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Substituting Marantz 32 amplifier bias transistors sans data sheet

MisterFishey

MisterFishey

Super Member
I am unable to correctly set the bias in my Marantz 32 amplifier, and one of the channels has a frequent popping sound, and I suspect the differential transistors and/or the bias transistors since they are quite old by now, and one channel was (somewhat competently) repaired by the previous owner's tech. The transistors in question are the Motorola NPN S33690 and SS47 transistors and the Motorola PNP S33691 and SS48 transistors, all of which are in the TO-39 "flying saucer" package. These are denoted on the attached amp board schematic as Q206 (SS48), Q208 (SS47), Q209 (S33690), and Q210 (S33691). I cannot find data sheets for these transistors anywhere, despite many many Google searches. The S33690/S33691 do not even show up in Google search results at all. I've made a few transistor substitutions in the past, but none without a data sheet.

First, I analyzed schematics for slightly later Marantz receivers and amplifiers to see if the circuits were similar, so maybe I could get a useful transistor name. Unfortunately, the circuits were very different, so no luck there.

I figured the next logical step was to measure the transistor hfe values on my multimeter (for both channels, out of circuit, of course), which came out as follows:

Transistor: Channel A hfe / Channel B hfe
(PNP) SS48: 56 / 126
(NPN) SS47: 127 / 46*
(NPN) S33690: 35 / 45
(PNP) S33691: 56 / 126

* This transistor appears to have been replaced as it is not Motorola branded, so I am not assuming it is indicative of the transistor intended to be there.

Based on these readings, I concluded that the approximate hfe of the SS47/SS48 transistors is around 125, and the approximate hfe of the S33690/S33691 transistors is around 40-50. However, these figures are somewhat meaningless without Vce and Ic, and I realize this. I do not know these specifications for my multimeter, but based on comparisons of known transistors to known specifications, it measures at roughly 30-50% between the min and max listed hfe. Not too helpful, I know. :no: Regardless, these are fairly low-gain transistors.

If it helps at all, the output transistors were replaced with the ON-Semi MJ21195 TO-3 outputs, since the output transistors were mismatched when I got the amp. The bias problem existed before and after the replacement.

From here, I'm not sure where to go. :dunno: Any suggestions? I would be glad to get any voltages or other information that would help. Thanks for reading my verbose post! :thmbsp:
 

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I don't know if this will help at all but I searched through all of my Motorola transistor books and all of the other interchange books I have and this is what I found.

I found a reference in a 1974 Radio Shack substitution guide for a SSA48 transistor, RS #276-2022.

That part references PNP transistor:2N2605, Small signal amplifier: low level, low noise, high gain

The specifications for the 2N2605 are:

Vceo 45 volts
Vcbo 60 volts
Vebo 6 volts
Ic 30 mA
Pd 400 mW

This is all outside my experience, I don't know if any of this helps or not. I hope it might provide a clue to finding answer to your situation

Good luck
 
catrafter said:
I would start by comparing the C to E voltage of Q207 in both amps.
Click to expand...

Good idea. CR203 might be conducting excessively. One would think that R224 could overcome any problem of this sort, but the circuit is arranged to prevent the suicide mode and that makes it difficult to pin down which component is bad. 1N541 is an obsolete germanium point-contact diode that might be difficult to sub. I did see a few in the findchips.com report, however.
 
catrafter said:
I would start by comparing the C to E voltage of Q207 in both amps.
Paging Mr. ecluser!:D

Tom
Click to expand...

Done. I got some interesting results for Q207: :scratch2:

Channel: Vc / Ve (with reference to ground)
Ch. A: -3.8mV / 0V
Ch. B: 14.5mV / 0V

I double and triple checked these readings. Ve is likely zero in both channels since the bias pots are turned all the way down on both channels, so Ve is going straight to ground.

EDIT: BinaryMike, just saw your post, this would indicate a problem with the CR203 diode on Channel A, I assume.

ChrisMarantz said:
I don't know if this will help at all but I searched through all of my Motorola transistor books and all of the other interchange books I have and this is what I found.

I found a reference in a 1974 Radio Shack substitution guide for a SSA48 transistor, RS #276-2022.

