency response. The volume control has the usual "loudness compensation" circuit to boost bass and treble at lower listening levels. The feedback has a series capacitor C181 to boost bass, and the bass control is a "cut" circuit with C213 to bring it back ~ flat with the control at mid-range. Treble is a "cut" control via feedback. C217 is a treble "cut" and also protects the output transformer against transients like static crashes from lightning. Have I convinced you to leave it alone yet?
Thanks for the breakdown. I had considered the first stage bias, but didn’t know what I’d do yet. If I do leave it original to the schematic, what can I do to place the DAC into the circuit?
Replaced the selenium rectifier. Schematic says 140mA, so 330ohm resistor should get me in the ballpark of the right b+.
Added RCA jacks, but didn’t have any shielded wire, so I’ll redo that if it’s noisy. This didn’t have any tape or aux in, so it’s parallel with the phono input. Should be fine as there’s no RIAA, just 47k input resistance, and the turn table doesn’t work anyway.
Replaced the very far drifted carbon comp resistors (hopefully they don’t have to be carbon composition), and just the electrolytics that seemed bad. Added a new power cord with a ground wire. Hopefully I can test tonight.
Added RCA jacks, but didn’t have any shielded wire, so I’ll redo that if it’s noisy. This didn’t have any tape or aux in, so it’s parallel with the phono input. Should be fine as there’s no RIAA, just 47k input resistance, and the turn table doesn’t work anyway.
Replaced the very far drifted carbon comp resistors (hopefully they don’t have to be carbon composition), and just the electrolytics that seemed bad. Added a new power cord with a ground wire. Hopefully I can test tonight.
Finished it up today. Brought it up on the variac slowly and all good. ~295V once warmed up, but unfortunately 335V from a cold start.
Thanks for the tips on this. It’s sounding great!
A picture of it being tested on the concrete for safety. Also, I named the streaming device appropriately.
Thanks for the tips on this. It’s sounding great!
A picture of it being tested on the concrete for safety. Also, I named the streaming device appropriately.