Marantz 2275 P700 Component Selection Affecting Bias Setting

[richiem46]

I read a number of threads about bias issues with the P700 amp boards in the Marantz 2275 and in other Marantz units. A lot of great information freely shared by other members. I came away not completely clear on the best approach to restoring the P700 board and thought I would start another thread on the subject.

I just restored a 2275 P700 board and replaced all the transistors and some of the diodes on the board including the output transistors. I replaced H713 and H714 with MJE15032G and MJE15033G transistors and I could only bias the board down to 20MV at the range limit of the adjustment pot R742. The 2275 parts list states R742 is a 1K pot but the unit installed was a 2K pot. Installing a 1K put only makes matters worse bias-wise. I am assuming somewhere along the way there were engineering changes made to the P700 board that account for the change in the R742 value.

What I did to allow the bias to be set properly was replace the jumper R743 with a 1.8K resistor using the 1K R742 bias pot I had installed per the parts list. With this combination, I was able to dial in the bias to 10MV per the service manual. A more elegant approach I guess would be to use a 3K pot for R742 and leave the R743 jumper in place .In another thread I read that using KSC2073TU and KSA940TU for H713/H714 instead of MJE15032G and MJE15033G that I used would also solve this issue. I also observed that the bias transistor H709 is installed under H713 on the P700's I have worked on VS. the location shown in the service manual and on the P700 I am currently working on.

Another variation is that on some P700 boards H710 is tucked under the H713 heatsink while on others it is just sticking up from the board. Not sure which is the proper location for it and why there is a variation in the position. H710 is in series with the bias pot R742 so it's operating temperature directly affects the bias setting. Tucking it under the H713 heat sink next to the bias transistor H709 makes sense to me but wondering why the location varies. Another thread recommends replacing H710 with two 1N4148 diodes in series, which would eliminate the thermal compensation that the original thermistor provided.

The service manual does not indicate if you should allow the 2275 to warm up before setting the bias, which is interesting in that operating temperature will clearly affect the bias setting with the original H710 installed. A lot of variations to consider, hoping others have suggestions on the best course of action to take with refurbishing the 2275 P700 board.

Sorry for being a bit long-winded, I will try to summarize:

1. What is the best approach to positioning/replacing H710?

2. When replacing the Bias pot what is the best value to use?

3. What transistor pair to use for H713/H714?

4. Any other suggestions…….

Thanks everyone for listening!

Rich

 

[Dingman]

Crickets.
I can't answer your questions, I can only say I've done similar mods (replacing resistor with a pot) to get bias where it needed to be.

 

[richiem46]

I did some further digging and here is what I came up with. I am sure it is not definitive but given the variations on the 2275 P700 it is a loosely-planted stake in the ground.

I found two variations of the 2275 P700 board schematic, one with H719 installed and one without it.

The P700 Board schematic with H719 is shown below. Jumper R743 and H710 are not in series but R743 provides direct connection between the Collector of H711 and the Base of H713. My suggestion to replace jumper R743 with a fixed resistor is not appropriate in this configuration.




The 2275 P700 Boards I have seems to match the schematic below, no H719 present and R743 jumper in series with H710. The connection between the Collector of H711 and the Base of H713 is direct, not passing through jumper R743 so in this case replacing R743 with a fixed resistor to get downward bias adjustment range is ok. I looked for 3K bias pots on Mouser but could not find one with the proper adjustment/lead orientation combination so I am going to leave in the 2K factory bias pot after a thorough cleaning and replace jumper R743 with a 1K resistor.




The H710 question is a bit confusing. Several threads and parts lists state it should be replaced with a 1N4148 diode. Other threads state it should be two 1N4148 diodes in series, or two 1N4148 diodes in parallel depending on which thread you reference. With the P700 board I am working on I am going to replace H710 with one 1N4148 diode along with the 1K resistor substitute for R743 and use the original 2K bias pot R742. Hopefully the combination will provide the bias range required to set 10MV. It will make my question about warm-up time and the positioning of H710 moot.

Update to my summary questions assuming P700 board in question matched the secong schematic without H719:

1. What is the best approach to positioning/replacing H710?

Use a single 1N4148 diode to replace H710.

2. When replacing the Bias pot what is the best value to use?

A 3K put would be ideal but I used a 2K pot with jumper R743 replaced with a 1K resistor.

3. What transistor pair to use for H713/H714?

I used MJE15032G/MJE15033G as recommended by several other folks and I already have them installed. No doubt using other substitutes will affect some of the other component choices I mentioned.

4. Misc suggestions:

Replacing H710 makes my positioning and warm-up questions moot.

Please feel free to comment on my suggestions.

Rich

 

[keitht]

I did the p700 boards on an 1150. My understanding is the 2275 and 1150
share a design. I used the kit sold by love, peace and music. I don't change the
Outputs. Had no issues at all with adjustments. Maybe I was just lucky.
But I was pretty pleased with the result. The only real issues I had were PS
related. A case of not following through. I knew it needed to be done,
but just didn't.

 

[goldswimmerb]

I read a number of threads about bias issues with the P700 amp boards in the Marantz 2275 and in other Marantz units. A lot of great information freely shared by other members. I came away not completely clear on the best approach to restoring the P700 board and thought I would start another thread on the subject.