That part references PNP transistor:2N2605, Small signal amplifier: low level, low noise, high gain

The specifications for the 2N2605 are:

Vceo 45 volts
Vcbo 60 volts
Vebo 6 volts
Ic 30 mA
Pd 400 mW

This is all outside my experience, I don't know if any of this helps or not. I hope it might provide a clue to finding answer to your situation

Good luck
Click to expand...

:ntwrthy: :ntwrthy: :ntwrthy:

SSA48, SS48, both by Motorola? I would say they are likely a match. Thank you so much! :music:

The specs look good on the data sheets I found for the 2N2605. After doing a little bit of searching, the 2N3906 looks like a good match, but the Vceo(max) is too small - the spec is 40V and I measured 40.3V across the SS48 in the amp, so I'm not going to use that.

EDIT: I found a PN2907 transistor that is spec'd for higher voltages than the 2N3906, so I will likely go with that one. :thmbsp: I'll just track down the NPN compliment on Mouser, since Digikey appears to not have it.
 
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We wanted voltage readings from E to C on Q207, *not* ground-referred. They should be in the neighborhood of 1.2V. It would also be a good idea to see what the range is, by adjustment of R224, but don't let output stage bias current get away from you.
 
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BinaryMike said:
We wanted voltage readings from E to C on Q207, *not* ground-referred. They should be in the neighborhood of 1.2V. It would also be a good idea to see what the range is, by adjustment of R224, but don't let output stage bias current get away from you.
Click to expand...

I apologize, Vce is as follows with the bias turned all the way down:

Channel A: 3mV
Channel B: -14mV

Pretty far away from 1.2V on both counts. I'll get additional readings with different bias settings in a little bit, since I'll need to lay the heatsinks flat to adjust the bias.
 
If that voltage stays so low, then Q207 may be shorted. But we could have other problems -- like possibly no current flowing through Q206 and Q208. You might want to check this by looking at the voltages across those 56R emitter resistors. I would expect maybe 10~20mA in that path, which would produce roughly one half to one volt across those resistors.
 
What is the voltage across the zeners, CR201 and CR202?

I think you should try to make a better channel with the good SS47 and the good SS48 for Q206 and Q208. I computed 20mA in those transistors (assuming Vbe of 650mV), so they have to dissipate ~800mW. The PN2907, at Pd max 600mW, will not survive. You will need a transistor in TO-126 or TO-220 case. Furthermore, Q206 and Q208 must have a Vce rating of at least 80V because, at clipping, the bases of Q209 and Q210 will have to swing from +40V to -40V.
 
BinaryMike said:
If that voltage stays so low, then Q207 may be shorted. But we could have other problems -- like possibly no current flowing through Q206 and Q208. You might want to check this by looking at the voltages across those 56R emitter resistors. I would expect maybe 10~20mA in that path, which would produce roughly one half to one volt across those resistors.
Click to expand...

The voltages across the 56R emitter resistors followed by the calculated current based on their actual resistance are as follows:

Resistor: Ch A / Ch B | Ch A / Ch B
R227: 1.20V / 1.16V | 20mA / 20mA
R228: 1.17V / 1.16V | 20mA / 20mA

I actually measured the Vce across Q207 again today, with much different results:

Ch A: 1.049V
Ch B: 1.049V

These seem to be far more reasonable figures, I don't know why I got the results I did last time. :dunno: At any rate, it looks like Q206 and Q208 are passing current fine.

ecluser said:
What is the voltage across the zeners, CR201 and CR202?

I think you should try to make a better channel with the good SS47 and the good SS48 for Q206 and Q208. I computed 20mA in those transistors (assuming Vbe of 650mV), so they have to dissipate ~800mW. The PN2907, at Pd max 600mW, will not survive. You will need a transistor in TO-126 or TO-220 case. Furthermore, Q206 and Q208 must have a Vce rating of at least 80V because, at clipping, the bases of Q209 and Q210 will have to swing from +40V to -40V.
Click to expand...

The voltages across zener diodes CR201 and CR202 are as follows:

Diode: Ch A / Ch B
CR201: 14.57V / 14.62 V
CR202: 14.63V / 14.56V

I was actually thinking about swapping the transistors to make one good channel, it isn't a bad idea. I really would like to replace these transistors once and for all, though.

