The Pioneer "Series 20" Gear Thread!

[jpalkhivala]

My A-27 was recently overhauled and recapped

 

[moewillis]

i was not aware of this series. i now have an m-25 to look for!!

 

[AvFan]

I've had my A-27 for a few years now following a 2+ year search and I really like it. If you are not familiar with the A-27 it is combo of the Series 20 M-25 amp and C-21 preamp and Spec 1 preamp. I got lucky as it hadn't been used in several years, it had never been worked on and was stored in the dry high desert of Southern California. Regardless, I had the main and phono preamp power supplies rebuilt due to age and the fact that both run in Class A (main for the first 3 watts) and get hot. A bonus was the face was nearly flawless. It features both MM (with variable loading) and MC (100ohms only) phono inputs, very flexible tone controls and a nice headphone amp. I bought a pretty nice tube integrated about 1 1/2 years ago but I've gravitated back to the A-27 for its convenience and the fact it sounds "right" to me.

 

[hennessy]

I do have a C-21, M-25, and two A-27s. I did rebuild them from scratch, except the other A-27. Did some mods, too. I would dare to disagree that A-27 is a C-21 and M-25 combo, with tone circuit thrown in. There are similarities, sure, like power output section, and electrical design. C-21 and M-25 have much better components inside, especially A-27 phono section somewhat stripped down comparing to C-21. On the other hand, A-27 has MC head amp, but C-21 doesn't. Generally, M-25 and A-27 are somewhat similar, but C-21 does not have much in common with A-27.

 

[cnolanh]

I do have a C-21, M-25, and two A-27s. I did rebuild them from scratch, except the other A-27. Did some mods, too. I would dare to disagree that A-27 is a C-21 and M-25 combo, with tone circuit thrown in. There are similarities, sure, like power output section, and electrical design. C-21 and M-25 have much better components inside, especially A-27 phono section somewhat stripped down comparing to C-21. On the other hand, A-27 has MC head amp, but C-21 doesn't. Generally, M-25 and A-27 are somewhat similar, but C-21 does not have much in common with A-27.

I wonder if one of those differences is that the M-25 has (apparently) potted transformers and it looks like ones in the A-27 are bare. And with all the fins on the M-25, there's probably more surface area on its heat sinks. But the A-27 sure looks like an amazing integrated amp. I'd love to hear one sometime.

 

[hennessy]

I think they are not bare. I think they are the same. I cannot find the original M-25 board pictures, only those after I overhauled them. Anyway, usually pots of that age are replaced. Here Bourns 3339 and 3386 were used @M-25, and 3299 @A-27.

A-27 power amp board in progress.

Finished.

One of M-25 boards. Not final. Later Orange Drops and some other caps were replaced with FKP

 

[AvFan]

I do have a C-21, M-25, and two A-27s. I did rebuild them from scratch, except the other A-27. Did some mods, too. I would dare to disagree that A-27 is a C-21 and M-25 combo, with tone circuit thrown in. There are similarities, sure, like power output section, and electrical design. C-21 and M-25 have much better components inside, especially A-27 phono section somewhat stripped down comparing to C-21. On the other hand, A-27 has MC head amp, but C-21 doesn't. Generally, M-25 and A-27 are somewhat similar, but C-21 does not have much in common with A-27.

Thanks for the clarification on similarities, or the lack of, between the various Series 20 models. I'm curious, what makes the C-21's MM phono better than the A-27's? And how did you light up the controls on your A-27?

 

[hennessy]

It is not bad, it is just component selection does not justify somewhat high A-27 price, and killer looks. For example, this is original A-27 cartridge loading board (Duplicate). Ceramics, with characteristics all over (I measured when pulled them out).

C-21 is all polystyrenes, choke-full of them. I did not need to replace C-21 most caps. Sadly, I did not photograph C-21 insides, but as I remember, I just replaced power supply caps, power resistors to beefier ones, put schottkies in, replaced output caps, etc.
A-27 has total 4 (four) polystyrene capacitors, located @phono board.
I promised I'll write up the A-27 rebuild, just have to sit and do it.
I did A/B the rebuilt A-27 driving one side speaker, and original untouched driving the other side from the same source(mono/stereo). There is difference. Even I can hear it. The most compelling reason to rebuild those - reliability improvement, and high heat mitigation. Everything in it runs scorching hot, even pre-drivers.