I just restored a 2275 P700 board and replaced all the transistors and some of the diodes on the board including the output transistors. I replaced H713 and H714 with MJE15032G and MJE15033G transistors and I could only bias the board down to 20MV at the range limit of the adjustment pot R742. The 2275 parts list states R742 is a 1K pot but the unit installed was a 2K pot. Installing a 1K put only makes matters worse bias-wise. I am assuming somewhere along the way there were engineering changes made to the P700 board that account for the change in the R742 value.

What I did to allow the bias to be set properly was replace the jumper R743 with a 1.8K resistor using the 1K R742 bias pot I had installed per the parts list. With this combination, I was able to dial in the bias to 10MV per the service manual. A more elegant approach I guess would be to use a 3K pot for R742 and leave the R743 jumper in place .In another thread I read that using KSC2073TU and KSA940TU for H713/H714 instead of MJE15032G and MJE15033G that I used would also solve this issue. I also observed that the bias transistor H709 is installed under H713 on the P700's I have worked on VS. the location shown in the service manual and on the P700 I am currently working on.

Another variation is that on some P700 boards H710 is tucked under the H713 heatsink while on others it is just sticking up from the board. Not sure which is the proper location for it and why there is a variation in the position. H710 is in series with the bias pot R742 so it's operating temperature directly affects the bias setting. Tucking it under the H713 heat sink next to the bias transistor H709 makes sense to me but wondering why the location varies. Another thread recommends replacing H710 with two 1N4148 diodes in series, which would eliminate the thermal compensation that the original thermistor provided.

The service manual does not indicate if you should allow the 2275 to warm up before setting the bias, which is interesting in that operating temperature will clearly affect the bias setting with the original H710 installed. A lot of variations to consider, hoping others have suggestions on the best course of action to take with refurbishing the 2275 P700 board.

Sorry for being a bit long-winded, I will try to summarize:

1. What is the best approach to positioning/replacing H710?

2. When replacing the Bias pot what is the best value to use?

3. What transistor pair to use for H713/H714?

4. Any other suggestions…….

Thanks everyone for listening!

Rich

The VBE of H709 is most important for proper biasing, something like a KSC3503 should do nicely as it tends to have a higher VBE (I believe MJ182 or similar may work as well, but it has been a while). For proper tracking I have seen people move H709 to rest under the driver transistor heat sinks, however, the factory location has never given me any major troubles. I usually let units warm up for 5-10 minutes before I start adjusting anything.

If i recall the 2275 uses a near identical bias section to the 2270.

 

[richiem46]

I did the p700 boards on an 1150. My understanding is the 2275 and 1150
share a design. I used the kit sold by love, peace and music. I don't change the
Outputs. Had no issues at all with adjustments. Maybe I was just lucky.
But I was pretty pleased with the result. The only real issues I had were PS
related. A case of not following through. I knew it needed to be done,
but just didn't.

I used kits from peace, love and music also, pleased with parts and support. I had no bias issues with the basic kit for on 4 previous P700 boards. When I used the complete kit which replaces all the transistors except the outputs is when the bias issue was present. The kit uses a MJE243G transistor to replace H709, the bias transistor. Other posts indicate that using different transistors for H713/H714 also affect the bias. It would be interesting to try the different combinations to determine the effects they have on the bias setting.

Later in the day I will test the HFE on the transistors I removed VS. the replacements and report what I measure. That is more appealing to me rather than disassembling the P700 boards again! It might provide some useful information.

The good news is that the 2275 with the upgraded P700 boards plays beautifully.

 

[richiem46]

Here are the HFE measurements I took on the old and new Transistors from the P700 boards. I had spare replacements which I measured in place of the newly installed parts rather than removing the ones actually installed.

Old Bias Transistors H709 HFE values
2SC496 HFE=69 HFE=114

New Bias Transistor H709 HFE Values
MJE243G HFE=116 HFE=131

Old Driver Transistors H713/H714 HFE Values
2SC382 HFE=86 HFE=69
2SB537 HFE=118 HFE=110

New Driver Transistors H713/H714 Values
MJE15032G HFE=85 HFE=91
MJE15033G HFE=182 HFE=183

The only observation I can make is that the new transistor HFE values are more consistent than the old ones I removed and they tend to be higher. I am not sure what bearing this has on the Bias setting but it surely must.

I also tested eight pairs of the H701/H702 transistors that I removed from P700 boards out of curiosity. These are suppose to be matched and are heat-shrinked together so they thermally track. I did not take note of the HFE values but what I observed is that the old transistors were no longer closely matched, in most cases significantly. Replacing them with new matched transistors is definitely indicated as most threads and upgrade "kits" recommend.

 

[rBuckner]

On the bias transistors the more important characteristic is the Vbe. That is the voltage from base to emitter in operation. The lower the better in my experience.

The diff pair were never factory matched; else they'd have a paint dot on them. Well, they should have!

 

[goldswimmerb]

On the bias transistors the more important characteristic is the Vbe. That is the voltage from base to emitter in operation. The lower the better in my experience.

The diff pair were never factory matched; else they'd have a paint dot on them. Well, they should have!

I've actually found the opposite, higher VBE equates to a wider bias adjustment range. I've had many where KSC2690 wouldn't work at all and KSC3503 worked perfectly.

 

[patfont]

I am pretty sure I use 2.2K pots, and have not had a problem dialing in.

 

[rBuckner]

Just had a 2270 that had different outputs from side to side and bias wouldn't go below 25mV. I changed the jumpers to 5.6 ohms and it adjusted fine.