I didn't even consider the power dissipation necessary, good catch! Better than figuring it out after the transistors burn themselves to a crisp. I did some hunting and it looks like the KSE44H11 meets the bare minimum voltage spec, with the 2SC3063 looking like a better choice, if I can find a complement and if I can buy some before the stock at Digikey runs out. I'll do some hunting at Mouser as well, they have a larger selection of transistors than Digikey.
 
MisterFishey said:
I am unable to correctly set the bias in my Marantz 32 amplifier...
Click to expand...

What do you mean by this statement, exactly? 1.049V across Q207 seems too low to obtain forward bias in the output section, so I'm guessing that you see no bias current as measured by the voltage across R230 and R231. Correct? Or does the manual specify some other way of setting bias?
 
Thanks for the zener voltage. I am a little fooled by the schematic. In principle, at standby, both sides of R216 should be at 0V. I computed ~30mA in Q203 with R215 = 56 Ohms, if the input stage is balanced. However, I computed 3mA in R217. I think that R215 should be a 560 Ohms resistor. Vce of Q203 would be approximately 13V. The amp can work with R215=56 Ohms, but the input stage would be greatly unbalanced, almost all the current would flow in Q201, and a very small current would flow in Q202. This can be the cause of the popping sound that you have.

But I see that I was correct with the 20mA in the pre-driver stage.

Digikey has a good stock of 2SA794(R) and 2SC1567(S). The 2SC1567(S) are inexpensive, so you can buy more 2SCs to find two good matched pairs, although it is not really important.
 
BinaryMike said:
What do you mean by this statement, exactly? 1.049V across Q207 seems too low to obtain forward bias in the output section, so I'm guessing that you see no bias current as measured by the voltage across R230 and R231. Correct? Or does the manual specify some other way of setting bias?
Click to expand...

The manual specifies a very bizarre way of setting the bias. I quote verbatim from the service manual:

Bias adjustment test:

1. Preset the bias adjust potentiometer R224 on each amplifier board fully CCW.
2. After discharging the filter capacitors using the bleeder resistor, disconnect all red and blue wires from the capacitor terminals.
3. Turn Line Switch to ON and slowly advance variac while observing the voltmeter and wattmeter. The relay K301 should energize at 105 volts or less.
4. Turn Line Switch to OFF. Reconnect all red and blue wires to filter capacitors.
5. Turn Line Switch to ON and advance variac to 120 volts. Observe wattmeter reading. Adjust the Channel A amplifier board potentiometer R224 to increase wattmeter reading to a point halfway between the initial reading and 20 watts.
7. Adjust the Channel B amplifier board potentiometer R224 to increase wattmeter reading to 20 watts.
8. Turn the Line Switch to OFF. Remove shorting plugs from the input jacks.

Since I don't have a variac or a wattmeter (and I figured there must be an easier way), I did some comparison of the bias setting procedure on Marantz 2235B and 2250 receivers, and saw that measuring the voltage across R230 and R231 on the Marantz 32 was the same location of the pins for measuring the bias on the receivers (the voltage across both 0.2 ohm wirewound resistors, which have a name that I don't recall at the moment). The circuits are obviously not 100% analogous, but I figured it was a good place to start, and the measurements I was getting in the amp (in the mV range) were reasonable values for uncalibrated bias.

The bias specified in the receiver manuals is 8.0mV, whereas I cannot keep the bias below 15-20mV on either channel in the amp. In addition, it takes roughly 10 minutes for the bias to stabilize at all in the amp. It starts at around 5mV, but then climbs steadily to 15-20mV. I'm well aware that it is not abnormal behavior for the bias to change as the amp warms up, but this much change? :scratch2:
 
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ecluser said:
Thanks for the zener voltage. I am a little fooled by the schematic. In principle, at standby, both sides of R216 should be at 0V. I computed ~30mA in Q203 with R215 = 56 Ohms, if the input stage is balanced. However, I computed 3mA in R217. I think that R215 should be a 560 Ohms resistor. Vce of Q203 would be approximately 13V. The amp can work with R215=56 Ohms, but the input stage would be greatly unbalanced, almost all the current would flow in Q201, and a very small current would flow in Q202. This can be the cause of the popping sound that you have.