Those panel lights? I photo-shopped them! Kidding. That's one happy A-27 shining from joy having survived my butchering. Using some yellow SMD diodes and light path distribution. The installation is completely reversible - can be removed without trace. It is really practical; I appreciate it when listening in the dark through headphones. That's another area where A-27 shines.

 

[AvFan]

The tech I use recommended rebuilding the phono, along with the main power supply, in my A-27 because of the high heat and age. No complaints from me when he suggested the work. I like the phono in my A-27 and if the better parts in the C-21 yield better sound that is good for those who are lucky enough to own the C-21. While I have no direct comparisons I would suggest either are in the upper tier of phono stages from the late '70s. I enjoy experimenting with other gear though and use a Budgie tube phono preamp because I can roll tubes, that along with its flexible loading, tailor the sound to my preference.

I'd like to see how you added the panel lights for the reason you noted. The single source light isn't much help in the dark.

 

[hennessy]

The other things that should be done IMHO on A-27 would be output reseating with new compound, and TO-126 Q9-12 and 21-24 on Volume board heatsinking. Those run extremely hot
Regretfully, I do not have pictures of panel light installation. It was just a fly-by job on a whim.

 

[Hauxon]

Does anyone know if the M-25 has common ground? I'm considering buying this amp to drive my Polk SDA speakers. They need the amp to have common ground for the SDA system (with interconnection cable) to work. Many dual mono designs do not have common ground so I'm thinking the M-25 might possibly be without common ground.

Thanks, Hrannar

 

[hennessy]

It does have common output negative connection (common speaker ground). The schematics say so, and the picture is here.
The very very top two nuts are speaker negative posts. Their "S" shape silvery buses are grounded, in addition to black wire common grounding to the chassis center.
Left white and grey right are positive speaker post wires connected to the lower pair of the top nuts.

 

[Hauxon]

Thank you!

 

[tuberadios69]

Oops! edited with corrected dates. I found my original invoices for the M-22's:
I recently acquired a working Pioneer Series 20 F-28 tuner. This tuner has been a grail unit for me for many, many years. It sounds wonderful! I also have an M-22 I bought back in 1986 when I worked for Pioneer. I found a bunch on an inventory sheet one day in 1986 while perusing a B-stock list. Seems many units, particularly unsold M-22's, had been sent to Long Beach when US subsidiaries US Pioneer (Moonachie, NJ, home audio division) and Pioneer America (Long Beach, CA, car audio division) were merged, transferring to Long Beach in 1982/1983. Despite being an awesome amp built like a you-know-what, the Series 20 line and M-22 did not sell well due to the perception (wrong) that a Japanese mid-fi giant couldn't make hi-end gear. We know better, now. Two of my mid-Atlantic accounts bought a few dozen M-22's when I presented the opportunity (late 1986 or early 1987) and quickly sold them to their friends, employees and some lucky consumers. As a result, I bought 2 for myself and a friend. I still have mine although I let it languish in my attic for many years when I became a tube guy. Someday I'll bring it up on my variac and see if the caps are okay and reform. It is sitting in the spare bedroom right now. I am using the F-28 in my main system with a Terrestrial Digital 3-element yagi in the attic. It sounds as good as I remember, maybe better! Here's a pic:

 

[IanT417]

Ive got a very nice perfectly running Series20 C-21 Preamp and series20 tuner F-28. Probably going to list them publicly soon. Maybe. I just dont know what could fill their shoes. Sorry no m-25 amp to go with
I run a SA-9500ii

 

[IanT417]

Loved reading all the submissions and seeing the gear photos. Here is my contribution to the archives of Series20.
The C-21 and F-28. Still hoping to find an Amp one day. For now the SA-9500ii is towing the line.