Diagnostic help request for MQ-101

[Refugee]

I have an MQ-101 that has some problems.
Symptoms:
Right channel is functioning properly.
Left channel has no bass boost. As far as I can tell it is passing the signal fine and the mid and hi controls are working. When the L bass control is adjust from or to setting one(flat) there is a slight thump in the left channel only. There is no boosting of the low frequencies at any of the settings.

What I have already checked:
It is not the interconnect cable and it does follow the channel when I swap things around so it is not the amp or the pre - it is defiantly the left channel on the 101.
I have cleaned the switches/controls on the 101 and verified them with a multimeter in the ohm setting including the 'input attenuator' switch on the rear of the unit.
I have checked the muting Relay (K1) on the power board and it seems to be working fine on both channels
I have noticed that (unlike the working channel) when I engage the bass boost on the left channel (any position except pos1)a reading of .07 VDC jumps to .36 VDC. This voltage change occurs at several locations in the circuit and does not happen in the working channel. I will include a picture of the schematic highlighted where this voltage rise occurs.
I have checked the voltages on the transistors on both boards. They are as follows: The odd # transistors on the top row are from the bad board (L). The even numbered transistors on the bottom row are their counterparts on the working board (R).

Q1 = 41.8V, 44.5V, 46.42V	Q3 = 75.5V, 46.49V, 45.86V	Q5 = 1.36V, 10.57V, 11.47V	Q7 = 9.78V, 1.36V, .80V	Q9 = 28.6V, .798V, .17V
Q2 = 41.8V, 44.3V, 46.19V	Q4 = 75.5V, 46.14V, 45.5V	Q6 = 1.36V, 10.57V, 11.47V	Q8 = 9.79V, 1.36V, .82V	Q10= 27.3V, .818V, .19V

These readings were taken as described on the schematic. I did not notice any change when I engaged the L bass control above setting one 1.
I have also taken voltages from the power supply board and have attached a picture of the readings.
So what do you experts say?

Other questions:
There are two tantalum polarized electrolytic caps on each (L+R) board in this unit ( C7,8 and C29,30). Did they use tantalum for a good reason or should I replace with audio grade aluminum electrolytic caps ( one is in the signal path and one goes to ground)? I do plan on replacing the caps on the power board. I might keep the Mylar caps until I get the unit working properly and then decide if they need changing, or just change them while I am in there.
Any suggestions on best practices with maintenance/recap of this unit is appreciated. I am itching to hear my ML-1C speakers as designed for the first time!
Thanks!!

 

[Refugee]

Here is a better copy of the service manual with the schematic all on one page.

 

[w3jn]

C7 and/or C9 and/or C23 is leaky, putting DC on the feedback networks. The reason you get the thump and jump in voltage is that network is all DC grounded by the bass boost switch in its first position (see circled area below). Then when you go to position 2 it's no longer grounded and the voltage - which is coming from one of the afore-mentioned leaky caps - rises. That could be the reason for your lack of bass boost. However I'd also look at C29.

I wouldn't replace tantalums with electrolytics in this case, as you want very low DC leakage or it's gonna mess up the gain in this whole feedback network.

 

[Refugee]

Thank you for the quick reply!
You pointed to both tantalum caps as possible causes. I will order the same type for replacements.
As for the mylar caps - do you suggest sticking with mylar replacements or ... ??

I will order replacements for the suspects you pointed out and see if that fixes the problem and follow up with the results. I will also change the electrolytic caps in the PS as due diligence.

 

[w3jn]

If you find a bad Mylar, replace it with a Mylar. You want the same value - this isn’t a place to fudge values. What is critical here is the Q of the filter. If you have a leaky cap, or one with high ESR the Q will be lower and it’s not gonna work right. I had a leaky cap in the 30 hz eq circuit on a C31V recently that messed up the 30 hz boost on one channel.

The power supply caps are another story. Nothing too critical there.

 

[Refugee]

I tend to refurbish old equipment, not update. That being said, I am stressing about the tantalum capacitors that are in the circuit. There is a lot on this, and other sites, about the 'evils' of tantalum capacitors in audio circuits so I may swap them for aluminum or film later, but only after I have the unit fully functional and can benchmark the original configuration. This seems to be the only way to test for oneself to see if updates are really improvements.
I have decided that I will start by replacing them with tantalum even though one is right in the signal path. When ordering these caps I notice some categories that I don't understand and would like some advise about. I am also having trouble finding tantalum caps under 10% tolerance.
The caps in question when looking at the schematic in the above post are C7,8 and C29,30. Which type should I use in these positions in the circuit?

See snip from Mouser page below:

 

[w3jn]

I wouldn't stress too much about this. C7/C8 is the only tant in the signal path. C29,30 are part of the low freq gyrator R/C network. C7/C8 will have some effect on the HF boost/cut, but very little in comparison to the other caps in that circuit. It should be close in value to .47 but a 10% should be fine. Better than the tolerance on an electrolytic, which is generally -20/+100. I think most tants are 10 or 20%.

