Armstrong A17=621 repair
I'm restoring a vintage Armstrong 621 Receiver/amplifier and am looking for information on the amplifier section A17 used in a number of models.
The unit I have is similar to the circuit posted by Jim's Audio Pages and Websites but not identical. It has presumably a later unit that has been "subject to continuous development"
In the amplifier section (still marked A17) the driver transistors are plastic cased similar in style to a TO126 but larger and with no metal tab, instead of the original metal TO5 types. Two are marked 16505 (or possibly I 6505) RERCA509, the other (obviously the complimentary) is marked 16506. There are obvious differences in the use of resistors, (mainly additions) but the basic board and the schematic have obvious similarities to that posted on the web.
Searching the web has not brought up any information on these transistors.
The thread "Overhaul of an Armstrong 625...." has provided much useful information, so this does not need to be repeated, but I would be interested to know if anyone has the actual schematic of this amp, or information on these transistors. Or indeed any other changes that might have been made.
'Not sure about your case description there when you say "like TO126.. but no metal tab". I think they would both have a metal surface like the TO126 at least, but anyway, it's possibly like Motorola's case 90, which I link to with an early MJE3055 case example, just for comparison of the dimensions. MJE3055 datasheet and application note, data sheet, circuit, pdf, cross reference | Datasheet Archive
As Jim Lesurf points out, Armstrong 600 series went through a range of transistor types in their product lifetime. I have a 626 Receiver that is fitted with BD239B/40B, IIRC, as A17 drivers in the same Mullard circuit ca. 1970. It reminds me of the local, very similar Playmaster 132 DIY design also based on that. Doubtless, mods had to be made to get the circuit performing correctly again after the substitutions which might explain different components around this area.
FWIW, I would restore the standard schematic parts if it's necessary to replace these oddballs and source the original drivers, assuming they are still available as per the 'Overhaul' thread.
I found a record of RCA plastic power transistors p/nos RCA6505 and 6507 in an RCA substitution list but tbh, no further confirmation. It's a guess, but I think The RERCA509 label gives the source away.
Certainly that looks like the case.
AS you say, rather than chase my tail any further I will revert to the original design, but will probably substitute easily obtained semis from Jaycar.
I'm not after the best sound, just a clean working amp.
Have you emailed Jim Lesurf? He is quite human, and may have information which does not appear on his website.
MJE3055 and MJE3055T is a transistor that makes me cry.
Nono, nothing wrong with the transistor, it's actually a fine one, but Tandberg used the earliewr version, MJE3055 wich had its B and E in switched places opposed to "all" later transistors with the same case, and the TO220 transistors as the MJE3055T.
Some elder Tandgbergs (Aren't they all?) used the MJE3055 in the elder case, and no substitute is available. Too bad. Some of theese Tandberg really need them.
to others with a similar code. That is really confusing. :bawling:
Of course, TO126 is reversed compared to all larger plastic power devices now available but back in the 1970s, case 90 was just like them -
with optional pinouts becoming available where alternatives to other supplier's TOP66 styles (precursor to TO220) were required.
TO220(TOP66) has it's lead arrangement and spacings to fit straight into TO66 mounting positions as a substitution. Similarly, TOP3
(now TO218) fits TO3 mountings. However, the collector lead had to be trimmed and the leads bent at a specific length to do this.
It just shows that not everything remains certain over time and part/specification numbers are not enough to define components.
When I searched Ferranti (UK) versions of the Armstrong A17c drivers in my version, Ic(max) was 4 amps but current devices from ST are only 2!
Be wary, rjb, of PB2's comments on the driver ratings in the thread you referred to. Those 2N3773s are hungry for base current.
Darn timeouts on edits.
In above post, I meant to take note of PB2's comments, not to doubt them and the board is A17 (no c suffix)
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