rlisin
Quad 4 life
This weekend I took out my Superscope R-1270 for cleaning and running a it bit. Thread is worthless without pics, so here they are:
and figured I'd try adjusting it. According to some sources the amp section is based on Marantz 1070 amp, but I think it's actually based on Marantz 2235 receiver: both are quite similar, but R-1270 has the same outputs as 2235, and 1070 has different outputs.
Anyway, service manual for R-1270 is not available (for free anyway), so I tried to figure things out based on manuals for 2235 and 1070. I'm working on an assumption that Marantz engineers would not make any major changes to the curcuits between 2 very similar amplifier boards that use the same output transistors, so that the adjustment procedures would be similar or the same for Superscope R-1270 and Marantz 2235.
Here's the amp board and outputs of Superscope R-1270:
Here's the adjustment procedure and amp board layout for Marantz 2235:
DC offset was easy. There are 4 trimming pots on top of the the amp board, the inner pots adjust DC offset (like in Marantz 1070 and 2235). DC offset was quite healthy (fluctuating within 5mV for each channel), but I adjusted it to within 0.3 mv, because why not.
Idling current is a bit more tricky and I need advice.
The outer trimming pots on top of the the amp board adjust idling current, again like in Marantz 1070 and 2235
So far I figured out that I don't have to bother with not-so-accessible pins at the bottom of the amp board (which the SM for both Marantz 2235 and 1070 identifies as measurement points for idling current), because it's the same as measuring across the emitter resistors, which are easier to access. Using the board from 2235 as an example, adjusting across pins J713 and J717 would be the same as adjusting across resistors R756 and R758 (for 8 mV), or I could just adjust across one of them for 4 mV. Is that correct?
Well, that's what I did and nothing exploded - yet.
But I also noticed that the value of these resistors is different between my R-1270 and Marantz 2235. In 2235 resistors R756 and R758 are 0.22 ohm, in R-1270 their counterparts are 0.5 ohm. So the question is, how should I change the value of the idling current measured across these resistors? Is is as easy as double the value of resistors, double the idling current across them? So, if idling current in Marantz 2235 is 8 mV across two 0.22 ohm resistors (4 mV across one), would it be the same as around 16 mV across two 0.5 ohm resistors (8 mV across one)?
For now I used this reasoning plus a small safety margin and adjusted for 6 mV across one emitter resistor in my R-1270. If anyone can confirm my above reasoning, I will readjust for 8 mV. If I'm dead wrong, tell me now, before things start blowing up
Idling current was much higher before adjustment, around 30 mV in one channel, around 40 mV in the other.
and figured I'd try adjusting it. According to some sources the amp section is based on Marantz 1070 amp, but I think it's actually based on Marantz 2235 receiver: both are quite similar, but R-1270 has the same outputs as 2235, and 1070 has different outputs.
Anyway, service manual for R-1270 is not available (for free anyway), so I tried to figure things out based on manuals for 2235 and 1070. I'm working on an assumption that Marantz engineers would not make any major changes to the curcuits between 2 very similar amplifier boards that use the same output transistors, so that the adjustment procedures would be similar or the same for Superscope R-1270 and Marantz 2235.
Here's the amp board and outputs of Superscope R-1270:
Here's the adjustment procedure and amp board layout for Marantz 2235:
DC offset was easy. There are 4 trimming pots on top of the the amp board, the inner pots adjust DC offset (like in Marantz 1070 and 2235). DC offset was quite healthy (fluctuating within 5mV for each channel), but I adjusted it to within 0.3 mv, because why not.
Idling current is a bit more tricky and I need advice.
The outer trimming pots on top of the the amp board adjust idling current, again like in Marantz 1070 and 2235
So far I figured out that I don't have to bother with not-so-accessible pins at the bottom of the amp board (which the SM for both Marantz 2235 and 1070 identifies as measurement points for idling current), because it's the same as measuring across the emitter resistors, which are easier to access. Using the board from 2235 as an example, adjusting across pins J713 and J717 would be the same as adjusting across resistors R756 and R758 (for 8 mV), or I could just adjust across one of them for 4 mV. Is that correct?
Well, that's what I did and nothing exploded - yet.
But I also noticed that the value of these resistors is different between my R-1270 and Marantz 2235. In 2235 resistors R756 and R758 are 0.22 ohm, in R-1270 their counterparts are 0.5 ohm. So the question is, how should I change the value of the idling current measured across these resistors? Is is as easy as double the value of resistors, double the idling current across them? So, if idling current in Marantz 2235 is 8 mV across two 0.22 ohm resistors (4 mV across one), would it be the same as around 16 mV across two 0.5 ohm resistors (8 mV across one)?
For now I used this reasoning plus a small safety margin and adjusted for 6 mV across one emitter resistor in my R-1270. If anyone can confirm my above reasoning, I will readjust for 8 mV. If I'm dead wrong, tell me now, before things start blowing up
Idling current was much higher before adjustment, around 30 mV in one channel, around 40 mV in the other.