If you have a cap tester, you can buy a batch then match 'em.... but again I wouldn't worry about this too much.

 

[Refugee]

Thanks. I went with 5% .47 and 3.3 that I found in the 'solid leaded' section (so standard not polymer) on Mouser.

https://www.mouser.com/ProductDetail/80-T353A474J35AT-TR

https://www.mouser.com/ProductDetail/74-199D335X5035B6B1

and also 2) 200mf@160V Chemi-con caps to replace the multi-can, and 2) 220mf@25VDC Nichicon UKZ1E221MHM for the PS board.
Digi-key had Mylar replacements for all the originals that will fit well.
When I get the parts in I will change the suspected caps one at a time and post which one failed. Then I will pull the boards and shotgun the rest of the caps in and let you guys know how it comes out.

 

[Refugee]

Below are the parts I got for the repair/refurb/recap.
I'm sure there are some that would second guess the decision not to upgrade the polyester (Mylar) caps to Polypropylene and to replace the tantalum with new tantalum instead of changing to aluminum electrolytic. I was very tempted to upgrade, but I realized that I really just want to preserve the unit and get it up to the original spec as far as the filter frequencies and curves - not change it. The only upgrade is in the tolerance of the caps being 5%, the type and values remain the same.

 

[w3jn]

No reason to go such high voltage.... won't hurt anything but it's a 75V circuit at most. Perhaps the lead spacing is the factor here?

 

[Refugee]

All the original Mylar caps were 250V. The couple of 400V caps I ordered were due to fit and availability.
Strange…I did not even question the 250V choice by McIntosh. Maybe they chose the higher voltage for the same reasons I ended up with a few 400V - availability, lead spacing. Were there many options for Mylar caps in the early 70’s?

In a case like this, would you go down in voltage if you could get the type and fit you needed?

I have it so ingrained in my thought patterns that you can only match or exceed voltages for capacitors. The thought of using a lower voltage than the existing caps never occurred to me.
Seems like lower voltage might have some advantages like price, and lower ESR.

 

[w3jn]

Yeah, I'd go with a bit lower voltage if the fit was better but it never hurts to go up in voltage. Your general strategy is 100% correct - match original voltage or go up. In this case perhaps Mac used what they had lying around, or what they wanted wasn't available in lower voltages, or they figured the higher voltage caps would last longer? I dunno.

I don't think the ESR between mylar caps is going to make a lot of difference in this circuit. It's not THAT high a Q circuit - you want good quality, low D factor caps in the tone control elements, but "better" is many times the enemy of "good enough". There's minimal AC current flowing and a bit of ESR isn't gonna impact things much. I'd go with what you ordered, you're not constructing the detonator on a nuclear weapon here

 

[Refugee]

I'd go with what you ordered, you're not constructing the detonator on a nuclear weapon here

LOL. I can drive myself crazy looking for parts - that’s for sure! Especially capacitors, each one comes with 100 choices and 1000 opinions.
I will keep it in mind in the future though that maybe the original parts V is not 100% gospel.
There is more refurbishing going on here than design, if you know what I mean.
McIntosh engineers are not people I tend to question.

 

[Refugee]

So I rank ordered the caps that I thought were the culprits and started with the tantalum cap C7.
That was the one!
The unit seems to be working fine. I also changed the one on the working board (C8), to keep things balanced.
It will be a while before I change out the other caps as I will have to get the L and R boards out to do a good job of replacement. I have recently moved and have not set up a proper bench. I will post a thread on the full recap when the work won’t need to do be done on my kitchen table.
Contrary to some opinions, the MQ-101 makes a huge difference with the ML-1C speakers. I have been using the bass setting on the 101 at position 2 for modern rock and up to setting 3 for classical/acoustic music. The eq is very different than the bass control on the C-26 pre amp. I was running the pre with some loudness dialed in before I hooked up the eq, even when at louder volumes, but with the eq I turn off the loudness at these volume levels. I like the mid and hi controls in the flat position (3).

Thanks @w3jn for the help and moral support. LOL

Caps changed:



In the rack:

 

[Refugee]

I always hear about tantalum caps blowing up. Now I know that they can get leaky like any other coupling cap.

 

[w3jn]

Woohoo! What a beautiful setup. Glad you got it working!

I'm not a fan of tantalums. Replaced way to many and repaired too much damage caused by them shorting in professional shortwave receivers, where they were quite popular.

 

[trister]

Refugee, did you ever replace out the rest of the caps? How did it go?

 

[Refugee]

@trister
I did not. I ordered the caps that I listed but as the unit is working well I have not replaced the caps with the new parts; only the pair of tantulum caps that were causing the problem.