Just picked up a used DH200 for almost nothing. Have a ST70 that needs new tubes so thought I’d stay in the family and try a classic SS.
After checking fuses, DC offset and bias I hooked her up and except for less mid-bass than my tired ST70, it really sounded pretty good.
Gonna recap, improve the input line to the boards, etc. Most of what I have read here about smart mods and tuneups.
Examining it more closely I’m finding a few anomalies that perhaps someone here can help me with.
The schematic shows a 2.2 ohm resistor on the left input to ground. First, why? And second, why only on the left? Mine is on the right, I’m guessing this was kit built and the assembler mixed up his right and left. Should I move it to the prescribed left input? I’m going to replace the input and speaker terminals with gold plated units so it won’t be much trouble. But I’m confused by this apparent circuit imbalance in a unit often referred to as two identical mono amps.
Lastly, on the bench I can adjust each bias to 275ma but it moves around quite a bit. I have the inputs shorted. It stays in the 275 vicinity but wavers between about 240 and 290, both sides. Will recapping influence that or shall I look for something else in particular? The pots don’t seem noisy and bias changes smoothly with pot adjustments.
Thanks in advance for any advice.
Edit: The schematic shows the 2.2 ohm resistor (R45) on the left input, but the assembly instructions call for it to be installed on the right input.
“Select the 2.2 ohm (red-red-gold) resistor. Cut each lead to 1 1/2" or less. Connect one end to the separate solder lug installed under input socket RS. (S). Connect the other end to the short (ground) lug #2 of socket RS.”
Jeff
After checking fuses, DC offset and bias I hooked her up and except for less mid-bass than my tired ST70, it really sounded pretty good.
Gonna recap, improve the input line to the boards, etc. Most of what I have read here about smart mods and tuneups.
Examining it more closely I’m finding a few anomalies that perhaps someone here can help me with.
The schematic shows a 2.2 ohm resistor on the left input to ground. First, why? And second, why only on the left? Mine is on the right, I’m guessing this was kit built and the assembler mixed up his right and left. Should I move it to the prescribed left input? I’m going to replace the input and speaker terminals with gold plated units so it won’t be much trouble. But I’m confused by this apparent circuit imbalance in a unit often referred to as two identical mono amps.
Lastly, on the bench I can adjust each bias to 275ma but it moves around quite a bit. I have the inputs shorted. It stays in the 275 vicinity but wavers between about 240 and 290, both sides. Will recapping influence that or shall I look for something else in particular? The pots don’t seem noisy and bias changes smoothly with pot adjustments.
Thanks in advance for any advice.
Edit: The schematic shows the 2.2 ohm resistor (R45) on the left input, but the assembly instructions call for it to be installed on the right input.
“Select the 2.2 ohm (red-red-gold) resistor. Cut each lead to 1 1/2" or less. Connect one end to the separate solder lug installed under input socket RS. (S). Connect the other end to the short (ground) lug #2 of socket RS.”
Jeff
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The resistor is there so that the chassis does not float, thus its only needed on one channel, does not matter which one.
The bias is okay to drift around the little bit that you show. I do not think re-capping will make any difference on the bias
I do not think it necessary to change the input jacks as long as they make a solid conenctions. You'll know if the connections are compromised, channel loss or imbalance, hum etc.
The bias is okay to drift around the little bit that you show. I do not think re-capping will make any difference on the bias
I do not think it necessary to change the input jacks as long as they make a solid conenctions. You'll know if the connections are compromised, channel loss or imbalance, hum etc.
Thank you, Rick.
I have read almost completely through the 118 page "Hafler DH200/220 Mods" thread.
Perhaps I should have posted this there, but when I saw the date range I thought it might be closed.
I think I get the grounding issue now, and will attend to that. I'm not sure I understand the reduction of impedance by increasing the guage of the ground bar between the filter caps. I plan on installing the inrush limiter. New 5550s and 5401s are on their way, as well as matched MOSFETs, if I discover the necessity to replace. There is a bit of DC offset: -24 mv on the left and +30mv on the right. I know that isn't a lot but over 50mv differential, I'd really like to see 0.
Any other mods I note in that thread that doesn't involve topology changes are on my list. My objective is reliability and safety for my (priceless to me) Altec VOTTs. The mids and highs are terrific compared to my ST70.
Any idea where to look regarding the lack of bass?
Thank you for any other advice offered.
Jeff
I have read almost completely through the 118 page "Hafler DH200/220 Mods" thread.
Perhaps I should have posted this there, but when I saw the date range I thought it might be closed.
I think I get the grounding issue now, and will attend to that. I'm not sure I understand the reduction of impedance by increasing the guage of the ground bar between the filter caps. I plan on installing the inrush limiter. New 5550s and 5401s are on their way, as well as matched MOSFETs, if I discover the necessity to replace. There is a bit of DC offset: -24 mv on the left and +30mv on the right. I know that isn't a lot but over 50mv differential, I'd really like to see 0.
Any other mods I note in that thread that doesn't involve topology changes are on my list. My objective is reliability and safety for my (priceless to me) Altec VOTTs. The mids and highs are terrific compared to my ST70.
Any idea where to look regarding the lack of bass?
Thank you for any other advice offered.
Jeff