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Old 2nd Jul 2020, 2:28 pm   #1
bikerhifinut
Octode
 
Join Date: Nov 2013
Location: Penrith, Cumbria, UK
Posts: 1,993
Default Rogers Ravensbrook series2 amp. Before attempting a capacitor replacement.

I am about ready to start doing a cap replacement on a Rogers ravensbrook Series2 amplifier.

I carefully powered it up and did a quick set of voltage measurements around the PSU side of things. I noted a small amount of 100Hz hum but as its basically down to a bare PCB just now and the mains transformer etc doesn't have a mains earthed steel case around it I think some hum could be to insufficient earthing. So careful when poking the meter probe in. And of course the capacitors are pretty old, most having been there since 1971.

So I did a quick voltage check, First with the mains voltage setting at 240V (which would be its usual setting) and again quickly at 220V to check how the regulation was doing.
The regulator is a simple one, consisting of a 43V 1W zener diode, bypassed by a 120uF capacitor to the base of a PNP Germanium power transistor which has its collector on the negative side of thee rectifier and emitter outputting to the signal earth power rail. Theres a 390 ohm resistor between base and collector. The main reservoir capacitor is a weedy by modern standards 1000 uF but memory serves me that increasing this value causes fairly serious issues and I feel that the regulation is the main hum "filter".
The meter shows that the regulation seems fine as it stays stable at just less than 43V on the 240V tap and a shade above at 44.1V on the 220V tap. No signal and a pair of 8 0hm speakers connected. This puzzles me a bit as I thought the zener and pass transistor would hold the output voltage steady at whatever input voltage. Insight here would be appreciated, keep it simple please as I can be hard of understanding.

The pass transistor should be a 2N3611, this blew many years ago and the best germanium power transistor I could lay hands on (or rather my dad who blagged it off an instrument tiffy at his work) was a proper OC28. The spec isnt quite as good as a 2N3611 in terms of power handling but its rated at 6 times the maximum current it's expected to pass and as far as I can see it just about copes with the voltages its exposed to. It hasnt blown yet but I am wondering if the low level background 100Hz could be down to it breaking down a bit? If so I wondered if any decent PNP TO3 transistor would be ok including silicon, I am assuming that good PNP silicon power transistors werent readily (cheaply) available when this amp was in production. I havent looked for a 2N3611 as yet.

So the plan is to firstly replace just the PSU caps and power supply decoupling caps and then give it a listen and maybe bung the scope on it as a check.

Would it be wise to replace the speaker coupling caps too at this point? I would imagine after 49 years they will be pretty dried up.

I'll replace the rest of the electrolytics in stages once the power supply is sorted out. I am very aware of the hazards of replacing all the caps at once and then facing the possiblity of tracing a fault not knowing where I boo booed!

On the face of things this isnt a major resuscitation from a non working state, luckily, but this particular amp has great sentimental value for myself and Father and that's why I am going very carefully.

I hope it's not cheeky or rude but can we keep the comments and replies focussed on my queries or relevant to each stage of the refurbishment please? I can also assure everyone that I have sourced a full complement of electrolytics at or as close as current preferred values allow.

Andy.
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