But I see that I was correct with the 20mA in the pre-driver stage.

Digikey has a good stock of 2SA794(R) and 2SC1567(S). The 2SC1567(S) are inexpensive, so you can buy more 2SCs to find two good matched pairs, although it is not really important.
Click to expand...

I double checked the service manual schematics, parts list and change log, and R215 is supposed to be 56 ohms, but I see what you are saying. It would balance out the input stage better. I'll add some 560 ohm resistors to my next parts order and see if the popping noise goes away. I'm actually in the process of moving my order over to Mouser since they have a much larger transistor selection, so I should be able to find subs pretty quick for the 2SA794(R) and 2SC1567(S). :yes:
 
MisterFishey said:
The bias specified in the receiver manuals is 8.0mV, whereas I cannot keep the bias below 15-20mV on either channel in the amp. In addition, it takes roughly 10 minutes for the bias to stabilize at all in the amp. It starts at around 5mV, but then climbs steadily to 15-20mV. I'm well aware that it is not abnormal behavior for the bias to change as the amp warms up, but this much change? :scratch2:
Click to expand...

In this kind of circuit, CR203 and Q207 should be in close thermal contact with Q209 or Q210.

The current in R230 comes from Q209 (~6mA) and Q1. I would adjust to 25mV across R230, the "effective emitter resistor" of Q1.
 
MisterFishey said:
I double checked the service manual schematics, parts list and change log, and R215 is supposed to be 56 ohms, but I see what you are saying.
Click to expand...

Unless I didn't read the schematic correctly. Can you confirm that R210=7.5k, R217=5.1k?

If my reading is correct and R215 is 56 Ohms, then the current in Q202 is approximately 0.1mA, and the current in Q201 is approximately 0.5mA. Since the tail current is 0.63mA, either transistors in the differential pair are at the edge of saturation.
 
ecluser said:
In this kind of circuit, CR203 and Q207 should be in close thermal contact with Q209 or Q210.
Click to expand...

CR203 and Q207 are located on the back side of the amplifier PCB and should be in contact with the heat sink. On the units I have seen usually both components are not in contact with the heat sink. I add some thermal grease and readjust them so they make contact.
 
MisterFishey said:
The bias specified in the receiver manuals is 8.0mV, whereas I cannot keep the bias below 15-20mV on either channel in the amp. In addition, it takes roughly 10 minutes for the bias to stabilize at all in the amp. It starts at around 5mV, but then climbs steadily to 15-20mV. I'm well aware that it is not abnormal behavior for the bias to change as the amp warms up, but this much change?
Click to expand...

There's good evidence now (from research) that bias current adjustment doesn't necessarily optimize crossover distortion performance. The theory is that one should rather adjust for 26mV across each emitter resistor, or 52mV across the pair. That would yield 130mA in the Marantz 32, which seems reasonable if it's stable. But these older amps do tend to drift quite a lot more than we're used to seeing in later designs. If you can improve bias drift by improving thermal coupling to the driver transistors, go for it. In general, bias drift is a fundamental weakness of class AB transistor power amplifier design.
 
ChrisMarantz said:
CR203 and Q207 are located on the back side of the amplifier PCB and should be in contact with the heat sink. On the units I have seen usually both components are not in contact with the heat sink. I add some thermal grease and readjust them so they make contact.
Click to expand...

I believe ecluser has this right, in that it's better to seek thermal feedback from the driver transistors instead of the output devices when you're working with a CFP output circuit. What is your experience with bias drift after the suggested modification?
 
ChrisMarantz said:
CR203 and Q207 are located on the back side of the amplifier PCB and should be in contact with the heat sink. On the units I have seen usually both components are not in contact with the heat sink. I add some thermal grease and readjust them so they make contact.
Click to expand...

I've never worked (I mean physically) on this model but the heat sink you are talking about... is it for the power transistors (Q1 and Q2) or for the driver transistors (Q209 and Q210)? Usually the thermal sensistive components (diode, transistor) of the bias stage are thermally coupled to the power transistors in amplifiers. This is correct in most amplifiers but the topology of the model 32 dictates to favour a good thermal coupling to the driver transistors.
 
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